Abstract
Studies were made of the liquid within a stress corrosion crack in 70:30 brass (CDA Alloy No. 260) propagating in 15N ammonium hydroxide containing 8 g/l dissolved copper. The liquid in the crack was isolated for study by removing the sample from the solution, immersing the sample in liquid nitrogen to freeze the liquid within the crack, fracturing the sample, and removing the liquid once it had thawed. Spectrophotometric analysis of the liquid indicated a copper concentration of 14.5 g/l and a zinc concentration of 11.5 g/l after one hour exposure and a crack length of 5 mm. The relative dissolution rates of copper and zinc were 0.57 compared to the 2.3 ratio of these elements in the alloy. The total corrosion rate in the crack was estimated to be 4 × 10−6 g/cm2/min. The corrosion rate of a bulk sample under identical experimental conditions was 5.4 × 10−6 g/cm2/min. The pH within the crack showed no measurable change during crack propagation. The Pourbaix diagram was calculated for pure copper and pure zinc immersed in 15N ammonium hydroxide containing 8 g/l dissolved copper. Potential and pH measurements on stressed samples indicated that zinc should dissolve with the formation of Zn(NH3)4+2 and that the stable copper species are cuprous oxide and a cuprous ammonia complex. Dissolved copper and zinc greatly increase the conductivity of 15N ammonium hydroxide. The tendency of 70:30 brass to undergo stress corrosion cracking (SCC) in concentrated ammonium hydroxide is a result of dezincification at the base of the crack and consequent weakening of the matrix. The high conductivity of the solution within the crack presumably enhances local cell action within the crack.
