Doel 3 & Tihange 2 - Some Peer-reviewed Scientific Papers & Reports

332

tt. Pircher / ttydrogen corrosion of pressure-uessel steels

800

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6 OCr-O5MoSteel

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30Cr-0S.o,eel

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200

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T

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T T T T T T TT 200 345 621 896 207 ~B3 758

50

100

150

Hydrogen portial pressure in ba r Fig. 1. Operating limits for steels in hydrogen service.API-Nelson diagram [3].

surface decarburization (dashed lines) and internal de- carburization with cracking due to the formation of methane (solid curves).

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3. Hydrogen solubility and di ffusion

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Fig. 2 shows the concentrations of hydrogen (defined by the Sievert solubility constant S) which are soluble in the lattice of ferritic ferrous materials [11]. From the figures stated it is possible to infer the hydrogen con- tents cn for different equilibrium pressures by way of using the Sievert equation c . =S The solubility of hydrogen in steel decreases sharply as the temperature drops. Shut down of high-pressure reactors after a prolonged period of operation at elevated temperatures tends to result in hydrogen oversaturation of the materials, which has to be removed by way of effusion. The latter is governed by the laws of diffusion which make it necessary to take into account also decreasing diffusion coefficients with decreasing tem- perature. If excess hydrogen remains in the material after cooling down to the ambient temperature, delayed cracking occurs, similar to flaking as a result of metal- lurgical hydrogen introduced during steelmaking (fig. 3).

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Fig. 2. Solubility constant and diffusion coefficient of hydro- gen in low-alloysteels [11].

4. Hydrogen- induced cracking

liquid media is favored by promotors. Fig. 4 displays an absorber [6] used to dry wet sour gas at the exploitation site prior to feeding it into the pipeline for conveyance

As shown in fig. 3, a very similar pattern is obtained whenever the pick-up of large hydrogen quantities from