Electrochemical dissolution of MCrAlY coatings and oxide layers on Ni-based superalloys in 3.5% NaCl and aqua-regia solutions

Abstract

Superalloys are widely used for aerospace industry. At the same time, more and more superalloy scraps are produced. Therefore, the recycling and reuse of superalloy scraps is of great significance. Purification smelting after surface impurity removal is an application trend of superalloy regeneration technology. Electrochemical methods can be used to remove metal impurities such as oxide layers and coatings. The electrochemical dissolution behavior of a Ni-based superalloy with coating and oxide layer has been studied in 3.5 wt% NaCl solution and aqua regia solution. The results show that pitting corrosion occurs on the surface of the alloy in 3.5 wt% NaCl solution, forming a "occlude corrosion cell" and leaving a lot of pitting corrosion. The oxide layer itself is a passivation film with high stability in 3.5 wt% NaCl solution. However, in aqua regia solution, the alloy underwent strong corrosion, and the surface coating and oxide layer of the alloy fell off in large quantities. The results show that the oxide layer like a “coating” with high corrosion resistance, which prevents the reaction between the alloy and the electrolyte. However, due to the poor density of the oxide layer, the electrolyte flows into the alloy matrix along the oxide layer gap, and electrochemical reaction occurs with the matrix. In aqua regia electrolyte, the oxide layer is thus peeled off. In this study, the dissolution behavior of the coating and the oxide layer in different electrolyte solutions was clarified, and the dissolution model of the coating on the alloy surface and the spalling mechanism of the oxide layer were established.