13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- The corrosion pit initiated in the result of reaction between 12Cr stainless steel and H2S gas in humid air. Intergranular fracture surface was induced by hydrogen occurred in the corrosion reaction.Fig.3 also shows plane bending corrosion fatigue testing results for T type welded joints of high 17-4PH stainless steel 5000ppm H2S and 99.9% CO2 gas in a humid air[25]. The remarkable reduction of corrosion fatigue strength of T type welded joints can be observed. The similar results were obtained on AISI410 and AISI4330 steel [26]. The higher the ultimate tensile strength of the steel, the larger the influence of H2S gas environment was. In 5000ppm H2S and 99.9% CO2 environment corrosion fatigue crack initiated from corrosion pit at welded toe. Striation was predominant at crack propagation area. Typical fracture surface of 17-4 PH steel in humid H2S gas environment is shown in Fig. 4. The following reaction can be considered. Figure 3. S-N curves of T type welded joints of 17-4pH T type welded joints in H2S and CO2 gas environment [Ebara et al. [26]]. Plane bending, 20Hz, R=0.05 Figure 4. Corrosion fatigue fracture surface of 17-4pH T type welded joint in humid air(353K, 90% relative humidity) containing 5000ppm H2S gas[Ebara et al.[26]]. 294.3MPa,1.4x10 7cycles a) corrosion pit at crack initiation area b)3mm from initiation area, Arrow shows crack propagation direction
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