Developing Scale Inhibitor Dosage Models

Robert J. Ferguson
French Creek Software, Inc.
Kimberton and Hares Hill Road, Box 684
Kimberton, Pennsylvania U.S.A.

Presented at WaterTech '92, Houston, Texas
NACE EUROPE '93, Sandefjord, Norway

Published in Industrial Water Treatment Magazine

Click here to get more.

Abstract
Feeding the minimum effective inhibitor dosage can reduce operating costs for chemical treatment, minimize treatment chemical discharge to the environment, and in some cases, prevent under­feed of a scale inhibitor. Common sense indicates that the same scale inhibitor dosage is not required for all waters and systems. One size does not fit all. Water treatment companies have capitalized on this general concept since the introduction of first computerized water chemistry evaluation and treatment recommendation systems in the 70's.(1,2,3) Dosage models have been developed by the industry for scale control in applications ranging from long residence time open recirculating cooling tower systems, to the ultra low treatment levels required in very short residence time once through utility surface condenser cooling systems.

This paper discusses the parameters critical to developing an effective dosage modulation model for scale inhibitors from laboratory data, field data, or a combination of both. The paper draws upon the concept of induction time as a basis for the mathematical models used to develop predictive models from actual data. The models are based upon the concept that threshold effect inhibitors do not prevent scale formation, they only delay the inevitable. The models are in agreement with current theories and treat scale inhibitors as agents which extend the induction time before crystal formation and/or growth on existing active sites occurs in the case of calcium carbonate, and as dispersants which control particle size in the case of calcium phosphate.

The models predict the dosage required to inhibit deposition until the treated water has passed through a cooling system. This delay can vary from 3 to 15 seconds in a large volume once through condenser cooling system, to days in open recirculating cooling systems.

Thermodynamic driving forces and system operating conditions are used by the models to describe the kinetics of scale formation, growth, and the impact of inhibitors upon induction time.

Similar models to those discussed have been used successfully to optimize scale inhibitor treatments in once through utility cooling systems and open recirculating cooling systems since the late 70's.

Introduction
The models described in this paper were developed from a combination of field observations, common sense, and laboratory data. Model development began in the early 's. At this time, utilities using man­made impounded lakes as a source for condenser cooling water were beginning to develop condenser scale problems. The lakes were originally filled with water of a low scale potential. The heat load from condenser cooling, and the lack of blowdown, caused the lakes to concentrate with time. Calcium carbonate scale became an economically significant problem after several years of operation. Deration was encountered due to high condenser back pressure in addition to the heat rate penalty associated with increased back pressure due to condenser deposits. Treatment with scale control agents would have cost more than the problem if treatment were implemented using the once through cooling technologies of the time. New technology was needed if the treatment of these systems for scale control was to be economically feasible. Dosage optimization studies were conducted to determine if ultra low dosages, on the close order of 0.001 to 0.2 mg/l active, could effectively prevent calcium carbonate scale.

The initial studies were conducted using well instrumented test heat exchangers. Tests were run in parallel. One exchanger was treated at a high level to assure that scale would be controlled. The parallel exchanger remained untreated. Fouling factors were monitored continuously on the exchangers. Fouling could be measured a day or two prior to the formation of a visible deposit.

The time required for measurable deposit formation was noted. This time was used to determine the minimum time between dosage reductions during the dosage optimization studies. Dosage reductions were made after a minimum of two of these time periods had elapsed.

It was observed that ultra low dosages were effective in preventing scale formation in the short residence time condensers. The cooling water was present in the condensers for less than 10 seconds in all of the systems evaluated. Models were developed based upon the initial studies at seven (7) locations. The once through cooling system data was later expanded to longer residence time open recirculating (cooling tower) systems. Laboratory studies filled data gaps.

During the evaluation of the data it was found that several parameters were critical to dosage: time, temperature, and the degree of supersaturation. System cleanliness was also found to be important. This paper discusses the models and their practical application to cooling water scale control. The method outlined in this paper has been used to develop models of minimum effective inhibitor dosages from laboratory data, field data, and combinations of both. The models provide a natural path for bringing research data into the practical arena of the operating engineer or water chemist.

The use of the models is described in case history format, after a brief description of the impact of individual parameters upon the minimum effective scale inhibitor dosages.

Induction Time: The Key To The Models
Reactions do not occur instantaneously. A time delay occurs once all of the reactants have been added together. They must come together in the reaction media to allow the reaction to happen. The time required before a reaction begins is termed the induction time.

Thermodynamic evaluations of a cooling water scale potential predict what will happen if a water is allowed to sit undisturbed under the same conditions for an infinite period of time. Even simplified indices of scale potential such as the Langelier saturation index can be interpreted in terms of the kinetics of scale formation. For example, calcium carbonate scale formation would not be expected in an operating system when the Langelier saturation index for the system where 0.1 to 0.2 . The driving force for scale formation is too low for scale formation to occur in finite, practical cooling system residence times. Scale would be expected if the same system operated with a Langelier saturation index of 2.8 . The driving force for scale formation in this case is high enough, and induction time short enough, to allow scale formation in even the longest holding time index cooling systems.

Induction time has been modeled for economically important crystals such as sucrose. Models follow a formula similar to equation
  _________________________________________1
EQUATION 1_______ Induction Time = _____________________
________________________________k [Saturation Level ­ 1]P­1

where

Induction Time is the time before crystal formation and growth occurs;
k is a temperature dependent constant;
Saturation Level is the degree of super­saturation;
P is the critical number of molecules in a cluster prior to phase change.

Gill and his associates demonstrated that commercially available scale inhibitors extend the induction time for calcium carbonate scale(4). Their paper points out several critical parameters which impact the induction time prior to crystal growth:

The degree of supersaturation.
The temperature.
The presence of active sites upon which growth can occur.
The inhibitor level.

Gill's study used saturation level as the thermodynamic driving force for scale growth. Saturation level calculations performed using a computerized ion pairing method eliminate most of the assumptions inherent in simplified indices(5). They account for common ion effects which can increase the apparent solubility of a scale forming specie such as calcium carbonate. Driving forces for scale formation calculated using the ion pairing method are transportable between systems because they base their calculations upon free ion concentrations rather than the total analytical values. This is the heart of the ion pairing, or ion association method, which subtracts ion pairs (e.g. CaSO4, CaHCO3­) from the total analytical value to estimate the free ion present and available to react in forming seed crystals, or in driving growth on existing substrates.

The remainder of this paper uses ion association model saturation levels for the driving force for scale formation. Table 1 provides a working definition of the term saturation level for calcium carbonate and tricalcium phosphate.

Critical Parameters
The parameters contributing to equation 1 are included in the basic relationships used for inhibitor dosage modeling. Major data values required include the time period during which scale formation must be prevented, the degree of supersaturation which is the driving force which must be overcome, the temperature at which the inhibitor must function, and the pH of the cooling water. The surface area of active sites also impacts the dosage requirement.

These parameters have the following impacts upon dosage:

Time ­ The time selected is the residence time the inhibited water will be in the cooling system. The inhibitor must prevent scale formation or growth until the water has passed through the system and been discharged. Figure 1 profiles the impact of induction time upon dosage with all other parameters held constant.

Degree of Supersaturation ­ An ion association model saturation level is the driving force for the model outlined in this paper, although other, similar driving forces have been used. Calculation of driving force requires a complete water analysis, and the temperature at which the driving force should be calculated. Figure 2 profiles the impact of saturation level upon dosage, all other parameters being constant.

Temperature ­ Temperature affects the rate constant for the induction time relationship. As in any kinetic formula, the temperature has a great impact upon the collision frequency of the reactants. This temperature effect is independent of the effect of temperature upon saturation level calculations. Figure 3 profiles the impact of temperature upon dosage with other critical parameters held constant.

pH ­ pH affects the saturation level calculations, but it also may affect the dissociation state and stereochemistry of the inhibitors(8). Inhibitor effectiveness can be a function of pH due to its impact upon the charge and shape of an inhibitor molecule. This effect may not always be significant in the pH range of interest (e.g. 6.5 to 9.5 for cooling water).

Active sites ­ It is easier to keep a clean system clean than it is to keep a dirty system from getting dirtier. This rule of thumb may well be related to the number of active sites for growth in a system. When active sites are available, scale forming species can skip the crystal formation stage and proceed directly to crystal growth.

Other factors can impact dosage such as suspended solids in the water. Suspended solids can act as sources of active sites, and can reduce the effective inhibitor concentration in a water by adsorption of the inhibitor. These other factors are not taken into account in the models in this paper. Table 2 summarizes the factors critical to dosage modeling, and their impact upon dosage.

Data Base
The dosage models used as examples in this paper were developed from data collected in field studies(6), laboratory studies, published data, or a combination of these sources.

Examples in this paper include data from sidestream evaluation of the minimum effective dosages in utility surface condensers.(6,7) In these studies, two parallel fouling probes were used to develop estimates of the minimum effective dosages for the phosphonates amino­tris­methylene phosphonic acid (AMP), 1,1­hydroxy ethylidene diphosphonic acid (HEDP), and polyacrylic acid (PAA). One probe was over­treated at a level where no calcium carbonate deposition would be anticipated. The parallel probe was not treated, and the time required for a measurable deposit to form determined. This was deemed the minimum period between dosage adjustments for the test. (Note: A minimum test duration of twice the time required for fouling was allowed to pass between dosage adjustments). Dosages were decreased until failure, as indicated by a measurable deposit formation.

Inhibitor dosages were then decreased to the minimum effective level on the condenser cooling systems to confirm that the dosages did indeed prevent scale. Condenser cleanliness was monitored by heat transfer. This work was done in the late 70's when sub­ppm treatment levels and ultra low dosages were just beginning to be used in utility once through cooling system scale control programs.

A dosage model is only as good as the data from which it is derived. The most generally applicable models include data points over the anticipated ranges for critical parameters. For example, a model developed using data in the temperature range of 30 to 40 ºC might be totally useless in predicting a dosage for a system operating at 70 ºC.

