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PRCI PR-186-917
- Assessment of the Effects of Surface Preparations and Coatings on the Susceptibility of Line Pipe to Stress Corrosion Cracking
- Report / Survey by Pipeline Research Council International, 02/24/1992
- Publisher: PRCI
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L51666e
Cortest Columbus Technologies, Inc.
Need: Within the last decade, a number of gas pipeline companies throughout the world have identified the presence of Stress-Corrosion Cracking (SCC) on their pipelines. While the cause of the SCC resulted from a multitude of factors, there appeared to be a commonality relating to pipeline coatings. Although SCC has occurred under polyethylene/PVC tapes and enamel coatings, not a single incident has been recorded where SCC has occurred on a fusion-bonded-epoxy coated line. The three major factors that were thought to account for the difference in the SCC performance of the various coatings were; (1) resistance of a coating to disbonding, (2) the ability to pass current through the coating and thereby protect the disbonded regions and (3) the surface preparation of the line pipe prior to coating application. These factors were addressed in this three-year program for PRCI.
Benefit: This report examines the relationship between coatings and surface preparation and the likelihood of subsequent stress corrosion cracking. Standard Cathodic disbondment tests, measurements of potential gradients beneath disbonded coating, electrochemical measurements of current passages through coatings and actual SCC tests are described and results reported.
Result: The following are the conclusions of the overall program, which are based on the results of the laboratory and field research. It should be noted that all of the laboratory work was performed using sections from steel pipe that had previously shown susceptibility to SCC and the test environment was an environment known to support cracking. 1) The primary factors affecting the relative SCC susceptibility of pipelines with different coatings are (a) the disbonding resistance of the coating and (b) the ability of the coating to pass cathodic protection (CP) current when it disbonds. 2) Based on these primary factors, FBE coating coated pipelines would be expected to exhibit good resistance to SCC. The FBE coating evaluated exhibited a high resistance to disbonding and an ability to pass CP current once disbonding occurred. 3) Pipelines coated with polyethylene tape (PET) would be expected to exhibit poor resistance to SCC. The PET coating evaluated exhibited poor disbonding resistance and shielded the CP current in the disbonded areas. 4) Pipelines coated with a coal-tar enamel (CTEN) coating would be expected to exhibit intermediate resistance to SCC. The CTEN coating evaluated exhibit intermediate resistance to disbonding but readily passed CP current in the disbonded areas. 5) The surface condition of the pipeline is a secondary factor affecting SCC. Grit blasting may be beneficial or detrimental to SCC susceptibility, depending on the intensity of blasting. 6) Grit blasting was found to be beneficial to the disbonding resistance of the FBE and CTEN coatings but did not appear to have a beneficial effect on disbonding resistance of the PET coating. 7) Grit blasting is beneficial to the native potential of the pipe; new grit blasted surfaces were found to have free corrosion potentials that were more negative than the cracking range and the application of CP would not be expected to move the potential into that range. However, this beneficial potential effect is not maintained if corrosion products form on the pipe surface. 8) Very intense grit blasting (high Almen strip levels) were found to be required to increase resistance to SCC. 9) SCC of FBE coated pipelines is possible, although the occurrence of SCC with an FBE coating is less likely than with a CTEN or PET coating.
Cortest Columbus Technologies, Inc.
Need: Within the last decade, a number of gas pipeline companies throughout the world have identified the presence of Stress-Corrosion Cracking (SCC) on their pipelines. While the cause of the SCC resulted from a multitude of factors, there appeared to be a commonality relating to pipeline coatings. Although SCC has occurred under polyethylene/PVC tapes and enamel coatings, not a single incident has been recorded where SCC has occurred on a fusion-bonded-epoxy coated line. The three major factors that were thought to account for the difference in the SCC performance of the various coatings were; (1) resistance of a coating to disbonding, (2) the ability to pass current through the coating and thereby protect the disbonded regions and (3) the surface preparation of the line pipe prior to coating application. These factors were addressed in this three-year program for PRCI.
Benefit: This report examines the relationship between coatings and surface preparation and the likelihood of subsequent stress corrosion cracking. Standard Cathodic disbondment tests, measurements of potential gradients beneath disbonded coating, electrochemical measurements of current passages through coatings and actual SCC tests are described and results reported.
Result: The following are the conclusions of the overall program, which are based on the results of the laboratory and field research. It should be noted that all of the laboratory work was performed using sections from steel pipe that had previously shown susceptibility to SCC and the test environment was an environment known to support cracking. 1) The primary factors affecting the relative SCC susceptibility of pipelines with different coatings are (a) the disbonding resistance of the coating and (b) the ability of the coating to pass cathodic protection (CP) current when it disbonds. 2) Based on these primary factors, FBE coating coated pipelines would be expected to exhibit good resistance to SCC. The FBE coating evaluated exhibited a high resistance to disbonding and an ability to pass CP current once disbonding occurred. 3) Pipelines coated with polyethylene tape (PET) would be expected to exhibit poor resistance to SCC. The PET coating evaluated exhibited poor disbonding resistance and shielded the CP current in the disbonded areas. 4) Pipelines coated with a coal-tar enamel (CTEN) coating would be expected to exhibit intermediate resistance to SCC. The CTEN coating evaluated exhibit intermediate resistance to disbonding but readily passed CP current in the disbonded areas. 5) The surface condition of the pipeline is a secondary factor affecting SCC. Grit blasting may be beneficial or detrimental to SCC susceptibility, depending on the intensity of blasting. 6) Grit blasting was found to be beneficial to the disbonding resistance of the FBE and CTEN coatings but did not appear to have a beneficial effect on disbonding resistance of the PET coating. 7) Grit blasting is beneficial to the native potential of the pipe; new grit blasted surfaces were found to have free corrosion potentials that were more negative than the cracking range and the application of CP would not be expected to move the potential into that range. However, this beneficial potential effect is not maintained if corrosion products form on the pipe surface. 8) Very intense grit blasting (high Almen strip levels) were found to be required to increase resistance to SCC. 9) SCC of FBE coated pipelines is possible, although the occurrence of SCC with an FBE coating is less likely than with a CTEN or PET coating.
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