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ABSTRACT 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. INTRODUCTION Supermartensitic stainless steels are intended to replace higher alloyed materials, not only as a flowline material in several recently explored North Sea oil and gas fields, but also in the topside equipment of the production platforms and also onshore. Due to the fact that no long term service experience of these materials is existing up to the present, life time assessment of such welded components made of such steels becomes increasingly important, which has, however, also to take into consideration hydrogen assisted cracking [ 1]. As a part of life time determination hydrogen assisted cracking has to be modelled for which hydrogen transport data are essential [ 2], [ 3]. Due to the fact that only a few data for hydrogen diffusion coefficients in supermartensitic stainless steels have been published [ 4] - [ 6], hydrogen permeation has been investigated more quantitatively in a previous contribution [ 7]. By respective permeation experiments it has been shown that severe high hydrogen concentrations might be absorbed by the investigated supermartenstic stainless steel during cathodic charging at current densities representative for cathodic protection, but also, if the material becomes activated during free corrosion in the buffered NACE TM 0177-96 solution. It has been verified that hydrogen was only reversibly trapped in the quenched and also in the as received material. As a further result, the effective diffusion coefficient and the subsurface concentration in the as received and the quenched steel differed by an order of a magnitude. This effect was attributed to the precipitation of chromium carbides alongside the former austenite grains and the martensite laths. As a specific item, reliable values of the effective diffusion coefficient of hydrogen in the investigated steel were evaluated which were assigned to a respective scatterband. In combination with the respective subsurface concentrations such values now allow consistent numerical calculations of the hydrogen distribution as a part of modelling hydrogen assisted cracking. For those reasons the quantitative investigations of the hydrogen transport behaviour in supermartensitic stainless steels have been continued and further values dependent on heat treatment and also for a higher alloyed matedal are presented in this contribution. EXPERIMENTAL As reported previously, a simplified Devanathan test set up [ 7] - [ 9] has been used. Hydrogen detection has been performed by oxidation at the anodic side at a constant potential of +200 mV- SCE in 0.1 N sodium hydroxyde within the passive region of the test material. Due to its capability to build hydrides [ 10], [ 11] and its by far higher solubility for hydrogen as compared to
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- Europe > United Kingdom > North Sea (0.24)
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- Production and Well Operations > Production Chemistry, Metallurgy and Biology > Corrosion inhibition and management (including H2S and CO2) (1.00)
- Facilities Design, Construction and Operation > Pipelines, Flowlines and Risers (1.00)
- Health, Safety, Environment & Sustainability > Health > Noise, chemicals, and other workplace hazards (0.99)
ABSTRACT Cast Cu-AI-Ni- bronze valve components for naval seawater supply facilities have frequently been subjected to crevice corrosion, for example, in contact with PTFE. The extent of such local degradation leading to leakage depends on environmental conditions including seawater chemistry as well as local potentials, temperature and flow velocities. The present investigation is based on an actual failure case of a cast Cu-9% A1-5% Ni-5% Fe (UNS C 95800) alloy valve exhibiting crevice corrosion at the contact areas to the PTFE sealing. For identification of incubation times and extent of crevice corrosion at various free mixed potentials and temperatures of the pipe-valve system a "Remote Crevice Assembly,(RCA)"- mock up was engaged including the cast material and PTFE as a small local anodic crevice location and the corresponding wrought 90% Cu-10% Ni pipe material (UNS C70600) as a large cathode. With aerated and stirred Baltic Sea water as a fluid the effect of fluctuating free potentials and anodic as well as cathodic temperatures was investigated. As a result, increasing potentials at the start of the 24 h test runs are providing increased corrosion net currents and reduced incubation times for start of crevice corrosion. A maximum mixed potential o f - 1 0 0 mV(/Ag/AgCl ) for avoidance of crevice corrosion at anodic temperatures up to 45°C for the investigated assembly is established. Cathodic protection at -850mV efficiently avoided crevice corrosion. Increasing cathodic temperatures are initially providing accelerated anodic net currents followed by a subsequent net current reverse due to start of pitting corrosion at the pipe material at its respective pitting temperature. The failure mode of the cast anode material is characterized by early grain boundary attack on secondary phases followed by general dissolution of the grains after the marked current increase at crevice corrosion start. As a conclusion, the application of cast Cu-A1-Ni-Fe valves in the present composition and microstructure is to be questioned, in particular, at higher ambient temperatures as prevailing in southern areas. INTRODUCTION Local crevice corrosion of Cu-A1-Ni marine alloys has recently been focussed upon by various publications, o-4) . The reason is, that valves of seawater supply systems are exhibiting crevices in contact with their gasket materials thus providing favorite conditions for crevice corrosion by seawater, in particular at elevated ambient temperatures. Although, in most cases, general corrosion rates seem to be under control, it becomes increasingly evident, that in case of local crevice corrosion the resulting leakages may lead to malfunctioning together with unsafe vessel operations and last, not least, unpredictable life time cycles of the systems. The quantitative assessment of crevice corrosion susceptibility as a support to material and design decisions of actual valve-pipe seawater systems should therefore be integrated in particular, at new developments. It has been shown in earlier work by various authors, (5-s) that for quantitative information on crevice corrosion the continuous registration of corrosion currents in a "Remote Crevice Assembly,(RCA)" is providing the start times (incubation times) for significant anodic dissolution in the crevice and the average corrosion current representing the crevice corrosion rate. This implies a sufficiently low anodic dissolution current at the flee surface outside the crevice so that the measured net crevice corrosion current is nearly equivalent to the mass loss in the crevice during the test. It has, however, also been shown, that at the onset of pitting corrosion at the outside surface, the directions of the net currents are reve
- Well Completion > Well Integrity > Subsurface corrosion (tubing, casing, completion equipment, conductor) (1.00)
- Production and Well Operations > Production Chemistry, Metallurgy and Biology > Corrosion inhibition and management (including H2S and CO2) (1.00)
- Facilities Design, Construction and Operation > Pipelines, Flowlines and Risers > Materials and corrosion (1.00)