MAINTENANCE OF METAL STRUCTURES
Not grinded paint, pH1
In copper mining, the metal structures of the copper electrowinning plants are permanently exposed to acid salts and sulfuric acid mist that settle on the structures. At night, when the temperature drops, the high humidity inside the plant condenses and activates the corrosive action of these agents. The coatings that satisfy this condition correspond mainly to epoxy phenol novolac or highly crosslinked vinyl esters, which must be reinforced with agents that provide impermeability and optimize their chemical resistance. In the presence of sulfuric acid, the epoxy phenol novolac resin changes to red color, which does not mean degradation or deterioration of the film.
In order to conceal this intrinsic and inherent behavior of the resin, these coatings are often supplied in red, using iron oxide as a pigment.
By using pigments that are not reactive to sulfuric acid and in a color different from red, it allows the paint to act as an indicator of those sectors with greater concentration and acid accumulation, being able to identify the most aggressive areas from afar.
The evaluation of the red anti-acid paint was to determine the acid penetration rate in the 10 years it had been in operation. The original paint thickness was 500 microns and the pH on the surface of the paint was 1, very acidic. In the first 200 microns, a white powder with a pasty appearance (ferric sulfate formation) was observed that corresponded to acid contamination. With a 250 micron roughing, the paint had pH 7, red color and very good adhesion to the substrate. The maintenance of paints with these characteristics should consider washing with pressurized water or steam, lowering the contaminated paint level to uncontaminated paint, washing and neutralizing, drying, application of an adhesion bridge and finishing with a layer of phenolic epoxy paint specifically designed for these purposes. The work plan and products required for this type of maintenance, should be studied on a case to case basis. Vinylester coatings are more suitable for high temperatures in acid oxidizing environments.
