Effect of oxygen content on corrosion resistance of steel

To explore the influence of oxygen content on the corrosion resistance of steels, carbon and weathering steels with different oxygen contents of (20×10⁻⁶–200×10⁻⁶) were smelted. Using scanning electron microscopy inclusion analysis, polarization curve test, and full immersion test, the change rules of inclusion type, shape, quantity, size, and corrosion resistance under different corrosion environments with changing oxygen content in steel were investigated. The findings indicated that with increasing oxygen content in the steel, the inclusions in the steel changed from long strip MnS and Al₂O₃ to granular silicate inclusions, and the total number and mean size of all inclusions increased gradually; for instance, with the oxygen content increasing from 20 × 10⁻⁶ to 60 × 10⁻⁶ and then to 195 × 10⁻⁶, the number fraction of MnS decreased from 69.9% to 23.7% and then to 5.8%, the number fraction of silicate increased from 3.4% to 54.9% and then to 73.2%, the total area fraction of inclusions increased from 0.01% to 0.04% and then to 0.25%, and the equivalent circle diameter (ECD) increased from 0.78 µm to 1.15 µm and then to 4.65 µm, respectively. The pitting potential demonstrated a positive tendency, with an overall increase of about 40 mV. The full immersion corrosion rate first decreased and then increased, following the cubic function change rule. When the oxygen content increased from (20×10⁻⁶–30× 10⁻⁶) to 60 × 10⁻⁶, the corrosion rate of carbon and weathering steel decreased by 53% and 24%, respectively. The corrosion resistances of carbon and weathering steels were enhanced, and the corrosion rate of weathering steel was evidently lower than that of carbon steel, which is below 3 mm·a⁻¹. With the corrosion time prolonging from 24 h to 48 h and then to 96 h, the corrosion rates of carbon and weathering steels decreased significantly, and the corrosion gradually slowed down. From the analysis, there were mainly long strip sulfides in the steel with an oxygen content of (20×10⁻⁶–30×10⁻⁶). When the steel plate experienced pitting corrosion, the sulfides exposed on the pit wall rapidly dissolved, speeding up the corrosion process. The long strip sulfides were reduced in the steel at an oxygen content of (60×10⁻⁶–85×10⁻⁶), and most of them were replaced by silicate composite inclusions that did not easily induce pitting corrosion, and the corrosion propagation was restrained, which is demonstrated by good corrosion resistance. At an oxygen content of 195 × 10⁻⁶, the average ECD of inclusions in the steel and the total area fraction of inclusions was four times and six times that of the steel with an oxygen content of 60 × 10⁻⁶, respectively. This large number of inclusions as the source of pitting corrosion leads to the steel matrix being vulnerable to severe corrosion, which is characterized by poor corrosion resistance. Thus, at an oxygen content of (20×10⁻⁶–100 × 10⁻⁶), the corrosion resistance of steel in the full immersion corrosion environment was improved due to the combined effect of the reduction of long strip sulfide that can easily induce corrosion and the increase in solid solution oxygen to increase the matrix potential. At an oxygen content of (100×10⁻⁶–200 × 10⁻⁶), the corrosion resistance of the steel was weakened due to the sharp increase of inclusions. Therefore, economical corrosion-resistant steel can be developed by increasing the oxygen content appropriately.