PITTING POTENTIALS OF ALUMINIUM ALLOYS AND THEIR USE TO DETERMINE THE PROBABILITY OF PITTING CORROSION

摘要

The pitting potential has been used historically to measure the susceptibility of metals to pitting corrosion. These measurements were used in most cases as either a qualitative and/or relative measure of pitting corrosion susceptibility. Where the absolute pitting potential directly or the difference between the open circuit and pitting potentials are compared. There is no work that the authors are aware of that has attempted to use the measurement of pitting potentials quantitatively. Pitting potentials are known to change with changes in both the electrode area and the potential scan rate, with decreases in the pitting potential when the electrode area increases and when the potential scan rate decreases. This behaviour suggests that the pitting potential may be a statistical measurement and could be employed to determine pitting corrosion probabilities. A large study of pitting potentials was undertaken at DSTO for two aircraft aluminium alloys in an attempt to map the pitting corrosion likelihood with changes in exposure electrolyte composition, for example the concentration of chloride, inhibitors and pH. This approach was found to be difficult due to the inherent qualitative nature of the measurement and the correlation between the short term electrochemical testing and long term field exposure. The failure of this method led to the development of a probabilistic method based upon the metastable pitting behaviour of aluminium alloys. This method was successful but required a significant amount of detailed electrochemical testing. Due to this the use of pitting potentials was revisited and a new simple method to determine the probability of pitting corrosion on aluminium alloys was developed and is reported here.