The process of blister formation on electrogalvanized sheet steels

Electrogalvanized sheet steels are the base material in the production of automobile bodies and appliances. The coated sheet steels act as substrate materials to complex coating systems. When small defects known as blisters form at the zinc steel interface the coating integrity can be adversely effected. The formation and growth of blisters on electrogalvanized sheet steels was examined in this study.;Blister growth was examined by evaluating models of the zinc-steel system, analyzing the internal gases in blisters, monitoring the growth of blisters, and evaluating the amount of hydrogen absorbed by the preplating electroprocesses.;The plate model was found to best describe the zinc-steel system. The gases within the blisters were found to be hydrogen and methane. The methane is the reaction of hydrogen with interfacial carbon. Blister growth monitoring demonstrated that blisters do not grow once they have formed. The zinc coating was seen to deform via a grain boundary sliding mechanism. It was found that the permeation of hydrogen through a steel membrane is reduced by 90% by pulsing between cathodic and anodic states. It was also determined that the sequence of pulsing has an effect on the amount of hydrogen absorbed.;The formation of blisters can be summarized in the following manner. The presence of areas of poor adhesion at the zinc steel interface act as initiation sites for blister growth. Hydrogen which is injected into the steel during the plating process will diffuse into the initiation site. If carbon is present at the initiation site the formation of methane is favored. When the internal gas pressure builds up the coating is deformed. Plastic deformation can occur by flow or grain boundary sliding. Deformation will continue until the pressure is relieved by a void or a crack in the coating.