热浸镀锌板的锌花颜色差异原因

Reasons of Color Difference for Spangles of Hot Dip Galvanized Sheets

热浸镀锌板表面出现的亮色与暗灰色锌花是一种表面缺陷,采用扫描电子显微镜,能谱仪及X射线衍射分析仪,观察并分析了亮色与暗灰色锌花处的镀层横截面组织与化学成分。结果表明:亮色锌花处镀层最外层几乎不含Al元素,暗灰色锌花处镀层最外层含有大量Al元素及少量Fe元素。产生暗灰色锌花的主要原因是锌液中微量元素在晶界处堆积,腐蚀晶界,降低镀层的耐蚀性,使镀层中Al元素及基体金属中的Fe元素与外界O结合生成宏观上呈暗灰色的金属化合物Al2O3与Fe3O4。在夏天高温或潮湿环境中,暗灰色锌花更加明显。建议在夏天高温环境中生产的镀锌板放置于低温且干燥的专用仓库中,以保证镀锌板的表面质量。

Bright and dark grey spangles on the surface of hot dip galvanized sheets is a kind of surface defect.Scanning electron microscopy,energy dispersive spectroscopy and X-ray diffraction were used to observe and analyze the cross-sectional microstructure and chemical composition of the coating at the position of bright and dark grey spangles.The results show that there was almost no Al in the outermost layer of bright spangles,while a large amount of Al and a small amount of Fe were detected in the outermost layer of dark grey spangles.Microelements in the galvanizing zinc preferred to accumulate in the grain boundaries and thus corroded the grain boundaries,which damaged the corrosion resistance of coating.The element Al in the coating and element Fe in the matrix would combine with O and form metallic compound Al2O3 and Fe3O4,showing macroscopic dim gray morphology.Dark grey spangles were easier to appear in hot summer or humid environment,therefore,the hot dip galvanized sheets produced in summer should be placed in low temperature and dry warehouse to ensure the surface quality.