Introduction

SCC is one form of environmentally assisted cracking (EAC). EAC is a generic term that describes all types of cracking in materials in which the environment and stress act together to reduce the strength or load-carrying capacity of the material. Other forms of EAC include hydrogen embrittlement, sulfide stress cracking, and corrosion fatigue. EAC is an ongoing integrity concern for many industries including oil and gas, nuclear power, and chemical process. It affects most common construction materials including carbon steels, stainless steels, and copper-based alloys.

The first reported incident of external SCC on natural gas pipelines occurred in the mid-1960s, and numerous failures have occurred since that time.² SCC failures have also been reported on liquid pipelines, and SCC continues to be an integrity concern. It is now recognized that there are two forms of external SCC on underground pipelines: high-pH SCC (also referred to as classical SCC) and near-neutralpH SCC (also referred to as low-pH SCC). A characteristic of both forms of SCC is the development of colonies of up to thousands of longitudinal surface cracks in the body of the pipe that link up to form long, shallow flaws. In some cases, growth and interlinking of the stress corrosion cracks produce flaws that are of sufficient size to cause leaks or ruptures of pipelines.

The high-pH form of SCC is intergranular, and there is usually little evidence of general corrosion associated with the cracking. A concentrated carbonate-bicarbonate (CO₃- HCO₃) solution was identified as the most probable environment responsible for this form of cracking.³ The nearneutral- pH form of SCC is transgranular and is associated with corrosion of the crack faces, and in some cases with corrosion of the external surface of the pipe as well. This form of cracking occurs in near-neutral-pH (6 lt; pH lt; 8) dilute carbon dioxide (CO₂)-containing electrolytes and was first observed beneath polyethylene-tape coatings on TransCanada PipeLines Limited's (TCPL's)⁽²⁾ system in the 1980s.^4,5

⁽²⁾ TransCanada PipeLines Limited (TCPL), 450 1 Street SW, Calgary, AB T2P 5H1 Canada.

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