Insight into TiN inclusion induced pit corrosion of interstitial free steel exposed to aerated NaCl solution

The mechanism of step-like corrosion pit induced by TiN inclusion in Ti-stabilized interstitial free steel was systematically investigated. There was neither micro-crevice nor local stress in the steel matrix adjacent to the TiN inclusion, but a significant Volta potential difference was detected between the conductive TiN and steel matrix. The corrosion pits induced by the TiN inclusion had a step-like structure, composed of an inner pit and a shallow ring-like corroded region (RCR). Three stages, initiation, transition, and propagation stages, involved in the formation of the step-like corrosion pits were observed. The three stages were manifested with the initiation of inner pit, the deepening of inner pit, and RCR formation, respectively. The inner pit was triggered by the Volta potential difference in the initiation stage. In the transition stage, the extreme gradients of H+ and Cl− occurred through the hydrolysis of Fe2+ and electromigration of Cl− in the deepening inner pit. Finally, the extreme gradients of aggressive ions between the inner pit and NaCl bulk solution resulted in the formation of RCR surrounding the inner pit in the propagation stage.