The influence of external tensile stress on corrosion and trench formation of low alloy steel in a low H 2 S content sour corrosion environment was investigated. These experiments were conducted with steel for pipelines, and electrochemical methods were used. The results showed that external stress increases the amount of corrosion weight loss and trench depth by promoting the anodic dissolution reaction, and stress concentration was proven to be one of the driving forces for trench formation due to localized corrosion. Based on the experimental findings, the mechanism of trenching was discussed from the viewpoint of promotion of the anodic dissolution reaction by dislocations.

TMCP (thermo-mechanical controlled process) linepipes have been long used for severe sour environment, but recently sulfide stress cracking (SSC) caused by local hard zones has become a concern. In order to clarify the hardness threshold that leads to SSC, four-point bend (4PB) SSC tests as per NACE TM0316 were conducted under several H 2 S partial pressure conditions. For 1 bar and higher H 2 S partial pressure conditions, the surface hardness threshold (at 0.25 mm from surface) observing 4PB SSC specimens without SSC cracking was approximately correlated to a maximum acceptable hardness level of 250 HV0.1. By suppressing the hard lath bainite (LB) and obtaining the soft granular bainite (GB) microstructure, stable low surface hardness of 250 or less HV0.1 was achieved, resulting in superior SSC-resistant property. On the other hand, it was found that SSC crack propagated when the surface hardness increased with increasing the volume fraction of LB microstructure. In the case of 16 bar H 2 S partial pressure condition, the crack growth rate increased in the sour environment, and hydrogen embrittlement by H 2 S was promoted. However, in the 4PB SSC test at 16 bar, since the shape of localized corrosion is semicircular due to low localized corrosivity, it was considered that the stress concentration and transition to crack were suppressed. This may be the reason why the SSC susceptibility was similar to 1 bar condition, especially in the 4PB SSC test using the samples with lower surface hardness level of 250 or less HV0.1.

The temperature dependence of the yield stress, effective stress and activation volume on the Cu content in steel was assessed using polycrystalline ultra-low carbon steels with 0.5, 1 and 2 mass% Cu added. A small hump was seen in the effective stress-temperature curve for ultra-low carbon steel, which has also been reported for high-purity singlecrystal iron. The effective stress was found to decrease with increasing the Cu content. The activation volume was found to be linearly related to the temperature at low temperatures for all types of specimens considered. The activation volume was found to increase abruptly at a certain temperature, which increased with increasing the Cu content. The change in the relationship of the activation volume to temperature is discussed in this paper with respect to the thermally activated process of dislocation glide from the Peierls mechanism to the interaction between dislocations and solute Cu atoms.

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