Elsevier

Corrosion Science

Volume 48, Issue 12, December 2006, Pages 4378-4385
Corrosion Science

Short Communication
Hydrogen embrittlement of high strength pipeline steels

https://doi.org/10.1016/j.corsci.2006.02.011Get rights and content

Abstract

A comparison was made between three API grade pipeline steels (X60, X80 and the X100 grade) from the point of view of their susceptibility to hydrogen embrittlement. The main aim was to determine whether the development of higher strength materials led to greater susceptibility to hydrogen embrittlement. This was achieved by straining at 2.8 × 10−5 s−1 after cathodic charging. The results showed that there is a distinct susceptibility to loss of ductility after charging and this tends to increase with the strength level of the steel at a charging current density above 0.44 mA mm−2. All three steels exhibited fine cracks parallel to the major rolling direction after charging and an increasing amount of brittleness on the fracture surface.

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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