Development of Phosphate Treatment for Hot-dip Zn-6 Al-3 Mg Alloy Coated Steel Sheet

摘要

Hot-dip Zn-6% Al-3% Mg alloy (hereafter referred to as "Zn-Al-Mg") coated steel sheet provides superior resistance to corrosion and is used in a wide range of applications including building materials and automobile parts. Zn-Al-Mg coated steel sheet, however, has a high metallic gloss in common with other hot-dip Zinc or Zinc-alloy coatings, thus anti-glare treatment may be necessary depending on the location of use for outdoor use. When painted by the user, a high level of corrosion-resistance after painting is required for use in salt corrosive environments. In order to resolve these problems, we developed a new phosphate treatment for Zn-Al-Mg coated steel sheet, which allows formation of fine phosphate crystals with superior adhesion to the coated surface. Atmospheric exposure tests conducted over a period of two years showed that the phosphate-treated Zn-Al-Mg coated steel sheet maintains good anti-glare property with no significant corrosion, and remaining phosphate crystals were observed. This result appears to be due to high weatherability not only of the hot-dip Zn-Al-Mg coated layer but also of the phosphate-treated film. Cyclic corrosion test of painted specimens showed that the maximum blister width on the crosscut part of the developed material was % or less of that of chromate-treated Zn-Al-Mg-coated steel sheet and conventional Zn-coated steel sheet. This appears to be due to the suppression of the under-film corrosion, which readily progresses in coatings containing Al, resulting from the excellent paint adhesion of the developed material. The developed phosphate-treated Zn-Al-Mg coated steel sheet provides not only corrosion resistance but also anti-glare property and paintability.