Effect of TiN Inclusion on Pitting of an Ultra-Pure Ferritic Stainless Steel

Through electrochemical test and immersion test, using scanning electron microsopy (SEM), transmission electron microscope (TEM) and atomic force microscope (AFM) to characterize the size of pitting pits, the micro morphology of TiN inclusion, and the potential difference and height difference between TiN and substrate of an ultra-pure ferritic stainless steel. And the mechanism of TiN on the occurrence and expansion of pitting corrosion of the sample was analyzed. The results showed that corrosion potential and pitting potential of the sample were 94 mV and 353 mV, respectively. The average width and depth of pitting pits were 203.6 μm and 114.74 μm, respectively. TiN inclusion existed in the form of single or cluster. The potential of TiN was 65 mV higher than that of the substrate. The pitting corrosion occurred preferentially at the interface between TiN and substrate, and the substrate became the anode for micro galvanic corrosion. The autocatalytic process caused by “activation in the hole-passivation out of the hole” accelerated the expansion of pitting corrosion.