Hydrogen embrittlement of clad steel pipes: Experiments and FE modelling
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Pipes with an inner layer of corrosion resistant alloy, so called clad pipes, are increasingly used in the oil and gas industry as en economical viable option for corrosion management, combining the mechanical properties of the structural steel with corrosive properties of the clad. This however offers new challenges with respect to integrity management and degradation assessment, due to an inhomogeneous material combination and a complex interface regions. Hydrogen embrittlement is among the challenges that need to be addressed. The present PhD work investigates the hydrogen induced degradation of clad steel pipes. This is accomplished by a combination of experimental fracture mechanical testing and numerical simulations. The overall objective has been to establish basic knowledge on the hydrogen embrittlement of clad steel pipes, while developing a coupled mass diffusion and cohesive zone modelling approach for the prediction of hydrogen induced fracture initiation. The fracture mechanical test results indicate and overall reduction in fracture toughness for clad pipes compared to the conventional carbon steel pipes, attributed to element diffusion during production. The presence of a Ni-interlayer between the clad and base material was found to limit element diffusion during production, and thus the detrimental interface degradation, while the Ni-interlayer itself represents a soft zone preferable for crack propagation. Pipes without a Ni-interlayer revealed strong susceptible to hydrogen embrittlement. The presence of a Ni-interlayer was found to reduce the fracture toughness for testing in air, while it increased the fracture toughness for testing under in situ hydrogen charging. It was concluded clad pipes with a Ni-interlayer should be considered the preferred choice of subsea pipelines in areas of aggressive environments where there is considerable risk of hydrogen induced cracking. The numerical simulations revealed a significant dependency on the choice of mass diffusion input parameters and boundary conditions on the resulting hydrogen distribution and fracture initiation toughness of the clad steel pipe. Both hydrogen in lattice and hydrogen trapped at dislocations were found to be possible sources of embrittlement. Asymmetrical notch opening and plastic zone size was revealed for all simulations, confined to the undermatching base material. The deleterious effect of material mismatch was found to be less severe in hydrogen environment. The model is able to qualitatively predict the detrimental effect of hydrogen on the fracture initiation toughness of clad pipes, giving reasonable results when compared to experimental findings. Further effort should be considered in developing the models ability to provide a reliable description of the interface hydrogen content and distribution.