Investigation into the effects of metallic coating thickness on the corrosion properties of Zn–Al alloy galvanising coatings

• DOI: 10.1016/j.corsci.2006.07.006
• Publication Date: 2006-12-06T12:11:17Z
• Authors: David J. Penney; James H. Sullivan; David A. Worsley
• Keywords: 0103 physical sciences; 02 engineering and technology; 0210 nano-technology; 01 natural sciences

Galvalloy (4.5%Al/95.5%Zn) coatings were produced on a continuous coil coating line with varying metallic coating thickness from 7.8μm (120gm-2) to 48μm (325gm-2). Coating thickness was controlled by adjusting the air knives. A higher volume fraction of the primary zinc phase was observed metallographically for the thinner coatings. The faster cooling initiates greater steel coating interfacial nucleation of solid primary zinc phase. The scanning vibrating electrode technique (SVET) was used to measure the cut-edge corrosion properties. It was found that increasing the coating thickness resulted in a decrease in the level of zinc removed from over a 24-hour experiment in 5% NaCl solution. The same trend was also observed in external zinc runoff measurements. The increasing corrosion observed at lower coating weights results from greater undercutting and self-corrosion of these coatings that are further away from a eutectic composition and the presence of crevice corrosion due to zinc dendrite interconnectivity.