Disbonded coating

A. Eslaini, B. Fang, R. Kania, B. Worthingham, J. Been, R. Eadie, W. Chen, Stress corrosion cracking initiation under the disbonded coating of pipeline steel in near-neutral pH environment, Corros. Sci. 52 (2010) 3750-3756. 

Fig. 11.4 Bode magnitude plots corresponding to disbonding coating (Fig. 11.le). Parameters ...

Coatings Surface preparation Protection under disbonded coatings... 

Coated pipe is somewhat more difBcult to evaluate, because corrosion only occurs at coating hohda and sometimes under disbonded coating. Coating defects must be carefiilly examined and the condition of the pipe documented. 

Pipelines—oil, gas, water, wastewater Internal corrosion primarily at the bottom (6 00) position Dead ends and stagnant areas Low points in long-distance pipes Waste pipes—internal corrosion at the liquid/air interface Buried pipelines—on the exterior of the pipe, especially in wet clay environments under disbonded coating Aerobic and anaerobic acid producers, SRB, manganese and iron-oxidizing bacteria, sulfur-oxidizing bacteria... 

Figure 10.3 Typically deep, sharp-sided pits usually found under disbonded coatings or hard accumulations on a pipe surface. When disturbed the area may smell of hydrogen sulfide, a telling sign of microbial activity. (Courtesy of MACAW S Pipeline Defects, published by Yellow Pencil Marketing Co.)...

When condensed moisture and water are collected under disbonded coatings, at least two scenarios can occur ... 

The phenomenon of coating disbondment plays a major role in this type of problem. The protective properties of a coating are greatly dependent on its ability to resist disbondment around defects." The protective properties of the coating are compromised when water enters the gap between the (disbonded) coating and the metallic surface. A corrosive microenvironment will tend to develop in such a situation. Depending on the nature of this microenvironment, the CP system may not be able to protect the surface imder the disbondment. Only when the trapped water has a high conductivity (e.g., saline conditions) wiU a protective potential be projected under the disbondment. In the absence of protective CP effects, the surface will corrode imder the free corrosion potential of the particular microenvironment that is established. 

Jack, Wilmott, and Sutherby identified three primary corrosion scenarios that could be manifested under shielded disbonded coatings on buried steel pipelines, together with secondary transformations of the primary sites (Table 11.5). A brief description follows. 

TABLE 11.5 Primary Corrosion Scenarios and Transformations at Disbonded Coating Sites for Steei Pipeiines Buried in Aiberta Soii... 

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