Stress Corrosion Cracking of Linepipe Steels in Near-Neutral pH Environment: Issues Related to the Effects of Stress

W. Zheng,1 R. Sutherby,2 R. W. Revie,1 W. R. Tyson,1 and G. Shen1

1,Research scientists, CANMET/Materials Technology Laboratory, 568 Booth St. Ottawa, Ontario, Canada, K1A 0G1.

2,Senior integrity specialist, TransCanada Transmission, 801 - Seventh Avenue S, Calgary, Alberta, Canada, T2P 3P7.

(This paper is based on a recent ASTM publication by the same authors in Environmentally Assisted Cracking: Predictive Methods for Risk Assessment and Evaluation of Materials, Equipment, and Structures, ASTM STP 1401, R. D. Kane, Ed., American Society for Testing and Materials, West Conshohocken, PA, 2000. Slight modifications have been made in the text.)

ABSTRACT

Stress corrosion cracking (SCC), a form of environment-assisted cracking, is known to be one of the important failure mechanisms for oil and gas pipelines. The role of stress in the SCC of linepipe steels in near-neutral pH environment, as in other SCC systems, is complex. In this paper, the current issues related to the effects of stress on the development of both axial cracking and circumferential cracking are reviewed. As a mitigative method, hydrostatic testing is widely used for pipes suspected to contain cracks. The long-term beneficial effect of compressive stress introduced by hydrostatic testing is also discussed.

INTRODUCTION

The transgranular stress corrosion cracking in pipelines discovered in the mid-1980s on the Canadian system is known to be associated with a dilute ground water chemistry [1], which is very different from the well-known carbonate-bicarbonate solution thought to be responsible for the intergranular form of cracking of line pipe steels first reported four decades ago [2]. As a result of a nation-wide public hearing held by the National Energy Board (NEB) of Canada [1], much attention has been given to the stress corrosion problem on the Canadian pipeline system. However, a great deal remains uncertain in terms of the roles played by the key factors such as stress and steel metallurgy.

The available evidence from laboratory work [3-9] suggests that stress fluctuation, even minor, is necessary for crack initiation and growth. The source of stress can be both operational, resulting from the internal pipeline pressure, and secondary resulting, for example, from localized bending or axial tension in the pipe, as in the case of circumferential SCC. In fact, the level of secondary stress can be the dominant source of loading and result in cracks at high angle to the axis of the pipe. In this paper, the issues related to the role of stress and stress fluctuation on the development of axial and circumferential SCC as well as the beneficial effects of residual stress produced by high-pressure hydrostatic testing are discussed.  

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