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Preferential weld corrosion (PWC) remains a major operational challenge that jeopardizes the integrity of oil and gas production facilities. It is the selective dissolution of metal associated with welds, such that the weld metal (WM) and / or the adjacent heat-affected zone (HAZ) corrode rather than the parent metal (PM). Corrosion inhibition is conventionally used to mitigate this problem however several indications suggest that some corrosion inhibitors may increase PWC. Furthermore, it is not possible to detect systems that are susceptible to PWC and or to understand the apparent ineffectiveness of some corrosion inhibitors at high flow rates. Consequently, the aim of this research is to assess the suitability of submerged jet impingement method to study flow accelerated preferential weld corrosion, which is critical to safe and economic operations of offshore oil and gas facilities. In this research, a submerged jet-impingement flow loop was used to investigate corrosion control of X65 steel weldment in flowing brine, saturated with carbon dioxide at 1 bar, and containing a typical oilfield corrosion inhibitor. A novel jet-impingement target was constructed from samples of parent material, heat affected zone and weld metal, and subjected to flowing brine at velocities up to 10 ms- 1 , to give a range of hydrodynamic conditions from stagnation to high turbulence. The galvanic currents between the electrodes in each hydrodynamic zone were recorded using zero-resistance ammeters and their self-corrosion rates were measured using the linear polarisation technique. At low flow rates, the galvanic currents were small and in some cases the weld metal and heat affected zone were partially protected by the sacrificial corrosion of the parent material. However, at higher flow rates the galvanic currents increased but some current reversals were observed, leading to accelerated corrosion of the weld region. The most severe corrosion occurred when oxygen was deliberately admitted into the flow loop to simulate typical oilfield conditions. The results are explained in terms of the selective removal of the inhibitor film from different regions of the weldment at high flow rates and the corrosion mechanism in the presence of oxygen is discussed.

The aim of this research proj ect was to produce compositionally modulated zincnickel and zinc-nickel-manganese coatings usi ng a single bath process by variation of the applied electroplating current density. These could then be considered as possible replacements for electroplated cadmium. Zinc-nickel electrodeposits from both a simple bath and one containing the complexant tris(hydroxymethyl)methylamine (TRIS) were produced using either bath or by selective electroplating onto mild steel, 2014-T6 aluminium and 2000 series aluminium alloy connector shells. Zinc-nickelmanganese coatings were electroplated onto mild steel, using both DC and pulse electroplating from sulphate based baths containing either the complexant TRIS or sodium citrate. The coatings and the various electrodeposition processes were evaluated by thickness measurements, cun'ent efficiency calculations, composition detennination using energy dispersive X-ray anal ys is and SEM to examine the morphology. Corrosion characteristics were investigated by neutral salt fog exposure, alternate immersion, atmospheric trials, and electrochemical immersion tests including potential monitoring, galvanic and polarisation measurements. Other properties such as microhardness and surface electrical conductivity were also investigated. Compositionally modulated zinc-nickel and ziJlc-nickel-manganese were successfully deposited from single baths with nickel contents ranging from 5-20% Ni and manganese contents varying from 0-12% Mn. Current efficiency measurements showed the efficiency to be low especially for the zinc-nickel-manganese coatings, which exhibited a distinct morphology when electroplated from the citrate bath. Corrosion testing indicated that both coatings had a lower corrosion resistance than cadmium especially in accelerated tests, with a reduction in corrosion resistance being seen as the nickel content was increased; due to a decrease in density of the electroplate. This, and increasing through-thickness porosity promoted ennoblement of zinc-nickel and zinc-nickel-manganese leading to the coatings becoming nonsacri ficial to the substrate in accelerated tests. Zinc-nickel-manganese showed less white rust than zinc-nickel and exhibited lligher micro-hardness than zinc, cadmium and zinc-nickel. Electrical measurements indicated that both coatings were sufficiently conductive to meet the surface conductivity requirements for use on electrical connector shells.

No description available.

The majority of UK’s intermediate level radioactive waste is currently stored in 316L and 304L austenitic stainless steel containers in interim storage facilities for permanent disposal until a geological disposal facility has become available. The structural integrity of stainless steel canisters is required to persevere against environmental degradation for up to 500 years to assure a safe storage and disposal scheme. Hitherto existing severe localised corrosion observances on real waste storage containers after 10 years of exposure to an ambient atmosphere in an in-land warehouse in Culham at Oxfordshire, however, questioned the likelihood occurrence of stress corrosion cracking that may harm the canister’s functionality during long-term storage. The more corrosion resistant duplex stainless steel grade 2205, therefore, has been started to be manufactured as a replacement for the austenitic grades. Over decades, the threshold stress corrosion cracking temperature of austenitic stainless steels has been believed to be 50-60°C, but lab- and field-based research has shown that 304L and 316L may suffer from atmospheric stress corrosion cracking at ambient temperatures. Such an issue has not been reported to occur for the 2205 duplex steel, and its atmospheric stress corrosion cracking behaviour at low temperatures (40-50°C) has been sparsely studied which requires detailed investigations in this respect. Low temperature atmospheric stress corrosion cracking investigations on 2205 duplex stainless steel formed the framework of this PhD thesis with respect to the waste storage context. Long-term surface magnesium chloride deposition exposures at 50°C and 30% relative humidity for up to 15 months exhibited the occurrence of stress corrosion cracks, showing stress corrosion susceptibility of 2205 duplex stainless steel at 50°C.The amount of cold work increased the cracking susceptibility, with bending deformation being the most critical type of deformation mode among tensile and rolling type of cold work. The orientation of the microstructure deformation direction, i.e. whether the deformation occurred in transverse or rolling direction, played vital role in corrosion and cracking behaviour, as such that bending in transverse direction showed almost 3-times larger corrosion and stress corrosion cracking propensity. Welding simulation treatments by ageing processes at 750°C and 475°C exhibited substantial influences on the corrosion properties. It was shown that sensitisation ageing at 750°C can render the material enhanced susceptible to stress corrosion cracking at even low chloride deposition densities of ≤145 µm/cm². However, it could be shown that short-term heat treatments at 475°C can decrease corrosion and stress corrosion cracking susceptibility which may be used to improve the materials performance. Mechanistic understanding of stress corrosion cracking phenomena in light of a comprehensive microstructure characterisation was the main focus of this thesis.

This Master’s thesis is related to corrosion degradation processes on metal surfaces exposed in different environments. The theoretical part is concern with basic principles in electrochemical and atmospheric corrosion. Practical part describes experimentation of corrosion process of aluminium (99,5) and low carbon steel (11321) in two different atmospheric environments. The conclusion of thesis includes evaluations of the results from practical part and comparison with the theoretical part.