Electrochemical Characterization of Trivalent Chromium Process (TCP) Coatings on Aluminum Alloys 6061 and 7075

The electrochemical characterization of trivalent chromium process (TCP) coatings formed on AA6061-T6 and AA7075-T6 is reported. The coating provided corrosion protection to both alloys during full immersion tests (room temperature) in air-saturated Na2SO4 and Na2SO4 + NaCl. Evidence of corrosion protection was the 10-100x increase in the polarization resistance and decrease in the corrosion current density for the coated alloys in both electrolyte solutions as compared to the uncoated controls. The data indicate the coating on AA6061 provides protection primarily through cathodic inhibition while the coating on AA7075 provides both anodic and cathodic protection. The coating provides corrosion protection even in the presence of Cl− as no pitting was detected on AA6061 at potentials up to 0.4 V vs. Ag/AgCl (1.2 V vs. OCP) and the pitting potential for AA7075 was shifted positive by ∼400 mV as compared to the untreated controls. The electrochemical parameters were used to calculate an apparent coating porosity (i.e., defect parameter). The apparent porosity decreased in order of AA2024 > AA7075 > AA6061. A model is presented that describes the coating as a partially blocking layer with some hydrated channels and or defects through which ions and dissolved oxygen can reach a low fraction of the underlying metal surface.