SCC/HE of Bolts/Fasteners in Marine Environments
By Jose Ramirez on Tuesday, February 26th, 2013
In the oil and gas industry, bolted connections are often used for high integrity systems and are thus vital for the long term performance of the given structure. As the oil and gas industry explores and develops reservoirs in deeper waters where operation and intervention costs are exceptionally high, factors such as bolt/fastener design, proper material selection, materials and bolts/fastener processing, and installation of bolts/fasteners become more important to provide the required service life for the various exposure and service conditions.
Fasteners can fail as result of one or a combination of overload, corrosion, fatigue, corrosion fatigue, or environment assisted cracking (EAC) in the form of stress corrosion cracking (SCC) or hydrogen embrittlement (HE). SCC is a localized corrosion attack that propagates rapidly leading to failure of components and potentially the associated structures. SCC / HE are failure mechanisms resulting from the synergistic effect of the environment, presence of tensile stresses, and material susceptibility.
In marine environments, chlorides can act as a catalyst for chloride SCC. Hydrogen generated by corrosion reactions or cathodic overprotected systems can induce HE. Potential sources of tensile stresses in bolts/fasteners include stress applied during tightening or torquing, and residual stress from different manufacturing processes (forging or casting, surface treatment, heat treatments, forming, machining, and cutting and shearing). Mechanical properties and resistance to corrosion and EAC are critical factors for the selection of materials for bolts and fasteners in the marine application. The corrosion protection of the bolts depends on the intrinsic corrosion resistance of the bolting material and the effect of any supplementary protection measures. Such measures can include coating systems and/or cathodic protection. Available materials for subsea high-strength bolts or fasteners includes high-strength low alloy steels, stainless steels, nickel-based alloys, Cobalt-Ni alloys, beryllium-cupper alloys, and titanium alloys. High strength steels, mainly AISI 4140 and 4340, remain the most common materials for subsea fasteners.
As higher-pressure systems are being built in subsea environments large diameter fasteners made with higher-strength materials are being used. High-strength low alloy steels may provide adequate corrosion resistance in cathodic protected systems, but their resistance to SCC or HE is a concern. High-strength steel may be more susceptible to HE in seawater under cathodic protection, and the susceptibility increases with strength.
The SCC resistance of these materials to particular environments is generally expressed in maximum hardness limits.Therefore, adhering to specifications and quality assurance should be a prime concern in bolt/fastener procurement. Selectionfor subsea application still relies in qualification testing for the specific application. The oil industry challenge is to develop reliable test methods for materials qualifications and to assess the risk to HE. Experiences in the field indicate that the main drivers for bolting material HE have been susceptible microstructures, CP, and applied load levels.