Cathodic disbonding

The production of OH ions according to Eq. (2-17) or (2-19) in pores or damaged areas is responsible for cathodic disbonding [9,10], where the necessary high concentration of OH tons is only possible if counter-ions are present. These include alkali ions, NHJand Disbonding due to the presence of Ca ions is extremely slight (27]. 

With anodic polarization, the anodic partial reaction predominates at defects so that OH ions formed according to Eq. (2-17) are combined in the corrosion 

S-8 Total adhesion loss of a 500-jUm-thick coating of EP (liquid lacquer), 0.2 M NaCI, galvanostatic Jy = - 1.5 /iA m , 5 years at 25 C. Left coating with a pin pore loss of adhesion due to cathodic disbonding. Right pore-free coating loss of adhesion due to electro-osmotic transport of H O. In both cases the loose coating was removed at the end of the experiment. 

The disbonding rate decreases with time [35], which can be attributed to the consumption of OH ions by reaction with adhesive groups. This consumption is obviously partly compensated for by the formation of OH ions through oxygen reduction these permeate inward from the outer surface of the coating. If this permeation is hindered by an aluminum foil gas seal, the disbonding rate falls off 

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A steel surface can be wetted and corroded by permeation of corrosive agents (see Section 5.2.2) and by cathodic disbonding (see Section 5.2.1.5). In all cases the corrosion rates are negligibly small (see Table 5-2). In the case of cathodic... 

Saponification Paints are most commonly used to protect steel from corrosion by seawater in marine applications and soil in the case of buried structures. Additional protection is often supplied by the application of cathodic protection to the steel. Any paint coating used in conjunction with cathodic protection must be resistant to the alkali which is produced on the steel at defect sites in the coating. The amount of alkali generated depends on the potential to which the steel is polarized. Some paint binders such as alkyds and vinyl ester are very susceptible to saponification, and should not be used on cathodically protected structures. Cathodic disbondment testing should be undertaken if the relevant information is not available. 

Paint stripping by water is most likely to occur from cathodic areas, the phenomenon of cathodic disbonding sometimes observed on steel protected by external anodes or impressed current being a particularly... 

The coating is applied to protect the steel from corrosion due to the acid or alkaline condition of the soil surrounding the pipe in service. Usually, the process requires three layers. First, an epoxy powder is applied to achieve adhesion to the pretreated metal and therefore resistance to cathodic disbondment. Second, a tie layer of polyolefin copolymer is applied and third a thick layer of polyethylene is cascaded, which in effect protects the epoxy from physical damage. 

Determination of resistance to cathodic disbonding of coatings for use in marine environments... 

There has been some discussion as to whether acoustic signals result from the fracture that occurs when adhesion Is lost at the paint-metal Interface during blistering and cathodic disbonding. In the present study, very sensitive Acoustic Emission measurements have been made In systems exhibiting these types of breakdown. 

When the epoxy coated panel containing artificial holidays was allowed to cathodically disbond, areas of greater than one square centimetre became detached in less than 7 days. Once again at no time were realistic A.E. transients observed. 

Analysis showed that time to failure (TTF) did not correlate well with the results of the EIS or the cathodic disbondment individually. However, when the... 

Cathodic disbondment is associated with over-the-ditch applications of tape and, to a lesser extent, with shop- or railhead-applied tape. Moisture penetration into the overlap is also a potential problem. For this reason, and to minimize damage during handling, a double wrap is preferred. The potential problems with polyethylene tape have been avoided in many cases, and there is a long history of successful applications. 

Two principle mechanisms that are discussed as possible corrosion protection mechanisms on mild steel are discussed in short. ICPs may induce the formation of a passive oxide [206]. The ICP will be reduced as a consequence of passivation and will be reoxidized by oxygen reduction. Consequently, the ICP may promote the cathodic oxygen reduction on the polymer surface rather than at the metal-polymer interface. On the basis of the good corrosion results gained by the combination of a molecular adhesion promoter and the subsequent electrodeposition of the polymethylthiophene film Rammelt and coworkers [207] concluded that the essential aspect of the corrosion protection by ICPs could be the local separation of iron oxidation and oxygen reduction. This would eliminate the local pH increase at the metal surface and subsequent cathodic disbondment. 

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