Corrosion behaviour of galvannealed steel sheet

This paper analyzes the influence of galvannealed coating on the corrosion behaviour of press-formed and painted galvannealed steel sheet (GA) against the painted cold rolled and closed annealed (CRCA) sheet evaluated by electro impedance spectroscopy (EIS), salt spray (SST) and combined cyclic corrosion test (CCT). EIS study showed a significant decrease in the polarization resistance from 6.38 to 0.8 Ω/cm2 in case of CRCA painted sheet and from 6.07 to 4.07 Ω/cm2 in case of GA painted material revealing the superior corrosion resistance of galvannealed sheet. This was also confirmed from the scribed samples exposed to salt spray test. The corrosion product was also very low in GA painted compared to CRCA at scribed areas. Studies on under-paint corrosion from a deep scratch or creep-from-scribe in combined cyclic corrosion tests (CCT), simulating drying and wetting conditions in actual service life, have shown drastic reduction in creep-back for GA material. Salt spray test (SST) exposure of 720 h on painted and cross-scribed samples has also demonstrated the ability of GA material to resist corrosion that inevitably occurs on stone chipping on outer surface of auto body panels. Red rust formation started just after two cycles and propagated from crosscut portion with further test cycles. The initiation of red rust across the crosscut in GA material was not visible even after 65 cycles. The corrosion rates of CRCA painted samples after 30 and 45 cycles were 5.17 and 11.56 mpy, respectively, while in the case of GA painted, it was only 0.412 and 0.732 mpy, respectively. In case of CRCA, the corrosion rate increased with the passage of time. After 45 cycles of testing, GA painted sample has shown almost 16 times more corrosion resistance than the painted CRCA material.