Models should be derived from data over the range of water chemistry anticipated as well as over the range of saturation level anticipated. If a calcium carbonate scale inhibitor model will be used in waters ranging from a calcium level of 40 ppm to over ppm, this range should be covered from laboratory and/or field sources. The saturation level range anticipated should also be bracketed (e.g. 1.0 to 250 saturation level for calcite).

Although field data is the source of choice, field conditions can rarely be adjusted to cover the temperature, pH, time, and water chemistry ranges desired. The use of static laboratory tests designed to elucidate the variation of dosage with any of the parameters can be used to supplement field data. Field data, although desirable, is not always necessary for the development of a preliminary correlation. As demonstrated in the calcium phosphate deposit control example, dosages predicted by laboratory tests can be directly applicable to field conditions. Each model developed should be compared to field results to assure that a correlation exists between the test data, the model, and actual field results.

Development Of A Model
A modified version of Equation 1 provided the basis for model correlation. Dosage was added as a factor to the equation on the right side to produce Equation 2.

____________________________________________________DosageM
Equation 2 ______________________Induction Time = ________________________
______________________________________________k'[Saturation level ­ 1]P­1


The temperature dependent rate constant k' was found to correlate with the Arrhenius relationship (Equation 3).


Equation 3 ______________________k' = A e­Ea/RT


Saturation levels were calculated from water analysis input using a computerized ion association model. The time used for the correlation is the time to failure in laboratory tests, the residence time in a heated state for utility once through cooling systems, and the holding time index in open recirculating cooling systems.

Equation 2 was rearranged to solve for dosage in the first order. Regression analysis was used to estimate the coefficients.

Field Correlation
The test of any model is its applicability to operating systems. Two examples are presented in this paper as an indication of the value of dosage models in suggesting an initial inhibitor treatment level.

Example 1: CaCO3 Scale Control in Utility Once Through Condenser Cooling Systems
In the late 70's the efficacy of ultra­low scale inhibitor dosages was demonstrated in systems serviced by man­made impounded lakes. These system typically started up with a low to moderate hardness water of low to borderline scale potential. The lakes concentrated with time to create a very scaling condition. In many cases, acid cleaning was required to prevent condenser related capability loss, in the absence of treatment.

The minimum effective dosages for these systems ranged from 0.01 to 0.2 ppm active phosphonate, depending upon the water chemistry, temperature, and residence time during which scale deposition or growth had to be prevented. The efficacy of these low level treatments was demonstrated in many of the midwest and south central United States central station power plants where condenser cooling water was supplied by man­made impounded lakes(6,7), and continues to be demonstrated and optimized using on­line real time control(2,3). Real time optimization is an economic necessity in many of these lakes due to the high changes in pH encountered over even a twenty four (24) hour period. pH fluctuations of 1.2 pH units have been reported. As depicted in figure 4, this equates to a ten fold change in dosage requirement in a single day.

Table 3 summarizes the water chemistry, scale potential indices, and dosage recommendation for 100% active HEDP for a single analysis and set of operating parameters for a typical utility once through cooling system as outlined in one of the initial dosage minimization studies(9). The treatment level recommended is comparable to that found effective in the original published study.

It is of interest to note that the model which recommended an accurate treatment level for a short residence time utility once through system also recommends a reasonable treatment level for an open recirculating cooling system with a residence time which can be calculated in days. The model used for this comparison has been found to provide reasonable treatment recommendations for both short and long residence time cooling systems. Figure 5 profiles a typical system.

Example 2: Calcium phosphate Deposition Control
Calcium phosphate inhibitors have been successfully modeled by the method described in the paper. The same basic formula was used for modelling calcium phosphate deposition as was used for the calcium carbonate inhibitors.

Figure 6 indicates the preliminary correlation for a copolymer in common use as a calcium phosphate inhibitor. The model was applied to an open recirculating cooling system water chemistry to determine if the dosages recommended were comparable to those effective in operating systems. The treatment program had undergone extensive dosage optimization to determine the most appropriate treatment level. The recirculating water chemistry, operating parameters, and dosages for the system are outlined in Table 4. The system typically operates at approximately six (6) cycles of concentration.

The model predicted the final dosage within approximately ten (10) percent of the final optimized value. Use of the model would have provided a reasonable initial treatment level for the on­site optimization studies. An initial dosage three times the optimized level was used as a starting point in the actual study.

It is interesting to note that the coefficients calculated for the phosphonate calcium carbonate inhibitors HEDP and AMP were of a comparable order, indicating the same inhibition mechanism. The order for the calcium phosphate scale inhibition by a copolymer is different, indicating a different scale inhibition mechanism.

Summary
Laboratory and field dosage optimization data can be converted to a mathematical model using standard statistical methods and a relationship derived from theoretical models for induction time. The models provide a practical method for collating laboratory and field data for a scale inhibitor. The correlations developed can then be used to predict the dosage for cooling systems based upon water chemistry and operating parameters without the necessity for laboratory or in­depth field studies to determine the minimum effective dosage. Dosages predicted by models developed in this manner are typically accurate as long as the system parameters and water chemistry data are within the range of the data used to develop the models. The examples presented in this paper are by necessity limited. The basic models described in this paper have been used successfully in systems ranging from short residence time, low scale potential systems, to high residence time, high scale potential systems for calcium carbonate control. The phosphate models have been used extensively as an integral portion of treatment recommendation systems for multi­functional, alkaline phosphate corrosion and scale control programs.

As with any predictive method, dosage recommendations from such models should be evaluated by an experienced water treatment chemist prior to implementation in an operational cooling system. Predicted dosages should be used as a guideline, not as an ultimate treatment recommendation due to factors which may not be taken into account by the models.

References

1.  C.J. Schell, "The Use of Computer Modeling in Calguard to Mathematically Simulate Cooling Water Systems and Retrieve Data," paper no. IWC­80­43 (Pittsburgh, PA: International Water Conference, 41rst Annual Meeting, ).

2.  R.J. Ferguson, O. Codina, W. Rule, R. Baebel, "Real Time Control of Scale Inhibitor Feed Rate," paper no. IWC­88­57 (Pittsburgh, PA: International Water Conference, 49th Annual Meeting, ).

3.  S.R. Payne, B.W. Perrigo, R.M. Post, T.P. Clay, "Application of a Self­calibrating, Microprocessor­driven Metering Device to a Utility Once Through Cooling System," paper no. IWC­90­46 (Pittsburgh, PA: International Water Conference, 51rst Annual Meeting, ).

4.  J.S. Gill, C.D. Anderson, R.G. Varsanik, "Mechanism of Scale Inhibition by Phosphonates," paper no. IWC­83­4 (Pittsburgh, PA: International Water Conference, 44th Annual Meeting, ).

5.  R.J. Ferguson, "Computerized Ion Association Model Profiles Complete Range of Cooling System Parameters," paper no. IWC­91­47 (Pittsburgh, PA: International Water Conference, 52nd Annual Meeting, ).

6.  R.J. Ferguson, "A Kinetic Model for Calcium Carbonate Deposition," CORROSION/84, Paper no. 120, (Houston, TX: National Association of Corrosion Engineers, ).

7.  R.J. Ferguson, "Practical Application of Condenser Performance Monitoring to Water Treatment Decision Making," paper no. IWC­81­25 (Pittsburgh, PA: International Water Conference, 42nd Annual Meeting, ).

8.  W.M. Hann, J. Natoli, "Acrylic Acid Polymers and Copolymers as Deposit Control Agents in Alkaline Cooling Water Systems," CORROSION/84, Paper no. 315, (Houston, TX: National Association of Corrosion Engineers, ).

9.  B.W. Ferguson, R.J. Ferguson, "Sidestream Evaluation of Fouling Factors in a Utility Surface Condenser," Journal of the Cooling Tower Institute,2, ():p. 31­39.

TABLE 1: MAJOR FACTORS INFLUENCING DOSAGE

FACTOR IMPACT Time Dosage increases with residence time.

Degree of Supersaturation 

Dosage increases with saturation level. Temperature Dosage increases with temperature due to its impact upon reaction rate.

This temperature impact is independent of any impact of temperature upon saturation level. pH Dosage may be pH dependent due to the impact of pH upon the inhibitor dissociation state and stereochemistry.

This pH impact is independent of any impact of pH upon saturation level. Suspended solids Dosage requirements may increase as suspended solids increase due to absorbtion of the inhibitor on the solids.
Active sites Dosage requirements increase if active sites for scale growth are present.

It is easier to keep a clean system clean than it is to keep a dirty system from getting dirtier.

TABLE 2: SATURATION LEVEL DEFINITION

Saturation level is the ratio of the Ion Activity
Product to the Solubility Product.

For calcium carbonate:

____________(Ca)(CO3)
______SL = _____________
______________Ksp'

For tricalcium phosphate:

___________(Ca)3(PO4)2
_____SL = _____________
_____________Ksp'

A water will tend to dissolve scale of the
compound if the saturation level is less than 1.0

A water is at equilibrium when the Saturation
Level is 1.0 . It will not tend to form or
dissolve scale.

A water will tend to form scale as the Saturation
Level increases above 1.0 .

