L51464 Kinetics of Stress-Corrosion Cracking in Pipe Steel at 175F

Abstract

The kinetics of stress-corrosion crack (SCC) growth in gas transmission pipelines is not well understood. Several factors in pipelines act to make the SCC situation less pessimistic than it might at first appear. First, SCC cannot occur in a pipeline as long as the protective coating remains intact. Second, even if breaks in the coating do develop with time, it is unlikely that the electrochemical potential existing in that region will be constantly in the critical range for cracking. Third, unlike the laboratory specimens cited earlier, the steel will not be subjected to a constant rate of strain. Rather, it will be subjected to an essentially constant stress (perhaps with small fluctuations of a low frequency) that does not exceed 72 percent of the specified minimum yield strength (SMYS). This will result in a very low rate of strain (due to creep and stress cycling) and a rate that will diminish with time. Fourth, the temperature of the pipeline will usually be lower than the value of 175 F that is frequently used in laboratory experiments; stress corrosion cracking rates diminish substantially as temperature is lowered. In addition, the critical potential range for cracking is narrower at lower temperature. The study of the effect of crack depth on crack-growth rate employed a pre-cracked cantilever-bend specimen. The method of loading the specimen and monitoring the crack growth is illustrated schematically. The stress at the crack tip was calculated by assuming the beam depth to be equal to the amount of un-cracked material below the notch and ignoring any stress concentrating effect of crack. Addition of the direct-current electric-potential (d-c E.P.) wiring for measuring crack growth required no alteration of the electrochemical cell or the loading fixture. Direct-current leads (14 gauge copper wire) were attached to the specimen outside the cell and potential probe leads (24 gauge iron wire) were spot-welded across the notch mouth. Upon completion of each test in the crack-depth-effect series, the specimen faces were polished and etched metallographically to confirm that intergranular stress-corrosion cracking (IGSCC) had occurred. The specimen was then fractured at a low temperature to reveal the total amount of crack growth by IGSCC.