Zinc alloy is mainly composed of zinc; it also contains about 4% aluminium and trace amounts of copper and magnesium. Zinc alloy dies castings are used in many applications, such as automobiles, architectural hardware, and household appliances, by plating a protective or decorative protective layer. Zinc alloy dies casting plating than other substrate plating is more difficult. This is because zinc alloy’s chemical stability is poor and easily corrupted in acid and alkali solutions. The workpiece in the die-casting moulding substrate left bubbles, cracks, pinholes and other defects. The main methods to overcome the defects are pretreatments, such as billet inspection, mechanical grinding and polishing, oil removal, etching, activation and pre-plating.
Inspecting the blank before plating is an important step to ensure the plating quality of zinc alloy die castings. Zinc alloy dies castings in the moulding of the surface left gates, flying edge, horizontal lines, burrs, knots and other defects. This must be strictly inspected to prevent defective products from entering the plating process. It is difficult to mechanically polish or levels the rough parts, which will bring quality problems to the removal plating, reducing the rework rate and production costs.
Mechanical grinding and polishing
The tool for grinding should be a soft and flexible wheel. Because of the soft texture of zinc alloy die castings, it is necessary to use a grinding wheel made of white cotton cloth (with diamond grit stuck on its working surface) for polishing. When pressing the workpiece to the working surface of the grinding wheel, the abrasive grains can be stuffed into the wheel so that the cutting ability of the abrasive grains can be weakened, thus not causing changes in the geometry of the workpiece, and the surface is smoother after grinding. It is not suitable to use coarse grit grinding wheel directly to avoid causing the bottom of the workpiece (the following porous, loose structure), grinding to remove the dense layer while preventing soft metals from sticking to the grinding wheel, reducing the smoothness of the surface to be ground. Therefore, the working surface of the grinding wheel needs to be coated with the red polishing paste so that the numerous grinding grains are covered by the polishing paste, and the grinding performance can be weakened. Thus the surface obtained is flatter and smoother.
The pressure of the working surface on the grinding wheel should be small and even. The dense layer of zinc alloy die-casting surface directly affects the plating quality, so it must be protected when polishing. If the grinding operation is too much force, it will destroy the dense layer and generate a lot of friction, heat and even scorch. Polishing is to polish the residual fine defects further when polishing and improve the smoothness of the substrate surface. The polishing wheel should be softer, preferably a tweed cloth wheel. The polishing wheel speed is higher than the polishing of some goods and uses a white polishing paste. White polishing paste in calcium oxide and magnesium oxide particles is fine and round, with no sharp edges, is very suitable for zinc alloy die-castings and other soft metals and requires a high degree of finish polishing.
When polishing, the workpiece should be lightly polished and placed, and the amount of polishing paste used should be appropriate; that is, less diligent use is appropriate. If the amount of polishing paste with too much, in addition to waste, but also sticks in the groove or gap in the workpiece, bringing difficulties in removing oil. But the polishing paste should not be too little. Otherwise, it is easy to pour light when there is no polishing paste, so the workpiece surface local overheating and pockmarks, affecting the plating quality. If necessary, polishing can be divided into two processes, that is, first with red polishing paste rough polishing, and then white polishing paste (or powder) fine polishing. It should be noted that polishing and buffing cannot destroy the dense surface of the substrate and reveal the loose porous layer. Otherwise, it isn’t easy to obtain a good bonding layer.
After the workpiece is ground and polished, it should be cleaned with organic solvent as early as possible. Otherwise, after a long time (more than 24h), the residual polishing paste will age and be difficult to remove. Organic solvent grease removal is an ordinary dissolution process of saponified grease and non-saponified grease. This grease removal on the metal generally does not produce corrosion, but the removal of oil is not complete. Because the organic solvent attached to the surface of the workpiece evaporates, part of the oil is left on the metal surface, so chemical or electrochemical degreasing is also required to meet the requirements of complete oil removal.
After the organic solvent degreasing, the workpiece must be dry before entering the chemical or electrochemical degreasing. Otherwise, it will make the surface of the workpiece more difficult to remove the oil. This is because the organic solvent, cleaning oil, has been diluted, but most of the oil is still attached to the surface of the workpiece. If not evaporated, it will contaminate the chemical or electrochemical degreasing solution, thus affecting the workpiece degreasing effect.
