The qualification rate of zinc alloy die-casting parts has a great relationship with the pre-treatment process.
1) Zinc alloy die-casting material is zinc-aluminum alloy, and its activity is much higher than that of steel. It is susceptible to corrosion and acid-base instability.
2) The surface of zinc alloy castings suitable for electroplating is only 0.05-0.1 mm thick, and the porous layer below is not suitable for electroplating. There are also defects such as alloy segregation, shrinkage, and pinholes on the surface, which is not conducive to electroplating. The steel parts have a compact surface with no defects such as holes and pinholes.
3) Zinc alloy parts are mostly cavity parts. The shape of the part is complex. After pre-treatment and pre-plating, it must pass through a strong acid plating solution. Therefore, the inner hole of the inner hole portion must be previously coated to prevent corrosion of the post plating solution, and the inner portion of the steel part, even if it is not plated, does not cause corrosion.
In view of the above characteristics of zinc alloy die-casting parts, a set of pre-treatment and pre-plating processes suitable for zinc alloy castings must be developed to complete the processing tasks of the parts.
(1) Polishing and polishing of polished steel parts is relatively simple. As long as the surface conditions are acceptable, the parts should not be deformed significantly. The hardened layer on the surface of the zinc alloy die-cast part cannot be destroyed, so the grinding and polishing should not be too hard, and the amount of grinding should be kept to a minimum to ensure the integrity of the hardened layer and to meet the required roughness.
(2) After degreasing and polishing, the polishing paste on the surface of the part is removed with a trichloroethylene solvent, and the chemical dewaxing water suitable for the zinc alloy part is used to remove the polishing paste under the following conditions: ultrasonic. Strong alkali degreasing agents can be used for steel parts, while zinc alloy parts cannot use strong alkali degreasing agents, because strong alkali dissolves aluminum quickly, and there are shrinkage holes on the surface of the parts. These shrinkage holes will affect if they are not properly cleaned during the plating process. Coating, then adhesion of the coating.
(3) Electrolytic degreasing of steel parts can be used as anode degreasing in strong alkali solution. The properties of zinc alloy parts determine that they cannot be degreased as anodes in strongly alkaline solutions and can only be used as cathodic degreasing in weakly alkaline solutions with a current density of 3.5/-2 minutes. In order to remove the black film after the degreasing of the cathode, after the cathode is degreased, the black film should be removed by the method of anode degreasing for 0.5-1 minutes.
(4) In the hydrochloric acid and sulfuric acid solutions, the activated oxide film on the surface of the steel parts can be removed. Zinc alloy parts can only be removed in weak acids. In production, 1% to 2% hydrofluoric acid solution is usually used. Hydrofluoric acid has good etching properties, but its acidity is weaker than that of ordinary acids and dissolves silicon. In this acid, the corrosion should not be too long (5-30s 0 years old), the cleaning should be clean.
(5) Copper cyanide or dark nickel can be used for pre-plating of steel. The main purpose is to prevent the copper layer from being replaced when the acid bright copper is thick.
When zinc alloy parts are pre-plated, all parts of the part need to be plated to prevent reaction with the solution during the thickening plating process, so that plating cannot be performed. Zinc alloy parts are complex in shape, and solutions with high coverage should be used for pre-plating. General copper cyanide plating. Cyanide pre-plated copper requires high free sodium cyanide, low copper ions and weak alkalinity.
It is best to establish a prepreg process before the copper pre-galvanized alloy parts. The pre-immersion process uses 0.5%~l sodium cyanide solution, which does not need to be cleaned and goes directly into the tank.
If a nickel precoat is used, the usual dark nickel process can be used for steel parts. Zinc alloy parts must be compounded with a neutral nickel plating process. The dioxin is soaked in a 3%-5% citric acid solution and placed in a can. "The thickness of the pre-plating must be guaranteed to prevent the actinomycetes from penetrating into the zinc due to the thin coating. Zinc is brittle. Pre-plated copper cyanide is usually thicker in copper pyrophosphate and less in the coke. The coating is fine. After pre-plating of copper cyanide, it can be thickened before nickel plating, and the thickness should be 5-7 microns.
If the parts can be pre-treated and pre-plated according to the above requirements, the parts can be thickened according to the general plating process.
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