Corrosion protection mechanism of aluminum triphosphate modified by organic acids as a rust converter

Water-borne rust-resistant coatings were prepared by using aluminum triphosphate as the rust conversion agent and a styrene acrylic emulsion, and the mechanism of rust prevention was studied by electrochemical impedance spectroscopy. With increasing soaking time, aluminum triphosphate is further decomposed into orthophosphate to improve the corrosion resistance of the matrix. Within six hours, with increasing immersion time, the impedance value of the conversion film increased, and the maximum impedance of the conversion coating was 954.4 Ω cm2, indicating that the speed of rust transformation to form a protective film was slow and that the corrosion resistance of the conversion film was low. Hydroxyethylidene-1,1-diphosphonic acid (HEDP) and tannic acid were added to modify aluminum triphosphate, and all of them had a chelating effect. Through synergistic effects, the impedance of the modified conversion film reaches 32,495 Ω cm2, which improves the rust conversion and antirust performance and obviously improves the shortcomings of the early antirust performance of aluminum triphosphate used alone. The results of infrared spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy show that the modification with the organic acids involves adsorption on the aluminum triphosphate particles through physical and chemical adsorption. Electrostatic repulsion can prevent the tripolyphosphate ions from agglomerating, making them uniformly dispersed and improving the bonding of the coating film. The modified formulation has better corrosion resistance and rust transformation ability than that of aluminum triphosphate alone.