Sacrificial aluminum-indium anode cathodic protection is commonly used for protecting offshore and subsea structures. As a consequence, all metallic fasteners are hydrogen charged under high tensile stresses at the thread roots. This means that material selection must prevent the occurrence of hydrogen stress cracking (HSC) under these conditions. For most equipment where low strength fasteners are sufficient a low alloy steel can be used provided a maximum yield strength and hardness are specified. When higher strength fasteners are required, low alloy steels must be replaced by corrosion resistant alloys which must be qualified with regard to their susceptibility to HSC. In this work, precipitation-hardened nickel alloys from API 6ACRA, work hardened austenitic stainless steels and nickel alloys with yield strengths up to 200 ksi (1240 MPa) have been tested under cathodic protection using a specific test methodology. The objective was to quantify their resistance to HSC and identify their limits of use when exposed to cathodic protection. The results are presented and discussed in the light of current knowledge on HSC for subsea fastener applications.
Bolted connections for subsea flanges and other components must be reliable as they are often used for pressure containing components (subsea Christmas trees, connectors …). When possible, the primary choice for bolting is low-alloy steel with a limitation of actual yield strength (135 ksi) and a maximum hardness of 34 HRC to prevent HSC. In some cases, the use of low-alloy steel is prohibited and corrosion resistant alloys (CRAs) must be used. This is the case when higher strength is required and also for sour service resistance under thermal insulation. In addition, CRAs do not require temporary corrosion protection coatings.
There is limited literature dealing with high strength subsea fasteners. Eskalul et al. listed possible alloys for use as subsea fasteners with yield strength above 150 ksi. Among these they highlighted alloys with good resistance to environment-assisted cracking (EAC) including some martensitic stainless steels (SS), precipitation-hardened (PH) nickel alloys and cobalt alloys.1,2
