Effects of Nb and Cr on the Corrosion Characterization of Al-Containing Transformation-Induced Plasticity Steels in Neutral Chloride Solutions

Abstract

Electrochemical measurements were used to characterize the effect of Nb and Cr on the corrosion behavior of two types of Al-bearing transformation-induced plasticity (TRIP) steels in naturally aerated sodium chloride (NaCl) solutions over a wide concentration range. In all cases, the open-circuit potential shifts toward more negative values with time until the potential acquires a steady-state value, which depends on both steel composition and chloride concentration. The sample with higher Al content (1.32 wt%) displays significantly more negative corrosion potential (Ecorr), larger corrosion current density (icorr) in polarization curves and lower fitted film resistance (R1) values in electrochemical impedance spectroscopy (EIS) plots. On the other hand, the TRIP sample with lower Al content (0.006 wt%) but containing micro-additions from both Nb and Cr as alloying elements greatly enhances the formation of surface film, offering better corrosion resistance. For both steel samples, Cl– ion concentration has a critical effect on the corrosion rate. The rate or icorr increases and R1 decreases with increasing Cl– ions in solution up to a threshold value beyond which the corrosion rate starts to decrease. For Nb-containing TRIP steel, the critical Cl– concentration is lesser than its value for the Nb-free sample. This indicates that Nb micro-alloyed in the TRIP steel favors the formation of oxide films with better insulating character. However, an increasing Al level in the steel has a deleterious effect and leads to the formation of a less stable surface film with lower protection ability. Scanning electron microscopy (SEM) micrographs, at various magnifications, confirm the obtained results.