Table 3: Utility Once Through Cooling System Example

Lake Water Analysis Deposition Potential Indicators Cations Saturation Level Calcium (as CaCO3) 120.0 Calcite (CaCO3) 3.94 Magnesium (as CaCO3) 34.0 Aragonite (CaCO3) 3.84 Sodium (as Na) 14.00 Silica (SiO2) 0.03 Potassium (as K) 0.00 Calcium phosphate (Ca3(PO4)2) 0.00 Iron (as Fe) 0.10 Anhydrite (CaSO4) 0.00 Ammonia (as NH3) 0.00 Gypsum (CaSO4 * 2H2O) 0.00 Aluminum (as Al) 0.00 Fluorite (CaF2) 0.00 Boron (as B) 0.00 Brucite (Mg(OH)2) 0.01 Anions Simple Indices Chloride (as Cl) 35.0 Langelier 1.07 Sulfate (as SO4) 13.0 Ryznar 6.27 "M" Alkalinity (as CaCO3) 120.0 Practical 6.64 "P" Alkalinity (as CaCO3) 0.0 Larson-Skold 0.39 Silica (as SiO2) 7.0 Treatment Recommendation Phosphate (as PO4) 0.0 10% HEDP (mg/L) 0.20 Fluoride (as F) 0.0 Parameters Nitrate (as NO3) 0.0 pH 8.40 Other Temperature (oC) 20.0 Calculated TDS 254 Residence Time (Seconds) 5.60

Open Access Article
This Open Access Article is licensed under a
Creative Commons Attribution 3.0 Unported Licence DOI: 10./D4RAK (Review Article) RSC Adv., , 14, -

Current and emerging trends of inorganic, organic and eco-friendly corrosion inhibitors†

Mahmoud A. Ahmed *ab, Sherif Amin b and Ashraf A. Mohamed *a
aChemistry Department, Faculty of Science, Ain Shams University, Cairo , Egypt. : ;
bVeolia Water Technologies, Cairo , Egypt

Received 5th August , Accepted 23rd September

First published on 8th October

Abstract

Effective corrosion control strategies are highly desired to reduce the fate of corrosion. One widely adopted approach is the use of corrosion inhibitors, which can significantly mitigate the detrimental effects of corrosion. This systematic review provides a thorough analysis of corrosion inhibitors, including both inorganic and organic compounds. It explores the inhibition mechanisms, highlighting the remarkable inhibitive efficiency of organic compounds attributed to the presence of heteroatoms and conjugated π-electron systems. The review presents case studies and investigations of corrosion inhibitors, shedding light on their performance and application potential. Moreover, it compares the efficacy, compatibility, and sustainability of emerging environmentally friendly corrosion inhibitors, including biopolymers from natural resources as promising candidates. The review also highlights the potential of synergistic impacts between mixed corrosion inhibitors, particularly organic/organic systems, as a viable and advantageous choice for applications in challenging processing environments. The evaluation of inhibitors is discussed, encompassing weight loss (WL) analysis, electrochemical analysis, surface analysis, and quantum mechanical calculations. The review also discusses the thermodynamics and isotherms related to corrosion inhibition, further improving the understanding of inhibitor's behavior and mechanisms. This review serves as a valuable resource for researchers, engineers, and practitioners involved in corrosion control, offering insights and future directions for effective and environmentally friendly corrosion inhibition strategies.

1. Introduction

Corrosion is a complex phenomenon that takes many forms, including uniform and localized corrosion, where atmospheric, galvanic, microbiological, pitting, crevice, erosion, intergranular, and stress-cracking corrosion types are most common.1 Corrosion causes irreversible degradation of materials, affecting not only the structural integrity of buildings, bridges, and infrastructures but also posing risks to human safety and life.2,3 In addition to the direct costs associated with replacing corroded parts and maintaining equipment, corrosion has indirect economic consequences.4 Corrosion-related equipment breakdowns reduce industry efficiency and cause lost productivity. Furthermore, corrosion's detrimental consequences on the environment should not be ignored. Corrosion-induced leaks and spills have the potential to pollute soil, water, and air, causing ecological disruption and posing health risks. When toxic substances from corroded materials leak into the surroundings, it can have lasting impacts on the ecosystem, necessitating extensive cleanup and mitigation activities. Because of the serious consequences of corrosion, governments and corporations all over the world are focusing their efforts on developing innovative corrosion avoidance and management approaches.5 While corrosion cannot be completely eradicated, several approaches can greatly reduce its incidences and consequences.6 These approaches often include changing the potential, surface coating, improved structural design, proper material selection, modifying the surrounding environment, and the use of corrosion inhibitors as potent protection measures, as shown in Fig. 1.7–9

Corrosion inhibitors are one of the popular approaches that have been thoroughly explored and are commonly employed for mitigating various types of uniform and localized corrosion. These inhibitors are chemical species that interact with the material's surface and/or change the characteristics of the surrounding environment to substantially boost a material's corrosion resistance in a particular environment. When applied in tiny concentrations, these inhibitors prevent or retard corrosion without substantially altering the concentration of other corrosive agents. An ideal corrosion inhibitor should be economical, simple to use, eco-friendly, and highly efficient at low dosages. Corrosion inhibitors found applications in various industrial, and pipe-line protection applications, including the oil and gas industry, cooling systems, potable water production, and the processing of metal surfaces before coating application, such as protecting reinforced concrete structures.10,11 Corrosion inhibitors can be classified according to their chemical composition (organic or inorganic), their oxidizing/non-oxidizing properties, or their application field (descaling, pickling, cooling water systems, acid cleaning, among others). Corrosion inhibitors stop or retard the anodic oxidation and/or cathodic reduction processes and may lead to the formation of a protective layer or film on the exposed metal surface. Thus, inhibitors may be classified as: anodic, cathodic, or mixed inhibitors. However, the most used classification scheme is that based on chemical composition. Organic inhibitors work mainly by adsorption mechanisms, whereas inorganic inhibitors typically work with electrochemical mechanisms. When it comes to performance, inorganic inhibitors have been long used since they work better over a wider temperature range and for longer periods. However, despite their relatively higher cost, organic inhibitors are thought to be safer.12–14 The performance of organic inhibitors relies on their composition, which typically includes various polar groups such as –OH, –OCH3, –COOH, –COOC2H5, –NH2, –CONH2, among others.14–16 These groups contain heteroatoms and non-bonding and π-electrons that enable extensive interactions.17,18 When these organic inhibitors contact a metallic surface, they can form adsorption layers or protective films that shield the metal from its corrosive environment and prevent its oxidation. Organic inhibitors can bind to metal surfaces by chemisorption and/or physisorption. The former involves coordinate-type bonding, e.g., sharing the inhibitor's π– and non-bonding electrons with the metal d-orbitals, whereas the latter involves physical electrostatic attraction between the inhibitor and the metal surface.13 Inorganic inhibitors such as chromates, molybdates, phosphates, nitrites, nitrates, borates, and silicates were commonly used to combat corrosion.19,20 These substances interact with metal surfaces with various mechanisms that prevent corrosion. Due to cost considerations, more feasible alternatives such surface active chelates, gluconates, polyacrylates, polyphosphates, carboxylates, and phosphonates gradually replaced many of the older inhibitors. Some of the latter substances act as precipitating inhibitors that typically produce precipitates at the metal–environment interface, while others can act as passivators, scavengers, corrosion poisons, or blockers.13,21,22 Moreover, some old inhibitors, e.g., chromates, have frequently raised environmental concerns due to detrimental impacts on soil and aquatic life.23,24 In response to these concerns, researchers are actively exploring environmentally friendly inhibitors as alternatives to traditional inhibitors. These eco-friendly alternatives offer numerous advantages, including ready availability of resources, non-toxicity, renewability, friendly synthesis processes, cost-effectiveness, and development of environmentally acceptable products.24 Some well-researched eco-friendly substitutes for harmful corrosion inhibitors include natural polymers, polysaccharides, amino acids and their derivatives, and Arabic gums.25,26 Green corrosion inhibitors may generally be divided into two primary groups: inorganic or organic green corrosion inhibitors. A typical example is biopolymers, which are naturally occurring compounds synthesized by cells of plants and animals, offering eco-friendly appropriate substitutes for a range of industrial uses. In contrast to synthetic polymers, they are biodegradable and do not accumulate within living organisms. Prominent examples of biopolymers encompass polypeptides, polysaccharides such as cellulose, starch, and chitosan, natural rubber, nucleic acids such as RNA and DNA, and lignin.27 The incorporation of heteroatoms within the complex structure of biopolymers confers enhanced adsorption capabilities and plays an integral role in corrosion inhibition.28,29 Consequently, extensive research has focused on exploring the anticorrosive features of biopolymers.29 Furthermore, corrosion inhibitors are generally added to materials such as plastics, paper, etc., for metal rust protection. The process of releasing corrosion inhibitors from materials to the metal surface will also have an impact on its corrosion inhibition.30 Fig. 2 shows comparison between various types of inhibitors.

This review delves deeply into the world of corrosion inhibitors, aiming to shed light on their various types, mechanisms, performance, and potential applications. To better understand the inhibitor's efficiency and mode of action, the review critically analyzes the numerous mechanisms involved in corrosion inhibition. Further, the review evaluates and compares the performance of various corrosion inhibitors, both organic and inorganic, through insightful case studies and investigations. Furthermore, this review highlights the use of emerging eco-friendly corrosion inhibitors derived from natural resources such as biopolymers, plant extracts, and drugs as potential corrosion inhibitors. To comprehensively evaluate a corrosion inhibitor's performance, the review highlights various analytical tools such as WL analysis, electrochemical analysis, and surface analysis. Additionally, the review discusses thermodynamics and isotherms related to corrosion inhibition, which advances our comprehension of inhibitors' behaviors and mechanisms. Furthermore, the review highlights the importance of computational studies in predicting new inhibitor's performance, thereby saving time and effort. This review bridges the gap between theory and practice, allowing researchers and practitioners to make informed decisions, develop effective corrosion inhibition strategies, and pave the way for a future in which viable inhibitors protect materials and structures from the detrimental effects of corrosion.

