METALLURGICAL FACTORS INFLUENCING THE CORROSION OF ALUMINUM, AL-CU, AND AL-SI ALLOY THIN-FILMS IN DILUTE HYDROFLUORIC SOLUTION

The corrosion behavior of sputter-deposited Al, Al-Cu, and Al-Si alloy thin films in dilute hydrofluoric (HF) acid solution was investigated. These materials maintain a thin aluminum oxide film in dilute HF solutions and, consequently, are susceptible to localized corrosion. Pit densities increase for the alloys with Cu and, to a lesser extent, Si additions. Open circuit potentials (OCP) are more positive for such alloys relative to the OCP of pure Al. Metastable pits in Al-Cu alloys are formed in Cu-depleted zones at grain boundaries which are galvanically coupled to adjacent theta-Al2Cu precipitates. Metastable pits in Al-Si alloys are formed in the Al matrix which is galvanically coupled to adjacent elemental Si nodules. Theta-Al2Cu has different electrochemical characteristics than Al, even though both maintain a thin Al oxide in dilute HF solutions. Theta-Al2Cu has a more positive OCP than pure Al and facilitates cathodic reactions at enhanced rates relative to pure Al. Hence, its presence raises the potential of the adjacent pure Al grain boundary to potentials which increase the probability of metastable pitting. Evidence is also presented which suggests that metastable pit growth may be cathode limited. A new hypothesis describing one mechanism by which theta-Al2Cu supports cathodic electron transfer reactions is discussed.