Effect of phosphate and chromate pigments on sacrificial corrosion protection by Al-Zn coating and delamination mechanism of pre-painted galvalume steel

The effect of Mg₃(PO₄)₂ and SrCrO₄ pigments in paint layer on the corrosion behavior of 55 mass% Al-Zn coating at the cut edges and the delamination mechanism of the organic paint at cross-cut areas of pre-painted galvalume steels were examined using wet/dry cycle corrosion tests. The Mg₃(PO₄)₂ pigment did not inhibit white rust formation at the cut edges and the delamination of the organic paint. On the other hand, the SrCrO₄ pigment suppressed white rust formation and improved the delamination resistance of the organic paint. Observations of the cross sections of the cross-cut area indicate that delamination was the result of the formation of corrosion products in the Al-Zn layer under the organic paint layer, thus the SrCrO₄ pigment inhibited the delamination of the organic paint layer by reducing the corrosion rate of the Al-Zn layer. The Kelvin probe force microscopic measurements revealed that while the addition of the anti-corrosive pigments increased the corrosion potential of the Al-Zn layers, it did not affect the electrochemical properties of the steel substrates. The increases in the corrosion potential can be attributed to the barrier effect of the corrosion products containing chromate pigments on the Al-Zn layer against anodic dissolution. From these results, it is reasonable that chromate pigments effectively increase the delamination resistance of organic paint layers on pre-painted galvalume steels under atmospheric corrosion conditions. In addition, phosphate pigments would improve the corrosion resistance of Al-Zn layers in atmospheric environments with low chloride concentrations.