An evaluation of a novel method for the inhibition of sulphide stress corrosion cracking in steel
Sulphide stress corrosion cracking, (SSCC) is one of the major problems encountered by the petroleum industry throughout the world. The problem is likely to increase in severity for the North Sea oil and gas industries as the fields get older and platforms are moved to deeper waters. This is because the hydrogen sulphide concentration increases as the fields get older and deeper water explorations require higher strength steels. The protective measures taken at present to combat SSCC are not adequate. Therefore a novel method was developed to inhibit SSCC in steel. This method is based on using an active hydrogen evolution sulphide electro-catalyst, more active than steel, as a coat on the surface of the steel, such that the hydrogen evolution will take place on the catalyst surface, instead of on the corroding steel surface. Therefore, the amount of hydrogen diffusing through the steel is greatly reduced. Hence, SSCC in the steel is effectively inhibited. Electrochemical and mechanical experimental studies were carried out to confirm the validity of this method A computer-aided literature survey on SSCC and its prevention in oil- and gas-well equipment is presented. The viability of three sulphide electro-catalysts, NiCo₂S₄, MoS₂ and WS₂ for this method were studied in various experiments, namely, electrochemical polarization, hydrogen diffusion studies and corrosion weight loss measurements. The experimental studies carried out in NACE solution, consisting of a 5 percent (mass/volume) NaCl and 0.5 percent (volume/volume) acetic acid, with a continuous flow of H₂S at 1 atmospheric pressure, indicated that hydrogen evolution performances are in the following order: in the absence of H₂S, NiCo₂, S₄ > WS₂ , > MoS₂> EN 42 steel in the presence of H₂S, MoS₂> WS₂> NiCo₂S₄> EN 42 steel MoS₂was found to be the most stable catalyst in the sour corrosive environment. Evans diagrams, constructed to predict corrosion rates, indicate that the corrosion current ratio of the MoS₂ - EN 42 steel couple and EN 42 steel did not change significantly when the catalyst loading was reduced. The hydrogen diffusion studies confirmed that an MoS₂/ FEP (fluoro ethylene polymer) adherent coat with higher catalyst to FEP ratio was the most effective of three adherent coats. The corrosion weight loss measurements showed that the corrosion rates of steel coupons partially coated with MoS₂/ FEP coat were higher than those of uncoated coupons for up to 50 hours but thereafter they reduced significantly below those of uncoated coupons. Mechanical studies carried out to eväluate the effectiveness of this method were helped by a literature survey on stress corrosion test methods and interpretation of results. Slower straining/loading rate tests and sustained load tests were selected to study the changes in various mechanical parameters on different types of specimens when protected with MoS₂ / FEP coat. In addition to these tests, Charpy impact tests were also carried out. The mechanical parameters measured on the specimens are: a) for compact tension specimens - stress intensity factor at failure - total energy required for fracturing the specimen - average energy consumed for unit length of crack extension - crack opening displacement - crack growth rate - time to failure b) for three-point bend specimens - crack opening displacement c) for Charpy V-notch impact test specimens - fracture energy All these mechanical parameters confirm the effectiveness of the MoS₂ / FEP coat to inhibit SSCC in steel. Scanning electron microscopic examinations of the specimens also confirmed the viability of the novel protective method. The sour-corrosion fatigue tests showed that the MoS₂ / FEP coat could be used effectively in environments where a cyclic loading pattern is inevitable. These studies confirm that the proposed protective technique could be used effectively in the oil and gas industries to inhibit SSCC.