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.

The phenomena known as `internal oxidation' can play a major detrimental role in the failure of carburised components such as bearings and gears. Internal oxidation leads to a degradation of the surface layer often leading to surface break up and fatigue. This work is concerned with a detailed understanding of the formation of internal oxidation leading to modifications to composition or process parameters to eliminate or reduce internal oxidation. Experimental steels for the most part have been used in this study with Si content varying from 0.11 to 0.77 mass percent. A commercial carburising process at David Brown Heatch Ltd. consisting of a number of process stages with varying C potential and treatment temperature has been used in the study. Scanning Electron Microscopy (SEM) , Cross sectional Transmission Electron Microscopy (TEM), Energy Dispereive Spectroscopy (EDS) and Electron Probe Microanalysis (EPMA), Glow Discharge Optical Emission Spectroscopy (GDOES), Quantitative Image Analysis techniques were applied to characterise the specimens. A model has been proposed that explains the formation of the characteristic morphology observed in internal oxidation and is based upon competitive processes concerning the diffusion rate of alloy species within the bulk material and the differences in the free energy of formation of the various oxides types. The observed morphology of the internal oxidation zone at different carburised exposure time, in range of 0.25-16.6h, related to the penetration depth and density of the internal oxides in the internal oxidation zone, with particular emphasis on the relative importance of oxygen partial pressure at reaction and free energy of formation of oxides have been studied. The research indicated that the internal oxides grew fast in the base process. Three elements Cr, Mn and Si were oxidised at this stage and formed complex oxides within the grains and on the grain boundaries. Further, as carburising time increased, existing oxides grew and new oxides nucleated again along grain boundaries. In the boost process, only Si was oxidised. Si oxides penetrated to a greater depth along the grain boundaries. The generally two-zone morphology characteristic was found in the internal oxidation zone of carburised steel. Outer zone: larger size complex oxides which contain higher concentrations of Cr, Mn and some Si on the grain boundaries or within the grain; Inner zone: intergranular Si oxides on the grain boundaries. Small dispersed oxide paticles were observed in both zones. Different oxides were formed in the internal oxidation zone as complex oxides, sometime as agglomerated oxide phases, and intergranular oxides. These complex oxides were identified as Cr1.5Mnj.5 O4C, rMnO4, Mn2SiO4 and MnSiO3. The intergranular oxidation was mainly Si oxides, such as Si02. The agglomerated oxide phases were observed usually as the Cr-Mn complex oxides with Si oxides or Mn-Si complex oxides growing around them. The role of Si is critical in that its solid state diffusion coefficient in Fe is considerably higher than that for Mn and Cr whilst the free energy of formation of Si oxide is lower than that for Mn and Cr. It affects internal oxidation, not only on the morphology but also the rate of penetration. For the specimen with low Si bulk content, the internal oxidation zone consisted of larger complex oxides elongation close to the surface, with intergranular oxidation remote from surface. The penetration depth of the internal oxides increased with increasing bulk Si content. In this case, oxygen diffused not only through the metal lattice but also at an enhance rate along the internal oxides/metal matrix interface. There was a peak value, after which as Si increased in the bulk metal, more intergranular oxidation was formed instead of the larger oxides. A continuous layer was formed parallel to the surface that reduced further diffusion of oxygen. The penetration depth of the internal oxides decreased with further increases of bulk Si content. The higher the carbon potential, the lower was the penetration depth of the internal oxides and the less dense the internal oxidation zone.

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