Microstructural effects on the formation and degradation of zinc phosphate coatings on 2024-Al alloy

The formation of zinc phosphate (ZPO) coatings on 2024-T3 aluminum alloy was studied using scanning electron microscopy (SEM), scanning Auger microscopy (SAM) and X-ray photoelectron spectroscopy (XPS), with an emphasis on microstructural effects involving second-phase particles and the alloy matrix. Surface polishing results in an Al–Cu–Mg particle surface that contains metallic Cu as well as an overlayer of aluminum and magnesium oxide, while larger amounts of aluminum oxide are present on the Al–Cu–Fe–Mn particle and matrix. When dipped in an acidic ZPO coating solution, the oxide covering the Al–Cu–Mg particle is etched most easily, and metallic Cu near the surface makes that region most cathodic, allowing more coating deposition compared with the other regions. The oxides on the Al–Cu–Fe–Mn and matrix regions are similar, thereby confirming that the observed differences in ZPO coating characteristics at these two regions arise from their underlying electrochemical characteristics. Immersion of a coated 2024-Al sample in corrosive NaCl solution for extended periods indicates that the ZPO provides better protection to the second-phase particles than to the matrix.