Cathodic debonding

There is a need for an accurate assessment of the mechanism of cathodic debonding of thick adherends. This would contribute to better control over the selection of formulations resistant to attack. One of the objectives of this work was to develop an accelerated screening test. The screening test was used to evaluate the relative performance profiles of a number of commercial adhesives which were recommended for marine applications. It is hoped that a screening test such as this one could be used to isolate measurable primer properties which can be related to the long term prospectus of the primer, and the variability of this prospectus under permutation of substrate type or additives. 

Cathodic debonding requires electron-conducting oxides and an interface that allows electrochemical reactions to occur. The latter usually requires the presence of ions and water molecules at the interface and therefore will dominate at high relative humidity. 

At low humidity and in the presence of insulating oxides such as MgO or AI2O3, cathodic debonding cannot occur and anodically driven debonding will prevail, which is called filiform corrosion (FFC, introduced by Sharman in 1944 [40]) in the corrosion community, as this type of debonding is character-... 

There are complications however. The cathodic potential effectively stops corrosion on the metal substrate but it also contributes to early debonding of adherends, the development of leak paths under seals, and the blistering and peeling of coatings. Degradation of protective coatings is a basic life-limiting problem for underwater equipment exposed to a cathodic potential. 

The rate of the cathodic reaction is not the only factor influencing the debonding however. Consider the IN NaOH solution. 

NaOH solution and in 3.5%(Wt.) solution of NaCl. Thus, the upper curve, showing the volume change in the presence of hydroxide ion, will be typical of the behavior of the primer at the debond during cathodic delamination. 

Figure 45 Spring-loaded double cantilever beam specimen undergoing cathodic delamination in seawater. The spring lowers the rate of decay of G with debond distance.

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