The sealing of an anodic coating on AA2024-T3 with a trivalent chromium process coating (TCP-HF) was investigated. Specimens were anodized in 9.8 wt% H 2 SO 4 at 15 V DC (23 min) producing an oxide coating weight of ca. 1200 mg ft −2 and a thickness of ca. 5 μm. Elemental analysis revealed the sealant forms across the outer oxide layer and within the pores up to a depth at least approaching 1 μm. Anodic and cathodic currents in potentiodynamic polarization curves were suppressed by 5-10× and low frequency impedance (Z 0.01 Hz ) values were 5× higher for anodized specimens sealed with TCP (∼5 × 10 6 ohm-cm 2 ), as compared to the unsealed anodized controls. During continuous exposure to 3.5 wt% NaCl, Z 0.01 Hz values remained unchanged and greater than 10 6 ohm-cm 2 after 5 days for the TCP-sealed specimen and decreased by 10× for the unsealed anodized control after just three days. A 14-day (336 h) neutral salt-spray exposure produced no significant pitting, discoloration or major coating detachment on anodized specimens sealed with TCP. Overall, the results indicate that the TCP sealant increases the barrier properties of the anodic coating and functions to provide anodic and cathodic corrosion protection.

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.

We report herein on the electrochemical behavior and corrosion resistance of as-prepared aluminum alloy, A360 (AlSi10Mg), prepared by additive manufacturing (AM), with and without a trivalent chromium process (TCP) conversion coating. Selective laser melting (SLM) was the 3D printing process used for the alloy build. The coating system performance, in terms of corrosion suppression, was assessed through electrochemical measurements and accelerated degradation testing, specifically a 14-day continuous neutral salt-spray (ASTM B117) exposure. The results indicate that a TCP conversion coating can be formed optimally by solution processing (degreasing and deoxidation) the alloy (x-z plane perpendicular to the build direction) with its native surface roughness (as-prepared) followed by immersion for 15 min in the TCP coating bath. The conversion coating suppresses both anodic and cathodic currents, increases the polarization resistance, and provides both anodic and cathodic corrosion protection to the as-prepared alloy, as evidenced from potentiodynamic polarization curves and other electrochemical data. The TCP-coated specimens exhibited good corrosion resistance during a 14-day neutral salt-spray exposure with corrosion intensity values (g/m2-year) ~13x lower than values for the uncoated, as-prepared alloy specimens.

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