Strain Hardened UNS N08935 Bar for Subsea Bolts, Hydrogen Embrittlement and Localized Corrosion Testing

Abstract Fasteners, studs or bolts, used in subsea applications need to possess high strength as well as withstand cathodic protection. In addition, sufficient corrosion resistance is needed which is of specific importance in the splash zone, where localized corrosion is a risk when the contact to the cathodic protection is broken. Such conditions are challenging for traditionally used materials, such as precipitation hardened Ni-base grades and super duplex, and therefore strain hardened super austenitic grades may be a viable option. UNS N08935 is a very structure-stable and cost efficient super austenitic grade with PRE of 52 that now is available as cold drawn bar. Testing was performed on both 120 ksi (827 MPa) and 180 ks i (1241 MPa) strength levels. The results from mechanical and localized corrosion resistance for both 14 mm and 35 mm cold worked bar from full scale heats are presented and show very high CPT. Hydrogen embrittlement was tested according to NACE TM0198 Method C to demonstrate superior resistance compared to precipitation hardened Ni-base grades such as Alloy 718. Furthermore, SSRT with cathodic charging was performed at 4°C in order to investigate hydrogen embrittlement at service conditions.