Electrochemical, chemical and morphological characterization of galvannealed steel coating

The intermetallic phases present in galvanneal coatings are of great importance to their final performance. In the present study the phases in a galvanneal coating on carbon steel for use in the automotive industry were electrochemically and morphologically characterized. The electrochemical techniques employed were potentiodynamic polarization, potentiostatic polarization and coulometric stripping. This last technique has been widely used for characterization of galvanneal coatings. Additionally, X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) were carried out for chemical and morphological characterization of the phases in the coating. The results indicated that for the zinc richer phases the corrosion mechanism was similar, consisting of Zn selective dissolution and leading to enlargement of cracks and pores initially present in the coating. Two main morphologies were associated with the major phases in the galvannealing coating, namely an elongated rod-like phase and a flat and more compact type. Their chemical composition indicated that they were related to the zeta and delta phases, respectively. Three current density peaks were obtained in the anodic polarization curves and they were related to the zeta, delta and gamma-1 phases in the galvanneal coating. The cracks present in the coating previous to immersion in the stripping solution increasingly enlarged with exposure time to the stripping solution favoring the exposure of the various phases to the electrolyte and led to their concurrent corrosive attack. The corrosion product deposited on the galvanneal coating was mainly composed of a zinc rich product with a platelet-like morphology.