Materials Performance

MAY 2015

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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43 NACE INTERNATIONAL: VOL. 54, NO. 5 MATERIALS PERFORMANCE MAY 2015 With the increase in disbonding depth to- ward the disbondment bottom , the CP shielding is more apparent. The shielding effect can be mitigated by application of more negative CP potentials. Figure 3 shows the time dependence of the distributions of local solution pH under disbonded coating (disbonding thickness of 120 μm) at various disbonding depths, where the steel was either at corrosion po- tential or at CP potentials of –0.875 and –0.975 V vs. SCE, respectively. Prior to CP application, the solution pH is ~7.5, the value of the prepared solution, at all prob- ing positions. Upon CP application, the so- lution pH is elevated. Moreover, when the CP potential is more negative, the solution pH is further elevated at individual probing positions. For example, at the open holiday, the steady-state solution pH is 8.5 at –0.875 V vs. SCE and 11.0 at –0.975 V vs. SCE. However, the CP driven pH elevation be- comes less obvious with the increasing disbonding depth, especially at the dis- bondment bottom. Figure 4 shows the distributions of local potential under disbonded coating at var- ied disbonding depths from the open holi- day, where the CP potential of –0.875 V vs. SCE is applied under various disbonding thicknesses. Identical to previous results, the CP is shielded from reaching the dis- bondment. Only at the open holiday, the measured value is the same as the applied CP potential. Under the coating disbond- ment, the potential tends to be less nega- tive. Moreover, with the increase in dis- bonding thickness, the CP shielding effect becomes less significant. For example, at the disbonding thickness of 120 μm, the local potential at the probing position of 30 mm is ~ –0.810 V vs. SCE. When the dis- bonding thickness is increased to 240 and 360 μm, the potentials at the same position are –0.855 and –0.865 V vs. SCE, respec- tively. Therefore, as the coating disbond- ment becomes wider (i.e., with an increased disbonding thickness), the CP shielding ef- fect is less significant. Figure 5 shows the distributions of solu- tion pH under disbonded coating at varied disbonding depths from the open holiday, where the CP potential of –0.875 V vs. SCE is applied under various disbonding thick- nesses. The applied CP is able to elevate solution pH, especially at the open holiday. With the increasing disbonding depth, the FIGURE 4 Distributions of local potential under disbonded coating at varied disbonding depths from the open holiday where the CP potential of –0.875 V vs. SCE is applied under various disbonding thicknesses (a) 120 μm, (b) 240 μm, and (c) 360 μm. Probing Potential and Solution pH under Disbonded Coating on Pipelines

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