Corrosion, hydrogen uptake and environmentally assisted cracking of flexible pipe steel armour wires

Abstract

Flexible pipes are widely used in oil and gas applications. They consist of several metallic and polymeric layers, including a layer of tensile armour wires made from high-strength carbon steels. The armour wires are closely packed and the environment in the annulus contains gases that have permeated from the bore such as CO2, CH4, H2S and H2O. This creates a corrosive environment in the flexible pipes, but the large surface area of steel compared to the free volume of the annulus promotes formation of protective films of iron carbonate or sulphides. Corrosion may however cause damage if there are actively corroding areas in the steels or if the annulus is flooded with seawater due to outer sheath damage. The retrieval of damaged pipes have shown features consistent with hydrogen damage, and hydrogen embrittlement is therefore considered a possible failure mechanism, either by cathodic polarization from a corrosion protection system when the outer sheath is breached, or from hydrogen produced in the corrosion processes. In this PhD project the environmentally assisted cracking of tensile steel armour wires was investigated by measuring hydrogen uptake in six flexible pipe steel armour wires with different microstructures in simulated environments and by mechanical tests on smooth and notched samples exposed to air and simulated environments. The wires were characterized to link the hydrogen uptakes and effect of hydrogen on mechanical properties to microstructural properties. The wires had pearlitic-ferritic microstructures with carbon contents from 0.28 to 0.83 wt% and were plastically deformed to different degrees. Two of the materials with higher carbon contents had almost completely pearlitic microstructures, with lamellar pearlite, while the other materials had globular or partly globular carbides. The shape and distribution of carbide affects the tortuosity of the hydrogen diffusion path, and a tortuosity factor was estimated to distinguish this effect on the effective diffusion coefficients from the effect of hydrogen trapping on the effective diffusion coefficients. The interface between cementite and ferrite is a well-known hydrogen trap, and to investigate whether this is a significant factor on the hydrogen uptake and diffusion, the interfacial area between ferrite and cementite was also estimated for each material. Electrochemical permeation tests were conducted under cathodic polarization to -12 mA cm−2 while exposed to deaerated NaOH. The permeability to hydrogen was highest for the materials with finest grain size and lowest for the materials with largest grains, which is an indication of enhanced permeability of hydrogen on grain boundaries. The diffusion coefficients showed a tendency to decrease with the increase of ferrite-cementite interfacial area, thus confirming the occurence of trapping on the ferrite-cementite interfaces. The overall uptake of hydrogen in lattice and reversible traps was calculated using the permeation flux of hydrogen and the diffusion coefficients, but the hydrogen uptakes did not show a strong correlation with the grain size nor ferrite-cementite interfacial area. A clear difference in hydrogen uptake was observed between the two less deformed materials compared to the materials with a more deformed microstructure. Electrochemical permeation tests were also conducted in a simulated flexible pipe annulus environment, where the wire materials were exposed to artificial seawater with H2S and CO2 on the hydrogen entry side of the permeation cell. The hydrogen permeability did not increase with finer grain size in these conditions. Protective film formation was prevented by continuous pumping of electrolyte in and out of the corrosion compartment, which kept the iron content low (< 10 ppmw). Ferrite dissolved preferentially, leaving retained carbides on the surface. As the retained carbide area increased, the corrosion rates increased while the hydrogen uptake decreased. The proposed mechanism for the decreasing hydrogen uptakes with increasing carbide area is that the hydrogen adsorbed far from ferrite will be prevented from absorbing in ferrite due to the low solubility and diffusivity of hydrogen in cementite. The material with the lowest carbon content and very fine carbide distribution had relatively stable corrosion rate and relatively high hydrogen uptake. For the other materials, the rank of hydrogen uptake followed the rank in corrosion rate and carbon contents. Whether the hydrogen uptakes were directly increasing with the carbon contents or the corrosion rates could not be concluded. Tensile tests were conducted for both smooth and notched samples in air and in 3.5% NaCl while polarised to -1.4 V vs. Ag/AgCl, sat. KCl. The samples had increasing susceptibility to hydrogen embrittlement with increasing carbon content, but the notched samples showed a particular increase in hydrogen embrittlement susceptibility for the lamellar materials. It was seen that the presence of hydrogen lead to more crack initiation points, when comparing the fracture surfaces of samples tested under cathodic polarisation to samples tested in air. For the notched samples, the maximum load and crack-tip opening displacement at maximum load decreased for most of the materials when exposed to cathodic polarisation compared to testing in air. The material with lowest carbon content and highest ductility did not have significantly reduced properties in the hydrogen charged environment.