Influence of pre- and post-treatments on formation of a trivalent chromium conversion coating on AA2024 alloy

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Article

Qi, J.¹  Nemcova, A.¹  Walton, J. R.²  Zhou, X.¹  Skeldon, P.¹  Thompson, G. E.¹ 

[1] Univ Manchester, Sch Mat, Oxford Rd, Manchester M13 9PL, Lancs, England

[2] TSTC Ltd, 5 Grosvenor Terrace, Teignmouth TQ14 8NE, England

关键词: Aluminium alloy;    Trivalent chromium conversion coating;    Corrosion protection;    Transmission electron microscopy;    Rutherford backscattering spectroscopy;    X-ray photoelectron microscopy;   

DOI  :  10.1016/j.tsf.2016.08.044

来源: Elsevier

【 摘 要 】

A comparison has been made of the influence of two alloy pre-treatments and two coating post-treatments on the formation, composition and corrosion protection of a trivalent chromium conversion coating on AA2024-T351 alloy. The investigation employed analytical electron microscopies, ion beam analysis, X-ray photoelectron spectroscopy (XPS) and electrochemical tests. The pre-treatments used alkaline etching followed by de-oxidizing in either nitric acid or a commercial de-oxidizer. The conversion coatings were formed in SurTec 650 chromitAL and revealed two-layers, comprising an inner aluminium-rich layer and an outer chromium- and zirconium-rich layer, with a Cr:Zr atomic ratio in the range similar to 0.73-0.93. XPS indicated a chromium-enriched near-surface region that contained similar to 2 at.% of Cr(VI) species. Potentiodynamic polarization and electrochemical impedance spectroscopy revealed an improved corrosion protection for a pre-treatment that left copper-rich sponges, probably de alloyed S phase, and fewer residues of other intermetallic particles on the alloy surface. Post-coating immersion treatments in deionized water at 20 degrees C or 40 degrees C resulted in a significant difference in the zirconium species in the region adjacent to the coating surface that is accessible to XPS, with oxide and hydroxide dominating at the respective temperatures. (C) 2016 Elsevier B.V. All rights reserved.

【 授权许可】

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