The effect of Ni2+ on zinc phosphating of 2024-T3 Al alloy

• Published in: Surface & coatings technology 2004-10, Vol.187 (2-3), p.208-215
• Main Authors: AKHTAR, A. S, SUSAC, D, GLAZE, P, WONG, K. C, WONG, P. C, MITCHELL, K. A. R
• Format: Article
• Language: English
• Subjects: Applied sciences; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Heat treatment; Materials science; Metals. Metallurgy; Other topics in materials science; Physics; Production techniques; Thermochemical treatment and diffusion treatment
• ISSN: 0257-8972; 1879-3347
• DOI: 10.1016/j.surfcoat.2004.02.044

Zinc phosphate (ZPO) conversion coatings formed on 2024-T3 aluminum alloy, and on high-purity aluminum, after dipping in coating baths containing different amounts of Ni2+, have been studied by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy, scanning Auger microscopy (SAM) and electrochemical measurements. Significant variations are observed in coating morphology and in stability against corrosion for the 2024-Al alloy as the amount of Ni2+ in the coating solution increases through the 0-2000 ppm range. By contrast, for high-purity Al, the most marked changes in coating morphology occur only as the Ni2+ concentration reaches 2000 ppm. The presence of additional elements, and especially Cu, in the 2024-Al samples result in an increase in the number of cathodic sites and hence more coating is formed for corresponding conditions. Additionally, the existence of second-phase particles in the 2024-Al sample leads to variations in the coatings at the microstructural level across the surface. These differences include local variations in the amount of Ni2+ in the coatings, as well as the local coverage.