Abstract
The effect of tarnish films on stress corrosion cracking behavior of pure copper and α-brass (Cu-10.1% Zn) was studied with respect to fracture strain of tarnish films formed in ammoniacal solutions of various concentrations (0.05-0.07M) at 70°C. Pure copper and α-brass were chosen since the former shows transgranular cracking whereas the latter does intergranular cracking. The onset of the fracture of tarnish film was detected from 1) the corrosion potential change upon straining at a strain rate of 1.0×10-4min-1 after 50hr immersion, and 2) the current change upon straining at the same strain rate after 23hr polarization at various potentials. The fracture strain was determined from the true strain at which the corrosion potential or the current starts to change. The fracture strain for the films on pure copper was found in the range of 1.1×10-3-1.4×10-3 and for the films on α-brass in the range of 1.3×10-3-1.6×10-3. The tarnish films on pure copper was observed to fracture within grains of substrate while those on α-brass predominantly at grain boundaries of substrate. The crack propagation rate and the time to failure estimated by tarnish rupture theory utilizing the experimentally determined values of the fracture strain, and the creep rate during SCC tests were in good agreement with those observed under constant load.
