The phosphating of zinc and zinc alloys is often used in electro-galvanizing, hot-dip galvanizing, die-casting zinc and some alloys.
The zinc-based phosphating solution can be adjusted on the basis of the iron and steel phosphating solution, and cations of iron, manganese and nickel can also be added to adjust the formation and growth process of the crystal nucleus, and a uniform zinc-based phosphating film can be obtained.
A. The suitability of zinc and zinc alloy phosphating
The materials for phosphating treatment of zinc and zinc alloys are mainly hot-dip galvanized sheets, electro-galvanized parts and castings. For the phosphating treatment of zinc and zinc alloys, the treatment time should be appropriately extended, and the phosphating solution should maintain a high content. The phosphating of galvanized parts of iron and steel is best done at room temperature with a zinc phosphate solution containing an accelerator. Under this condition, the dissolution of zinc is slow and will not damage the galvanized layer.
Special attention should be paid to the fact that galvanized steel parts cannot be phosphated together with steel parts. The phosphating treatment of zinc-aluminum alloy parts is difficult, because the presence of aluminum ions will stop the process of phosphating film formation. In order to avoid the influence of aluminum on the phosphating process, a normal temperature phosphating solution of zinc can be used to minimize the dissolution of aluminum, or an appropriate amount of sodium fluoride or sodium fluorosilicate can be added to the solution to precipitate aluminum from the solution. , or dipping the zinc-aluminum alloy parts in 60% (mass fraction) sodium hydroxide solution before phosphating, so that the aluminum on the surface is selectively dissolved, and then phosphating.
B. The formula and process conditions of zinc and zinc alloy phosphating solution
C. Zinc and zinc alloy phosphating process
The phosphating process of zinc and zinc alloy is: degreasing and degreasing → cold water washing → pickling and rust removal → cold water washing → surface activation → phosphating → cold washing → passivation → drying.
The surface of zinc and zinc alloy should be activated first before phosphating. The activation can be dipped in titanium-phosphate solution or sprayed with insoluble zinc phosphate slurry to improve the surface activity of the workpiece and promote the formation of phosphating film. After phosphating, the phosphating film on the surface of the workpiece should be passivated.