Curing of inorganic composition coating of aluminum – phosphate system

Abstract

Known methods for applying an inorganic composite coating are accompanied by its heat treatment at a temperature of at least 300 °C, which requires the use of furnaces. Recent developments have allowed to reduce the temperature of heat treatment to 90 °C for at least 3 hours or to 105 °C for at least 1 h, which allows the use of portable heating equipment. However, the use of any equipment with such prolonged heat treatment is not acceptable when applying corrosion protection to large structures, especially when their dismantling is impossible. The results of studies on the chemical curing of an inorganic composite coating that does not require heat treatment are presented. It is proposed to spray a hardener solution onto a composite coating having a more alkaline pH instead of heat treatment. When a hardener reacts with acidic residues of an inorganic binder on the surface of a dried coating, monosubstituted phosphates undergo a transition to poorly soluble two-and practically insoluble tri-substituted phosphates not only on the coating surface, but also on the inner surface of the pores, which leads to almost complete insolubility of the coating. Treatment of the coating with an aqueous solution of monosubstituted potassium phosphate, two-substituted potassium phosphate or potassium pyrophosphate gives the coating water resistance. The inorganic composite coating acquires a high protective ability (more than 1300 hours in a salt fog chamber) on steel parts after treatment with only aqueous solutions of mono-substituted or two-substituted potassium phosphate.