2. Corrosion inhibition mechanisms

The inhibition of metal corrosion can be achieved through one or more of the following mechanisms:

2.1 Adsorption mechanism of corrosion inhibition

There are multiple paths in which corrosion inhibitors can adsorb onto metal surfaces. These include strong chemisorption bonds due to chemical interactions of the metal surface and the inhibitor, as well as weak physical adsorption forces, such as hydrogen bonding, electrostatic attractions, or van der Waals interactions. Although, recent studies have shown that van der Waals interaction has little impact on identifying preferential sites compared to electrostatic interactions, it still plays a role.31,32 Commonly, organic inhibitor molecules comprise π-electrons and/or lone-pair electrons of heteroatoms (N, O, S, P, …), where the latter can participate in protolytic equilibria yielding positively or negatively charged moieties depending on the medium pH.33,34 Metallic species that experienced partial oxidation acquire positively charged surface sites that can attract negatively charged counterions like chloride and sulfate, as well as electron-pair donor moieties from inhibitors. In some cases, however, these counterions adhering to the metal surface result in negative charges that strongly interact with protonated groups of some organic inhibitors.35 Additionally, the inhibitor's electron-pair donor moieties can participate in coordinate bond formation with metals' low-energy empty orbitals. The adhesion forces between the substrate and the inhibitor molecules demonstrate greater potency in chemisorption than in physisorption processes. Consequently, chemisorption boasts heightened adsorption energy, thereby establishing itself as a superior approach for corrosion inhibition.36 Furthermore, an important aspect of some adsorption processes is referred to as retro-donation, which involves electron transfer from occupied metal orbitals to unoccupied anti-bonding orbitals of the inhibitor's heteroatoms. This retro-donation results in a synergistic chemical bonding effect.13 Fig. S1† depicts possible corrosion inhibition mechanisms via chemisorption and physisorption pathways involving organic inhibitors.36 Furthermore, the inhibitor's adsorbed molecules may create a protective layer on the metal surface, which might serve as a physical barrier.37

In another instance, XPS examination was carried out to identify the adsorption of Isatin-CS self-assembled monolayers (SAMs) on Q235 carbon steel. The physical and chemical interactions are evidenced through the presence of two types of nitrogen in the XPS data, reflecting that the Isatin-CS SAM are adsorbed onto the surface of steel via both mechanisms38 In another study, the N (403 eV)/N (398.1 eV) peak area ratio stays the same at varying inhibitor concentrations. This indicates that when the steel surface is fully coated, the proportion of molecules in different orientations is constant. Hence for nitrogen-functional inhibitors, adsorption can happen via electrostatic bonding between the steel surface and the N group39 XPS analysis results confirm the chemisorption of DAPO (2,5-bis(4-dimethylaminophenyl)-1,3,4-oxadiazole) on the MS surface. The existence of an N–Fe bond complex reflects that DAPO was chemisorbed onto MS surface, corroborating the thermodynamic findings. Furthermore, the addition of DAPO promotes the generation of a robust and insoluble oxide layer (Fe2O3, FeOOH) on the MS surface, thereby promoting its corrosion resistance.40 A similar study showed that the peak at 710.28 eV (N–Fe/S–Fe) also reflects that the inhibitor interacts with the metal surface to form coordination bonds.41

The majority of organic inhibitors adhere to the target metal surface by displacing water on the surface and creating a tight barrier, as reveled by many studies.42 The physical barrier prohibits corrosive substances like oxygen, water, and aggressive anions from approaching the metal surface. These barriers can affect both anodic and cathodic processes, thus slowing down both the chemical and electrochemical corrosion processes. The corrosion inhibitor's chemistry plays a crucial role in its inhibition performance. The inhibitor's specific chemical structure, functional groups, electron density, and molecular weight can all affect its ability to adsorb onto the metal surface and form a protective layer or film.43

To summarize, in corrosion inhibition by adsorption, the inhibitor's molecules or their ions adhere to anodic or cathodic sites, resulting in the blocking of active zones, a shift in anodic and/or cathodic potentials, and/or the formation of a protective barrier or film.43

For instances, the researchers examined the mechanism of Euphorbia heterophylla L. extract as a potent inhibitor for mild steel (MS) when exposed to a 1.5 M HCl solution and proved physical adsorption as the dominant mechanism behind the inhibition process.44 Furthermore, polyaspartic acid (PASP) was explored as an environmentally friendly MS corrosion inhibitor in a 3% NaCl solution.45 By establishing an adsorption layer on the metal surface, PASP demonstrated a moderate inhibitory efficacy of 61% at a concentration of 2.0 g L−1. However, at a concentration of 0.5 g L−1 PASP, the addition of zinc ions further increased the inhibitory efficiency to 97%, indicating a synergistic effect between PASP and Zinc ions. As shown in Fig. S2a–d,† when compared to a blank specimen, scanning electron microscopy (SEM) images verified that a protective inhibitor coating (PSAP or PSAP/Zn) was present on the MS surface. Zinc ions, in particular, produced a thicker PSAP/Zn protective layer that functioned synergistically as a cathodic inhibitor.45 In another study,46 the researchers explored the effectiveness of PASP and threonine (Thr) in preventing corrosion within simulated cooling water. The findings demonstrated that the PASP-Thr exhibited superior corrosion inhibition compared to PASP alone. This synergism can be attributed to the ability of PASP-Thr to create a protective film on the carbon steel surface, utilizing a combination of chemical and physical–chemical adsorption approaches, as shown in Fig. S2e.† The unique characteristics of PASP-Thr, such as its abundance of polar groups, and considerable molecular weight facilitate strong adherence and uniform coverage on the carbon steel surfaces.46

Thermodynamic information can be utilized to determine the type of adsorption exhibited by a corrosion inhibitor, whether it is chemisorption or physical adsorption. When the absolute value of ΔG0ads is larger than 40 kJ mol−1, chemisorption takes place, signifying the creation of a chemical bond between the inhibitor and the metal surface.47 In contrast, physisorption happens when the absolute value of ΔG0ads is lower than 20 kJ mol−1, suggesting an electrostatic interaction between the inhibitor's molecules and the metal surfaces.16 However, in many cases, the adsorption mechanism was found to be a combination of both physical and chemical interactions between the inhibitor and the metal surface.16,47

2.2 Electrochemical mechanisms of corrosion inhibition

Through an electrochemical mechanism, corrosion inhibitors smoothly suppress corrosion on metal surfaces by suppressing the anodic and/or cathodic reactions occurring during a corrosion process. This is achieved by inhibiting the cathodic reduction reaction rate and/or preventing the anodic oxidative metal dissolution.32,48 Corrosion inhibition employing cathodic inhibitors involves several mechanisms. Firstly, these inhibitors increase the overpotential of the cathodic reaction, making corrosion more difficult to occur. By raising the overpotential, the rate of corrosion is effectively decreased. Cathodic inhibitors, e.g., polyphosphates, zinc salts and cerim(III) salts, also work by blocking the cathodic reaction through deposition at cathodic sites, e.g., through the formation of insoluble compounds, or by increasing the metal liability to hydrogen.37,49 This blocking action effectively hinders corrosion by lowering the availability of the cathodic reactant. The presence of cathodic inhibitors acts as a barrier, disrupting the flow of electrons and ultimately reducing the overall corrosion rate. Conversely, anodic protection takes place when a corrosion inhibitor is adsorbed on the surface and forms an oxide film on the metal surface. This protective film acts as a protective barrier, inhibiting the anodic reaction. Anodic inhibitors can be categorized into non-oxidizing ions (silicates, tungstates, and phosphates) which demand oxygen for protection, and oxidizing anions (e.g., nitrites, and chromates) that can protect metals without external oxygen. Regardless of the type, anodic inhibitors may cause pitting issues and accelerated corrosion rate if concentrations are too low. Therefore, monitoring inhibitor levels is crucial.

For instance, polarization curves of MS in simulated cooling water (SCW) with various concentrations of silicate or phosphate inhibitors demonstrated that the cathodic Tafel slopes decreased, while, it was observed that an increase in the concentration of SiO32− or PO43− led to an enhancement in the values of anodic Tafel slope, with phosphate having a greater effect. This implies that both SiO32− or PO43− act as anodic inhibitors by impacting the anodic reaction of metal dissolution.50 Another study researched the synergized inhibition of Ce4+/melamine on the corrosion behavior of aluminum alloy (AA) in a 3.5% NaCl solution.51 The results showed that the corrosion current density of the sample was significantly lowered compared to the blank or single inhibitor samples. Additionally, examination of Tafel lines of polarization curves of samples in different inhibitor's concentrations showed that the anodic branches of all curves exhibited similar behavior, but with a decrease in the corrosion current density (Icorr) attributable to a reduction in the cathodic current. These findings suggest that Ce4+/melamine solution acts as a cathodic-type inhibitor.51 However, in a study of N80 steel corrosion in a concentrated tetrapotassium pyrophosphate solution and its corrosion control by vanadate, the addition of 0.5 wt% NaVO3 caused a drop in the passive current density by 10–100, showing that NaVO3 functioned as an anodic inhibitor.52

2.3 Summaries of corrosion mechanisms

Corrosion inhibitors can be classified into different categories based on their mechanism of action, including anodic, cathodic, and mixed inhibitors. Anodic inhibitors work by inhibiting the anodic metal dissolution reaction. Cathodic inhibitors work by inhibiting the reduction reaction, which is a crucial step in corrosion processes. Mixed inhibitors work by reducing both the anodic and cathodic reactions, leading to a more comprehensive corrosion inhibition. The synergistic interplay between the anodic and cathodic processes can result in a higher overall inhibition efficiency. Based on the mode of protection, inhibitors can form a passive layer, adsorb on the metal surface, or form a protective layer. Fig. 3 illustrates a scheme of corrosion inhibitor's role.

3. Type of corrosion inhibitors

In the realm of inhibition, we can differentiate between organic and inorganic inhibitors. Inorganic inhibitors exert their influence by retarding or preventing the anodic and/or the cathodic reactions of a corrosion cell. Conversely, organic inhibitors possess a multifaceted nature, displaying adsorption action, cathodic, anodic effects, or a mix of them. By classifying inhibitors into these categories, we gain a clearer understanding of their mechanisms and functionalities.

3.1 Inorganic inhibitors

An anodic inhibitor causes passivation by coating the metal surface with a protective layer, e.g., oxide layer. Anodic or passivation inhibitors include nitrite, silicate, phosphate, chromate, and molybdate, among others. On the other hand, cathodic inhibitors operate through various approaches, such as cathodic precipitation, cathodic poisons, or oxygen scavenging. Cathodic inhibitors include salts of magnesium, calcium, zinc, and others. Table 1 lists some of the common inorganic corrosion inhibitors along with their important characteristics.
3.1.7. Summary of inorganic inhibitors. Inorganic inhibitors perform an essential role in protecting metals in harsh circumstances. Their corrosion resistance, nonvolatility, and thermal stability make them preferred over organic inhibitors in certain applications. Understanding the impact of these inhibitors on the cathodic and anodic polarization branches are keys to predicting their corrosion prevention performance. By altering the reactions in each branch, inhibitors can either impede or prevent the corrosion process, making them valuable tools for safeguarding metals. There are three main forms of inorganic inhibitors: anodic, cathodic, and mixed inhibitors. However, environmental concerns and the high concentrations often required hinder their application. Further, the utilization of anodic inhibitors at low levels can induce the stimulation of corrosion, notably the formation of pits, constituting a substantial hazard. Considering these factors, the selection and utilization of inorganic inhibitors require careful assessment and consideration.

