Research on Mechanism of Surface Blackening of Hot-galvanized Steel Pipe and Discussion on Relevant Solution

Studied are the mechanism and contributing factors of surface blackening of the hot-galvanized steel pipe.And relevant solution proposal for the said problem is discussed via the NSS test and the high temperature/high humidity test.As a result it is revealed that the origin of the pipe surface blackening is directly related to the content and distribution of the lead in the surface zinc layer of the pipe,i.e.,in areas with high lead content,the corrosion potential is higher,whereas in areas with low lead content,the corrosion potential is relatively lower.When the corrosive medium is present,electrochemical corrosion will occur so as to,accelerating corrosion in areas with low potential,resulting in enrichment of corrosion products,and increase of oxygen content in the areas.The enrichment of corrosion products cause the apparent color of this area to become dark,and formation of black striped surface markings.When GDC-008-type one-component passivation solution is,and the curing temperature of the passivation film reaches over 90 ℃,and the curing time is over 3 min,a surface passivation film with good corrosion resistance can be obtained,and thus the problem of surface blackening of the galvanized steel pipe can be effectively solved.