Research on Electroplating of Zinc Alloy Die Castings


Because zinc alloy die castings are formed in one step, the production efficiency is high, and zinc is widely distributed in nature. Therefore, in industry, especially in the automobile industry, it is suitable for structural parts and decorative parts with less stress and more complicated shapes. Zinc alloy die castings are widely used. Because zinc alloys have poor chemical stability and are prone to cracks, important grade tables are generally not used, so copper/nickel/chromium multi-layer anticorrosive decorative coatings are required.

   For zinc alloy die castings, zinc alloy materials with an aluminum content of about 4% should generally be used to increase the pass rate of electroplated products.

   When electroplating zinc alloy die castings, pay attention to the following characteristics of zinc alloy die castings and take appropriate measures.

   (1) The surface of the die casting is a dense surface layer, about 0.05mm to 0.1mm. Below the surface layer is a loose and porous structure. For this reason, when polishing and polishing, do not throw away all the surface layer and expose the loose bottom layer. Otherwise, electroplating is very difficult and will reduce the corrosion resistance of the product.

   (2) Die-casting parts change from a molten state to a solid state during the die-casting process. Because of the different freezing points during cooling, segregation tends to occur on the surface of the die-casting parts, causing some parts of the surface to produce aluminum-rich phases or zinc-rich phases. For this reason, do not use strong alkali and strong acid to remove oil and etch during pretreatment. Because the strong alkali can dissolve the aluminum-rich phase first, and the strong acid can dissolve the zinc-rich phase first, thereby forming pinholes and micro-pores on the surface of the die-casting part, and will leave strong lye and strong acid solution, so that when plating After coating, it is easy to cause peeling and air bubbles.

   (3) The shape of zinc alloy die castings is generally more complicated, and a solution with better dispersion and covering ability should be used during electroplating. In order to prevent the replacement of the positive potential metal by zinc in the recessed or masked place, which will make the bonding force poor, the pre-plating layer should choose the plating solution with good dispersing ability and covering ability.

   (4) The plating layer used is preferably a bright plating layer, and the polishing process is avoided or the polishing workload is reduced as much as possible. On the one hand, it is not easy to polish because of the complicated shape, and on the other hand, the thickness of the coating can be guaranteed to ensure the quality.

   (5) If a copper layer is used for the coating layer, its thickness should be slightly thicker, because when copper is plated on the surface of the zinc alloy, the copper diffuses into the zinc and forms a brittle copper-zinc alloy intermediate layer, the copper layer The thinner the diffusion effect occurs faster, so the thickness of copper must be at least 7μm or thicker.

   (6) The multi-layer chromium plating layer is a cathodic protection plating layer of zinc alloy, so the plating layer must have a certain thickness to ensure that the plating layer has no pores. Otherwise, because the electrode potential of the zinc alloy is relatively negative, the white powdery corrosion product of basic zinc carbonate is likely to be produced in the humid air. Therefore, the appropriate coating thickness must be selected according to the use conditions of the product.

   (7) If the die-casting process is inappropriate, or the die-casting mold is designed unreasonably, causing cold lines, shrinkage, porosity or pinholes on the die-casting surface, then even if measures are taken in the electroplating process, a satisfactory coating is often not obtained.

   Electroplating process on zinc alloy die castings:

   (1) Blank inspection

   There should be no cracks, bubbles, looseness, scratches and other defects that affect the quality of electroplating on the surface of the parts. If the defects are not serious, it depends on whether these defects can be eliminated by polishing and polishing. If they cannot be eliminated, electroplating is not allowed.

   (2) Polished

   Polishing is to remove defects such as burrs, parting lines and flash on the surface of the blank. The methods include: cloth wheel polishing, rolling polishing and vibration polishing. Do not dry sand when polishing. The polishing pressure should not be high, and the drum speed should not be high, so as to avoid excessive grinding. The particle size of cloth wheel polishing should be 0.045mm~0.069mm with butter paste as auxiliary abrasive. The newly bonded abrasive should be poured with acute angles, and the grinding speed should be less than 1200m/min.

   (3) Rolling

   Rolling is carried out in a drum filled with abrasives and rolling liquid. The abrasives and parts are generally 2.5:1. The rotating speed of the drum should not be too high, generally 5-6 rotations are better.

