Examination of tarnish films formed during SCC of α-brass in ammoniacal solutions

The tarnish films formed on the sample surfaces during the stress corrosion cracking (SCC) of unannealed and annealed alpha brass (an alloy of 67%Cu-33%Zn) in ammoniacal solution were examined. The color and structure of the films varied depending on the solution pH, temperature and copper concentration of solution. Black tarnish film formed easily in the pH range where the fastest cracking was observed at all temperatures applied. Green tarnish film formation was noticed at higher pH values than the fastest cracking range. Longer failure times were observed in this case. Reddish brown powdery or flaky tarnish film formed at pH values lower than the fastest cracking range and no cracking occurred in this case. All fracture surfaces were yellow-red. The samples cracked faster in higher copper concentrations due to faster oxidation. The tarnish films were examined by SEM and analysed by EDS. Zinc depletion was observed on the sample surfaces in all colors of tarnish film.