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Studies on phosphate and silane coatings on aluminum alloy

University of British Columbia

The work in this thesis involves studies of two chromium-free corrosion protection coatings on 7075-T6 aluminum alloy. In Part I, emphasis was put on finding new coating recipes for forming an effective phosphate coating, while in Part U a search was made for evidence for direct Si-O-Al bonding at a particular organosilane/metal interface. An initial review is made of the effects of changing parameters involved in forming a zinc phosphate coating layer by spraying. Various coatings were evaluated by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and adhesion tests. The coating solution reported represents a modification of that previously used for coating the same system by the dipping method (J. Mat. Sci. 31 (1996) 565). For forming an adequate phosphate layer by spraying it was necessary to add the accelerators KCIO3 and NaNCh, and to spray for 1 min. at 85°C (instead of 5 min. at 75°C for dipping). The polishing pre-treatment of the substrate also has a significant effect on the phosphate process: the coating formed on a surface polished by 1200 grit aluminum oxide sandpaper showed good adhesion to paint, whereas that polished with 1200 grit silicon carbide sandpaper, under otherwise identical conditions, failed to form an adhesive phosphate coating. In general, conditions for spraying are less easy to control than for dipping, but directions are indicated for obtaining promising coatings by the former approach. The interface of y-glycidoxypropyltrimethoxysilane (y-GPS)/aluminum alloy was studied by the XPS bias potential technique and by secondary ion mass spectrometry (SIMS). Consistently with earlier work in this laboratory, a chemical shift was observed in narrow scan A12p spectra from negatively biased samples, compared with those that were grounded (after correcting for the expected effect of the bias potential). The change in the electrical properties of the aluminum oxide was previously hypothesized to indicate the presence of direct Si-O-Al interfacial bonding to the y-GPS. In this thesis a further search was made to support this hypothesis by making static SBVIS investigations of this interface. Samples which showed the bias potential effect also showed radicals at mass 71 in SIMS. Such species appear interpretable as AlSiO+ and therefore are apparently indicative of direct Si-O-Al bonding at the interface.

Thesis/Dissertation

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2009-03-23

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http://hdl.handle.net/2429/6350

Chemistry

University of British Columbia

UBCV

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