Localized corrosion promoted by protective coatings on ARMCO iron in deaerated acid solutions

The screening effects of some insulating materials like Nylon 66, PTFE and PVC had been tested in corrosion processes of carbon steels with different carbon contents and of ARMCO Iron in deaerated acid solutions showing that all the types of screen in all the experimental conditions induce a fast process of localized corrosion at the three phases contact metal-screen-solution and that the attack morphology depends on geometry contact. Also the varnishes up to now tested on ARMCO Iron, exhibit the same behaviour except a glycerophtalic Pb 3O 4 charged resin. The generality of this phenomenon has led to formulate the hypothesys that the activation of the localized corrosion process may be due to the interaction of double layers generated in the solution by the metal and by the insulating material. The electrochemical experiments have shown a correlation among the ratio of the coated surface perimeter to the exposed surface area and the electrode potential, the current density and the polarization resistance.