Properties of Colored Phosphate Coatings for Corrosion Protection of Steel

Phosphate coatings obtained from solutions based on the drug containing Mn(H₂PO₄)₂∙2H₂O (proportion of phosphoric acid, in terms of P₂O₅ 46-52 % ; mass fraction of manganese not less than 14 %), used to protect steel products from corrosion. Phosphating solutions are widely used to produce protective films at low temperatures. To obtain colored coatings, it is proposed to introduce the dyes procyon olive green and methylene blue in the amount of 8 g/L into phosphating solutions. Colored phosphate films unevenly cover the surface of steel samples. The protective and physical and mechanical properties (thickness, heat resistance, wear resistance, breakdown voltage value) of colored phosphate coatings obtained on steel by the cold method are studied. The protective properties of phosphate coatings strongly depend on their thickness and the nature of the crystal structure, since the thickness and structure determine the porosity of the coatings, and, consequently, the freedom of access of the aggressive medium to the metal surface. It was found that phosphate coatings can withstand short-term heating up to 100 °C, after which their protective ability is greatly reduced. Colored phosphate films produced on steel by the cold method have low values of the coefficient of friction, but this disadvantage can be eliminated by impregnation with lubricants. The breakdown voltage of colored phosphate coatings is about 200 V. Its electrical insulation properties can be improved by impregnation with oil and bakelite lacquers.