Two zinc-based pigments provided good anti-corrosion protection, while a calcium ferrite performed poorly in an eight-year study of anti-corrosion pigments in an actual urban setting, an Argentinian research team has reported.
"Evaluation of eco-friendly anticorrosive pigments for paints in service conditions," published in the September 2010 issue of Progress of Organic Coating, reports on the team’s effort to compare new anti-corrosive pigments that are replacing chromate, “as chromates are, or will be, forbidden,” lead researcher Cecelia Deyá said in an interview.
The research was carried out by CIDEPINT, a coatings technology research center based in LaPlata, Buenos Aires. The goal of the project was to evaluate nine corrosion protective pigments in outdoors condition—in this case, in the urban-industrial environment of LaPlata.
The pigments were tested in solvent-borne paints with epoxy and alkyd resins. The researchers formulated the paints, which contained 30% by volume of anticorrosive pigment, with respect to the total pigment content.
The researchers noted that classical anticorrosive coatings have historically contained inhibitors based on hexavalent chromium or lead compounds—both of which have come under increasing regulation worldwide for their health and environmental hazards.
“The use of zinc phosphate in anticorrosive paints is widespread and has been well documented,” the researchers note. However, they add, zinc phosphate proved to have a lower performance than zinc chromate, leading to additional modifications with molybdenum, aluminum and other elements.
The pigments drew mixed reviews from the researchers.
After four hours of immersion, they noted, “steel in contact with calcium polyphosphate had a corrosion potential more negative than that of the steel immersed in the supporting electrolyte (blank). The same results were obtained with calcium ferrite, while in the case of aluminum and zinc polyphosphates, the corrosion potential was similar to the blank.
“However,” they added, “only steel in the blank test rusted. This fact evidenced that the metal activity was similar in all these cases but the presence of the pigments induced the formation of a protective film.”
“The corrosion potential of the steel immersed in the other pigments’ suspensions was at least 70mV more positive than the blank and they never reached uncoated steel corrosion potential, although the values decreased during the essay.”
The team gave its highest performance rating to zinc molybdenum phosphate, zinc polyphosphate and aluminum polyphosphate, which showed “good protective behavior, independently of the resin employed in this paper.” Zinc pyrophosphate, however, only showed good anticorrosion behavior in the epoxy paint.
Calcium ferrite did not fare so well, demonstrating “a low performance in outdoor tests despite the resin employed.”