| In continuous electroplating of steel strip using sulfate bath, the bath can be easily contaminated by ferrous ions dissolved from steel strip due to it low pH. When the phosphate conversion coating was applied subsequently on the electro-galvanized coating layer, which was plated from the contaminated bath, the surface defect called black mark could be formed on the phosphate surface. The formation of black mark on the phosphate surface seemed to be arisen from the uneven co-deposition of iron into the electro-galvanized layer, which would render non-uniform coating weight distribution of the phosphate layer. Because only the uppermost layer of the electro-galvanized coating affects the coating weight of the phosphate layer, it could be possible to prevent the formation of black mark if the ordinary high current density plating is followed by the final low current density plating, which would give uniform and low iron content in the electro-galvanized coating layer. |
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