COATINGS & LININGS FIGURE 1
ing range, therefore the disbonded coat- ing must allow the CP current to effec- tively penetrate the disbonded area. If CP is effective under a disbonded coating, the pH will increase. Since near-neutral SCC occurs in
dilute bicarbonate solutions, the pH range is from 5.5 to 7.5. If the coating is CP shielding, then the electrolyte envi- ronment under the coating will likely stay in this range. The potential for near- neutral SCC will be in the range of the native steel potential in a deaerated solu- tion. Both the potential and pH for this form of cracking imply that current from the CP system does not reach the pipe surface.2 High pH SCC occurs in a potential and a con-
range of –0.60 to –0.75 V(CSE)
centrated carbonate-bicarbonate solution with a pH range of 9.5 to 11.5. Coatings that allow some CP under the disbond- ment can change the pH and potential into the SCC range, but the SCC poten- tial range is relatively narrow. If CP is adequate, the potential will move out of the cracking range.
Steel Type Revie3 studied the effects of CP poten-
tials on higher-strength steels. The objec- tive of this study was to establish the ef- fect of CP to produce hydrogen that can cause cracking and in-service failures of high-strength pipeline steels, from X70 to X120, and to establish the effective- ness of CP in mitigating cracking (SCC). He concludes, "In other words, for each steel, the ductility is less in solution than in air, and the ductility decreases further with cathodic protection." The industry must consider the level of CP when using these steels because of the potential of hydrogen-assisted SCC. Grit blasting (required for fusion-
bonded epoxy [FBE] and most modern pipeline coatings) may help to relieve some surface stress and raise the surface potential so it is more negative than the KZIKSQVO ZIVOM