Characterization and Performance of a Zr/Ti Pretreatment Conversion Coating on AA2024-T3

The formation, structure and corrosion performance of a commercial Zr/Ti-based conversion coating (Bonderite M-NT 5200, Henkel Corp.) on AA2024-T3 was investigated. This coating bath consists of a mixture of fluorozirconic and fluorotitanic acids (ca. 1:3) at a pH of 2–3. The coating was formed on polished, degreased and deoxidized specimens by immersion, and was characterized by scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray analysis and Auger electron spectroscopy (depth profiling) for coating morphology and chemical composition. Electrochemical methods were used to assess the corrosion inhibition provided by the coating. The coating consists of a hydrated ZrO 2 /TiO 2 outer layer on the order of 30 nm thick (in vacuo) and an interfacial region of approximately 60–90 nm between the coating and the alloy surface. In the coating, the Ti levels are ca. 2 × greater than the Zr levels consistent with the concentration differences in the coating bath. The coating provides no significant corrosion protection to the alloy under the test conditions employed based on similarities in the open circuit potentials, anodic and cathodic polarization curves and polarization resistances for the uncoated and coated specimens measured in a naturally-aerated 0.5 M Na 2 SO 4 . Measurements were also made with two other conversion coatings for comparison. The 5200 provides less corrosion inhibition than does a commercial trivalent chromium process (TCP) coating (Bonderite M-CR T5900, Henkel) or a non-chromium process (NCP) coating (NAVAIR). For example, the polarization resistance, R p , for both of these conversion coatings was 10 × greater than that for the 5200. We suppose that the limited corrosion protection of provided by the 5200 coating is due to a thin and porous structure on this alloy.