Low Temperature Curing of Hydrogen Silsesquioxane Surface Coatings for Corrosion Protection of Aluminum
Felix Lampert*, Annemette Hindhede Jensen** and Per Møller† *
*Technical University of Denmark (DTU), Lyngby, Denmark
**SiOx ApS, Espergærde, Denmark
Editor’s Note: This paper is a peer-reviewed and edited version of a paper as part of a presentation delivered at NASF SUR/FIN 2015 in Rosemont, Illinois on June 9, 2015. A printable PDF version is available by clicking HERE.
Hydrogen Silsesquioxane (HSQ) has shown to be a promising precursor for corrosion protective glass coatings for metallic substrates due to the excellent barrier properties of the films, especially in the application of protective coatings for aluminum in the automotive industry where high chemical stability in alkaline environments is required. The coatings have been successfully applied to stainless steel substrates. However the traditional thermal curing of HSQ involves heating to elevated temperatures, which are beyond those applicable for most industrial applications of aluminum. In this study low temperature processes are tested and evaluated as possible alternatives to the traditional high temperature cure. Thin HSQ films are deposited on silicon wafers to model the degree of curing induced by the low temperature methods in comparison to thermal curing. Furthermore, the coatings are applied on aluminum substrates to evaluate the adhesion and corrosion resistance of the films.
Keywords: Aluminum, corrosion, electron beam curing, hydrogen silsesquioxane, HSQ, vitreous enameling
Vitreous enameling is a well-known coating technology for the corrosion protection of metals due to the outstanding barrier properties of silica glass.1 The E-pH diagram shown in Fig. 1(a) displays silica as a largely inert material in the pH range from 0 to 11-12 in aqueous media. A transfer of these inert properties in form of a coating is particularly interesting for metals such as aluminum, where a passive region is only observed in the range between approximately pH 5-8 (Fig. 1(b)). The narrow stability range limits the industrial use of aluminum. Besides a curing temperature above the liquidus temperature of vitreous enamel close to the melting temperature of aluminum and a large coating thickness, traditional vitreous enameling often becomes problematic with respect to tolerances and flexibility of the coatings.