The influence of the anion type on the anticorrosive behaviour of inorganic phosphates

The objective of this work was to study the efficiency of the anticorrosive pigments obtained by substituting the phosphate anion in zinc phosphate with other anions such as tripolyphosphate and pyrophosphate. A procedure to prepare the pigments is outlined and their anticorrosive properties were evaluated following the electrochemical behaviour of a steel electrode in pigments suspensions. In a second stage, solvent-borne paints with 30% by volume (v/v) of the pigment, with respect of the total pigment content, and PVC/CPVC (pigment volume concentration/critical pigment volume concentration) ratio 0.8 were formulated. Two resins were chosen as film forming materials: an alkyd and an epoxy one. The performance of the resulting anticorrosive paints was assessed by accelerated (salt spray and humidity chambers) and electrochemical tests (corrosion potential, ionic resistance and polarisation resistance). It was demonstrated that the substitution of the phosphate anion by tripolyphosphate or pyrophosphate anions may give rise to effective anticorrosive pigments for paints. The essays performed with pigments suspensions suggested that zinc pyrophosphate has the best anticorrosive properties. The anticorrosive behaviour of the pigments in paints was found to depend on the resin employed to formulate the coatings. Good correlation was obtained between accelerated and electrochemical tests.