The formation of cuprous oxide films on α-brass stress-corrosion fracture surfaces

Abstract

A study has been made of the nature of the fracture surface of α-brass specimens after failure by stress-corrosion cracking in pH 7·2 Mattsson's solution. The yellow colour of the stress-corrosion fracture surface points to a possible inadequacy in the “tarnish-rupture” model of the mechanism of this instance of stress-corrosion. The techniques of cathodic reduction and chemical dissolution have been used to remove any oxide present on the original fracture surface. On subsequent re-immersion in pH 7·2 Mattsson's solution, the stress-corrosion fracture surface remains untarnished, whereas the mechanical fracture surface rapidly acquires a black tarnish film. Comparison of plateau lengths obtained on further cathodic reduction has shown that for equivalent times of re-immersion, a much thicker film forms on the mechanical fracture region than on the stress-corrosion region of the fracture surface. The experimental results are interpreted as being inconsistent with the “tarnish-rupture” model but consistent with a stress-corrosion mechanism involving dezincification at the crack tip.