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In Situ Surface Studies Of Conversion Coatings For Steel And Aluminum

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Final rept. 15 Apr 1989-14 Sep 1992

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The primary goals of the work were to develop mechanisms of corrosion protection for cerium based surface layers on aluminum alloys and on polyacrylic acid PAA complexed zinc phosphate conversion coatings on steel. Atomic force. microscopy AFM using tunnel current control was developed and applied to several problems. The cerium based coatings on AI 6061-T6 are shown to consist of two principle components--a poorly ordered monohydrated aluminum oxide, and an insoluble cerium oxide which forms at areas concentrated with impurities and alloying elements. Electrochemical action during the surface modification process fosters the precipitation of cerium compounds which inhibit further attack. The addition of high molecular weight PAA to the phosphating bath can significantly improve both resistance to corrosion and top-coat adherence of zinc phosphate conversion coatings on steel. Raman spectra showed the compositions of both unmodified and PAA modified films to be zinc phosphate dihydrate. Single crystallite surfaces were imaged using AFM. The morphologies of the unmodified and modified films were in general quite similar, but subtle differences were apparent. Several other projects involving surface layers and adsorbates were carried out and are described.

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  • Metallurgy and Metallography

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