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Hydrogen might be introduced during fabrication welding or might be taken up from an environment during sour service or cathodic protection. Thus, hydrogen assisted stress corrosion and cold cracking is still a major topic regarding the reliability of welded steel components, as for instance offshore platforms and pipelines. In order to support conclusive testing and life time evaluation of welded steel components, a numerical model for hydrogen assisted cracking has been developed, particularly taking into consideration the geometrical effects of crack propagation on the respective hydrogen distribution alongside and ahead of the crack. Numerical calculations were based on finite element analysis of the hydrogen and stress-strain distribution by using a commercially available program. The model has been verified experimentally by slow strain rate experiments of supermartensitic stainless steels which are intended to be used more extensively as materials for welded flowlines in North Sea oil and gas production. As first results of such simulations the influence of the subsurface concentration provided by different H2S saturation levels in the NACE TM 0177-96 standard solution on crack propagation and the effect of crack shape on the hydrogen distribution profile are presented in this contribution.
Hydrogen might cause severe degradation of supermartensitic stainless steels, if they are activated during exposure to sour environments. Consistent and comprehensive data for hydrogen transport in these materials are thus required to support, in particular, modelling of hydrogen assisted cracking as a part of life time assessment of welded steel components. In addition to previously published diffusion coefficients and subsurface concentrations of a supermartensitic stainless steel further data dependent on heat treatment are provided by this contribution. Furthermore, a higher alloyed material has been investigated in the state as received and also in the quenched condition, in order to approach the influences of chemical composition on hydrogen transport in supermartensitic stainless steels. With respect to welding it turned out that the diffusion coefficient and the subsurface concentration are markedly dependent on heat treatment of the materials.
High temperature SSRT - ductility data from cathodic hydrogen charging of 10CrMo910 and 7CrMoVTiB1010 are implemented into a mathematical model of SCC at various operational conditions including temperatures, pH and dissolved hydrogen/oxygen contents as well as global stresses and material properties. The results show that, as a consequence of experimentally verified local acidification at initial anodic path corrosion, subsequent local hydrogen assisted cracking can be a controlling factor for SCC in high temperature water. As a particular effect at global stresses close to the yield point, operational temperatures around 270°C exhibit peak crack growth rates depending on dissolved hydrogen, oxygen, pH and global stress, which has been found to be consistent for both, experimental studies and mathematical modeling.
As an extension of recently presented stress corrosion cracking (SCC) models for pure nickel and iron in high-temperature water, the anodic path, hydrogen-assisted stress corrosion cracking of a model Cr-Ni alloy is presented, utilizing Alloy 600 (UNS 06600) mechanical properties. With the local anodic path corrosion as the crack initiation phase, the respective oxides precipitate while the crack tip solution acidify and establish local hydrogen ion reduction conditions. Depending on the applied global stress, local plastic straining then provides an active crack tip surface that transfers atomic hydrogen to the plastic zone ahead of the crack tip. Hydrogen-assisted crack propagation is then controlled by the calculated local hydrogen ion reduction charge and the plastic strain distribution ahead of the crack tip. The results show that for global stress levels close to the yield stress and at constant dissolved oxygen contents, the increasing dissolved hydrogen contents provide peak crack growth rates at hydrogen levels that decrease both with increasing temperatures and bulk pH. The peak crack growth rates also decrease with decreasing global stress as well as with increasing temperature and increasing bulk pH. The results are in accordance with published experimental investigations and operational experiences regarding the effects of dissolved hydrogen on stress corrosion rates in high-temperature waters of nuclear reactors. They are explained by the interaction between the electrochemical parameters of the respective reactions.