THE DISSOLUTION MECHANISM OF PROTECTIVE METAL COATINGS DURING THE CHEMICAL ETCHING OF HEAT ENGINE COMPONENTS

Abstract

The aim of this research is to analyse processes occurring on the surface of engine components during their chemical etching. The study is based on experimental data on the removal of aluminide coatings. Aluminide coatings consist of intermetallic phases (Ni_xAl_y, M_xCl_y, M_xSi_y) and compounds exhibiting various levels of chemical activity (α-Cr, α-W, Si, Y). The efficiency of removing defective coatings (the rate of the process and the quality of etched surfaces) is determined by the ratio of components in the etching solution, which either stimulate or inhibit electrochemical reactions within the «solution-coating-alloy» system. It is shown that Cl^- ions contribute to the active dissolution of the Ni_xAl_y coating matrix, whereas the adsorption of (NO₃)^- can have either a stimulating or a passivating effect. The dissolution rate of the β-NiAl phase is determined by the release of Al³⁺ ions into the etching solution. These ions are known to be the most electrochemically-active component of aluminide coatings. The dissolution of the γ΄-Ni₃Al phase is limited by the formation of hydrated Ni₂⁺ ions. The removal of the corrosion-resistant M_xCl_y and M_xSi_y phases occurs due to the weakening of their adhesion to the coating matrix. The behaviour of (Cr₂O₇)^2- ions is shown to be in excellent agreement with classical chemical concepts. Acting as effective depolarizers, (Cr₂O₇)^2- ions accelerate the etching process; however, they also inhibit the etching of the nickel alloy by being adsorbed on its surface. The high dissolution rates of coatings in the presence of (NH₄)₆Mo₇O₂₄, which does not exhibit pronounced oxidizing properties, are explained by a possible transformation of (NH₄)₆Mo₇O₂₄ into (MoO₂)²⁺ and (MoO₄)^2-, followed by its reduction to MoO₂ and subsequent oxidation to (MoO₂)²⁺.