Cause Analysis of White Spots on Zinc-plated Square Tube

White spots were discovered on square tube made of 08 steel plate after zinc-plated and then stored for about half a month.The number of white spots increased with time.Analysis methods such as OM,SEM and EDS were used to analyze the white spots.The results showed that the process of acid pickling was too heavy,so the surface of the steel plate ethibited serious intergranular corrosion and grains were peeled off.The poor uniformity of zinc-plating could not prevent corrosion completely,with the time increasing the chloride remained at joint gaps of grains could attack the zinc-plating too.