Introduction of plating decorative chromium on zinc alloy die casting

As we all know, zinc is an amphoteric metal, which is easy to corrode in acid and alkali solution, so that the adhesion between the coating and the substrate is significantly reduced. Therefore, in decorative electroplating, the pretreatment of zinc base alloy surface is the key. Usually, after grinding and polishing, the parts are dewaxed and degreased by chemical and electrochemical methods with sodium phosphate and sodium carbonate as the main components, then activated by sodium cyanide solution, pre plated with copper cyanide, and then plated with other kinds of plating. However, matrix corrosion is hard to avoid, and there are problems of dispersion ability and deep plating ability, such as groove, inner hole lack of plating, corner burning, porous porosity, even blistering and peeling.

In this paper, we will introduce the technology and technological process of plating decorative chromium on zinc alloy die casting, which can improve the technology and technological process, reduce environmental pollution, and make the bonding force between the coating and the substrate good.

2 process flow

Roughcast inspection → mechanical polishing → fine polishing → gasoline scrubbing → chemical degreasing → hot water washing → water washing → pickling activation → water washing → alkaline chemical nickel dipping → water washing → neutral nickel plating → water washing → bright nickel plating → water washing → composite chromium plating → recycling → hot water washing → drying → inspection

3 bath formula and process conditions

3.1 chemical oil removal

Phosphate free washing powder (high quality) 50-80g / L, sodium carbonate 5-10g / L, 60-100 ℃, until it is cleaned.

Phosphate free washing powder is a kind of unique surfactant. Its main components include anionic surfactant, non-ionic surfactant, stain suspending agent, whitening agent, color protecting agent, etc. Therefore, the degreasing liquid made of this kind of washing powder has the special function of removing wax and other dirt on the surface of zinc die casting. It can quickly decompose all kinds of dirt and achieve the decontamination effect. The workpiece can be completely removed after being put into the groove for about 5min, without causing matrix corrosion and keeping the surface clean and fresh. In addition, this kind of washing powder does not use phosphate as auxiliary agent, so it will not cause eutrophication of water body.

Hydrofluoric acid is a weak acid, which can be stabilized by adding H2SO (or HNO, HBO, etc.) into HF aqueous solution.

3.3 chemical nickel leaching

Nickel chloride is 4o-50g / L, sodium citrate is 90-100g / L, ammonium chloride is 45-55g / L, sodium hypophosphite is 1o-12g / L, pH value is 8. 5-9.5, 80-9o ℃, 5-10min.

There are two kinds of electroless nickel plating: acid and alkaline. In contrast, the effect of alkaline electroless nickel plating is better than that of Acid Electroless nickel plating. The latter is easy to corrode the substrate, and the nickel layer will bloom, which is overcome by this process.

3.4 neutral nickel plating

Nickel sulfate 160-180g / L, sodium citrate 180-200g / L, sodium chloride 10-15g / L, boric acid 2o-30g / L, tannic acid 0.4-0.6g/l, pH 6.5-7.0, room temperature 0.4-1.0a/dm, 15min, cathode moving.

The addition of tannic acid mainly masked Zn "and refined the crystallization of the coating, reducing the porosity of the nickel layer.

3.5 bright nickel plating

Nickel sulfate 250-300g / L, sodium chloride 15-20g / L, boric acid 3o-40g / L, 1,4-butynediol 0.4-0.5g/l, saccharin 0.8-1.0g/l, sodium dodecyl sulfate 0.05-0.10g/l, pH 4.0-5.1, 50-55 ℃, 2-3a / DM, 15min, cathode moving.

This formula is a general all bright nickel plating process.

3.6 composite chrome plating

Chromic anhydride 250-300g / L, fluosilicic acid 4-6g / L, sulfuric acid 1.0-1.5g/l, trivalent chromium 2-6g / L, 45-55 ℃, 15-30a / DM, 1.0-1.5min. The anode is lead tin alloy.

Composite chromium plating, also known as fluorosilicic acid chromium plating. The process is used to overcome the phenomenon of "yellow" in common chromium of zinc alloy die casting due to complex shape (i.e. many inner holes in the groove), while the cathode colloid film of fluorosilicic acid plating solution is much thinner than that of common chromium plating solution only containing sulfuric acid, and the passivation trend of chromium layer is smaller, so the bright coating can be obtained at low temperature and low current density, and the bright range is wider with the increase of liquid temperature. Therefore, the composite chromium plating process is particularly suitable for decorative plating of zinc alloy die castings.

4 key points of operation

(1) for smaller parts, rolling treatment can be carried out (without grinding and polishing). For the parts suitable for grinding and polishing, the force shall be uniform during operation, and the thickness of the surface of the zinc casting to be removed shall not exceed the thickness of its dense layer, because the lower part is porous and loose structure. If the polishing force is too much, the dense layer on the surface of the substrate will be destroyed. As a result, the polishing paste and bath liquid will enter into the exposed pores and cracks, which will affect the quality of the coating.

(2) when polishing, first use yellow polishing paste for rough polishing, and then use white polishing paste for fine polishing. The principle of using a small amount of polishing paste is to use it frequently. When a large amount of polishing paste is used, it will stick to the groove and inner hole of the workpiece, which will bring difficulties to decontamination, but not too little. Otherwise, it will overheat the surface and produce small pits, which will cause blistering, peeling and other defects of the coating.

(3) after the above grinding and polishing, the parts shall be brushed with gasoline as soon as possible to remove the residue of polishing paste, so as to avoid the solidification of polishing paste and affect the quality of chemical oil removal.

(4) high quality washing powder (B) should be used, with small granules, large density and less foam. It can be dissolved quickly in water, and the hand contact solution has no burning sensation. The solution is clear and smooth, and the effect of decontamination is obvious. Special attention should be paid: the water temperature of the detergent with enzyme should not exceed 60 ℃, otherwise, the enzyme will lose its activity and affect the degreasing effect. In addition, for the large volume of zinc die casting, you can use a brush or rag to stick washing powder to remove oil by hand.

(5) the pickling activation time should not be too long. When hydrogen bubbles appear on the surface of the workpiece, it should be taken out and cleaned in the flowing water. Otherwise, the surface is easy to produce corrosion product adhesion, that is, ash hanging and sparse