鋼中MnSの局部腐食性におよぼす熱処理の影響

耐溝食性電縫鋼管の研究 (III)

抄録

Carbon steel containing MnS inclusions was rapidly heated and cooled. The effect of this treatment on the localized corrosion susceptibility of those inclusions in 3% NaCl solution was studied. Specimens of which central zones were heated to 1, 100-1, 450°C in air and cooled rapidly were used for this study. Measurements showed that the corrosion potential of the heated zone was lower than that of the non-heated zone. In addition, anodic polarization of the heated zone was smaller than that of the non-heated zone. Localized corrosion was easily caused around MnS inclusions in the heated zone by dipping in 3% NaCl at 40°C, and the heated zone was selectively corroded by testing in artificial sea water for 2 months. The measurement of Mn and S content with EPMA across the MnS inclusion revealed that the matrix surrounding the MnS inclusion in the heated zone was enriched in sulfur. It is thought that this partial enrichment in sulfur is due to the rapid cooling to form the unprecipitated phase of the MnS inclusion melted by electric resistance welding. On the initiation of localized corrosion, this sulfur-enriched part acts as a micro anode for MnS inclusions and the residual matrix as a micro cathode. The effect of normalizing ERW steel pipe at 700-1, 100°C on grooving corrosion resistance of the weld was also studied. Localized corrosion around MnS inclusions in the specimen normalized at 900 and 1, 100°C for 30min did not occur after immersion in 3% NaCl solution at 50°C. No grooving corrosion occured in those specimens tested in artificial sea water at 40°C for 4 months. Electron microprobe examinations revealed that the sulfur-enriched part around MnS inclusion disappeared in normalizing.