The effect of chromate pigment on the protective properties of an epoxy prime coat

Abstract

Sheet steels with combined protective coating are widely used in world practice [1 ]. During forge-rolling, one first applies a zinc layer onto a steel sheet by the method of hot-dip galvanizing and, later on, a polymer coating consisting of an epoxy prime coat and an upper polyester layer. An important role is played by the prime coat, which is a promoter of adhesion between the hot-galvanized steel, on the one hand, and the upper polymer layer and anticorrosive modifier, on the other hand. The prime coat must preserve the metal from corrosion even in the case of through damages of the main layer and at the unprotected edges of the sheet. Epoxy prime coats are used for already protected galvanized steels before the spraying of the main polymer coating. The epoxy composition for anticorrosive coatings and prime coats contains epoxy-dianic resin, a plasticizer, a structurizing agent, and a solvent. After curing, such compositions adhere well with the metal and have a high corrosion and chemical resistance [2,3].Thickness of the layer of an epoxy prime coat (-20 ~tm) cannot guarantee a barrier protection of the metal. However, one introduces anticorrosive pigments into the prime coat which protect the metal electrochemically. One of the most generally used anticorrosive pigments for epoxy prime coats are chromate pigments [4]. They are rather efficient in neutral and weak acid media; nevertheless, producers of sheets with polymer coatings try to decrease the content of chromate pigments to 2% because of ecological requirements. At the same time, numerous investigators in the world are searching for less deleterious pigments which can replace chromates. This is possible on the basis of a detailed study of the mechanism of protective action of a chromate pigment dispersed in the volume of an epoxy prime coat.The aim of our investigation is to study the protective properties of an epoxy prime coat with a low content of strontium chromate in the presence of through defects. ExperimentalWe manufactured coatings based on the two-component epoxy composition Epilife of the H. Marcel Guest Ltd. firm (Manchester, England). It consisted of epoxy-dianic resin and polyamide curing agent. The anticorrosive pigment (strontium chromate) with a concentration of 2 mass % was additionally introduced into the composition.Using a special applicator (Sheen instrumentation draw-bar), we obtained a coating of thickness 20-30 ~tm after complete curing. We made a through defect 1 cm in length and 0.1 mm in width in a specimen with area of 3.14.10 -4 m 2 by a sharp scalpel. We structurized the obtained polymer coatings for 24 h at 20~ and additionally subjected them to heat treatment in a thermostat for 6 h at 60~We estimated the protective properties of the prime coats by the method of impedance spectroscopy in a corrosive solution (Table 1) imitating acid atmospheric precipitations in the industrially developed countries of Europe [5] by using a three-electrode electrochemical cell. We used a calomel reference electrode and an additional p...