3.2 Organic inhibitors

3.3 Adverse effects of inorganic and organic corrosion inhibitors

Inorganic corrosion inhibitors can have significant adverse effects, particularly on the environment and human health. One of the primary concerns is the environmental impact of these inhibitors. Many inorganic corrosion inhibitors, such as chromates and phosphates, can accumulate in the environment and lead to the contamination of water bodies, soil, and air. Chromates, for instance, are known to be carcinogenic and can pose a serious threat to both ecological systems and human health.144 The release of these harmful substances into the environment can have far-reaching consequences, disrupting delicate ecosystems and potentially exposing human populations to toxic substances through various exposure pathways, such as drinking contaminated water, ingesting contaminated foods or inhaling polluted air. In addition to the adverse environmental impact, inorganic corrosion inhibitors can also exhibit direct toxicity to living organisms. Substances like nitrite and zinc salts can be toxic to a wide range of organisms, including aquatic life, plants, and humans, depending on the concentration level and exposure route.145 Ingestion or inhalation of these inhibitors can lead to various health problems, such as respiratory issues, skin irritation, and gastrointestinal disorders. This toxicity affects not only the workers who handle these inhibitors but also the general public and the surrounding communities where these substances are used or disposed of.

Further, organic corrosion inhibitors, while offering alternative solutions to inorganic inhibitors, also can have their own set of adverse effects that must be considered. One significant concern is the toxicity and environmental persistence of some organic inhibitors, such as imidazolines and quaternary ammonium compounds. These substances can be toxic to aquatic organisms and may bioaccumulate in the environment, leading to long-term impacts on ecosystems. Another adverse effect of some organic corrosion inhibitors is their potential to cause foaming or fouling in various industrial systems, such as heat exchangers, pipelines, or boilers. This can lead to operational issues, reduced efficiency, and the need for additional maintenance and cleaning, which can be time-consuming and costly. Furthermore, the use or decomposition of organic corrosion inhibitors can result in the formation of potentially harmful byproducts. These byproducts may have adverse environmental or health implications, as they can be carcinogenic, ecotoxic, or have other undesirable properties. The disposal and waste management of spent or unused corrosion inhibitors also present challenges. These substances may require specialized treatment or containment to prevent environmental contamination, and improper disposal can result in the release of harmful substances into the environment, further exacerbating the risks to ecosystems and human health.146–150 To address these adverse effects and promote sustainable corrosion management practices, it is crucial to carefully evaluate the specific application, environmental conditions, and potential risks associated with both inorganic and organic corrosion inhibitors before selection and use. Proper handling, disposal, and the development of environmentally friendly alternative inhibitors can help mitigate the associated risks and ensure the safe and effective use of these substances.

3.4 Environmentally friendly corrosion inhibitors

4. Synergistic inhibition effects

Herein, the term synergy refers to the collaborative impact of multiple corrosion inhibitors working together to promote their ability to hinder corrosion. When these inhibitors are combined, their combined impact is more robust than when employed individually. This collaborative impact can be attributed to the many ways in which these inhibitors work, which complement and promote each other's actions. By employing these inhibitors together, they offer extra protection to the targeted metal surface by combining and/or boosting their mechanisms. Higher doses may be needed when using an individual inhibitor to reach the necessary level of inhibition. Nevertheless, it can be employed in synergy with another inhibitor to accomplish the necessary inhibiting impact at fewer dosages, saving costs in various applications. Further, the development of novel molecules or complexes amongst the inhibitors may result in synergistic interactions. These complexes can be formed either by the inhibitors interacting with the metal surface or by their chemical interactions with each other. Furthermore, synergism is a useful strategy for boosting inhibitors' suppressive potency and expanding their use in harsh circumstances. It is vital for both real-world use and the theoretical investigation of corrosion inhibitors. As mentioned in eqn (1), a synergism parameter (S1) is frequently employed to assess this impact:275(1)The θ1+2 parameter is estimated by = (θ1 + θ2) − (θ1 × θ2), where θ1 represents the surface coverage of one employed inhibitor, while θ2 is coverage of another inhibitor or other additive, and represents the sum of both employed materials. Whereas a value of S1 less than unity implies an antagonistic impact, a value larger than unity implies inhibitory synergism between the employed materials.

4.1 Synergistic impact between inorganic substances

Numerous research studies have consistently shown that achieving the desired level of corrosion inhibition often requires the use of high concentrations of inorganic substances, leading to significant costs. Additionally, the toxicity of some inorganic compounds raised environmental concerns. However, extensive investigation has been conducted in various scientific papers to explore the benefits of utilizing a combination of two or more compounds, Table 5. This synergistic effect has been demonstrated to enhance inhibition efficiency, enabling the usage of lower concentrations of inhibitors compared to employing a single compound even in harsh circumstances. For instance, the combined action of nitrite with phosphate, chromate, zinc, or other inorganic inhibitors has been extensively examined, demonstrating significant synergistic effects. These investigations have found that mixed inhibitors containing nitrite are highly effective, even in the presence of elevated chloride levels.281,282 The effectiveness matches that of using nitrite as a standalone inhibitor at higher concentrations. Further, growing environmental consciousness regarding the hazardous impact of nitrite ions has led to the need to reduce their usage. To address this concern, the blending of nitrite with non-toxic and eco-friendly inhibitors like molybdate is being pursued to minimize harmful impacts.58 In one study, the impacts of zinc, nitrite, and molybdate ions on the protection of MS in chloride-containing water were examined.283 The results demonstrated that molybdate effectively hindered corrosion with pH levels above 6. Similarly, nitrite prevents corrosion at pH levels of 4.5 and above but enhances corrosion below pH 4.5, regardless of the presence of cupric ions. However, when nitrite and molybdate are combined, they served synergistically as inhibitors with or without cupric ions at pH levels of 4.5 and above. This combination also minimized corrosion phenomena in the acidic range between pH 3.0 and 4.5, with a low level of molybdate and high levels of nitrite. The synergistic impact is accomplished by absorbing molybdate which protects the surface passive layer from aggressive anion attacks. To analyze the passive films created on MS during immersion in NaCl medium and high levels of either MoO42− or NO2−, researchers employed an XPS analysis.284 The results confirmed that both anions led to the development of nanometer-thick films (approximately 5 nm) on the surface, with Fe3+ ions being the dominant cation. The analysis suggested that an upper sub-layer consisting of Fe2(MoO4)3 was formed, followed by a layer of ferric hydroxide/oxide, may be γ-Fe2O3. XPS data from the film created by NO2− inhibitor supported the notion that it mainly comprised materials like γ-Fe2O3. Thus, the impact of NaNO2 and Na2MoO4 concentrations on inhibition performance was examined.58 At a level of 2 g L−1, they achieved 65% and 53% inhibitions, respectively, when these inhibitors were employed individually. Interestingly, upon combined use, a synergistic impact was noticed at the same lower levels (2 g L−1), with a notable synergism value of 8.6. Moreover, the combination of these inhibitors in a ratio of 1 : 1 at lower levels significantly boosted the performance to 93%.58

4.2 Synergistic impact between organic and inorganic substances

The widespread use of organic inhibitors is attributed to their eco-friendly features, cost-effectiveness, and ability to be employed in smaller quantities compared to inorganic inhibitors. As a result, these inhibitors have become the primary focus of research in various fields. However, the effectiveness of a single organic material is significantly impacted by factors such as the temperature, condition of the metal surface and its surrounding medium, and immersion time. In certain specialized industries, a sole organic agent may not meet the stringent requirements for corrosion prevention. To address this issue, combining organic and inorganic inhibitors, as well as incorporating organic inhibitors with trace cations, alkaline earth salts, halides, or other anions, can significantly boost the anticorrosion performance and stability of the targeted system, while also minimizing the overall usage of inhibitors.285,286 As an example, the synergistic mechanism of sodium tungstate and a Mannich base (C15H15NO) was examined.287 Their research revealed that the Mannich base initially attaches to the targeted surface, creating a film owing to its robust adsorption energy. Following this, tungstate ions are incorporated into the defects within the created layer by Mannich base, resulting in a tightly sealed film, as well as forming hydrogen bonds with hydrogen ions, which effectively block corrosive ions from penetrating the adsorption film. This process significantly minimized the existence of corrosive ions near the employed surfaces. In a research, the synergistic impact of sodium silicate and piperazine inhibitors on ST-14 steel was explored.288 The findings revealed that the combination of these two inhibitors greatly promoted the steel's resistance against corrosion, as demonstrated by PDP and EIS measurements, with improvements of approximately 87% and 76%, respectively. The most effective corrosion inhibition was achieved with a combination of 10–15 ppm sodium silicate and 2 ppm piperazine (PIP). Observing the interaction between the iron oxide layer and inhibitor molecules, it can be inferred that physical adsorption played a more prominent role in the film generation for both PIP and sodium silicate inhibitors. The researchers suggested that oxygen atoms of silica acted as bridges linking piperazine to metal ions at the surface defects. This connection enables the formation of a thicker and more impenetrable film at the anodic sites (as shown in Fig. 8a). Another study showed that the combination of sodium molybdate and benzotriazole (BTA) resulted in a protective layer consisting primarily of BTA-Fe and FeMoO4.290 This structure enhanced the density of the FeMoO4 corrosion inhibition film and facilitated the conversion of FeOOH into a stable Fe2O3 compound. Additionally, when the pH levels are maintained between 8.0–10.0, the inhibition performance reached 99%, and the system exhibited high stability and required a low concentration for optimal effectiveness. Furthermore, an inhibitor composed of gluconate, as well as small quantities of molybdate was reported.289 The synergistic impact between gluconate and molybdate was elaborated (Fig. 8b). PDP measurements revealed that the existence of gluconate made the corrosion of tested metal kinetically and thermodynamically unfavorable, surpassing the impact of molybdate alone. The SEM, FTIR, and XPS analysis revealed that gluconate acted as a bridge between iron and molybdate, leading to the formation of a protective layer that hindered corrosion, as shown in Fig. 6b.

