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

TitleMicrostructural effects on the formation and degradation of zinc phosphate coatings on 2024-Al alloy
Publication TypeJournal Article
Year of Publication2008
AuthorsAkhtar, AS, Wong, PC, Wong, KC, Mitchell, KAR
JournalApplied Surface Science
Volume254
Pagination4813-4819
Date PublishedMay
Type of ArticleArticle
ISBN Number0169-4332
Keywords7075-T6, ALKALINE STABILITY, aluminum alloy, ALUMINUM-ALLOY, Auger electron spectroscopy, BEHAVIOR, corrosion, CORROSION PROTECTION, INITIATION, MICROSTRUCTURE, NI2+, PARTICLES, PHASE, surface oxide, zinc phosphating
Abstract

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. (C) 2008 Elsevier B.V. All rights reserved.

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