Short CommunicationHydrogen embrittlement of high strength pipeline steels
Introduction
It was pointed out by Hinton and Procter [1] that, although sodium chloride solutions and sea water promote general corrosion of low and intermediate strength steels, these environments do not normally cause either stress corrosion or hydrogen embrittlement. However, the wide use of cathodic protection systems does increase the likelihood of environment-induced cracking. Since it was realised many years ago that increasing strength levels tend to decrease the resistance of steels to hydrogen embrittlement [2], [3], the work carried out in recent years to develop grades of pipeline steel of increased strength (API Grades X60, 80 and 100), for use with sour gas, raises once more the question of whether this introduces possible problems with hydrogen embrittlement. Speidel and Fourt [4], for instance, point out that hydrogen embrittlement may be induced under laboratory conditions at yield stress levels as low as 350 MPa, although service failures are absent from steels having yield stresses below 400 MPa. Of course, it has been recognised that microstructure is also an important factor and Boniszewski and Watkinson [5] clearly demonstrated, from extensive work on the heat-affected zones of welded steels, that hardness and microstructure both influence the susceptibility of low alloy steels to hydrogen embrittlement.
Section snippets
Experimental
Two of the pipeline materials tested had been manufactured in accordance with API-5L specifications and were graded X60 and X80. The third was one of the new generation X100 steels that are now being phased into service. All were essentially dead mild steels containing 0.1–0.15% carbon, 1.4–2.0% manganese, 0.2–0.5% silicon, 0.03–0.05% aluminium, 0.01–0.02% phosphorus and <0.003% sulphur and were the product of microalloying and controlled rolling of fully-killed steels to achieve high strengths
Discussion of results
There was significant loss in ductility in all three steels when tested after cathodic charging (Table 1 and Fig. 1) and the degree of embrittlement tended to increase with increasing current density, reaching a maximum above 0.10 mA mm−2. All these steels appear to suffer a similar degree of embrittlement, except at the highest current density used, where the degree of embrittlement tends to increase with strength level.
The reduction in ductility is a manifestation of damage done to the steels
Conclusions
Cathodic charging of three pipeline steels produced a loss in ductility that appeared to be independent of strength level, except at higher current density. The ductility loss was recoverable when a charged steel was left for seven days at ambient temperature after charging. The recovery must be attributable to desorption of hydrogen and the longitudinal microcracks resulting from the charging process only appear to have a significant effect on the measured ductility at higher charging current
Acknowledgement
The authors are grateful to the staff of the Chemical and Materials Analysis Unit, University of Newcastle for the electron microscopy assistance.
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