Pit Initiation in Aluminum and Aluminum Alloys

The growth of pits in passive metals and alloys exposed to chloride solutions is reasonably well understood and has generally been attributed to chemistry changes in occluded cavities in the metal surfaces.  The initiation of pits, however, is still a matter of some conjecture.  In particular the processes associated with the propagation of stable pits, versus pits that initiate but then apparently re-passivate are not particularly well understood.  It is well known that, many passive metals and alloys exhibit well-established relationship between the onset of stable pitting (as measured by an irreversible pitting potential) and chloride concentration [1].  The goal of this program is to study the local breakdown of passive films by onset of pitting in dilute chloride solutions where it is presumed that the pitting processes will be delayed and the specific interactions between chloride and passive films can be examined in more detail.  Pitting potential experiments on A1100 aluminum and A2024 aluminum alloys were performed in aqueous chloride solution, as a function of pH, from 10^-6 M to 3 M chloride, using conventional potentiodynamic techniques.  The results obtained for both alloys, in solutions ranging from 10^-3 M to 3 M chloride replicate those of McCafferty [1] that showed a specific relationship between pitting potential and chloride concentration.  However, the sensitivity of the pitting potential to chloride concentration shows a well defined inflection at chloride concentration of approximately 10^-3M.  Data will be presented comparing the pit distributions and morphologies at low, intermediate and high chloride concentrations.  The results will be discussed in terms of the specific interactions of chloride with passive films, transport of the chloride through the passive films, and the reaction of the chloride with the underlying metal surfaces.

1.  McCafferty, E. (2003). Sequence of steps in the pitting of aluminum by chloride ions. Corrosion Science, 1421-1438.

This project was supported by the National Science Foundation under grant #NSF-DMR 1309509