Pre-painted hot-dip 55 mass% Al-Zn coated steel sheets are widely used as a building material due to their superior corrosion resistance. In consideration of its long-term service, a pre-painted hot-dip 55 mass% Al-Zn-2 mass% Mg coated steel sheet with enhanced corrosion resistance has been developed in recent years.¹ It has been shown that the addition of 2 mass% Mg not only reduced the corrosion rate by 1/3 but also inhibited the delamination of organic paints from the cut-edge. As yet, however, the role of Mg in the cut-edge corrosion protection of the 55 mass% Al-Zn-2 mass% Mg coated steel has not been completely clear. In this study, we focused on the influence of Mg-containing corrosion products on the electrochemical properties of the coating layer and the steel substrate.

Hot-dip 55 mass% Al-Zn-2 mass% Mg-1.6 mass% Si and 55 mass% Al-Zn-1.6 mass% Si coated steel sheets were used in this study. The Al-Zn-Mg coating layer is composed of Al-rich, Zn-rich, and Mg-Si phases. Our previous study has reported that for the Al-Zn-Mg coating layer, the Zn-rich phase preferentially dissolves during the initial process of cut-edge corrosion.² In addition, Mg also preferentially dissolves from the Mg-Si phase in the Al-Zn-Mg coating layer.

The wet and dry cyclic corrosion test showed that the formation of the Zn-based corrosion products at the cut-edge for the Al-Zn-Mg coated steel was more inhibited than that for the Al-Zn coated steel. In addition, the sacrificial corrosion protection ability of the Al-Zn-Mg coated steel was higher than the Al-Zn coated steel. It was found that the dense corrosion products including Mg were formed on the steel substrate at the cut-edge for the Al-Zn-Mg coated steel 1 hour after the onset of the corrosion test. The Mg-containing corrosion products slightly reduced the cathodic current for the oxygen reduction reaction, and this is thought to be one of the factors for the improved cut-edge corrosion resistance.

1) N. Shimoda, Y. Morimoto, Y. Kubo, S. Fujii, and S. Shiragaki, Galvatech 2015 Proceedings, 802 (2015).

2) S. Tokuda, I. Muto, Y. Sugawara, M. Takahashi, M Matsumoto, and N. Hara, Corros. Sci, 129, 126 (2017).