No abstract

After forming a passive film coating on Type 304 stainless steel using a new method, we investigated the improvement of corrosion resistance. Coatings were formed using a combination of electrolytic polishing as a pretreatment process, chemical oxidation and cathodic electrolysis as intermediate processes, with curing applied as a final process. The passive film structure formed by the coating was assessed in terms of film thickness and element distribution. Corrosion resistance was assessed in terms of pitting corrosion potential. A model of the film formation is also discussed in this report.

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.

Hydrogen barrier coating is a promising technology for preventing hydrogen embrittlement in metals. In this study, characterizations of hydrogen barrier films coated on surfaces of austenitic stainless steel, SUS304 of the Japanese Industrial Standard (JIS), by wet coating processes applied electro-polishing and chemical oxidation method are carried out using cross-sectional transmission electron microscopy (TEM) analyses, and then slow strain rate tensile (SSRT) tests are performed in 1.1MPa hydrogen and nitrogen gases at room temperature. The hydrogen barrier films show 200–300nm total thickness of compositionally modulated Chromium oxide dense layer. The SSRT results reveal that both the elongation and reduction of area are decreased in hydrogen gas compared with those in nitrogen gas for the non barrier-coated specimens but no significant differences appear for the barrier-coated specimens. The fracture surface of the non barrier-coated specimen shows quasi-cleavagy cracking in hydrogen gas while that of the barrier-coated specimen shows only ductile dimple fracture in hydrogen gas, indicating that the coated films effectively prevent the hydrogen embrittlement of SUS304 stainless steel in hydrogen.

This study assessed surface treatment techniques applicable to welded joints of stainless steel pipes and vessels for a hydrogen-permeation barrier. Specifically investigated was electrolytic polishing using electrolyte compositions consisting of sulfuric acid, phosphoric acid, and methane-sulfonic acid to promote uniform passive film formation on a type 304 stainless steel surface. This study also examined oxidation treatment using a combination of chemical oxidation, cathodic electrolysis, and passivation. Hydrogen gas permeation suppression was confirmed for welded SUS304 steel substrates treated under selected conditions suitable for thickening, homogenizing, and densifying passivation films.