J. Japan Inst. Metals, Vol. 73, No. 11 (2009),
pp. 862-869

Self-Healing Effect by Phosphoric Zinc and Calcium Silicate Included in Organic-Inorganic Composite Coating on 55%Al-Zn Coated Steel Sheet

Akira Matsuzaki¹, Masayasu Nagoshi¹, Hisato Noro¹ Masaaki Yamashita¹ and Nobuyoshi Hara²

Abstract:

Recently, it has become necessary to improve the corrosion resistance of zinciferous coated steel sheets, especially 55% Al-Zn alloy coated steel sheets. However, it is known that cracks are easily generated on the surface of 55% Al-Zn alloy coated steel sheet during production processes. This is due to the hardness of the metallic coating. We studied non-chromate corrosion-preventive compounds for organic-inorganic composite coatings and found that the composite coating containing both Zn₃(PO₄)₂ and CaSiO₃ had the highest corrosion resistance among all the additives tested. The corrosion mechanism was investigated by the analysis of the specimens after corrosion test using SEM, AFM, TEM and EDX. It was found that the amorphous surface layer, which was approximately 30 nm in thickness and contained Ca, Cr, P, Si, C, and O, was newly formed on the steel surface exposed after deformation of the coating. It is presumed that Ca²⁺ and HPO₄^2- are formed by dissolution of the CaO component in CaSiO₃ and hydrolysis of ZnO₃(PO₄)₂, respectively, and they react each other to form an insoluble Ca-phosphoric compound, which might inhibit the corrosion of metals exposed in the cracks.

(Received 2009/6/12)

Keywords:

55% aluminun-zinc alloy, coated steel sheet, corrosion, inhibitor, chromate, chromate-free, self-healing, microstructure, cross-sectional analysis

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