Stress corrosion cracking of pipeline steel in concentrated carbonate, bicarbonate electrolyte has been studied in the United States for several years. More recently, stress corrosion cracking has come to the attention of Canadian natural gas suppliers. Canadian stress corrosion cracking occurs in cathodically protected pipes at sites of disbondment of protective coatings. A dilute carbonate, bicarbonate electrolyte of pH 7 has been found at sites of stress corrosion cracking. Conditions and mechanisms for crack initiation and propagation have been proposed, but duplication of growth in the laboratory remains difficult.

A system was designed and built to simulate underground conditions of environment and load, to grow stress corrosion cracks, and to determine the propagation rate. Although most previous research has focused on crack initiation, this program concentrated on propagation. Specimens, machined from excavated pipe, were precracked in fatigue and loaded in cantilever bending. Loading was controlled by a computer that was also responsible for data acquisition. The direct current potential drop method was used to monitor crack growth. Load levels and environmental conditions were selected based on field data. Some tests were enclosed in anaerobic chambers, and some were inoculated with bacteria cultured from field samples.

Stress corrosion cracks from the field were examined to establish typical fractographic features. Through-wall cracks were composed of several smaller cracks that had coalesced. Facets were found on the fracture surfaces, especially on those of small cracks.

Stress corrosion cracking was not produced on most specimens, although potential drop indicated growth between 20 and 100 μm. The lack of growth was attributed to several experimental difficulties which have since been resolved. However, facets similar to those of the field cracks were found on fracture surfaces of two test specimens. Measurement of the depth of the largest facetted region indicated an average growth rate of 1x10⁻³ mm/s. This compares favourably to estimates of growth rates from field cracks.