Chemical deposition treatment of magnesium materials and magnesium alloy workpieces

Magnesium alloy materials and artifacts of chemical deposition process is a complicated process, to improve their corrosion resistance and hardness, in this process, the first full surface pretreatment, and then zinc chemical deposition, and chemical deposition of nickel and phosphorus alloy, finally according to the use of other surface treatment, such as corrosion resistance of electroplating nickel, chromium, etc.The process is as follows: polishing → degreasing → pickling → surface activation → chemical zinc dipping → chemical deposition of nickel, phosphorus → electroplating of nickel or chromium.The specific operating conditions and solution formula are shown in the table.

Pretreatment

The chemical deposition of nickel and phosphorus on magnesium alloy is not an easy process. Compared with other chemical deposition, it is more difficult because the potential of the standard electrode of magnesium is -2.36V. If the electroless plating of nickel occurs directly, the replacement reaction will seriously reduce the adhesion of the coating and even lead to the decomposition of the bath.Magnesium and magnesium alloy will oxidize rapidly in air and water, forming non dense oxide film, but it has a great influence on the bonding force of coating.Therefore, the pretreatment of magnesium alloy before nickel is very important, the main procedures include polishing, degreasing, pickling, activation, chemical zinc dipping.

The purpose of polishing is to remove the corroded surface and obtain a smooth surface with luster, which can improve the bonding force between electroless plating and electroplating, and is also beneficial to improve the decoration.Degreasing is to remove the residual grease, dirt and polishing paste on the surface of the polishing, to ensure that the coating is firmly combined with the magnesium matrix, and to obtain good quality coating.Formula 1 in the table is suitable for simple shape of the workpiece, formula 2 is suitable for complex shape, structure of the product.

Activation is to thoroughly clean the oxides on the workpiece, remove the residue and hanging ash on the workpiece after pickling, improve the gloss, formula 1 is better than formula 2.Before activation there is a pickling process to dissolve the thin oxide layer on the workpiece, embedded dirt on the workpiece surface, etc.Formula 1 is a universal acidic and passivating formula with a faster processing speed than Formula 2.Formula 1 is superior to Formula 2 in terms of safety and environmental protection.

The standard electrode potential of magnesium is -2.36V, while that of nickel is -0.25V, which is a big difference.Therefore, direct nickel plating on the surface of magnesium alloy will result in displacement reaction, and lead to the plating solution is unstable, resulting in coating decomposition.So chemical zinc dipping is very key to ensure the smooth completion of electroless nickel plating work.The zinc dipping speed of formula 2 is slower than that of formula 1, but formula 1 needs heating, which costs a little more. Moreover, the zinc layer of formula 2 after dipping is more dense, which is better than that of formula 1, which is conducive to the deposition of nickel in electroless nickel plating.

Electroless nickel plating

The corrosion rate of magnesium alloy in the solution containing SO42- and Cl- is fast, but in order to reduce the production cost, it is necessary to add NISO4 to adjust the process parameters.Selection of zincing formula 2 is a prerequisite for the addition of NISO4.In order to prevent corrosion of magnesium alloy in electroless nickel plating solution containing SO42- by adding complexing agent and adjusting pH value to neutral or slightly alkaline, the replacement reaction can be inhibited to a certain extent.

Properties of electroless nickel plating

The microhardness of electroless nickel plating layer is from 500 to 600HV0.1, which increases to 1000HV 0.1 after heat treatment.According to OCT standard, and according to ASTM B571-79, ISO2819 thermal shock test.The coating did not fall off after 10 times of heating and rapid cooling at 250℃.After decorative nickel or chromium plating, no corrosion spots appeared in the neutral salt spray continuous spraying test for 300h.