Mechanical properties, corrosion resistance and immunity to environmentally assisted cracking are the key elements in selection of fasteners for sub-sea applications. While corrosion resistance and mechanical properties are attainable, finding materials that offer resistance to cracking in seawater service in areas of intense cathodic polarization or in galvanically coupled systems has been a major challenge.
High strength low alloy steels have had acceptable performance in cathodically protected systems. Their applications have been limited to maximum yield strength of 140 ksi (965 MPa) and a maximum hardness of 34 HRC. Development of deepwater has required larger size and higher strength fasteners than have been typically used for sub-sea applications. Costly interventions have necessitated the need for increased reliability for subsea systems including fasteners.
Laboratory testing to qualify several materials for high strength fastener applications for two projects has shown that, in the presence of cathodic protection, nickel alloys 718 and 725 offer the best resistance to hydrogen embrittlement and are available in the strength/size required for high pressure and moderate temperature applications.
The results of the study demonstrated the susceptibility of high strength alloy steel fasteners to hydrogen embrittlement in cathodically protected systems when the yield strength exceeds 140 ksi (965 MPa).
Keywords: Bolting, fasteners, high strength, cathodic protection, hydrogen embrittlement, stress corrosion cracking, subsea, seawater