The electrochemical properties (open-circuit potentials, anodic and cathodic polarization curve currents, and polarization resistances) were evaluated for AA7075-T6 alloys coated with three variants of a commercial trivalent chromium process (TCP) pretreatment coating. The coatings were formed on degreased and deoxidized aluminum alloy specimens. Measurements were made in oxygenated 0.5 M Na2SO4 and 3.5% NaCl. Comparison of coatings formed by immersion and spray was undertaken. The three coating variants were 650 chromitAL, versions E, V, and C. Similar concentrations of Cr were in all three coating baths but there were differences in Zr, Zn, S (likely as sulfate), and Fe among the three. TCP coatings formed by immersion exhibited electrochemical properties similar to those formed by spray. Overall, the greatest level of corrosion protection was provided by 650 E based on electrochemical data and results from a 14 d thin-layer mist (3.5% NaCl, 55°C) accelerated degradation test. The coating provides both anodic and cathodic protection in low-chloride electrolytes and functions as more of a cathodic inhibitor in high-chloride electrolytes. Rotating disk voltammetric data revealed the coating inhibits the reduction of dissolved oxygen by providing a diffusional barrier and possibly blocking sites for O2 chemisorption on the cathodically-active intermetallic phases.
Structure and chemical composition of different variants of a commercial trivalent chromium process (TCP) coating on aluminum alloy 7075-T6
