Cathodic delamination adhesion, coating

The relationship between performance reliability and adhesive formulation is not simple. The key step in improving the reliability of adhesives on cathodically protected substrates is fully understanding the cathodic delamination process. Various mechanisms have been proposed in the literature. A large number of investigators have focused attention on the damage hydroxide ion does to coating adhesion. 

In cathodic delamination, the delamination rate of an organic coating under a cathodic potential depends upon the applied potential, electrolyte solution, and metal substrate (22,25). Cathodic delamination tests were conducted for all three commercial paints. As an example, Figure 5 shows the cathodic delamination plots of control alkyd enamel and water-reducible ISPC coated on bare CRS coupons. Curves Sa and 5b represent delamination area for the control alkyd enamel and the water-reducible ISPC formulation, respectively curves 5c and 5d are the respective plots of delamination current for the two formulations. The delamination test was conducted in a 3% NaCI solution the alkyd coated CRS coupon served as a cathode and was polarized at -I.IV versus a saturated calomel electrode. A significantly slower delamination rate was obtained for the ISPC formula (curve 5b) as compared to the control alkyd enamel (curve 5a). At 44 hours of delamination time, the entire painted area of the working electrode (almost 20 cm x 20 cm) for the control alkyd paint had been delaminated, whereas the delamination area of the water-reducible ISPC was only as little as 1 cm indicating a remarkable coating adhesion improvement for the alkyd ISPC painted on bare CRS coupon. 

In cathodic delamination the adhesion of the coating fails and causes lateral blister growth as a result of a high pH at the delamination front [6,55,56]. The loss of... 

Figure 10. SIMS positive secondary ion images of the interfacial substrate surface of zinc phosphated steel after adhesion failure of epoxy-ester coating in cathodic polarization testing. Images obtained at the intact coating edge of the delaminated zone. Image diameter 400 fm. Image a mass 55 Fe+ image b mass 54 Zn+ image c mass 31 P+.

Adhesion of the cathodic E-coat to the plasma polymer surfaces is an important parameter in the corrosion protection of A1 alloys. In general, the adhesion performance of E-coat applied to plasma polymers was found to be far superior to that of the control panels. A-Methylpyrrolidinone (NMP) paint delamination was not observed after 120 min for E-coat on plasma polymer surfaces as compared to a maximal time for complete delamination of 5 min for E-coat on chromate conversion coating [2B] CC/E panels. The adhesion performance of cathodic E-coat on the plasma polymer surfaces could not be differentiated by the conventional tape test (ASTM D3359-93B), since E-coat on all of the combinations... 

When the steel/adhesive/PMT samples were top-coated by an alkyd layer (40 pm thick) very good protection was observed in delamination tests in chloride solution and under cathodic polarization. The delamination tests failed when the alkyd layer was deposited directly either on bare steel or on steel coated only with PMT. 

The overall scheme employed to pretreat unfilled polyetherimide substrates utilizing nitric acid as the impiegnator is given in Table IV. Unfilled polyetherimide substrates could be plated on all sides without delamination when this pretreatment scheme was employed. Furthermore, excellent adhesion was obtained for either an initial copper or nickel coating. Table V. Application of cathodic charging techniques to impregnator-pretreated polyetherimide... 

This involves the cathodic polarization of a small portion of painted metal. The area exposed contains a scribed line that exposes a line of imderlying bare metal. The sample is cathodically polarized for 15 min in 5 % NaCl. At the end of this period, the amoimt of delaminated coating is determined from an adhesive tape pulling procedure. 

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