Inhibitor properties of “green” pigments for paints

Zinc chromate is one of the anticorrosive pigments most frequently used in the formulation of primers. However, its environmental aggressiveness and toxicity severely restrict its use, and different green alternatives have been proposed in order to replace zinc chromate. In the last decade, the behaviour of zinc phosphate as anticorrosive pigment has been intensively researched. During this time, various modifications have been made to this family of pigments to improve its properties, and a “second generation” of phosphate pigments, incorporating elements such as molybdenum, aluminium, or iron, has been produced. In this paper, the inhibitive properties of zinc phosphate and three second-generation phosphates have been investigated, using zinc chromate pigment as a reference. Pigment extract solutions, at different values of pH, have been used as corrosive media. Carbon steel samples were immersed in such solutions and their corrosion rates were measured using electrochemical techniques. The data obtained suggest that zinc chromate provides the highest percentage of inhibition in neutral and basic solutions, but phosphate-based pigments showed better results in acid solutions. Given this performance advantage, together with their less harmful environmental impact, these phosphate-based pigments can be proposed as realistic alternatives to chromates in the formulation of protective paints for use in acidic conditions.