The effect of pH on the mechanism of corrosion and stress corrosion and degradation of mechanical properties of AA6061 and Nextel 440 fiber-reinforced AA6061 composite

The breaking load method was used to determine the degradation rate of mechanical properties of wrought T6 aluminum 6061, and Nextel 440 fiber-reinforced AA6061 composite, as a function of pH, by measuring their post-exposure residual strength. The materials were exposed to acidic NaCl at two initial pH values of 1.5 and 2. Two different exposure techniques were employed: one in which the pH of the solution was kept constant by refreshing the corrosive medium, and a second method in which the solution was not refreshed. The change in pH with exposure time was also investigated to provide a more comprehensive understanding of the role of H+ ions in the corrosion process. Ar. an initial pH of 1.5 the matrix showed a significant strengthening response, after an initial decrease in residual strength with time, if the solution was unrefreshed. If the solution was refreshed, the residual strength for both the corrosion and stress corrosion specimens continued to decline at an approximately constant rate. The specimens at an initial pH of 2 showed a much more gradual strengthening because of the slower initial reaction rates between the solution and the alloy. The Nextel 440 fiber-reinforced AA6061 composite was found to be considerably more susceptible to corrosion than the AA6061 matrix. From the results obtained. it was concluded that the pH of the solution played a crucial role in the degradation rate of mechanical properties for the AA6061 matrix and the Nextel 440 fiber-reinforced AA6061 composite. (C) 1998 Elsevier Science Ltd. All rights reserved.