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The effect of pH and role of Ni2+ in zinc phosphating of 2024-Al alloy. Part I: Macroscopic studies with XPS and SEM

TitleThe effect of pH and role of Ni2+ in zinc phosphating of 2024-Al alloy. Part I: Macroscopic studies with XPS and SEM
Publication TypeJournal Article
Year of Publication2006
AuthorsAkhtar, AS, Wong, KC, Mitchell, KAR
JournalApplied Surface Science
Volume253
Pagination493-501
Date PublishedNov
Type of ArticleArticle
ISBN Number0169-4332
KeywordsALKALINE STABILITY, aluminum alloy, ALUMINUM-ALLOY, CATALYSTS, COATINGS, CONVERSION COATINGS, FILMS, Ni2+ additive, NICKEL, OXIDATION, photoelectron spectroscopy, scanning electron microscopy, SPECTROSCOPY, STEEL, zinc phosphate
Abstract

Coatings formed on 2024-T3 aluminum alloy were studied by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) after dipping in zinc phosphating (ZPO) baths at different acidities, for different lengths of time, and with or without Ni2+ additive. The overall objective was to learn more about the role of Ni2+ on the ZPO coating mechanism, particularly since this additive is believed to improve corrosion protection for the Al alloy. Secondary phosphates dominate the coatings when the Ni-containing solution is adjusted to starting pH values of either 3 or 5, while tertiary phosphate is predominant at pH 4. AlF3 precipitates during the early stages of the coating process. Ni2+ has two main roles in the mechanism. First, the rate of increase in local solution pH is retarded by the slower kinetics of reactions involving Ni2+ compared to Zn2+,leading to thinner ZPO coatings when Ni2+ is present in the coating solution. Second, most Ni2+ deposition occurs during the later stages of the coating process, by nickel phosphate deposition and/or by formation of a Ni-rich oxide. (c) 2006 Elsevier B.V. All rights reserved.

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