Zinc Corrosion Inhibitor
Basemat Doolen
Corrosion is the unavoidable destruction of metals as well as alloys owing to their interaction with surroundings [1]. Even though ferrous materials are known for undergoing corrosion spontaneously in various acidic, alkaline medium [2], corrosion of nonferrous materials is not new. Numerous reports are available in the literature for corrosion of nonferrous materials such as aluminum and its alloys [3], zinc [4], and other materials [5, 6]. Zinc is frequently used for coatings other metals, predominantly steel [7]. Zinc will undergo severe corrosion at pH lower than 6 and above 12. Industrial practices include procedures where metals are being treated with acids during pickling, descaling, etc. Due to these processes, zinc tends to undergo severe corrosion [8, 9]. The corrosion in marine environments is initiated by interaction between metal surfaces and marine aerosols settled down on it [10, 11]. Furthermore, zinc is liable to undergo atmospheric corrosion. Proper corrosion prevention helps to reduce various damages, direct as well as indirect. Thus, material engineers and corrosion scientists intend to lessen the influence of corrosion on all walks of life under multiple situations. Consequently, there would be a decline in economic losses and enhancement of industrial safety, which eventually protects and preserves engineering materials.
Even though researchers have focused on corrosion inhibition studies by adopting various techniques, the utility of corrosion inhibitors has been advocated as the primary tool to retard corrosion rate. Organic inhibitors are heterocyclic compounds containing O, N, S, and P as heteroatoms. These heteroatoms are centers of high electron densities with a lone pair of electrons in them. Through these reactive centers, they quickly form a covalent/coordination bond with metal [12]. Consequently, they tend to form a protecting deposit on the metal surface. It will form a barricade between metal and corrosive which will avoid promoted dissolution of metal and hence material loss [13].
Corrosion inhibitors are practical and flexible means to mitigate corrosion. They are exclusively utilized in innumerable ways in the industrial segments. They are used as the first-line protector in the oil extraction, processing, and chemical industries. These inhibitors minimize the damage of metal, reduce the degree of hydrogen embrittlement, protect the metal. They reduce corrosion by either controlling anodic or cathodic or both the reaction. A pictorial illustration for the application of inhibitors in pipelines is depicted in Fig. 1.
In industries, every year, many organic compounds were being synthesized and screened for anti-corrosion performance of different engineering materials in various mediums [14,15,16,17]. The other methods employed at the industrial level include coatings, cathodic and anodic protection, in addition to the selection of materials [18,19,20,21].
As a part of our research work on the studies of corrosion behavior and corrosion inhibition of nonferrous materials [22,23,24,25,26,27] under static and dynamic conditions [28], we report herein an up-to-date review on corrosion inhibition strategy for zinc under various corrosive mediums.
Zinc is vulnerable to corrosion in an acidic environment. Therefore, pickling [29], descaling [30] are usually done with dilute mineral acids [31]. The accelerated corrosion of zinc results from the predominant cathodic reaction in a highly acidic medium [32]. However, the added inhibitor plays a crucial role by getting adsorbed onto the metal surface, thereby decreasing the speed of hydrogen evolution and protecting the metal surface [7]. Many researchers have comprehensively deliberated the corrosion performance of zinc and inhibition of its corrosion in the acidic environment [33].
HCl is one of the expansively recommended mineral acids for metal pickling [32, 34,35,36]. Even though the significant role of the acid here is to remove extraneous materials present on the surface of the material, due to its aggressive nature, even though it is dilute, it dissolves the materials to a considerable extent. Hence, to attenuate the metal dissolution, inhibitors are recurrently added to the acidic solutions. The inhibitor's efficacy depends on its structure, electrolyte composition, and the charge on the metal surface [37, 38]. Diverse categories of organic compounds have been successfully used as inhibitors.
Many researchers have considered the dissolution of zinc in HCl employing different classes of chemical inhibitors. Important among them are: ethoxylated fatty alcohols [39], catholyte containing amino-benzotriazole [40] (Fig. S1), aniline [41], 2-[4-(methylthio) phenyl] acetohydrazide and 5-[4(methylthio)benzyl]-4H-1,2,4-triazole-3-thiol [42] (Fig. S2), triethylamine, ethanolamine and triethanolamine [43] (Fig. S3), cetyltrimethyl ammonium bromide (CTAB), bromohexadecyl pyridine and nicotinic acid [9] (Fig. S4), semicarbazide, thiosemicarbazide and diphenylcarbazide [44] (Fig. S5), C26H22N8O4, C26H22N8O2, C24H18N8O2, C24H16Cl2N8O2, C24H16N10O6 [45] (Fig. S6).
Limited studies were accomplished on corrosion inhibition of zinc in nitric acid, sulfamic acid, and phosphoric acid. Nitric acid is strongly oxidizing and hence attacks most metals [56]. Even though phosphoric acid is quite mild, it is reported to corrode zinc significantly [57]. Sulfamic acid is a strong acid used as a cleaning agent to remove rust, algae, and hard water scale from cooling towers and condensers [58]. Inhibitors like organic phosphonium and ammonium compounds (Fig. S13) were tried and tested as anticorrosive agents for corrosion of zinc in 1.0 M H3PO4 [57]. In addition, isomers of toluidines in sulfamic acid and HNO3 [58, 59] (Fig. S14) and ethylamine were tried for corrosion hindrance of zinc in HNO3 [60]. The inhibitors mentioned above have shown good inhibition efficiency.
