Laser initiated corrosion pits on aluminum

A laser initiation technique was developed that could control both the time and the location of pit initiation on a pure aluminum electrode that was immersed in 1.0 M NaCl, pH 11 solution and held under an applied potential. A new technique which was based on the laser initiation technique was invented to initiate an ordered array of pits of up to 61 pits that were spaced 100 $\mu$m apart.A study of the shape of single pits by a 3-D post electrolysis SEM method revealed detailed characteristics of pitting corrosion, such as: pit repassivation and secondary pit nucleation within a primary pit. The study also showed that single corrosion pits tended to be round and polished during the early stages of growth ($<$90 s) for applied potentials of $-$0.7 to $-$0.52 V, SCE, which suggested that a precipitated electropolishing film was present on the pit surface. It was also found that the measured dependency of the current density and the radius of a single pit with time, agreed, for over 3 orders of magnitude of time, with a theoretical model which was based on the pit growth rate being limited by the diffusion controlled dissolution rate of a precipitated aluminum oxychloride film. Other experimental results were found to be inconsistent for a pit growth rate that was limited by the potential drop in solution. Therefore, for these conditions, it was concluded that the pit growth rate was limited by the diffusion controlled dissolution rate of a precipitated aluminum oxychloride film, despite the presence of a vigorous gas evolution reaction.Additional experimental results suggested that a condition for pit repassivation was the disappearance of the precipitated salt film. Once the salt film had disappeared, the pit stability was found to depend on both the (Cl$\sp-$) in the solution adjacent to the pit surface and the applied potential. The pit stability criteria was found to be independent of the pit size. However, larger pits were found to be more stable than smaller pits owing to the increased transport resistance for a larger pit. In conclusion, for this system, transport processes were found to be important in determining the pit growth rate and the pit stability.U of I OnlyETDs are only available to UIUC Users without author permissio