Investigation and discussion of characteristics for intermetallic phases common to aluminum alloys as a function of solution pH

This paper presents results for corrosion potentials, pitting potentials, and electrochemical characteristics for intermetallic particles commonly present in high strength aluminum-based alloys, for tests conducted in a 0.1 M NaCl solution of varying pH via the use of a microcapillary electrochemical cell. The intermetallics investigated were Mg2 Si, MgZn2, Al7 Cu2 Fe, Al2 Cu, Al2 CuMg, and Al3 Fe. Elaboration of the results reveals that the electrochemical behavior of such compounds varies markedly with pH, with attendant ramifications for localized corrosion and protection in Al alloys. Examples of this are shown for AA7075-T651, where it is shown that the localized corrosion morphology is drastically different upon the bulk alloy depending on the pH of the test environment. A stochastic pitting is observed at an acid pH, near-neutral conditions result in a deterministic-type pitting, and a general corrosion is observed at an alkaline pH.