Oxidation of the three-phase Cu–20Ni–30Cr and Cu–20Ni–40Cr alloys at 700–800 °C in 1 atm O 2

Abstract

The oxidation of two ternary Cu–Ni–Cr alloys containing approximately 30 and 40 at.% Cr, but with a similar Ni content, was studied at 700–800 °C in 1 atm of pure oxygen. Both alloys contain a mixture of three phases, where the phase with the largest copper and lowest chromium content (α) forms the matrix, while the phase with an intermediate content of Ni and Cr (β) and that richest in chromium (γ) are present in the form of particles dispersed in the α matrix. The kinetics of oxidation were rather irregular and presented two approximately parabolic stages which for the alloy with 40 at.% Cr were followed by a final nearly linear stage. Generally, the corrosion rates decreased by increasing the chromium content in the alloy under constant temperature and increased with temperature for a constant alloy composition. The scales formed on the two alloys were rather complex and consisted in most cases of an outermost copper oxide layer followed by a layer containing a mixture of oxides of nickel and copper as well as Cu–Cr and Ni–Cr spinel and finally by an innermost very irregular and convoluted but continuous Cr 2O 3 layer which protruded into the alloy and contained a number of Cu-rich metal islands.