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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40 MAY 2015 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 54, NO. 5 COATINGS & LININGS D This work monitors the local potential and solution pH under disbonded coating on a steel pipeline. The ap- plied cathodic protection (CP) can be shielded by coating disbondment. While the open holiday is at a CP po- tential and associated with an ele- vated solution pH, the disbonded re- gion, especially the disbondment bottom, remains at the corrosion po- tential and the original solution pH. Disbondment of pipeline coatings can occur by a number of mechanisms. 1 In addition to poor surface preparation and thermal cycling during pipeline operation, cathodic disbondment is an important mechanism that results in lost adhesion, which usually starts at a holiday. Cathodic protection (CP) at coating faults could ele- vate th e electrolyte pH at th e holiday through enhanced cathodic reduction of dissolved oxygen or water. 2 The alkaliza- tion of the local solution can weaken the bond of the coating primer to the steel, causing coating disbondment. The shielding effect of coating disbond- ment on CP penetration into the disbond- ing crevice has been investigated. 3-7 Para- metric effects, such as solution resistivity, size of the holiday, temperature, CP poten- tial, and disbonding geometry have been tested. CP can be shielded from reaching the disbonded crevice bottom. There are numerous environmental conditions that can affect the CP shielding behavior. The steel under a disbonded coating, especially the disbondment bottom, can experience corrosion by anodic dissolu- tion, while the open holiday remains under effective CP. The separation of anodic and cathodic reactions could facilitate the oc- currence of localized corrosion, which has frequently been observed on cathodically protected pipelines where the coating has disbonded. Fusion-bonded epoxy (FBE) is a com- monly used pipeline coating that is com- patible with CP. 8 However, there has been limited work to investigate the shielding effect of FBE induced by geometrical fac- tors (i.e., disbonding from a holiday) on CP, as well as the time dependence of this ef- fect. This work attempted to determine the CP shielding behavior under disbonded FBE through in situ probing of local poten- tial and solution pH distributions under the coating. The effects of CP potential as well as the disbonding thickness and depth were determined. Experimental Conditions Steel coupons and coating used in this work were X65 pipeline steel and FBE, re- spectively. The chemical composition of the steel (wt%) is 0.04 C, 0.2 Si, 1.5 Mn, 0.011 P, 0.003 S, 0.02 Mo, and balance Fe. Prior to testing, the steel surface was ground with 120, 240, 400, and 800 grit emery papers, followed by cleaning in distilled water and methanol. Figure 1 shows a home-designed exper- imental setup to simulate the crevice gen- erated by coating disbondment. The di- mension of the steel plate was 200 by 25 by Probing Potential and Solution pH under Disbonded Coating on Pipelines D. Kuang anD Y.F. Cheng, University of Calgary, Calgary, Alberta, Canada

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