Page 400 - Failure Analysis Case Studies II
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                                                    air pump
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            Figure 4 shows the microscopic structure  of  the crack and the results of  observation  thereof.
          Plastic deformation (hardness of inside surface of bent portion Hv = 25G300) attributable to cold
          working is evident in the corner section where the hydrogen charge of the steel was 3.5  ppm. The
          crack, which had propagated transgranularly in a brittle manner, was a cleavage fracture exhibiting
          features typical of hydrogen embrittlement cracking. Accordingly, it is considered that the following
          reactions (galvanic corrosion [2]) between zinc and ferritic stainless steel occurred in the tank during
          acid washing at the fabrication stage, causing the stainless steel to become hydrogen-charged.
                                    anode reaction: M + M”+ +ne
                                   cathode reaction: nH+ +ne-  -+ nH
           The cracking is believed to have occurred in the subsequent water proof testing when the material
          was subjected to tensile strain.
            Cold working becomes more pronounced as the inner radius of the corner portion shown in Fig.
          2 becomes smaller and, consequently, hydrogen embrittlement susceptibility is thought to increase.
           Further, it is conceivable that the amount of tensile strain occurring on the inside surface during
          water proof testing also increases with smaller inner radii.
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