Evaluation of a Trivalent Chromium Process (TCP) Conversion Coating on AA2024-T3 That Requires No Surface Pretreatment

Abstract

We report on the study of a new commercial trivalent chromium process (TCP) conversion coating, TCP-1, that requires no deoxidation or etching surface preparation of the aluminum alloy for optimum performance. The corrosion resistance provided by TCP-1 and a comparable conversion coating, TCP-2, both formed by immersion on AA2024-T3, were evaluated using scanning electron microscopy and energy dispersive X-ray analysis, digital microscopy, XPS depth profiling, electrochemical impedance spectroscopy and potentiodynamic polarization techniques. Accelerated degradation testing was also performed to determine how the laboratory electrochemical data correlate with the coating's corrosion protection during neutral salt-spray exposure and full-immersion testing.The results indicate that TCP-1 produces a protective coating on the metal that minimizes corrosion to an equivalent degree as TCP-2; a conversion coating that requires a conventional deoxidation step prior to application. Electrochemical data revealed that both TCP coatings increase the polarization resistance by ∼100x and suppress anodic and cathodic currents by ∼10-100x, as compared to the uncoated alloy. Specimens coated with either TCP coating exhibited little degradation, discoloration or corrosion during a 14-day neutral salt-spray (ASTM B117) exposure and a 14-day full immersion in 3.5 wt.% NaCl at 55°C. In summary, the stand-alone corrosion protection provided by TCP-1 to this alloy is as good as the protection provided by a standard TCP coating.

Investigation of the Trivalent Chromium Process Conversion Coating as a Sealant for Anodized AA2024-T3

“…Importantly, both are over 1000× higher than the value for a unanodized (degreased and deoxidized only) alloy. [25][26][27][28][29][30][41][42][43][44] The remainder of the curves in the mid (capacitive) and high frequency (solution and electrode ohmic resistance) regions are overlapping with no distinct differences.…”

“…Based on past studies of TCP conversion coating formation on aluminum alloys, it is presumed the sealant layer consists of relatively insoluble ZrO 2 and Cr(OH) 3 . [25][26][27][28][29][30]41,43,44 These chemical species are formed by hydrolysis of the coating bath precursors, H 2 ZrF 6 and Cr 2 (SO 4 ) 3 , under alkaline conditions according to the following two chemical reactions:…”

“…The electrochemical behavior of this particular TCP conversion coating on AA2024-T3 has recently been evaluated. 25 The key research questions addressed in the present work were (i) how does TCP seal the anodic coating and does the sealant penetrate the pores of the oxide, (ii) to what degree does the TCP sealant increase the corrosion resistance of the oxide coating, and (iii) how does any corrosion resistance enhancement predicted by electrochemical measurements compare with what is observed during accelerated degradation testing? There is published literature on the formation, structure and corrosion resistance of TCP conversion coatings applied to aluminum alloys, as exemplified in these selected citations.…”

“…There is published literature on the formation, structure and corrosion resistance of TCP conversion coatings applied to aluminum alloys, as exemplified in these selected citations. [25][26][27][28][29][30] Other than a Department of Defense report, we are unaware of any published literature on TCP sealing of anodized aluminum. 15 There has been one recent publication describing the sealing of anodic coatings on aluminum alloys 2618 and 7175 with a solution containing trivalent chromium Cr(III) and fluorozirconate Zr(IV) salts.…”