Effect of H₂S Existence, Surface Hardness and Applied Stress on Corrosion Groove Formation and SSC Initiation in Low Alloy Steel

In designing thermo-mechanical controlled processing (TMCP) sour linepipe, lowering surface hardness is one of the most important points to prevent sulfide stress cracking (SSC). In previous studies, it was found that surface hardness should be controlled to be lower value less than 270HV0.1 at 0.25mm from surface in 4-point bend (4PB) SSC test under 0.15 bar H2S partial pressure condition. Also, it was found that the high applied stress activated the anode reaction in 4 point bend (4PB) SSC test. The results indicated that the anodic dissolution enhanced by applied stress was dominant factor for corrosion groove formation. However, an initial corrosion groove formation behavior and a SSC crack initiation behavior are still not clear regarding the effect of H2S existence, surface hardness and applied stress which can affect a microscopic plastic deformation at surface region. In this paper, in order to clarify the effect of those factors on an initial corrosion groove formation behavior and a SSC crack initi ation behavior, 4PB SSC tests were conducted under 0.15 bar H 2S partial pressure condition using Grade X65 steel plate with relatively high surface hardness at 0.25mm from surface. Firstly, the effect of H 2S existence was investigated. Two different gas compositions (0.15bar H 2S +0.85 bar CO2, 0.15bar N 2 +0.85bar CO2) were used. Also, the effect of surface hardness was evaluated using 4PB test specimen taken from three different position (0.25mm, 1.0mm and 7.15mm from surface) in thickness direction. As a result, localized corrosion pit or groove formation was observed under the H 2S condition, while no local corrosion pit groove formation was observed under the N 2 condition. It was confirmed that local corrosion pit groove formation was a phenomenon peculiar to H2S sour environment. The harder samples collected on the surface side, the larger the amount of corrosion and the larger the depth of corrosion pit groove. The mechanism of corrosion groove formation and SSC crack initiation is also discussed in this paper.