Akira Matsuzaki

Microstructure of organic–inorganic composite coatings studied by TEM and XANES

Abstract Chromate coatings on Zn or Zn alloy coated steel sheets often include silica for the aim to improve corrosion resistance. In the case of dry-in-place chromate coatings containing acrylic resin (hereafter referred to as an organic–inorganic composite coating), an addition of silica, however, did not show an improvement in corrosion resistance. The microstructures of the organic–inorganic composite coatings were observed by transmission electron microscopy (TEM) and the chemical states of Cr were investigated by the total electron yield X-ray absorption near edge structure (TEY-XANES) method. TEM samples were successfully prepared by dry ultramicrotomy preventing water-soluble components in the coatings fromdissolving out. TEY-XANES revealed the chemical states of components even in the organic matrix. Using these methods, it was found that the addition of silica changed just the morphology of the chromium compound in the organic–inorganic composite coating but not the chemical state of Cr. This is a reason for the addition of silica being not effective at improving corrosion resistance. The combination of dry ultramicrotomy-TEM and TEY-XANES spectroscopy was proven to be a powerful tool for characterizing organic–inorganic composite coatings.

Newly developed chromium-free organic-inorganic composite coated steel sheet with excellent corrosion resistance

From the point of view of ecology, there has been a trend of measurable advances in coated steel sheets for electrical appliances. For further advancement, completely hexavalent chromium-free coated products need to be developed. Despite the large amount of research of chromium free coating being carried out for many years, there have never been products that could provide excellent corrosion protection with a low cost treatment until recently. Recently, in response to the increasing demands for ecological products, new chromium-free coated steel sheets were industrialized independently by each of the six major steel companies in Japan in 1998. Those products were adopted for chassis and cases of audio-visual and office equipment. Features and practical characteristics of one of the new products, GEO-FREONTIER COAT (GF) developed by NKK Corp. are to be presented.