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

H. Pitcher / Hydrogen corrosion of pressure-vessel steels

337

densi ty increases, and embr i t t l ement is observed. Wi thin the potent ial range below Un = - 0 . 8 V, t ransgranular internal cracking may take place in sensitive mater ials due to hydrogen pick-up (fig. 11) [17]. Hydrogen absorpt ion under cri t ical strain rates is of great technical impor tance when determining the degree of hazard under al ternat ing stress. In this respect , very of ten the corrosive effect of hydrogen does not concern the phase of crack ini t iat ion but rather the rate of crack propagat ion. Fig. 12 shows the results of tests dur ing which notched specimens were exposed to al ternat ing stress in gases of di fferent compos i t ions [12]. In hydro- gen of roughly 1 bar, crack propaga t ion is much faster than in air. It is wor thwhi le not icing that slight ad- di t ions of other gases than hydrogen produced st imulat - ing or inhibi t ing effects. An example i l lustrat ing st rain- induced HSCC as the cause of damage is shown in fig. 13 [13]. The gas cooler

Fig. 11. Internal cracks in a low-alloy steel, slow strain rate test in seawater; i = 4× 10-6//s, U H = --0.8 V.

Fig. 10 illustrates the exper imental results obtained from four structural steels of the strength level ranging f rom HE 355 S.U. to HE 690 S.U. which had been exposed to an average strain rate of 4 × 10-6/ / s unt i l fracture in aerated synthet ic seawater. The reduct ion of area after fract ion was used as a parameter to def ine the resistance to HSCC of the steels tested. For the free cor ros ion potent ial , no signs of a negat ive effect due to cor ros ion were noted. At cathodic potent ials the current

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Fig. 12. Effect of additions to hydrogen gas on the crack propagation rate of precracked carbon steel (YS = 207 N /mm 2) [12].

Fig. 13. Gas cooler subsequent to the occurrence of HSCC at its base.