Triazoles as a class of multifunctional corrosion inhibitors. Review. Part IV. Magnesium alloys

• Yu.I. Kuznetsov

A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninsky pr. 31, 119071 Moscow, Russian Federation

Abstract: This article continues the review of studies (2005–2020) on organic corrosion inhibitors (CIs) of the triazole class and their effect on the corrosion-electrochemical behavior of Mg alloys. In contrast to the protection of Cu, Zn, Fe, Al, their alloys, and steels with 1,2,3-benzotriazole (BTA) and its derivatives, the protection of Mg alloys is poorly studied; therefore, a wider group of triazoles is analyzed here. BTA, as a low-toxic reagent, has been successfully tested for replacing chromates in the electrolyte for AZ31B alloy anodization. An increase in C_BTA to 5 g/L increased the stability of the anodic film, which is explained by the participation of BTA in the formation of a thick and compact film on the alloy. Another option for protecting Mg with BTA is demonstrated by loading it into porous particles of a copolymer that are then sprayed onto the surface of the alloy, followed by coating the layer of encapsulated CI with an epoxy resin. The release of BTA from these particles begins with an increase in pH due to the cathodic reaction and corrosion. Triazoles are capable of inhibiting the corrosion of Mg alloys by blocking Cu-containing intermetallic compounds, impurities of iron and other metals that are more electropositive than Mg, which initiate local dissolution of the alloys. In a solution with pH 10, BTA^– anions act as a nucleating agent, which promotes the formation of a dense and highly crystalline Mg(OH)₂ film with a homogeneous nanostructure, passivating the AMlite alloy. This distinguishes the mechanism of its protection from that for Cu-containing aluminum alloys, which is associated with the formation of poorly soluble Cu(I)–BTA complexes. The passivating ability of 1,2,4-triazole (TA) and BTA in relation to technical Mg (Mg90) is small even in weakly alkaline solutions, but their derivatives (a mixture of substituted TA – IFKhAN-92 and 5-chloro-BTA), especially in mixtures with dioctyl phosphate CI are more effective. An increase in the efficiency of passivation can be achieved by adding vinyltrimethoxysilane to the mixed CI, which increases 8–9 fold the time until the appearance of foci of corrosion on Mg in a heat and moisture chamber and exceeds the efficiency of chromate protection. According to XPS results, the protection of Mg by these CIs is due to the formation of a thin film on it, consisting of a mixed Mg oxide-hydroxide with a chemisorbed nearly-monolayer of organic molecules. A new possibility of using triazoles to protect Mg alloys from atmospheric corrosion, which can be achieved by treatment of their surface in vapors of a low-volatile (“chamber”) СI at elevated temperatures, is noted. It was first shown for 5-chloro-BTA as an example.

Keywords: metal corrosion, adsorption, magnesium and its alloys, passivation, corrosion inhibitors, 1,2,3-benzotriazole, 1,2,4-triazole and its derivatives, encapsulated inhibitor

Int. J. Corros. Scale Inhib., 2021, 10, no. 1, 29-53 PDF (568 K)
doi: 10.17675/2305-6894-2021-10-1-2

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