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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45 NACE INTERNATIONAL: VOL. 54, NO. 5 MATERIALS PERFORMANCE MAY 2015 tribution from the holiday toward the dis- bondment bottom indicate that the applied CP is shielded, at least partially, under the disbonded coating (Figure 2). The poten- tials are consistent with the pH values. At the disbonding thickness of 120 μm, only the open holiday is under full CP potential. With the increasing distance from the holi- day (i.e., the increasing disbonding depth), the local potential tends to be less negative until the steady-state corrosion potential is reached. Due to the shielding effect, the steel under the disbonded coating is not under CP. The potential records also indi- cate that, in order to cathodically protect the steel under coating disbondment, the CP potential must be sufficiently negative. However, hydrogen evolution must also be considered. The shielding effect of coating disbond- ment on CP permeation is affected by the disbonding thickness, as shown in Figures 4 and 5. The measurements of both local po- tential and solution pH under disbonded coating show that the CP shielding tends to be mitigated when the disbonding thick- ness is increased, and the potentials under disbonded coating approach those at the open holiday. Moreover, the pH elevation under di sbondm ent i s more apparent. Thus, the geometrical factor of the coating disbondment plays an essential role in CP shielding. The CP shielding by disbonded coating is primarily attributed to the blocking ef- fect of coating disbondment on CP current. Under narrow disbonding gaps, the distri- bution of CP current is highly nonuniform at the open holiday and under the dis- bonded coating. This effect is further en- hanced by limited diffusion of conductive ionic species through the thin solution layer trapped under the coating. Thus, al- though the open holiday is under full CP, the disbonded region is shielded from CP either partially or completely. With the in- crease in disbonding thickness, the distri- bution of CP current can be improved around the holiday. The increased solution volume under the wider coating disbond- ment enhances the diffusion of species, fa- cilitating the CP permeation into the dis- bondment. Thus, the CP shielding effect depends not only on the disbonding geom- etr y, but also on the conductivity of the trapped solution under coating. The CP shielding by coating disbond- ment can result in cathodic polarization of steel at the holiday, while the steel at the disbondment bottom can be at its corro- sion potential, depending on the disbond- ing thickness and applied CP. The potential difference produces separate anode and cathode sites. The cathodic reaction occurs at the holiday, and the anodic reaction at the disbondment bottom. The disbond- ment can become full of corrosion product, which is difficult to diffuse through, further increasing the blocking effect on CP per- meation. This is the key mechanism result- ing in localized corrosion on pipelines that are under CP. This phenomenon has been demonstrated by frequent field experiences that extensive corrosion pits are found under disbonded coating on a cathodically protected pipeline. 9 Conclusions CP can be shielded by coating disbond- ment. With the increase in disbonding depth toward the disbondment bottom, the shielding effect is more apparent. The CP shielding can be mitigated by more nega- tive CP potentials. The geometrical factor of the coating disbondment plays an essential role in CP shielding. When the disbondment becomes wider, the shielding effect is mitigated. The CP shielding can result in separate anodic and cathodic reactions, w hich occur at the disbondment bottom and the open holiday, respectively. This is the key mechanism that causes localized corrosion under disbonded coating on a cathodically protected pipeline. References 1 C.G. Munger, Corrosion Prevention by Protec- tive Coatings, 2nd ed. (Houston, TX: NACE International, 1999). 2 J.J. Perdomo, I. Song, "Chemical and Electro- chemical Conditions on Steel Under Dis- bonded Coatings: The Ef fect of Applied Po t enti al , S o luti on R e si stiv ity, Cre v i c e Thickness and Holiday Size," Corros. Sci. 42 (2000): pp. 1,389-1,415. 3 D.T. Chin, "Current Distribution and Electro- chemical Environment in a Cathodically Pro- tected Crevice," Corrosion 55 (1999): pp. 229- 237. 4 A.C. Toncre, N. Ahmad, "Cathodic Protection in Crevices Under Disbonded Coatings," MP 19 (1980): pp. 39-43. 5 X. Chen, X.G. Li, C.W. Du, Y.F. Cheng, "Effect of Cathodic Protection on Corrosion of Pipe- line Steel Under Disbonded Coating," Corros. Sci. 51 (2009): pp. 2,242-2,245. 6 T.R . Jack, G. Van Boven, M. Wilmott, R .L. Sutherby, R.G. Worthingham, "Cathodic Pro- tection Potential Penetration Under Dis- bonded Pipeline Coating," MP 33 (1994): pp. 17-21. 7 A. Fu, Y. Cheng, "Characterization of Corro- sion of X65 Pipeline Steel Under Disbonded Coating by Scanning Kelvin Probe," Corros. Sci. 51 (2009): pp. 914-920. 8 A. Fu, Y.F. Cheng, "Characterization of the Permeability of a High Performance Com- posite Coating to Cathodic Protection and its Implications on Pipeline Integrity," Prog. Organ Coat. 72 (2011): pp. 423-428. 9 M. Baker, Jr., "Integrity Management Pro- gram—Stress Corrosion Cracking Studies, Final Report," Office of Pipeline Safety TTO- 8, Department of Transportation, 2004. D. KUANG is a Ph.D. student at the Univer- sity of Calgary, MEB, 2500 University Dr. N.W., Calgary, AB T2N 1N4, Canada, e-mail: dkuang@ucalgary.ca. His research interest is pipeline corrosion and coating disbondment under alternating current in- terference. Y.F. CHENG is a professor and Canada Research Chair in Pipeline Engineering at the University of Calgary, e-mail: fcheng@ ucalgary.ca. He is an internationally re- puted researcher in pipeline corrosion. Probing Potential and Solution pH under Disbonded Coating on Pipelines The CP shielding effect depends not only on the disbonding geometry, but also on the conductivity of the trapped solution under coating.

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