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

H. Pircher / Hydrogen corrosion of pressure-t~essel steels

336

Fig. 9. HSCC in town-gas pressure vessel; (a) macrograph of the surface, (b) transversemicro-section.

filled up with town gas not fully dried, to which tetrahy- drothiophene (THT) was added as an odorizer. To prevent corrosion through condensates at temperatures below the dew point , the spheres were coated wi th a triple epoxide resin system. Af t er eight years of service, one of the spherical tanks commi ss ioned in 1970 sprang a leak in the form of a crack approxima t e ly 400 mm in length beneath a welded joint . When the damage occurred, the sphere was operated at a service pressure of 8 bar, with the outside and gas temperatures being + 8 ° C. Surface crack tests per formed wi thin the framework of damage invest igat ions resulted in the detect ion of cracks in all four spheres in weld zones of the lower sphere halves, wi th the frequency and length of the cracks clearly increasing towards the bot tom-end pole. Fissur ing most ly or iginated in the weld metal or the heat -affected zone, wi th the crack path ending in the base metal (fig. 9). The dependence of the degree of damage on the durat ion of exposure to the odor izing gas was clearly percept ible. The damage was at t r ibutable to the fact that, in the first place, the coat ing used was not resistant to the di ffusion of humidi ty and gas const i tuents. Secondly, cont rary to the then val id knowledge, the THT odor izer is defini tely soluble in water, forming H2S by hydroly- sis. This al lowed an acid H2S-bear ing humidi ty film to form at the steel surface beneath the coating. Enhanced by the H2S promotor , hydrogen s temming from the cor rosion react ion was picked up, triggering hydrogen- induced stress cor rosion cracking in the steel. Since their repair, the gas spheres are filled with dried gas, wi th the odor izer being added at the t ime the gas is fed

into the low-pressure network. Thus, the cor ros ion hazard was el iminated by changing the medium.

7. Strain-induced HSCC

To obtain clean and active surfaces, use is made of the so-cal led cons tant -extens ion- rate test (CERT) .

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