Electrochemical and corrosion behavior of AK12M2 alloy in a model solution used in heating systems

• A.G. Rakoch¹, A.A. Lobach², E.P. Monakhova¹, V.V. Begnarskii², O.V. Volkova¹ and Tran Van Tuan¹

¹ Department of Metallurgy of Steel, New Production Technologies & Protection of Metals, College of Environmentally Sound Technologies & Engineering, National University of Science and Technology MISiS, Leninsky prosp., 4, 119049 Moscow, Russian Federation
² RIFAR Join Stock Company, Tehnologicheskij proezd, 18, 462630 Gaj, Orenburgskaya oblast, Russian Federation

Abstract: At present, AK12M2 alloy is widely used for die casting sections of heating radiators. AK12M2 alloy has a higher content of iron and copper compared to silumin AK12. These alloying elements can cause the formation of effective cathodes in silumin and, consequently, activation of corrosive processes, including the most dangerous local ones. This article presents, for the first time, data on the dependence of manifestation and intensity of the development of ulcerative corrosion of AK12M2 alloy in a model electrolyte used in heating systems on the oxygen content and temperature. In addition, it is shown that accelerated electrochemical tests can be used to assess the risk of manifestation and development of ulcerative corrosion of AK12M2 alloy in alkaline electrolytes (pH = 10). The following mechanisms of local types of corrosion of AK12M2 alloy in these electrolytes are assumed: 1) at room temperature with an oxygen content of 8.6 mg/l in solution and at a high temperature of 90°C at all oxygen concentrations in alkaline electrolytes, the initial preferential dissolution of aluminum from silumin results in enrichment of its surface layer with silicon. Its subsequent dissolution results in electrolysis of anions and formation of a composite film based on silicon and its dioxide. At room temperature, only alkaline electrolytes deoxygenated to 0.12 or 0.04 mg/l show predominant chemical etching of silicon from silumins and formation of a light gray amorphous silicon dioxide based film; 2) intensification of local corrosion at high oxygen concentrations in alkaline electrolytes on exposed nano- or micro-surfaces of AK12M2 alloy whose major fraction of the surface is covered with a dark gray composite film. Self-destruction of intermetallides occurs on these exposed areas. It initiates the nucleation of cavities and excretion of copper and iron that are effective cathodes causing the formation of ulcers.

Keywords: AK12M2 alloy, alkaline electrolytes, ulcerative corrosion, intermetallides, electrochemical tests

Int. J. Corros. Scale Inhib., 2022, 11, no. 3, 1115-1130
doi: 10.17675/2305-6894-2022-11-3-13

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