Protective coating systems for corrosive environments normally include a zinc rich primer. In the past decade mainly zinc epoxy primers have been applied on offshore constructions in the Norwegian sector. Zinc rich primers improve the corrosion creep resistance of organic coating systems, and the effect is believed to be due to cathodic protection of the substrate. In order for the zinc rich primer to provide cathodic protection the zinc particles must be electrochemically active and in electrical contact with the substrate. However, the epoxy binder may insulate the zinc from the substrate and may also protect the zinc from oxidation. In this work electrochemical properties of zinc epoxy primers have been investigated: Capacity for cathodic protection of the substrate and electrical contact between the zinc particles through the film. The electrochemical properties are correlated with results from cyclic corrosion creep testing.
Protective coatings used in corrosive environments usually include a zinc rich primer. The beneficial effect of zinc-rich primer on the durability of protective organic coatings is primarily assumed to be due to a cathodic protection mechanism. Ensuring that the zinc particles are electrochemically active and in galvanic contact with the substrate is then essential, and electrochemical properties of zinc rich primers have been investigated in several studies.1-9 Only the zinc particles in galvanic contact with the steel substrate will polarize the substrate. An epoxy binder is regarded as electrically insulating and protects metallic surfaces against corrosion. The zinc particles in a zinc epoxy primer may therefore be protected by the epoxy and insulated from the steel surface. However, investigations have shown that many zinc epoxies are electrochemically active1,3,9.
Topcoating the zinc rich primer has been claimed to decrease its ability to protect the substrate.2,4,10 The topcoat will insulate the zinc rich primer, passivate the zinc and prevent electrolytic contact between the steel substrate and the zinc dust particles. Active zinc will be limited to damages in the topcoat, normally with a very limited capacity for cathodic protection. Continuous zinc coatings, i.e. zinc coatings applied by thermal spraying or hot dip galvanizing, will have better capacity for cathodic protection.11 In order to approach the properties of pure zinc coatings, zinc rich primers with zinc loadings of 90-96% by weight have been used. According to the Norwegian Public Roads Administration especially primers with the highest zinc content, i.e. 96 % zinc, provide better corrosion protection than primers with 80-86% zinc by weight. It is very important that the first coat on the porous, 96 % zinc primer, is able to penetrate into and fill the pores of the primer. According to the paint suppliers blistering of the topcoat due to actively corroding zinc and cohesion of the primer is a challenge with such high zinc loadings.12
