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

Corrosion fatigue of a C-Mn steel

Summary available: p. i

Some aspects of crack initiation in mild steel subjected to fatigue stressing in a corrosive environment

Some aspects of the corrosion fatigue crack initiation in EN1 type mild steel have been studied. Results of the electrochemical investigations show that the application of both static and compressive stresses enhance anodic dissolution. Application of cyclic stress assists the breakdown of the surface film resulting in localised attack, Metallographic studies indicate that a sulphur enriched band of ferrite exists around the non-metallic inclusions and corrosion occurs in this contaminated band with and without applied stress. Cyclic stress enhances the formation and coalescence of micropits at and around the inclusions leading to the nucléation of cracks. Preferential attack also occurs at the ferrite - pearlite interfaces, at slip band - matrix interfaces, and at grain boundaries. All these modes of attack can lead to crack nucleation. It has also been found that non-metallic inclusions can act as sites for hydrogen blisters; this may assist the nucléation of cracks

Localized corrosion and component failures: Causes, mechanism and remedial measures

Amongst the various types of failures of the components caused due to corrosion, a majority of them are attributed to localized type of attack of the environments on the surfaces of metals/alloys. The present talk incorporates the discussions on causes, mechanism and remedial measures adopted to control the localized type of corrosion attack. Initially, a brief background about the theoretical aspects of corrosion of metals is given. This includes the various types of cells that cause corrosion of metals and also about the thermodynamical approach to understand the phenomenon of corrosion. The subjects related to five types of localized corrosion, namely pitting, crevice, stress corrosion cracking, corrosion fatigue and intergranular corrosion, who are considered to be the most dangerous type and cause catastrophic failures, are discussed a bit in detail. Their mechanism, causes and remedial measures adopted to control the problems are described. The lecture also touches in brief, the principles involved in failure investigations. This describes the importance of site visit, collection of sample of corrosion products, visual observations, mechanical and chemical tests and fractographic studies. Finally, some examples of case studies of failed components are incorporated to illustrate the application of knowledge in pinpointing the causes of failures and suggest the remedial measures to avoid the recur¬rences of such failures

Corrosion Damage Analysis and Material Characterization of Sherman and Centaur - The Historic Military Tanks

A study of corrosion damage and material characterization of two historic military tanks, the Sherman and Centaur is reported. Experiments were conducted to analyse surface corrosion and corrosion propagation from surface to sub-surface. Significant surface corrosion was found, this phenomenon was further facilitated by delamination failure mechanisms. Corrosion depth for the Sherman was approximately 110 µm, where sulphide inclusions were detected in the sub-surface analysis. The Centaur’s analysis showed corrosion pits at 100 µm depth. These pits possess random geometrical configurations with evidence of sulfur, sodium and calcium

Design overview of high pressure dense phase CO2 pipeline transport in flow mode

In open literature, there is little information available with regards to the engineering and technological issues for material corrosion, in relation to high pressure supercritical CO2 pipeline transport from single point sources, such as the power industry. A typical CO2 pipeline is designed to operate at high pressure in the dense phase. However, it is evident that although there is considerable experience of testing materials in lower pressure gaseous CO2 in the oil and gas industry, there is little understanding of the behaviour of pipeline materials when in contact with impure CO2 captured either from power plants or the oil and gas industry. In this particular project development, a dynamic dense phase CO2 corrosion rig has been built (conditions: ∼85 bar, 40 °C and up to 5 l/min flow rate) in flow mode, to understand the effect of impurities (SO2, O2, H2, NO2 & CO) present in captured CO2 on the pipeline transport materials. This unique facility in the UK was developed via the MATTRANS project funded by the E.ON-EPSRC strategic partnership (EP/G061955/1). The test rig includes different metallic materials (X grade steel: X60, X70 and X100) to assess the corrosion of pipelines, and different geometry components (tubes, plates, charpy and tensile coupons), to assess ageing and decompression behavior of polymeric seals (Neoprene, fluorocarbon, ethylene and Buna N) under water-saturated dense phase CO2 with different impurity concentrations (0.05 mol % SO2; 4 mol % O2; 2 mol % H2; 0.05 mol % NO2; 1 mol % CO). The dynamic data generated from this dense phase CO2 corrosion rig will give vital information with regards to pipeline suitability and lifetimes, when operating with dense CO2

