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

333

H. Pircher/Hydrogen corrosionofpressure-vesselsteels

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Fig. 3. Flakes in a hot-rolled 25 mm plate.

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after Riecke Time in h Fig. 5. Permeation rates of hydrogen in the pipefine steel grade X 60 for various solutions [11].

to the cent ral gas refining equipment . The spots marked on the shell are ul t rasonical ly detected flaws at t r ibuta- ble to hydrogen- induced cracking. At omi c hydrogen is present as a result of the cathodic react ion of an electro- lytic corrosion whose absorpt ion by the steel is favored by H2S as a promotor . The amoun t of hydrogen picked up by the steel surface under given cor ros ion condi t ions can be de- termined by means of permea t ion measurements . This is i l lustrated in fig. 5 where a C -Mn steel is used as an example under free cor rosion in three di f ferent aqueous solutions. The effect exer ted by the promo t or s - - H2S in the present case - - and by the pH value of the medium becomes evident . The decreasing hydrogen per- meat ion rate wi th t ime is due to the forma t ion of protect ive layers. Wi thout external tensi le stress existing, the great amount s of hydrogen absorbed by means of promo t or s tend to form bl isters near the surface and cracks inside the material at non-metal l ic inclusions or microst ruc- tures having a critical shape and hardness. In plates, cracking preferably occurs in planes paral lel to the rol led surface (fig. 6). Structures suscept ible to hydrogen- induced cracking are made up of di f ferent types and shapes of non- metal l ic inclusions, in par t icular manganese sulfides, and of hard banded structures in segregat ion zones.

Fig. 4. Absorber with HIC in service [6].