Influence Of S And W On Sulfuric Acid Dew Point Corrosion-resistance Of Low-alloy Steel

In this study, the corrosion experiments were performed in70℃,50%sulfuric acid solution, of which the longest corrosion time is up to48hours, for the sulfuric acid dew-point corrosion resistant steels with the Cu-Cr-Sb-W composition system. The corrosion rates of the steels were determined after6h,24h and48h corrosion and the macro-and micro-characteristics of corrosion product films were analyzed by means of SEM, XRD, and Auger electron spectroscopy. The polarization curves of the steels were tested in the same corrosion conditions. Through these experiments, the influence of S and W content on corrosion behavior of experimental steels was investigated.The experimental steel with ultra-low sulfur content (0.0009%S) is easily passivated in a70℃,50%sulfuric acid solution, thus it shows the lower corrosion rate after6hour corrosion, due to the formation of the passive film. However, with corrosion time extending to24h and48h, the passive film is damaged, resulting in corrosion rate increased significantly. When the sulfur content changes in the range of0.0022%～0.046%, the corrosion rate after6hour corrosion is increased with the S content increasing. The S content increasing makes natural corrosion potential increased and natural corrosion current decreased, it promotes the enrichment of corrosion-resistant elements in corrosion product film and the formation of the dense corrosion product film, resulting in lower corrosion rate of the steel. With the corrosion time extending to24h and48h, the influence of S content on corrosion rate was weakened greatly. The steel with low S content (0.0022%S) shows the little lower corrosion rate than those of steels with high S contents (0.024%S and0.046%S) after48h corrosion.When W content changes in the range of0.05%～0.28%, the corrosion rate of the steels decreases with W content increasing after6h corrosion. The W content increasing makes natural corrosion current decreased of the experimental steels. It also promotes the enrichment of corrosion-resistant elements in corrosion product film. After24h and48h corrosion, the passive films with metallic luster or protective product film is formed in experimental steels containing0.05%～0.15%W. The films are stable which allows the corrosion rate at a lower level for a long time. When W content increasing to0.28%, the passive film formed is unstable which results in a significant increase in corrosion rate after48h corrosion. Analysis for passive film shows that the element W is more easily enriched in passive film and the synergy effect of corrosion-resistant elements W, S and Cu and Cr, Sb can effectively improve the protection ability of the passive film.For the sulfuric acid dew-point corrosion-resistant steels with Cu-Cr-W-Sb composition system used in this study, the appropriate ratio of W and S can effectively improve the corrosion resistance of steel. When the steel containing0.16%W and0.0074%S is eroded in70℃,50%sulfuric acid solution, the passive film can form quickly and is with good stability, thus the steel shows the low corrosion rates, both after short time (6h) and long time (24h and48h) corrosion.