Studies on defects in galvanized coating on steel wire

Abstract

Causes for deposition of defective galvanized coating (lumpy deposition of coating during hot dip galvanizing) of carbon-steel wire have been investigated. It is established that the lumpy deposition is caused due to segregation of silicon on the surface of steel wire. Due to this change in composition of the steel at surface, the regions get modified and lie in Sandelin hyper reactive zone. This accelerates reaction of steel with molten zinc in those localized regions of the surface and leads to formation of lumpy deposition of zinc. Scanning electron microscopy, energy dispersive X- ray analysis and Raman spectroscopy of lumpy wire (LW) and normal wire (NW) coatings confirm the presence of silicon bearing phases present in the LW coating. It is observed that due to localized high concentration of silicon on steel surface, a diffused and porous zeta layer instead of normal compact delta and zeta layers is formed. Thus, incoherent deposition of zinc layers occurs on steel surface. In order to control this problem, it is suggested to maintain silicon equivalent in the steel below 0.04 wt% and conduct flash pickling of wires in oxidizing acid to dissolve silicon rich phases.