Abstract
The reaction of phosphoric acid containing tannin compounds of large molecular weight (e. g. chebulinic acid) or natural tannins with a steel surface, produces a coating which, compared to coatings produced by phosphoric acid and tannins when applied individually, confers to steel a high degree of protection against corrosion. Corrosion tests have been carried out to evaluate the stability and protective merits of the coatings. Measurement of the electrical resistance of the coating has been shown to give an indication of the effective life of the coatings. The measured electrical resistance has been shown to be mainly contributed by the complex constituents of the coating composition. With the help of pH-titration curves, the stability range of the complexes formed by the reaction of chebulinic acid, ferrous iron and phosphoric acid have been studied. Similar complexes are considered to be formed by natural tannins' and phosphoric acid during coating formation. It has been observed that the corrosion resistance and electrical resistance of the coatings and the stability of the complexes are closely related to the molecular weight of the tannin compound used. The chemical composition of the coating has been studied. It has been demonstrated that the presence of moisture is necessary for the oxidation of the coating compounds. Primary ferrous phosphate which constitutes the soluble part of the coating has been found to effectively inhibit the corrosion of steel and acts as an anodic inhibitor. The stable iron-tannin-phosphate complex of high electrical resistance and the inhibitive nature of primary ferrous phosphate are considered to be responsible for protecting steel from corrosion. They are considered to exist in layers on the metal surface in which primary ferrous phosphate is adjacent to steel whilst ferric-tannin-phosphate is present at the coating/air interface. The mode of formation of the coatings and a possible mechanism for the protection offered to iron by these coatitgs is suggested.