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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42 MAY 2015 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 54, NO. 5 COATINGS & LININGS potential/pH micro-probes installed at dis- tances of 30, 60, 90, 120, 150, and 180 mm from the holiday. The distance of the prob- ing position to the open holiday was de- fined as the disbonding depth. A near-neutral pH (7.5) bicarbonate so- lution was used to simulate the electrolyte trapped under the disbonded coating. The solution was 0.01 M sodium bicarbonate (NaHCO 3 ), and was purged with 5% carbon dioxide (CO 2 )/nitrogen (N 2 ) for 48 h prior to the test. The corrosion potential of the steel in the solution was measured as –0.755 V vs. saturated calomel electrode (SCE). Various CP potentials were applied to the coated steel through a Solartron 1280C † electro- † Trade name. chemical system, using SCE as the refer- ence electrode and the steel as the working electrode. All tests were conducted at 23 °C. Results Figure 2 shows the time dependence of the distributions of local potential under disbonded coating (disbonding thickness of 120 μm) at varied disbonding depths from the open holiday, where the steel was either at corrosion potential or at CP po- tentials of –0.875 V vs. SCE and –0.975 V vs. SCE, respectively. It is seen that, prior to CP application, the local potentials at all prob- ing positions are ~ –0.755 V vs. SCE, which is the corrosion potential of X65 steel in the test solution. When the potential of –0.875 V vs. SCE is applied, the potential at the holiday (i.e., 0 mm in the figure) is the ap- plied CP value. However, the potential at the position of 30 mm from the holiday is less negative (i.e., –0.800 V vs. SCE after 48 h of testing). With the increase in the dis- bonding depth (i.e., the probing position is farther away from the holiday), the local potential is less negative than that at the 30 mm position, but the potential difference is not distinguishable. At the CP potential of –0.975 V vs. SCE, the potential at the holi- day is still the applied value, but the local potentials at the probing positions are shifted less negatively. With the increase in disbonding depth, the potential becomes less negative. The disbonding depth at the local potential is not distinguishable as this CP level is increased to 150 mm. Thus, the applied CP can be shielded from reaching the coating disbondment. FIGURE 3 Time dependence of the distributions of local solution pH under disbonded coating (disbonding thickness of 120 µm) at varied disbonding depths from the open holiday where the steel was either at corrosion potential (a) or at CP potentials of –0.875 V vs. SCE (b) and –0.975 V vs. SCE (c), respectively.

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