Galvanic corrosion of the seam weld in Zn-Al coated steel pipes manufactured by electric resistance welding

摘要

Severe corrosion attack at the seam weld was observed in Zn-Al coated steel pipes of a closed heating system. This system was operating with tap water at about 70 degrees C and in the presence of an unknown amount of oxygen due to water reintegration. The corroded pipes showed a preferential corrosion of the seam weld along the longitudinal direction of the pipe. In order to investigate the cause of this attack, the electrochemical behavior of the seam weld was studied by means of an electrochemical micro-cell. This technique was used for the characterization of non-corroded and corroded pipe samples. The study proves that the damage mechanism is due to Zn-Al coating depletion at the seam weld and the potential coupling reversal between the coating and the steel substrate under service conditions. This caused strong galvanic corrosion between the steel exposed along the seam weld and the Zn-Al coating leading to the failure of the pipes.