Effect of oxygen on economical corrosion resistant steel

Carbon steels are widely used structural materials in vessel and marine engineering. Many studies acknowledge that the pitting corrosion of these materials are heavily subject to their metallurgic factors. Although much attention has been paid on inclusions and microstructure, little had been dealt with metallurgical processing including deoxidizing degrees. Deoxidization is one of the most important processes in steelmaking. Stronger deoxidizing degree helps to improve steel’s mechanical property and welding property. However, some studies demonstrated that weaker deoxidizing degree tends to improve pitting corrosion resistance. Manufacturing ordinary structural steels nowadays have been changed from mould casting to continuous casting. However, the deoxidizing degree of continuous casting steel may be different due to different deoxidization techniques. Particularly, the oxygen content in current steels has a fairly low level, which may be harmful to pitting corrosion resistance of steels. In this study, the influence of oxygen content in carbon hull steels on corrosion and mechanical properties of steel was investigated by mechanical properties tests and alternate immersion test. Results show increased oxygen content when there is duction of deoxidization of molten steel in the range permitted by continuous casting. Interestingly, the average corrosion rate of steel slightly decreases and an obvious enhancement of resistance to pitting is observed. The average pit depth corresponding to the high oxygen side of the pit depth-oxygen curve is about 22.7% lower than that of the low oxygen side. The mechanical and cold bending properties of tested steels are able to meet technical code requirements and can reach the level of Grade D hull steel. Findings of this study suggest that solid solution oxygen in steel plays a major role in improving pitting resistance. It can enhance the thermodynamic stability of iron, elevate the corrosion potential of the iron in the pit, and reduce the pitting rate. Therefore, using oxygen as a corrosion resistant element is an economic strategy to reduce the cost of corrosion resistant steel.