Evaluation of the Hydrogen Barrier Properties of Chromium Oxide Films Deposited on SUS304 Austenitic Stainless Steels

Austenitic stainless steels including type SUS304 of the Japanese Industrial Standard (JIS), which is similar to ASME Type 304 SS, are candidate materials for the various facilities of high-pressure gaseous hydrogen, such as a hydrogen station containers and piping. To prevent the hydrogen penetration into SUS304, we developed the passivation films mainly composed of chromium oxide and investigated the hydrogen barrier properties using the gas permeability test and the slow strain rate tensile (SSRT) test. The passivation films with a maximum thickness of 300 nm was formed on the surface of hot-rolled SUS304 (Ni equivalent 22.4) by a series of wet processing steps such as electro-polishing, chemical oxidation, cathode precipitation of chromium and passivation immersion. Cross-sectional views of TEM observation suggested that the film was amorphous like structure including many independent voids with a size of 10 to 20 nm. The hydrogen gas permeability test was performed in a hydrogen gas pressure of 400 kPa at 573 K, 673 K, and 773 K. The estimated hydrogen transmittance of SUS304 substrate with passivation coating was 2.8 × 10−13 mol / (m · s · Pa) at 773 K, while that without passivation coating was 2.2 × 10−11 mol / (m · s · Pa) at 773 K. SSRT test was performed in 110 MPa hydrogen and nitrogen gas atmosphere at room temperature, and strain rate was 4.17 × 10−5 s−1. The fracture surface of the specimen without passivation coating showed brittle like and relative reduction rate (RRA) was 0.61. On the other hand, the fracture surface of the specimen with passivation coating showed typical ductile like dimple structure and RRA was 0.88. As the passivation films did not peel, adhesion between passivation film and SUS304 surface seems to be well. From these considerations, we anticipate the developed passivation film can inhibit hydrogen embrittlement of SUS304.