Numerous studies have confirmed that the inclusion of transition metal cations can greatly boost the performance of organic corrosion inhibitors. For instance, the synergistic features of sodium lignosulfonate–zinc acetate (SLZA) in hindering corrosion of employed MS in a 3.5 wt% NaCl was examined.279 The findings from electrochemical data demonstrated that the joint action of ZA and SL effectively hindered uniform corrosion. The synergistic impact was quantified to be approximately 9, and the overall resistance exceeded 400 kΩ cm2 when both SL and ZA were employed. Moreover, the SLZA system exhibited an impressive inhibition of around 96%. Based on SEM-EDS and XPS analyses, the researchers proposed that the film composed of zinc-containing compounds and SL-based complexes played a critical role in hindering corrosion, as shown in Fig. 8c. In another research, the effectiveness of PASP as an inhibitor was significantly boosted by the inclusion of zinc ions, reaching a performance of 97% at a level of 0.5 g L−1. By adding Zn2+ the performance of polyaspartic acid was noticeably boosted as it hindered the cathodic sites of the localized corrosion cells.45 The zinc ions were found to adhere to the surface, replacing Fe+ ions. This led to the inducement of a PASP–Zn complex, which created a protective layer and effectively hindered corrosion. Analysis using AFM and EDX revealed the formation of a thick layer following the inclusion of zinc ions.45

Halide ions, especially iodide ions, have been utilized to enhance the performance of organic corrosion inhibitors. The order of synergistic impacts typically follows: Cl < Br < I, where I− ions exhibit the best synergistic impact, thanks to their larger size (216 pm) and ease of polarizability. The robust electronegativity of halides allows for the creation of bridges between the metal surface and the positive end of the employed inhibitor. This connection helps to extend their surface coverage, resulting in better protection against corrosion. For instance, The effectiveness of CS and KI additive in inhibiting St37 steel corrosion, in a 15% H2SO4 medium was examined.196 The addition of KI significantly augmented the CS inhibitory performance, reaching 92%. PDP and EIS analysis revealed that the CS–KI film was more robust and reflected boosted effectiveness over longer immersion periods. The inhibitory performance of CS declined at elevated temperatures, while the CS–KI combination reflected a boosted trend and achieved its best inhibition of 99.72% at 60 °C. A calculated synergism emphasized that the boosted performance of CS–KI was a result of a synergistic effect.196 Furthermore, the impact of KI on tannin anticorrosion performance was examined.291 The findings reflected that adding just 0.025% KI to the tannin solution minimized the anodic current density (Icorr) as revealed by electrochemical tests and reflected improved corrosion inhibition performance.

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4.3 Synergism between organic inhibitors

Numerous investigations have been conducted to look at how combining organic inhibitors impacts the effectiveness of inhibition. In one study, ppm xanthan gum (XG) exhibited a 74.24% inhibition; however, the addition of low levels of surfactants, namely Triton X-100 (TX), cetyl pyridinium chloride (CPC), and sodium dodecyl sulfate (SDS), marginally boosted the corrosion inhibition efficiency.208 UV-visible analysis emphasized the creation of a complex between Fe2+ and XG. Additionally, SEM images reflected distinct morphological changes in the presence of XG and XG-surfactant additives. Moreover, an inhibitor consisting of a combination of sodium alginate (SA) and chondroitin sulfate obtained from pig cartilage (CSPC) was developed.212 The synergistic impact of these two polysaccharides on corrosion inhibition under 1 M HCl was investigated. The results suggest that the CSPC and SA mixture exhibit a robust impact, outperforming individual inhibitors (with performance of 95.18% compared to 72.78%). Furthermore, quantum mechanical calculations demonstrated that the bond creation between tested metal and tested inhibitors occurred via charge transfer between the CSPC, SA and iron with evidence of partial retro-donation bonding type. Furthermore, the synergism of a combination of Anacardium occidentale (cashew gum) and Acacia Senegal (Arabic gum) in corrosion protection of MS study was elaborated.292 The inhibitor adsorption followed the Langmuir isotherm, reflecting a chemisorption behavior between the metal surface and the gums. This was emphasized by the values of the free ΔG of −16.47 and −15.61 kJ mol−1 at 303 K and 333 K, respectively. Dipole moment, molecular weight, and EHomo were found to affect the gum mixture binding energy, and GC-MS was used to assess the gum constituent's hydrophobicity, which was shown to be a contributing factor to its efficacy as an inhibitor in acidic environments.292
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In summary, mixed corrosion inhibitors are a suitable choice when strict corrosion inhibition performance is needed in challenging processing environments. Among the available options, organic/organic systems offer the advantage of being non-toxic and biodegradable, making them environmentally preferable over organic/inorganic and inorganic/inorganic systems. However, it is crucial to mention that even though the amounts of inorganic components used in these mixtures is small, they have the potential to bioaccumulate to dangerous levels over time.

4.4 Synergistic corrosion-inhibition mechanisms

Synergistic corrosion inhibition mechanisms exhibited by inhibitors combinations can work via, the gaps-filling, bond formation between different inhibitors, cooperative and complementary adsorption, and mutual interactions of inhibitors in the bulk solution. The gaps-filling approach stipulates that different inhibitor molecules act in a complementary manner, where one type of inhibitor component fills the gaps and defects within the protective film established by the other inhibitor, thereby synergistically enhancing the overall corrosion protection. The process starts with the adsorption of the organic inhibitor compound, such as Mannich bases or polysaccharides, onto the metal surface to form a primary adsorption layer, which serves as the foundation for the development of the protective film. In a later stage, the inorganic inhibitors begin to incorporate or bind into the organic inhibitor layer while filling the gaps and defects present within the organic film, creating a more uniform and complete surface coverage.

Furthermore, the inorganic inhibitor can interact with the organic inhibitor film, leading to the formation of complex metal–organic compounds. These complex compounds contribute to the enhanced stability and protective properties of the passive layer, making it more compact, dense, and resistant to the penetration of corrosive species. The complementary adsorption and filling of the film by various inhibitor components create a synergistic effect, wherein the weaknesses or deficiencies of one inhibitor type are compensated by the strengths of the other.

On the other hand, cooperative adsorption is another key mechanism for the synergistic effect of mixed organic corrosion inhibitors. In this mechanism, the two inhibitors adsorb on the metal surface in a sequential manner, where one inhibitor first chemisorbs onto the surface, creating a foundation for the second inhibitor to then adsorb on top of the initial layer. This cooperative adsorption can lead to the formation of a more compact, stable, and protective film on the metal surface, resulting in enhanced corrosion inhibition. The synergistic effect arises from the combined protective action of the two inhibitors, where they work in harmony to provide better coverage and protection compared to individual inhibitors.

However, complementary adsorption is another prospective mechanism for the synergistic inhibition effect, where the two inhibitors may preferentially adsorb on different sites of the metal surface, effectively covering a larger area and providing more comprehensive protection. For instance, one inhibitor may predominantly adsorb on the anodic sites, while the other inhibitor adsorbs on the cathodic sites, leading to the inhibition of both the anodic and cathodic corrosion processes. Furthermore, inhibitors mutual interactions in the bulk solution can contribute to the synergistic inhibition effect. These mutual interactions may include the formation of complex species or micelles, which can enhance the transport and adsorption of the inhibitors onto the metal surface. This improved availability and adsorption of the inhibitors on the metal surface can result in a synergistic inhibition of corrosion. Another mechanism of synergistic inhibition involves the cooperative adsorption of the mixed inhibitors on the metal surface. The adsorption of one inhibitor can modify the surface properties, creating more favorable sites for the adsorption of the other inhibitor. Additionally, the presence of multiple inhibitors can promote the formation of insoluble metal–inhibitor complexes on the metal surface. These complex species can act as a physical barrier, blocking the access of corrosive species to the metal surface and effectively inhibiting the corrosion process. In some cases, the synergistic effect may involve the combined action of anodic and cathodic inhibitors.

In summary, synergistic corrosion inhibition results in a more robust and impermeable protective barrier against corrosion, making the hybrid corrosion inhibitor system particularly ideal for harsh processing environments where strict corrosion inhibition performance is required. In addition to mutual interactions between inhibitors in the bulk solution that promote the inhibitors diffusion towards the metal surface, the underlying mechanisms governing the synergistic effect of hybrid/composite corrosion inhibitors involve complex interactions at the molecular level, such as gaps-filling, cooperative and complementary adsorptions, impermeable film formation, and the creation of inorganic–organic and organic–organic inhibitors' bonding. The nature and concentration of the inhibitors, the metal–environment system, and other variables can all affect the precise mechanism underlying the synergistic impact in mixed corrosion inhibitor systems. However, in most cases, multiple mechanisms work cooperatively.

