Stress Corrosion Cracking of Alloy 718 Bolts in Marine Atmosphere

Abstract Bolts made from 25Cr super duplex stainless steel (25Cr) can be accepted up to 110°C in marine atmosphere containing chlorides according to ISO 21457.1 Some piping systems operate at higher temperature and alternative alloys need to be used. UNS N07718 (718) is one alloy that has been presented as an alternative due to a combination of strength, Stress Corrosion Cracking (SCC) resistance, availability and cost. Several studies related to oil & gas focusing on sulfide stress cracking (SSC)/SCC in well fluid have been published.2,3,4,5,6,7 A detailed study related to the corrosion resistance of 718 in seawater has also recently been published.8 According to this paper, crevice corrosion was initiated in the temperature range 10-20°C in natural seawater. However, limited information about the SCC resistance of 718 in marine atmosphere at high temperature is available in standards or published literature. Due to this, a test program to investigate the susceptibility of SCC for bolts made from 718 exposed to marine atmosphere with surface temperature up to 250°C has been executed. In addition, a test with 25Cr bolts at 110°C has been included in the test program. The test approach follows the philosophy of ISO 15324:2000.9 However, the temperature, load application and test solution have been tailor made for the project. Real bolts under constant uniaxial load at 100% of actual yield strength (AYS) and four-point bend (4PB) samples machined from real bolts have been included in the test program. No stress corrosion cracking was observed on any of the 718 samples tested at 250°C, confirming its high resistance to SCC under these severe conditions. For the 25Cr samples tested at 110°C, no SCC was detected on the tested bolts or the 4PB samples. One crack was found in a bolt sample, but metallurgical investigation concluded that it most probably was pre-existing and mechanically induced before testing, not caused by the exposure environment.