   (4) Polishing

   Polishing is to reduce the surface roughness of the parts to ensure the quality of the coating. The method is similar to polishing, but the polishing material used is different from polishing. Cloth wheel polishing generally uses a whole cloth wheel, first use yellow polishing paste and then white polishing paste, and use it sparingly and frequently. The diameter of the polishing wheel should not be too large, and the speed should not be too high. Rolling and vibration polishing generally use softer materials such as corn cobs, pits, husks, and plastic chips, and add lubricant to roll or vibrate with the parts.

   The polished parts should be transferred to the next process as soon as possible to prevent the polishing paste from drying up and difficult to remove. The circumferential speed of the polishing wheel should not exceed 2100m/min. Generally, the low speed of 1100m/min~1600m/min is used for small parts.

   (5) Degreasing

   (1) Pre-degreasing. The polished parts should be cleaned or scrubbed with organic solvents or surfactants as soon as possible to remove residual polishing paste or oil stains. Commercially available dewaxing water can also be used for pre-degreasing. It has the characteristics of non-toxic, odorless, non-combustible, etc., and can effectively remove residue and oil stains on the surface of the parts. The process conditions: Phosphorus-free washing powder 50g/L~80g/L, temperature 60℃~90℃, sodium carbonate 5g/L~10g/L until clean. If ultrasonic is used together, the concentration of wax removal water can be 10g/L~20g/L, and the temperature should be 50℃~60℃.

   (2) Chemical degreasing. Chemical degreasing is often used to replace solvent degreasing or emulsification degreasing.

   Zinc alloy has strong chemical activity, so it is not suitable to use strong alkali to remove oil, the temperature should not be too high, and the time should not be too long. The chemical degreasing specifications for zinc alloy die castings are listed in Table 5-2-1.

Specification for chemical degreasing of zinc alloy die castings

   (3) Electrochemical degreasing. The electrochemical degreasing solution of zinc alloy must be weakly alkaline, and sodium hydroxide should be used as little as possible or not. Generally, the cathode electrode is used to remove the oil. The degreasing time should not be too long. See Table 5-2-2 for the electro-degreasing process specification for zinc alloy die castings.

Technical specification for electrolytic degreasing of zinc alloy die castings

   The degreasing fluid should be replaced regularly to prevent the replacement of foreign metals to a certain concentration and affect the adhesion of the coating. At the same time, iron-based and copper-based parts should be carried out separately. It is strictly forbidden to use anode electrolysis to remove the oil.

   (6) Weak erosion

   In 1%-2% hydrofluoric acid or 1%-2% sulfuric acid solution, at room temperature, the treatment is 5s-30s. The solution should not be too thick and the etching time should not be too long, otherwise the binding force will be poor. After being etched, clean it with cold water.

   (7) Activation

   In order to ensure the adhesion of the coating, it can be activated according to the following process:

   (1) If copper or brass is pre-plated with cyanide process, it can be activated by immersion in 0.5% to 1% sodium cyanide solution, and then electroplating without cleaning the charged bath.

   (2) If the neutral citrate process is used for pre-plating nickel, it can be activated by immersion in a 3% to 5% citric acid solution, and then electroplating without cleaning the charged tank.

   (8) Pre-plating

   Zinc alloy die castings must be pre-plated with a dense and well-bonded complete metal in a solution that is less corrosive to the parts to ensure the quality of the coating.

   (1) Pre-plated copper. Zinc alloy die castings are generally pre-plated with copper in a cyanide solution. The copper content in the pre-plating copper solution is relatively low, and the free sodium cyanide is relatively high. A high current shock is used to ensure that the deep recesses can be plated with copper and the tank is charged under the current.

   (2) Pre-nickel plating. Nickel can also be chemically immersed.

   (3) Pre-plated brass. Using cyanide pre-plated brass instead of cyanide pre-plated copper can make the adhesion of the coating better. The pre-plated copper layer should not be less than 7 microns. If the copper layer is too thin, it can diffuse into the zinc base to form a zinc-copper alloy intermediate layer and affect the bonding force.

   (4) Chemical nickel immersion. There are two types of chemical immersion (plating) nickel, acidic and alkaline. The alkaline chemical immersion (plating) of nickel for zinc alloy die castings has better effect than acid.

   In order to ensure the reliable combination of the zinc alloy die casting matrix and the coating, a combined pre-plating process of pre-plating copper cyanide first and then pre-plating neutral nickel can be used.

After zinc alloy die castings are pre-plated with a qualified pre-plating layer, the copper or nickel plating layer can be thickened according to the general process, and other metals can be electroplated according to the general process according to the requirements.