Structural integrity assessment of C-Mn pipeline steels exposed to sour environments

Oil and gas fields can contain significant amounts of hydrogen sulphide and the behaviour of C-Mn pipeline steels exposed to sour environments (i.e. those containing water and hydrogen sulphide) continues to be one of the most active areas of research in the oil and gas industry. This project is aimed at improving the procedures used to assess the significance of flaws in offshore pipelines and risers operating in such environments. Experimental work has focused on examining the behaviour of C-Mn pipeline steel in a sour environment with respect to both static and fatigue crack growth behaviour, for which there is a paucity of data. In particular, the critical influence of crack depth on the crack growth rate has been studied, in order to ensure that test methods and assessment procedures used in industry are appropriately conservative. Under cyclic loading conditions, an environmental crack depth effect has been demonstrated, whereby, shallow flaws appear to grow faster than deeper flaws at the same (low) value of ΔK. The observed behaviour is believed to be dominated by bulk hydrogen charging, i.e. hydrogen charging by absorption from the external surfaces of the specimen rather than at the crack tip, and a lower concentration of hydrogen exists in the centre of the specimen than at the edges. The novel data generated have been applied to real-life pipeline defect assessments to demonstrate the influence of the observed crack growth rate, with a view to developing an improved assessment method. Example engineering critical assessments have been performed for circumferential surface-breaking girth weld flaws located on the internal surface of a typical steel catenary riser, operating in a sour environment and subject to vortex induced vibration fatigue loads. Companies operating in the oil and gas sector will derive benefit from this research programme through the application of new validated test methods and the development of improved in-service assessment procedures

engineering thoughts on hydrogen embrittlement

Abstract Hydrogen Embrittlement (HE) is a topical issue for pipelines transporting sour products. Engineers need a simple and effective approach in materials selection at design stage. In other words, they must know if a material is susceptible to cracking, to be able of: selecting the right material and apply correct operational measures during the service life. Following ASTM F2078, HE is "a permanent loss of ductility in a metal or alloy caused by hydrogen in combination with stress, either externally applied or internal residual stress". In many cases, hydrogen can play a role in crack propagation, as for instance in Stress Corrosion Cracking (SCC) and Corrosion Fatigue (CF). Three parameters are required to cause failure: presence of hydrogen, tensile stress, and material susceptibility. The two previous ones are triggering the failure, while the root cause is usually material susceptibility. This is why material selection is the important step to safely manage engineering structural materials. As an example, material selection for sour service pipeline is the object of well-known standards, e.g. by Nace International and EFC: they pose some limits in the sour service of steels, with reference to surface hardness. These standards have shown some weak points, namely: In the definition of sour service; In defining the role of crack initiation and propagation, considering that in Hydrogen embrittlement, stress state and stress variations are very important. As for the second point, in hydrogen generation anodic processes shall be taken into account too. For instance, there is a relationship between corrosion resistance and crack susceptibility. In carbon and low alloy steels, cracking will not normally occur when there is a significant corrosion rate. If a brittle layer (or a brittle spot) is present on the metal surface, this one can initiate a crack

Корозійно-воднева деградація промислових нафтопроводів

Подано аналіз руйнування промислових нафтопроводів при корозійній і корозійно-механічній дії. Особлива увага приділена деградації внутрішніх поверхонь труб в умовах вуглекислотної, сірчановодневої і мікробіологічної корозії, а також сірчановодневого корозійного розтріскування і корозійної втоми.The fracture analysis of industrial oil pipelines at corrosion and corrosion-mechanical influence has been done. The special attention is given for degradation of internal pipe surfaces in the conditions of carbon dioxide, hydrogen sulphide and microbiological corrosion, and hydrogen sulfide stress corrosion cracking and corrosion fatigue as well