Influence of chloride ion concentration on the corrosion behavior of Al-bearing TRIP steels

The effect of a systematic increase of chloride ion concentration on the electrochemical corrosion behavior of two types of Al-bearing TRIP steels (T1 and T2) was studied in aqueous NaCl solutions. Several electrochemical techniques were used comprising open circuit potential measurements, potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS). Chloride concentration has a critical influence on the corrosion rate of the two tested steel samples. For both steels the corrosion rate first increased with increasing chloride content up to a certain critical concentration (CC), and then decreased in solution with chloride level higher than the threshold value. TRIP steel T1 microalloyed with Nb and Cr as compared to steel T2 not containing these two elements, exhibited lower corrosion rate and smaller CC value, indicating better corrosion resistance to chloride attack, albeit the Al content in T2 is 220 times higher than that in T1. This is because Nb alloyed with TRIP steel likely enhances the formation on the surface of a stable rust layer enriched with other passivating elements Al, Cu, Cr and Ni, which has higher corrosion resistance and hence improve greatly the passive performance of the TRIP sample. The ac impedance data are in good agreement with the OCP and dc polarization measurements. Surface examinations via scanning electron microscope confirmed well the obtained results.