5. Inhibition performance validation

5.1 Weight loss

The corrosion inhibitor's performance is a critical aspect of corrosion mitigation strategies.293–295 Weight loss (WL) methods are commonly utilized to determine the effectiveness of corrosion inhibitors in various environments. WL analysis is a relatively simple and cost-effective method compared to other advanced techniques and provides both a quantitative measure of the corrosion rate, and a realistic simulation of the actual corrosion conditions in a particular application. The WL method primarily involves exposing metal specimens to the corrosive environment under controlled conditions over a specific period.45,293 The typical procedure involves a series of steps for the preparation and analysis of the metal sample. Initially, the sample is sanded employing emery paper to prepare it for the experiment. After that, it is dried, washed with double distilled water, and degreased with acetone. Scale with ±0.01 mg sensitivity is employed to estimate the specimen's weight for the measurement. The recommended procedure calls for submerging the metal sample in various test solutions for a certain amount of time at a predetermined temperature. This is done with and without different concentrations of inhibitors. Following the experiment, the material is dried, cleaned, and weighed one more time.294 To ensure precision, the experiments should be executed in triplicate, and the average values should be considered. The inhibition efficiency ηw (%), surface coverage (θ), and corrosion rate (Cr) can be calculated using the following equations:296,297

For instance, the WL method was used to assess the corrosion rate of MS under acidic conditions, in the presence and absence of the corrosion inhibitor.293 Notably, they achieved a high inhibition efficiency of 89.5% when using 400 mg L−1 inhibitor. In another study, WL monitored the inhibition performance of zinc ions and PASP at varying concentrations, where the addition of 2 mg L−1 zinc ions minimized WL compared to the use of PASP alone.45 Moreover, another investigation explored the impact of temperature on inhibition efficacy using the WL approach.295 The study observed that higher temperatures minimized the adsorption of PESA onto the examined metal surfaces. Consequently, this resulted in minimized surface coverage and subsequently lowered efficiency. Similarly, because of severe etching and degradation or desorption of inhibitor molecules, higher temperatures caused a reduction in surface coverage.298

Overall, the WL method plays a crucial role in evaluating the performance of corrosion inhibitors. It provides valuable insights into the effectiveness of inhibitors under specific conditions, aiding in the design and optimization of corrosion protection strategies for various industrial applications.

5.2 Surface analysis

SEM is a powerful imaging technique that provides high-resolution images of the sample surface.299–301 It utilizes a focused electron beam to scan the sample, generating secondary electrons, backscattered electrons, and characteristic X-rays.286,302–304 The secondary electron images obtained from SEM provide detailed information about the surface morphology, such as surface roughness, the presence of cracks, pits, or other corrosion features.305–307 Additionally, backscattered electron imaging can reveal compositional variations on the surface, highlighting the presence of corrosion products or the distribution of the inhibitor.302 EDX is used in conjunction with SEM to provide elemental analysis of the sample.308–311 It detects characteristic X-rays emitted by the atoms in the sample when excited by the electron beam. In the context of evaluating corrosion inhibitors, EDX can help determine the effectiveness of the inhibitor in reducing the concentration or presence of corrosive elements, such as chloride ions or oxygen.312,313 By comparing the elemental composition of metal surfaces with and without the inhibitor, changes in corrosion product formation or inhibitor adsorption can be assessed.314,315

For instance, SEM images of steel submerged in the solution without corrosion inhibitors in simulated concrete pores with chlorine revealed many pits on steel surface, revealing that the steel has been severely degraded by the Cl− present.316 Similarly, SEM pictures of the steel reveal signs of severe corrosion and comparatively extensive surface fractures.317 The majority of surface flaws were the sites where corrosion started, resulting in the production of corrosion products that most likely covered specific localized regions of the whole material surface. These corrosion byproducts created a porous layer that encouraged more corrosion, leading to a serious assault with surface fractures and long void.317 Furthermore, the formation of an inhibitory film on MS surface and the thickening of this film were confirmed by SEM images, which reflected the adsorption of PASP and its interaction with zinc ions.45 Another research also utilized SEM images to analyze the effect of chitosan-5-HMF on the MS surface after exposure to 1 M HCl, as illustrated in Fig. 9a–d.318 Without any inhibitors, the surface of MS exhibited uneven damage, characterized by the formation of pits of various sizes. Further, significant alterations were noted when mild steel was placed in a solution containing 200 mg L−1 of chitosan, as shown in Fig. 7b. Notably, the formation of pits and voids was eliminated. Conversely, when using the inhibitors chitosan-5-HMF1 and chitosan-5-HMF3 (Fig. 7c and d), the surface of the matrix became notably smoother and improved. This suggests that the adsorption of chitosan-5-HMF species hindered the contact between mild steel and the corrosive electrolyte, resulting in improved corrosion resistance. This inhibitory effect is further evidenced in the contact angle images. In another study, an EDX mapping examination revealed a significant presence of Cl in the affected area owing to its corrosive activity.319 Conversely, a reduction in chloride signal owing to the generation of a protective layer on the MS was observed and this protective layer effectively hindered chloride ions from reaching the active sites of the MS.317 Moreover, the proportion of oxygen atoms in the blank solution, which corresponds to the rate of oxide formation on the MS surface, was found to be 10.07%.320 However, following the inhibitor's addition, this ratio minimized significantly to 2.87% owing to the protective nature of adsorbed octacalcium phosphate (OCP) inhibitor molecules.

To gain quantitative insights into the surface morphology of metals, the utilization of AFM is necessary.216 AFM allows for the examination of utilized specimen topography in a 3D fashion. By employing AFM, one can evaluate the surface topography of a sample and observe shifts in roughness resulting from corrosion or the presence of inhibitors.321 For both protected and unprotected samples, the average roughness values, expressed in nanometers and root mean square (RMS) are calculated and compared.322 The quantitative investigation demonstrates that the maximum peak-to-peak height values and the RMS roughness of the inhibitor-coated metal surface are smaller than those of the untreated metal surface.323,324 For instance, the initial roughness (Ra) measurement of polished mild steel was 7.96 nm (Fig. 9e). However, immersion in a 15% HCl solution without any inhibitor for 24 hours resulted in a significant increase in Ra to 973 nm (Fig. 9f). In the presence of SA, SA-g-PMMA/Fe3O4, and SA-g-PMMA/TiO2, the Ra values were measured at 414 nm, 282 nm, and 144 nm, respectively (Fig. 9g–i). Based on these findings, the SA-g-PMMA/TiO2 proved to be the most effective inhibitor, as it produced the smoothest surface. This suggests the formation of a protective layer or adsorption film on the metal surface, highlighting its superior inhibitory properties compared to the other inhibitors examined.216 Similarly, it was found that the MS that corroded in soft water had a rough, non-uniform appearance with big, deep holes; however, the MS surface was smoothed up after adding the inhibitor.297

XPS is a powerful approach employed in evaluating corrosion and corrosion inhibitors. When a high-energy X-ray beam is directed at a material's surface, photoelectrons are released, which is how XPS analysis works. These photoelectrons carry data about the elemental composition and chemical states of the atoms near the surface.301 XPS is useful in determining the chemical composition of the corrosion byproducts that are generated.325,326 XPS can also provide information about the chemical states of these elements.146 For example, iron may exist in different oxidation states as Fe(0), Fe(II), or Fe(III) species.327,328 By analyzing the binding energy of the iron photoelectron peaks, one can determine the oxidation state and the presence of different iron species on the corroding surface. By comparing the XPS spectra of inhibited and uninhibited surfaces, differences in the chemical composition and bonding states of the elements can be observed. For example, the presence of a corrosion inhibitor may result in the formation of a protective layer, which is evident from the XPS spectra showing new peaks or shifts in the binding energies associated with oxygen or other relevant elements.329,330 To confirm the adsorption of chemically modified hydroxyethylcellulose (CHEC) on the electrode surface, XPS analysis was conducted.331 Fig. 9j displays the high-resolution plots and broad scan. The O and C bands exhibited a significant exsitance in the MS specimen immersed in the 15% HCl medium with 50 μM CHEC, which is expected as they are major constituents of the inhibitor structure. The O 1s peaks at 531.74 eV and 533.22 eV correspond to the C–O and C–OH groups, respectively, that are present in the CHEC structure. The C 1s peak at 288.15 eV indicates the presence of C–O, while it is located at 285.89 eV in the uninhibited systems. The binding of inhibitor molecules on the metal surface is responsible for the C 1s peak's shift towards higher energy. Furthermore, XPS examination of the CHEC system indicates the existence of N at 399 eV, which supports interactions between carbon (C), iron (Fe), and N. These results provide compelling evidence for the efficient adsorption of CHEC molecules onto the steel surface, which blocks active sites and inhibits corrosion. The nitrogen heteroatom and the accessible orbitals in the iron's atomic structure share electron pairs during the chemical adsorption process.331 Furthermore, XPS demonstrated that the adsorption of various gum-ased inhibitors on MS surface involved both chemisorption and physisorption.294 Similarly, the XPS analysis supports the conclusion that the adsorption of GAMo, GAMau, and GASe on the MS surface involves both physisorption and chemisorption.294 The XPS results provide strong evidence for the effective adsorption and outstanding inhibition properties of chitosan Schiff base (CS-FGA) molecules on the M.S. surface. The N 1s spectra clearly exhibit two distinct adsorption peaks, with the peak at 399.1 eV indicating the presence of N–Fe bonds. This finding further confirms the chemical adsorption between M.S. and CS-FGA.39

5.3 Electrochemical analysis

5.4 Computational studies

Unlike the time consuming and equipment-intensive nature of experimental measurements, computational techniques provide a versatile and efficient means of evaluating the effectiveness of inhibitors. By analyzing the structural properties of corrosion inhibitors, computational approaches offer valuable insights into their potential impact. This predictive capability is particularly advantageous as it allows for the assessment of organic molecules' corrosion-inhibiting performance even before conducting time-consuming and resource-intensive experimental tests. The power of computational analysis lies in its ability to facilitate the design and development of efficient inhibitors. Through the utilization of computer software, specifically density functional theory (DFT), researchers can estimate reactivity indices and establish quantitative structure–activity relationships (QSAR). This allows for a systematic understanding of how different molecular structures correlate with inhibitor performance. Moreover, atomistic, and molecular simulations, including Monte Carlo (MC) and Molecular Dynamics (MD) simulations play a vital role in providing detailed insights. These simulations provide a deeper understanding of the likely orientations of inhibitor adsorption on the metal surface under investigation. By visualizing these interactions at the atomic and molecular levels, researchers can gain crucial information to guide the design of corrosion inhibitors with enhanced effectiveness.

5.5 Summaries of evaluation techniques

Corrosion detection and monitoring are crucial in various industries to prevent infrastructure damage and equipment failure. Electrochemical techniques provide a non-destructive and fast way to assess corrosion and evaluate the effectiveness of inhibitors. However, challenges like electrode polarization, surface roughness, and corrosion by-products can hinder these techniques. Microscopy, including SEM and AFM, offers high-resolution imaging to examine corrosion products, pitting, and surface topography. Spectroscopic approaches like infrared spectroscopy and XPS provide detailed information on molecular composition and chemical changes on material surfaces, aiding in inhibitor characterization and corrosion product identification. Computational studies using DFT and MD simulations help understand atomic-level interactions between inhibitors and metal surfaces, predicting corrosion reactions' thermodynamics and kinetics. By combining electrochemical, microscopy, spectroscopic, and computational techniques, researchers and engineers can comprehensively understand corrosion processes. This multidisciplinary approach facilitates the development of more effective corrosion prevention strategies, optimization of inhibitor formulations, and informed material selection in various industries. Fig. 10. Illustrate summary of characterization techniques.

