Nitrogen-containing six-membered heterocyclic compounds as corrosion inhibitors for metals in solutions of mineral acids – A review

Abstract

The current state of studies on metal protection from corrosion in mineral acid solutions by six-membered N-containing heterocyclic compounds is reviewed. Literature data on the protection of various metals in acid solutions by these compounds are summarized. The specific features of their protective action mechanism are discussed. The feasibility of metal protection by formulations containing these compounds, even in high-temperature corrosion, is noted. N-Containing six-membered heterocyclic compounds are prone to adsorption on metal surfaces from mineral acid solutions. Based on the free adsorption energy values of these corrosion inhibitors (CIs) on metal surfaces, it can be deemed with high probability that they are mostly bound to a metal by physical forces, but upon adsorption on its surface they slow down the cathodic and anodic reactions, and eventually inhibit its corrosion. Compounds containing S atoms or bulky substituents are most interesting. These compounds are adsorbed on metals more strongly and behave as more efficient CIs. It often happens that such CIs hinder metal corrosion in HCl solutions but do it much more weakly in H 2 SO 4 , HClO 4 , and H 3 PO 4. A known method for improving the protective effect of N-containing six-membered heterocyclic CIs in these media involves combining them with anionic additives, e.g., halide or rhodanide anions. The heterocycles discussed behave as CIs in "cold" solutions but lose these properties at higher temperatures. Examples of the use of six-membered N-containing heterocycles as CIs of various steels and non-ferrous metals (Al, Cu, Sn, Zn and their alloys) are available in literature. The industrial application of individual six-membered N-containing heterocycles and their derivatives for metal protection in acid media is of little promise and is unjustified. It is more appropriate to use these compounds as components of inhibitor mixtures. Mixed CIs containing these compounds can hinder corrosion even in such corrosive media as high-temperature HCl solutions or hot H 3 PO 4 solutions. The base for creating prospective mixed corrosion inhibitors for metals in acids should be searched for among six-membered heterocyclic compounds containing two or more nitrogen atoms or compounds obtained from natural raw materials. The bibliography includes 150 references.

Environmentally friendly inhibitor of the corrosion of mild steel: Commercial oil of Eucalyptus

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