It is not advisable to use strong alkaline solutions such as sodium hydroxide to remove oil, the operating temperature should not be too high, and the workpiece should not be immersed in the degreasing solution for a long time. This is because zinc is an amphoteric metal chemically active and easily corrupted. Therefore, a weakly alkaline chemical or electrochemical degreasing solution must be used, for example. Chemical degreasing solution formula and process conditions are: sodium phosphate 20 ~ 30g / L, sodium carbonate 10 ~ 20g / L, sodium dodecyl sulfate (or OP emulsifier) 0.5 ~ 0.8g / L, 50 ~ 60 ℃, 1 ~ 2min.
Electrochemical degreasing solution formula and process conditions are: sodium phosphate 25g / L, sodium carbonate 25g / L, 1.5 ~ 3.0A / dm2, 50 ~ 60 ℃, 10s. generally use the anodic degreasing method. This is because the anodic degreasing process is easy to remove the film generated on the surface of the zinc alloy substrate by electrolysis in a weak alkali. Still, the processing time should not be long. Otherwise, it will cause the surface of the workpiece to dissolve too quickly (this is related to the composition of the alloy). Although this phenomenon does not occur in cathodic degreasing, impurities in the solution may be adsorbed on the surface of the workpiece, and sponge-like deposits may be precipitated, which affects the bonding of the plating. If the method of cathodic degreasing is used, it is best to remove the workpiece after turning to anodic degreasing for 2s at the end to remove the deposits on the surface.
Zinc alloy dies castings will produce a layer of oxide film on the surface after degreasing. This film can not be treated with a strong acid solution because strong acid will make the zinc-rich phase preferentially dissolved so that the workpiece substrate surface pores the residual solution left in the hole is not easy to clean off, causing quality problems such as coating blistering, peeling. Therefore, a dilute solution of the weakly acidic hydrofluoric acid system must be used to remove it.
Recipe 1: hydrofluoric acid 10-15mL/L, boric acid 3-5g/L, room temperature, 20-30s.
Recipe 2: hydrofluoric acid 3mL/L, sulfuric acid 1mL/L, room temperature, 20-30s.
Recipe 3: hydrofluoric acid 2mL / L, nitric acid 5mL / L, room temperature, 3 ~ 5s.
In the leaching treatment, the concentration of the hydrofluoric acid mixture and the operating time must be strictly controlled. Short etching time, in addition to not exhausting the oxide film; too long, causes the substrate surface to corrosion, resulting in loose film and hanging ash. This hanging ash is difficult to clean off with water, which seriously affects the quality of the subsequent plating, which is one of the important reasons for the low yield of zinc alloy die-casting plating.
When the workpiece is after etching the hanging ash, you can use oxalic acid (15g / L) and sulfosalicylic acid (15g / L) cleaning solution, room temperature impregnation for about 5min, you can remove the corrosion film hanging ash, to effectively ensure the quality of etching.
Activation and pre-plating
To further improve the bonding of the zinc alloy die-casting plating layer, it is also necessary to carry out pre-plating pretreatment according to the following methods. Suppose it is determined to use pre-plating copper cyanide. In that case, the workpiece should be activated in a sodium cyanide solution with a mass concentration of 3 to 10 g / L. Pre-plating copper should be carried out directly without cleaning. The mass concentration of copper in the plating solution should not be high, while the mass concentration of free sodium cyanide should not be low. At the same time, to prevent the replacement reaction between the zinc in the deep recess of the workpiece and the copper ions with positive potential in the plating solution, the workpiece must be charged down the tank and plated with an impact current. Suppose it is determined to use pre-plating citrate-neutral nickel. The workpiece should be activated in a citric acid solution with a mass concentration of 30 to 50 g / L without cleaning direct pre-plating nickel. This neutral pre-plating nickel needs to improve the pH of the plating solution, the general pH control in 6.5 ~ 7.0, and maintain the mass concentration of nickel sulfate and sodium citrate ratio in 1.1: (1.0 ~ 1.3), which can prevent the replacement reaction of zinc and nickel ions to improve the bonding of the coating.
Pre-plating treatment greatly impacts the plating quality of zinc alloy die castings, and the main points of each process must be taken seriously in production. Specifically: check the blank; mechanical grinding and polishing can not destroy the dense layer on the surface of the workpiece, can not use strong acid, strong alkali for oil removal or etching, even after pre-plating should not be in strong acid, strong alkali solution plating, to prevent the plating solution into the substrate after cleaning is not clean, so that the plating blistering off.
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