Estimate of Hydrogen Density at Starting Point for Crevice Corrosion and Cracking of Cold Rolled Type 304 Stainless Steel

In the past work of authors, a cold rolled type304 stainless steel pipe with shot peening was prepared. A crevice was created between the outside of the pipe and an O-ring, and the pipe was applied stress by press fitting another part. Cracking from a crevice corrosion pit was observed in a chloride environment, and it was presumed to be Hydrogen Embrittlement-Stress Corrosion Cracking（HE-SCC）. Provided that the crack is caused by HE-SCC､it is thought that hydrogen is generated by corrosion reaction and the SUS304 pipe absorbs it more than its critical level. Generally, the hydrogen distribution in the steel is analyzed in the condition that the hydrogen concentration of the environment is constant as the high-pressure hydrogen gas environment etc. In the case of HE-SCC described above, however, the hydrogen concentration is presumed to change with pitting growth, and there is no example of analysis in such condition. In this work, cyclic corrosion tests were conducted using the samples with no press-fitting part. The hydrogen density at the initiation point of crevice corrosion and cracking was estimated by following technique. First, the amount of hydrogen diffused from the corrosion pit was measured using Thermal Desorption Spectroscopy（TDS ）, and the number of pits was measured. Then, the average hydrogen quantity in a pit was derived. On the other hand, it is thought that hydrogen is generated by the hydrolysis of a metallic salt and the cathode reaction of the hydrogen ion in the pit. Therefore, the amount of the hydrogen is proportional to the quantity of corrosion amount. Then, assuming that pitting corrosion takes the form of half sphere shape and grows with time, the simulation model was made that the hydrogen absorbed and diffused from the pit surface with the development of corrosion. The profile of the hydrogen density in SUS304 from pit initiation till crack initiation was calculated based on this model. The calculated profile of the hydrogen density was compared with the amount of hydrogen measured by cyclic corrosion test. As a result, the hydrogen concentration at the crack starting point was derived 0.3ppm. At this density level, He-SCC can occur in high strength SUS304.