6. Comprehensive economic analysis of the corrosion inhibitor market

Corrosion poses a significant economic threat, resulting in substantial financial losses. According to the World Corrosion Organization, approximately 25% of steel production is impacted by corrosion yearly, equating to five tons per second or 150 million tons annually. Recent studies have shown that leading companies in the oil and gas sector are spending around $1.372 billion yearly to address corrosion-related issues. Ignoring the issue of corrosion can have significant financial consequences for various sectors. Even highly developed states and countries with advanced technology are impacted by corrosion and its substantial impacts. For example, in , the USA incurred losses of around $276 billion as a result of corrosion-related effects, which accounted for approximately 3.1% of the gross domestic product. As of , the overall estimated cost of corrosion exceeded $2.2 trillion, rising to $2.5 trillion by , amounting to 3.4% of the global gross domestic product. In India, the yearly cost of corrosion is less than $100 billion, with South Africa reporting a direct cost of $9.6 billion.25,26 Implementing proper corrosion protection methods could potentially reduce these losses by 15–35%.25,26

The global corrosion inhibitor market is a dynamic and rapidly expanding industry fueled by the escalating demand for effective corrosion protection across a wide range of sectors. According to recent industry analyses, the global corrosion inhibitor market was valued at US$ 8.3 billion in and is expected to reach $13.34 billion by , with a Compound Annual Growth Rate (CAGR) of 4.1% during the forecast period. This substantial market growth is driven by the increasing focus on corrosion prevention in vital infrastructure projects, the surge in demand for corrosion protection in the oil and gas sector, and the growing emphasis on sustainable and eco-friendly solutions.

Fig. 11a and b show the development of corrosion inhibitors and its global market over time in a chronological manner. Organic corrosion inhibitors dominated the market in and accounted for over 60% of the total market share. The growing popularity of organic inhibitors, such as amines and imidazolines, is driven by their versatility and effectiveness in various applications. The organic sub-segment had the greatest corrosion inhibitors market share three years ago. The corrosion inhibitors market value for this sub-segment was .5 million back then. The organic sub-segment is expected to have a CAGR of 4.13% for the period that this report covers. The inorganic corrosion inhibitors market growth rate is expected to be 4.5% for this period. Green corrosion inhibitors is projected to witness the highest CAGR during the forecast period, as there is an increasing demand for environmentally friendly and sustainable corrosion protection solutions.

In , North America emerged as the dominant regional market, capturing over 34% of the global market share, primarily bolstered by its well-established industrial base and strict regulatory frameworks in the United States and Canada (Fig. 11c). On the other hand, Asia-Pacific stands out as the fastest-growing regional market, exhibiting a remarkable CAGR of over 8% throughout the forecast period. Meanwhile, Europe ranks as the third-largest regional market, with notable market presence in countries. Moreover, the evolving regulatory landscape and mounting environmental concerns are ushering in a shift towards the adoption of less toxic and environmentally friendly corrosion inhibitor products. This trend highlights the industry's continuous evolution towards sustainable and eco-conscious practices in response to global environmental challenges.

7. SWOT analysis

The review highlights the potential of synergistic effects between mixed corrosion inhibitors, particularly organic/organic systems, as a viable and advantageous choice for applications requiring robust corrosion inhibition performance in challenging processing environments. These mixed inhibitors offer the benefits of low environmental risk and high efficiency, positioning them as a preferred alternative to single-system inhibitors. While the review article provides a comprehensive technical overview, it may fall short in offering detailed formulation-level insights, limiting the ability of readers to fully understand the nuances and optimization potential of these inhibitor systems. Additionally, the limited coverage of real-world application data and long-term performance evaluation could undermine the confidence of end-users in adopting the discussed inhibitor technologies. Furthermore, the review's narrow focus on the technical aspects may overlook critical factors such as regulatory, safety, and practical implementation challenges, which are crucial for the successful deployment of corrosion control solutions in the real world. This lack of a more holistic perspective could hinder the review's ability to provide a comprehensive roadmap for the widespread adoption of the discussed inhibitor technologies. Opportunities for expanding the review's scope include incorporating more diverse global perspectives, exploring hybrid and synergistic inhibitor systems, integrating life cycle assessment and sustainability analysis, establishing industry partnerships, and broadening the coverage to include alternative corrosion mitigation techniques. Addressing these areas could enhance the review's relevance and applicability across different regions and industries. Potential threats to the widespread adoption of corrosion inhibitors include regulatory and environmental concerns, technological advancements in competing corrosion control methods, economic and market fluctuations, resistance to change in established industries, and the lack of standardized testing and evaluation protocols. Addressing these challenges will be crucial for the successful implementation of effective and sustainable corrosion inhibition strategies. Fig. 12 demonstrated the summary of SWOT analysis.

8. Conclusion and prospects

This review article provides a comprehensive and in-depth analysis of corrosion inhibitors, with a focus on both inorganic and organic inhibitors, as well as ecofriendly and biological macromolecules. The review highlights the inhibition mechanisms, with a particular emphasis on the efficiency of organic compounds due to the presence of heteroatoms and conjugated π electron systems. The review also presents case studies and investigations of corrosion inhibition, showcasing the performance and potential application of various inhibitors. One significant aspect that this review addresses is the growing trend of seeking eco-friendly alternative inhibitors derived from natural resources. The review provides a comparative evaluation of the environmentally friendly biopolymer inhibitors, considering their efficacy, compatibility, and sustainability. Furthermore, the evaluation of corrosion inhibitors is discussed, encompassing various analytical techniques such as weight loss, electrochemical, and surface analysis tools. This comprehensive evaluation enhances our understanding of inhibitor behaviors and mechanisms; however, there are some gaps that need to be filled, including:

(1) Optimizing existing inhibitors: researchers should focus on enhancing the effectiveness and stability of currently available organic, inorganic, and green corrosion inhibitors. This can be achieved through strategic molecular design, the incorporation of synergistic additives, and the development of inhibitor-based coatings and composite systems. Particular attention should be given to improving the long-term durability and reliable performance of inhibitor films under challenging environmental conditions.

(2) Synergistic inhibition in extreme environments: investigating the performance of mixed/hybrid/and composite inhibitors under harsh and demanding conditions, such as high temperatures, aggressive chemical environments, or elevated mechanical stresses, can unlock new avenues for corrosion control. Demonstrating the synergistic inhibition capabilities in these extreme scenarios can expand the applicability of these systems to challenging industrial settings.

(3) Enhanced mechanistic insights: delving deeper into the mechanistic aspects of synergistic inhibition using mixed inhibitors can yield valuable scientific contributions. Elucidating the precise interactions between the inhibitors, their influence on passive film formation, and the interplay between anodic and cathodic processes can provide fundamental knowledge to guide the design of more effective corrosion mitigation strategies.

(4) Multilayered inhibitor architectures: building upon the concept of synergistic inhibition, the prospect of designing multilayered inhibitor architectures on metal surfaces presents an intriguing direction. By strategically arranging different inhibitors in tailored sequences, it may be possible to create hierarchical protective systems with enhanced durability and self-healing capabilities.

(5) Bioinspired and biomimetic inhibitors: the pursuit of green and eco-friendly corrosion inhibitors has led researchers to explore bioinspired and biomimetic approaches. Drawing inspiration from naturally occurring processes and structures, scientists are investigating the development of inhibitors that mimic the self-healing, self-cleaning, or anti-fouling properties found in biological systems. For instance, the study of marine organisms and their inherent resistance to corrosion has the potential to yield novel biomimetic inhibitor formulations with enhanced performance and environmental compatibility.

(6) Multifunctional and responsive smart inhibitors: the next generation of corrosion inhibitors is expected to exhibit multifunctional and smart capabilities, addressing not only corrosion prevention but also other surface-related challenges, such as antifouling, anti-icing, or self-healing properties. These responsive and adaptive inhibitors would be able to sense and respond to changes in the environment, automatically adjusting their protective functions to maintain optimal performance under varying conditions.

(7) Application of computational modeling, machine learning, and artificial intelligence that significantly accelerate the discovery, optimization, and deployment of more efficient corrosion inhibitors.

(8) Scaling up green inhibitor production: to meet the growing demand for eco-friendly corrosion inhibitors, researchers and industry should collaborate to develop scalable extraction, purification, and formulation processes for naturally occurring inhibitor compounds derived from plant extracts, marine organisms, or other renewable sources. Simulation and modeling tools can play a crucial role in optimizing the production parameters and enhancing the industrial-scale viability of these green inhibitors.

(9) Sustainable production and life cycle assessment: as the focus on environmental sustainability intensifies, the development of corrosion inhibitors will need to be accompanied by sustainable production processes and comprehensive life cycle assessments. This will involve the use of renewable, biodegradable, and non-toxic raw materials, the optimization of manufacturing methods to minimize waste and energy consumption, and the implementation of circular economy principles to enable the reuse, recycling, or safe disposal of inhibitor-containing products.

In conclusion, this review article not only provides a comprehensive analysis of corrosion inhibitors but also highlights the importance of adopting environmentally friendly alternatives. It offers valuable insights and future perspectives for researchers and industrial sectors, ultimately helping to build effective and sustainable corrosion control solutions.

If you are looking for more details, kindly visit Corrosion and Scale Inhibitor.

Data availability

The data analyzed in this review article are from previously published studies. The specific datasets and sources are cited throughout the manuscript and listed in the reference section. Readers can access the underlying data from the original published sources as cited. The authors confirm that they did not have any special access privileges to these datasets.

Conflicts of interest

There are no conflicts to declare.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10./d4rakThis journal is © The Royal Society of Chemistry