Careful observation of Table 1 clearly shows that researchers adopted both non-electrochemical and electrochemical methods for corrosion rate measurements. The non-electrochemical method, popularly known as the classical method, mainly involves weight loss techniques [58,59,60]. On the other hand, potentiodynamic polarization (PDP) measurements and electrochemical impedance spectroscopy (EIS) techniques are extensively applied at laboratory levels. A detailed procedure for these is available in the literature [49, 50, 57].
Adsorption isotherm provides information regarding the mode of adsorption of inhibitors onto the metal surface. In addition, it helps evaluate thermodynamic parameters related to adsorption [62]. Langmuir, Flory-Huggins, Frumkin, Bockris-Swinkel, and Temkin were significant isotherms tested and tried to fit experimental data. Linear correlation coefficient (R2) values, close to unity, were taken to measure best fitment [63] (Fig. S15).
Adsorption of the inhibitor can take place either due to physisorption and chemisorption. Physical adsorption encompasses the electrostatic force of attraction between metal and inhibitor. Chemical adsorption is a consequence of the covalent/coordination bond between metal and inhibitor. The schematic representation of adsorption of inhibitor through physisorption and chemisorption is represented in Fig. 2.
When studies were done under identical conditions [42] among 2-[4-(methylthio) phenyl] acetohydrazide and 5-[4(methylthio)benzyl]-4 h-1,2,4-triazole-3-thiol, the latter turned out to be a very good inhibitor. This is because triazole derivatives are expected to have better adsorption capacity than hydrazide derivatives due to two nitrogen and one sulfur atom.
Corrosion inhibiting capacity of semicarbazide was not as much of thiosemicarbazide and diphenyl carbazide [44]. Diphenylcarbazide demonstrations improved efficiency due to the presence of aryl group and then comes thiosemicarbazide and semicarbazide. The efficiency of the inhibitor depends on molecular size and charge density on the active sites, and it increases with an increase in both [64]. Further, aryl groups are more protective than alkyl groups. So diphenylcarbazide tops the series. Among thiosemicarbazide and semicarbazide, thiosemicarbazide is a better inhibitor due to the presence of sulfur atom, which has more tendency for adsorption than oxygen atom [64].
The maximum inhibition efficiency was observed for PEG400 and (FPEA) [67]. From the surface studies, it was seen that PEG400 strongly adsorbed onto the zinc surface, and there was almost no presence of zinc oxide formed. However, the number of inhibitors tried, tested, and reported for their anticorrosive property in alkaline medium is relatively less. This is because zinc undergoes severe corrosion in a highly alkaline medium; electrochemical measurement for the corrosion is very challenging in such a situation.
Manov et al. [69] explored the corrosion activities of zinc with 2-hydrazono-3-bornan-emethylenedithiol disodium salt (Fig. S17) with chelating groups as inhibitors at pH 6. (mixture of 0.2 M Na2SO4 and 0.2 M NaCl). Inhibitor principally intimidated cathodic reaction. It is due to the chelation between zinc and organic molecules, which formed a protective organometallic layer on zinc. Another study was carried out in an aqueous chloride-sulfate medium [70] using Benzaldehyde thiosemicarbazone (BTSC) (Fig. S18) as the inhibitor. Due to highly electronegative nitrogen and sulfur atoms, a strong adherent layer was formed on the metal surface. It was ascertained by surface morphology studies by SEM, EDX, and FT-IR.
Almost a inhibitors showed excellent inhibition efficiency; among them, Ce3+ [73] showed maximum inhibition efficiency. This is because of the direct interaction between hydrated Ce3+ with hydroxide of the solution resulting in the formation of cerium-rich oxide and hydroxide layer.
The majority of literature specified that corrosion and inhibition studies are accomplished employing both classical and electrochemical approaches. Weight loss is one of the most commonly adopted and most reliable methods under classical techniques. It gives highly reproducible results and is still being used by many researchers. However, this method is time-consuming [63]. It will not provide much information regarding the nature of the corrosion inhibition process. Nowadays, researchers prefer fast electrochemical techniques. The potentiodynamic polarization method (PDP), (Fig S19a), which is quite fast, gives a lot of information regarding corrosion current density, corrosion potential, etc. All these electrochemical parameters are beneficial in arriving at the nature and mechanism of corrosion inhibition. The electrochemical impedance spectroscopy (EIS) method helps understand the mechanistic aspects of the corrosion and inhibition process [79] (Fig. S19b). It provides detailed information on the various resistance parameters that play a decisive role in the corrosion and inhibition process. From the carefully done experimental observation, it is possible to prove that inhibition efficiency evaluated from all three methods will agree with one another with less than 2% deviation.
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