Cathodic blistering

A consequence of ion migration is electrolytic blister formation. In the case of anodic blisters the coated surface shows pitting, whereas in the case of cathodic blisters there is no change in the metal surface or there is merely the formation of thin oxide layers with annealing color. 

Fig. 5>5 Cathodic blisters in the vicinity of a scratched cross steel pipe with 70 /tm Zn-epoxy resin +300 fim EP-tar, seawater f cu-cuso = to -1.2 V, 220 days, 20X-...

There are numerous publications [9,10,16,19-24] and test specifications [8,25] on the formation of cathodic blisters. They are particularly relevant to ships, marine structures and the internal protection of storage tanks. Blister attack increases with rising cathodic polarization. Figures 5-6 and 5-7 show the potential dependence of blister density and the NaOH concentration of blister fluid, where it is assumed that c(Na ) and c(NaOH) are equal due to the low value of c(Cr) [23]. 

Compared with cathodic blisters, which can be recognized by their alkali content, anodic blisters can be easily overlooked. Intact blisters can be recognized by the slightly lower pH value of the hydrolyzed corrosion product. The pitted surface at a damaged blister cannot be distinguished from that formed at pores. 

In general the population of cathodic blisters increases with cathodic polarization and the population of anodic blisters with anodic polarization. Both types of... 

Fig. S-6 Relation between the density of cathodic blisters and potential shot-peened steel sheet without primer ( ) and with about 40 /tm primer coat (Zn ethyl silicate + polyvinyl butyral) ( ) top coat 500 /tm EP-tar ...

After commissioning the protection installation, an on potential of = -0.82 V was imposed. In the course of a few years of operation, the protection current rose from 100 to 130 mA. The mean values were = -0.63 V and the values were t/ = -0.50 V. An inspection carried out after 5 years showed that cathodic blisters, whose contents were alkaline, had developed in the coating. The steel surface at these places was rust free and not attacked. Uncoated steel at the pores was covered with CaC03 deposits as a result of cathodic polarization. 

Figure 20-14 shows the protection of a gravel filter in the treatment of raw water. The 200-m internal surface was coated with 300 jlm of tar-pitch epoxy resin. Long-term experiments indicated that at = -0.83 V, no cathodic blisters were formed, but were formed at more negative potentials (see Section 5.2.1.4). TiPt anodes 400 and 1100 mm in length, 12 mm in diameter, and with an active surface of 0.11 m were installed [29]. 

Cathodic Blistering. In the absence of a purposely-imposed defect in the coating, the cathodic delamination phenomenon is known as cathodic blistering. An example of cathodic blistering as a function of time is shown in Figure 1. 

Figure 1. An example of cathodic blistering. The coating was a zinc chromate alkyd primer material. The electrolyte was 0.5M KCl and the potential of the metal was maintained at - 1.0 v vs, SCE.

Nguyen, T, Hubbard, JB, McFadden, GB. 1991. Mathematical Model for the Cathodic Blistering of Organic Coatings on Steel Immersed in Electrolytes. J. Protect. Coat. Linings, 63, 43. 

Cathodic blistering is not expected with high-resistance polyolefins and rubberized coatings however, where there are defects, local reduction in adhesion (cathodic disbonding) is apparent. Enamel linings have the best resistance against cathodic influence (see Section 5.4). 

T. N. Nguyen and J. B. Hubbard, A mathematical model for the cathodic blistering of organic coatings on steel immersed in electrolytes, /. Coat. Technol. 63(794) 43-52 (1991). 

---