The Effect of Surface Treatment on the Performance of a Trivalent Chromium Process (TCP) Pretreatment on AA7075 Aerospace Alloys for the Protection Against Filiform Corrosion

The performance of a trivalent chromium process (TCP) pretreatment conversion coating, applied to AA7075 aerospace aluminum alloys, for its ability to improve the filiform corrosion (FFC) of a polyvinyl-butyral coating, was studied as a function of surface treatment. Data from atmospheric FFC and full-immersion corrosion experiments are compared with surface characterization data to gain an understanding of the FFC kinetics on this alloy and mechanism by which the TCP coating provides inhibition. Specimens were prepared with three different surface treatments and studied with and without a TCP pretreatment conversion coating. A biphasic layer incorporating a zirconium-chromium mixed oxide outer layer and a fluoroaluminate interfacial layer was observed on all pretreated specimens. Enhanced corrosion protection by the TCP coating was observed when an acid pickle surface treatment was applied and the FFC filament propagation was stifled with an efficiency of 86%. A zirconium-rich outer oxide layer of approximately 22 nm, more than double the thickness of that measured for the other surface-treatments, is reported. Such layers are reported to be sufficiently dense so as to block ion transport from solution to the alloy surface. This acid pickle surface treatment is shown to effectively reduce the thickness of the native oxide layer and remove intermetallic particles from the surface enabling the formation of a more homogeneous TCP layer. Furthermore, Cu replating resulting from an acidic deoxidation step is shown to have no discernable effect on FFC.