A study of the protective properties of oxide films formed at reactive aluminum-copper and aluminum-beryllium alloys

Two Al alloys of interest to the aerospace industry were studied during this research. AA2219 is an Al/Cu alloy containing about 6%-wt Cu, forming Cu-rich particles in a Cu-poor Al-matrix. AlBeMet162 is an At/Be alloy consisting of ca. 38%-wt A1 and 62%-wt Be, forming intermeshed networks of Al and Be. Both alloys do not respond to standard anodization techniques in sulfuric or chromic acid, yielding pitted surfaces and friable anodic coatings. Corrosion protection can be achieved on AA2219 but a significant part of its fatigue strength will be sacrificed as a result of these anodization processes, while no corrosion protection results from the anodization of AIBeMetl62. Thus, the main goals of this work were preventing the dissolution of the more reactive alloy constituents and electrochemical formation of a passivating surface coating. Cyclic Voltammetry methods (CV) were used as a means of creating barrier and porous oxide layers on AA2219, AlBeMet162 and the individual phases making up these alloys. The CV data and measurements of Electrochemical Impedance Spectroscopy (EIS) were used to determine the thickness and dissolution activity of the barrier oxide layers formed. Changes in the barrier oxide thickness during EIS and CV experiments in neutral borate and phosphate buffer as well as H2SO4 and H3PO4 solutions provided a means for predicting whether a stable barrier or porous oxide film would likely be formed in a given electrolyte. This approach was shown to be usable even in the presence of an overlying porous oxide film. The CV data were also examined for an indication of the oxide growth mechanism, with a particular focus on the High Field and Point Defect models, indicating that the High Field Model more closely fits the data acquired. EIS data collected in 5%-wt NaCl solution were shown to give a good indication of the corrosion protection granted by anodic coatings formed on A1 alloys and thus to be a suitable method for accelerated corrosion testing. DC anodization at only 2.5 V in neutral buffers and acidic solutions as well as and AC/DC anodization at voltages exceeding 100 V in alkaline silicate solutions were also studied and shown to successfully form anodic oxides without dissolving the reactive constituents of AA2219. Accelerated corrosion testing and microscopic examination of corroded samples suggested that neither of these coatings supplied acceptable corrosion protection, but the silicate coatings appeared very promising, and further optimization of this approach is recommended.