Effect of Deoxidation Pretreatment on the Corrosion Inhibition Provided by a Trivalent Chromium Process (TCP) Conversion Coating on AA2024-T3

Abstract

Aluminum alloy surfaces are typically (i) degreased and (ii) deoxidized and or desmutted to expose a clean and low-oxygen surface in preparation for the application of a pretreatment conversion coating. We hypothesized that the corrosion inhibition provided by one type of conversion coating, trivalent chromium process (TCP), would depend on the deoxidation chemistry and treatment time due to effects on the specimen's surface cleanliness and surface roughness, pit density, and pit depth. To this end, the effect of treatment time in an acidic fluoride deoxidizer/desmutter on the corrosion inhibition provided by a TCP coating on AA2024-T3 was studied. Longer treatment times of 6 and 12 min produced greater surface roughness, pit density and pit depth than did shorter times of 0.5 and 2 min. As a consequence, the TCP coating formed less completely and with more defects over the former surfaces. Greater corrosion inhibition was found on the less damaged specimens as evidenced by decreased anodic and cathodic currents in polarization curves, increased polarization resistances, Rp, and more positive breakdown or pitting potentials, Epit, in a choride-containing electrolyte solution. Deoxidation in 0.1 M NaOH was also investigated because of the more uniform aluminum dissolution that occurs in this medium. These specimen surfaces remained relatively smooth with the TCP coating providing the greatest level of corrosion inhibition, as evidenced by a larger Rp and more positive Epit. This is attributed to reduced surface roughing and pitting as well as the formation of an inert oxide on the exposed copper intermetallics.