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PRCI PR-230-9413
- Low pH SCC: Environmental Effects on Crack Propagation
- Report / Survey by Pipeline Research Council International, 07/01/2000
- Publisher: PRCI
$173.00$345.00
L51791e
CANMET Materials Technology Lab
Need: Stress corrosion cracking of oil and gas pipelines in soil environment of near neutral pH is known to be different, in many aspects, from the classical type of pipeline SCC that occurs in a much more alkaline carbonate-bicarbonate condition. The distinct contrast in the cracking mode, for instance, with cracking at near-neutral pH being predominantly transgranular and the high-pH SCC an intergranular mode, indicates a fundamental difference in the cracking mechanisms. The term "SCC at near-neutral pH" derived from the field experience that there is a strong tendency for SCC to occur in the near-neutral range. For pipes coated with asphalt, the environment around the majority of the SCC colonies had a pH between about 6 and 7.5. The likelihood of finding SCC sites decreased as the pH of the liquid around the pipe approached 8. For tape-coated pipes, the most susceptible pH was 6.5. This finding suggests that carbon dioxide plays an important role in maintaining the pH of the soil water under the disbonded coating in the near-neutral range, since many of the SCC sites are associated with a clay or gravel soil where the pH of the bulk soil, when fully saturated with water, is somewhat alkaline (pH>8).
Benefit: This is the final report of a research project, PR-230-9413, funded by PRCl on the environmental effects on the propagation of transgranular stress corrosion cracks (TGSCC) of linepipe steels in soil environments of near-neutral pH. Supervision by the Ad Hoc Group on this project, chaired initially by Matt Cetiner of TransCanada PipeLines and then by Paul Wong of NOVA Gas Transmission Ltd., set the objectives and the scope of the work and was an integral component in the completion of this research. Three full-size ERW pipes, 508 mm (20 inches) diameter, hot-rolled Grade 359 (X-52), and a gravel-type soil were used. The pipe had been manufactured in the late 1950s and had been in service in a natural gas transmission pipeline for about 38 years. Crack growth was measured as a function of environmental variables and of loading severity, at the free corrosion potential (F. C. P.), at potentials anodic to F.C.P. and at cathodic potentials. At the free corrosion potential, the crack growth rate was found to be influenced by carbon dioxide. Growth rates of cracks with direct carbon dioxide admission tended to be higher than growth rates of other cracks.
Result: The results also confirmed the role of pressure fluctuation in causing crack growth. For a crack located in the ERW microstructure, growth was repeatedly reinitiated and arrested by setting the R to 0.85 and to 0.995, respectively. The general trend of growth confirmed that the growth rate at the corrosion potential depends on the rate of crack tip opening.
CANMET Materials Technology Lab
Need: Stress corrosion cracking of oil and gas pipelines in soil environment of near neutral pH is known to be different, in many aspects, from the classical type of pipeline SCC that occurs in a much more alkaline carbonate-bicarbonate condition. The distinct contrast in the cracking mode, for instance, with cracking at near-neutral pH being predominantly transgranular and the high-pH SCC an intergranular mode, indicates a fundamental difference in the cracking mechanisms. The term "SCC at near-neutral pH" derived from the field experience that there is a strong tendency for SCC to occur in the near-neutral range. For pipes coated with asphalt, the environment around the majority of the SCC colonies had a pH between about 6 and 7.5. The likelihood of finding SCC sites decreased as the pH of the liquid around the pipe approached 8. For tape-coated pipes, the most susceptible pH was 6.5. This finding suggests that carbon dioxide plays an important role in maintaining the pH of the soil water under the disbonded coating in the near-neutral range, since many of the SCC sites are associated with a clay or gravel soil where the pH of the bulk soil, when fully saturated with water, is somewhat alkaline (pH>8).
Benefit: This is the final report of a research project, PR-230-9413, funded by PRCl on the environmental effects on the propagation of transgranular stress corrosion cracks (TGSCC) of linepipe steels in soil environments of near-neutral pH. Supervision by the Ad Hoc Group on this project, chaired initially by Matt Cetiner of TransCanada PipeLines and then by Paul Wong of NOVA Gas Transmission Ltd., set the objectives and the scope of the work and was an integral component in the completion of this research. Three full-size ERW pipes, 508 mm (20 inches) diameter, hot-rolled Grade 359 (X-52), and a gravel-type soil were used. The pipe had been manufactured in the late 1950s and had been in service in a natural gas transmission pipeline for about 38 years. Crack growth was measured as a function of environmental variables and of loading severity, at the free corrosion potential (F. C. P.), at potentials anodic to F.C.P. and at cathodic potentials. At the free corrosion potential, the crack growth rate was found to be influenced by carbon dioxide. Growth rates of cracks with direct carbon dioxide admission tended to be higher than growth rates of other cracks.
Result: The results also confirmed the role of pressure fluctuation in causing crack growth. For a crack located in the ERW microstructure, growth was repeatedly reinitiated and arrested by setting the R to 0.85 and to 0.995, respectively. The general trend of growth confirmed that the growth rate at the corrosion potential depends on the rate of crack tip opening.
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