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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44 MAY 2015 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 54, NO. 5 COATINGS & LININGS solution pH tends to be the value of the originally prepared solution. As the dis- b on din g thi ckn e ss in crea se s, th e C P- induced pH elevation becomes more obvi- ous, even at the disbondment bottom. For example, at the disbonding thickness of 120 μm, the solution pH at the disbondment bottom (i.e., 180 mm from the holiday) is ~7.5. This indicates that the CP does not penetrate into the disbondment bottom. W h en th e di sbonding thickn ess i s in- creased to 240 and 360 μm, the solution pH at the disbondment bottom is 7.9 and 8.1, respectively. Thus, as the coating disbond- ment becomes wider, the CP-enhanced pH elevation is more appreciable. Discussion In deoxygenated near-neutral pH bicar- bonate solutions, the anodic and cathodic reactions during corrosion of pipeline steel are primarily the iron oxidation and reduc- tion of water, respectively. In the absence of CP, the steel corrodes at both the holiday and under the disbonded coating. Upon CP application, the cathodic reaction is en- hanced, resulting in the generation of OH – to elevate the solution pH. With the nega- tive shift of CP potential, the enhancement is more apparent. Thus, the solution pH is further elevated. When CP is applied on a coated steel electrode containing a holiday, the CP is primarily applied on the open holiday. The solution pH at the holiday is elevated with the negative shift of CP potential, as shown in Figure 3. The nonuniform distribution of solution pH from the open holiday to the d i s b o n d m e n t i n d i c a t e s t h a t t h e C P- induced pH elevation is not fully realized under the disbonded coating. The measurements of the potential dis- FIGURE 5 Distributions of solution 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 thicknesses (a) 120 μm, (b) 240 μm, and (c) 360 μm.

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