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Materials properties that are used in specific oil and gas environments are de-rated due to the risks associated with hydrogen embrittlement cracking. In oil production environments the concern is for the onset of stress corrosion cracking (SCC), while in seawater environments the concern is for Hydrogen Induced Stress Cracking (HISC). Both are hydrogen embrittlement phenomena with the distinction being the source of hydrogen for each. In SSC the source of hydrogen is from the presence of H2S in the oil production fluids, and in HISC the source of hydrogen is from the dissociation of water from the cathodic protection system. This paper is focused on the latter phenomena and aims to characterize the susceptibility of carbon alloy steels as applied in fastener applications, in a seawater environment under cathodic protection.
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High-strength aerospace aluminum alloys, such as AA7075-T651, are susceptible to environmental assisted cracking (EAC) under the right combinations of stress, environment, and microstructure. EAC presents a serious risk to structures and equipment operated in corrosive conditions. Studies of EAC in aluminum alloys have highlighted the importance of both anodic dissolution and hydrogen embrittlement to EAC initiation and propagation.1–4 The EAC response of alloys under variable atmospheric conditions is of particular importance for assessing material performance for aerospace applications.
Carbon and low alloy steels (CS and LAS, respectively) used for exploration and production in the oil and gas (O&G) industry are normally exposed to environments that may contain H2S in a wide range of concentrations. In aqueous solutions, H2S acts as a cathodic poison.1,2 A cathodic poison inhibits the recombination of atomic hydrogen to H2, and as a result, favors its absorption by the metal.1,2 In the presence of a susceptible microstructure and the simultaneous effect of applied or residual tensile stress, a crack can nucleate and propagate, when a critical concentration of hydrogen is reached in the metal.3 This environmentally assisted cracking (EAC) phenomenon is known as Sulfide Stress Cracking (SSC).2 SSC is commonly addressed as a case of hydrogen embrittlement (HE) damage.2
During crack growth and localized corrosion, fresh metal surfaces become exposed to the chemical and electrochemical environment. Subsequent alteration of the material in the nascent pit or at the crack tip zone will be initiated by the surface chemical processes that template the subsequent reactivity. Quantum chemical calculations can provide a physics-based method to simulate the adsorption phenomena that occur in systems possessing multiple chemical species (such as water, chloride, hydrogen-sulfide, inhibitors, etc.), various pH levels, temperatures and electrochemical conditions.
The presence of trenches on low alloy steels (LAS) in sour environments at the open circuit potential (OCP) has been reported since 1977. Originally, they were classified as deep and elongated localized corrosion pits. Recently, many authors have referred to them as “stress-induced grooves.