Abstract
Protection of metals in neutral media with pH 5.0–9.0 (in humid atmospheres and various aqueous solutions) can be achieved by formation of thin coatings (up to several tens of nm) on their surfaces due to adsorption and more complex chemical interactions of organic corrosion inhibitors (OCIs) with the metal to be protected. The review contains three sections. The first section deals with coatings formed in aqueous solutions, while the second one, with those formed in organic and water-organic solvents. Here we consider metal protection by coatings mainly formed by the best-known classes of OCI (carboxylates, organophosphates and phosphonates) and estimation of its efficiency. The third section discusses the peculiarities of protection of metals in the vapor-gas phase, i.e., by volatile OCIs, and a relatively new type of metal protection against atmospheric corrosion by the so-called chamber inhibitors. OCIs with relatively low volatility under normal conditions can be used as chamber OCIs. To obtain a protective coating on the surfaces of metal items, they are placed in a chamber inside which an increased concentration of vapors of a chamber OCI is maintained by increasing the temperature. This review mainly focuses on the protection of iron, steels, copper and zinc.
Highlights
According to ISO 8044-1989, corrosion inhibitors (CIs) (Appendix A: Table A1) are chemical compounds or their formulations “which, when present in a system at a sufficient concentration, reduce the corrosion rate of a metal without significantly changing the concentration of the corrosive agent”
Protection of metals in humid atmospheres and aqueous solutions with pH 5.0–9.0 can be achieved by formation of thin coatings on their surfaces due to the adsorption of organic corrosion inhibitors (OCIs) and by more complex chemical interactions with the metal to be protected
In a discussion on the formation of such coatings from aqueous solutions, the important role of the chemical structure of OCIs was shown for salts of carboxylic, phosphonic and dialkylphosphoric acids as an example
Summary
According to ISO 8044-1989, corrosion inhibitors (CIs) (Appendix A: Table A1) are chemical compounds or their formulations “which, when present in a system at a sufficient concentration, reduce the corrosion rate of a metal without significantly changing the concentration of the corrosive agent”. SAM blocks Cl− diffusion to the oxide film defects, preventing its breakdown in a wide range of passive potentials It has long been known [41] that the corrosion protection efficiency of metals (mild steel, Cu, Al) by alkyl monocarboxylates (ALCs) and α,ω-dicarboxylates with a straight chain NaOOC(CH2)nCOONa in neutral aqueous solutions depend on its length. In addition to ALC and the above-mentioned salts of SPhAn and its substituted derivatives, other monocarboxylates deserve attention, such as sodium oleate C17H33COONa (SOl) or sodium N-oleylsarcosinate (SOS) NaOOC-CH2N(CH3)C(O)C17H33, whose anions have an even greater hydrophobicity (logD = 5.48 and 4.03 at pH 7.4, respectively) than in the case of SFF (logD = 2.57) Their high protective properties in neutral media are well known both for ferrous and other metals (Cu, Zn, Al and their alloys) [7,21]. Such passivation has proven good for other metals, in particular, Al alloys [71]
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