A STUDY OF GALVANIC CORROSION DURING COULOMETRIC DISSOLUTION OF GALVANNEALED STEEL

摘要

Galvanostatic stripping is used as a coating structure analysis technique for galvannealed steel sheet. The thickness of the coating determined in this way is underestimated by about 20% compared to the thickness measured by chemical dissolution. This study has shown that pH, electrolyte composition and current density affect coulometric measurements. Because of additional galvanic current between zinc rich zinc-iron phases and more noble phases (iron rich zinc-iron phases), hydrogen evolves during anodic stripping. This is especially observed at low current density. Also the duration of coulometric anodic dissolution cannot give the actual coating thickness. Potentiodynamic analysis tends to show that potentials observed on galvanostatic stripping curves are the result of different iron-zinc phase layers.