Electrodeposition Of Nickel On Magnesium Alloy Surface

Magnesium alloys have many unique advantages; however the application of magnesium alloys in many situations is restricted by its poor corrosion resistance. At present, the main ways to improve the corrosion resistance of magnesium alloys include surface treatment such as chemical conversion coating, anodizing oxidation, organic coating, electroplating and electroless plating. Electroplating on magnesium is one of the technics with good prospects because electroplating can enhance corrosion resistance, wear resistance, solderability, conductivity and improve artistic appearance of magnesium alloys.The characteristic and application of magnesium alloy as well as the recent progress in metal protection coating and the modern technics of nickel electrodeposition on magnesium alloy are reviewed. A technics of nickel electroplating on magnesium alloys is developed. Cyanide copper plating on magnesium can be substituted by nickel preplating in this dissertation. The effect of pretreatment, composition and operating conditions of nickel solution on coating quality has been investigated. Formation mechanism and growth process of nickel electroplating coating on magnesium alloy has been discussed. The nickel coating with small grain size is obtained by adding organic additive and the effect of various technological parameters on the size of the nickel grains has been studied. The research results of the experiments show that:1. The optimum pretreatment process is: ultrasonic degreasing→alkaline rinse→acid pickling→alkaline activation→zinc immersion. The acid pickling is conducted under room temperature and for 0.5-3.5min. Activation is conducted in a solution described as follows: K₄P₂O₇: 200g·dm^-3; KF: 5g·dm^-3; Na₂CO₃:60g·dm^-3; activation time: 2min; temperature: 75℃; and the addition of wetting agent is a preferred. Zinc immersion is conducted in a solution with immersion time for 1～2min at the operating temperature of 65℃.2. Nickel solution with low concentrated ammonium complexes is selected for preplating on magnesium alloys. The nickel preplating is conducted in a solution with the following composition and operating conditions: NiSO4: 50～150g·dm^-3; additive A: 25～55g·dm^-3; additive B: 2.5～5g·dm^-3; pH: 4.5～5.5; temperature: 45～55℃; current density: 1～10A·dm^-2 and operating time: 5～10 min. Brightness range of said solution is 1～15 A·dm^-2, Current efficiency of solution is about 90% when cathodic current density is 5 A·dm^-2,and throwing power and covering power of this solution is 39.5% and 100%, respectively. Nickel coating obtained from this solution is compactness with high hardness and showed good adhesion and excellent corrosion resistance.3. Magnesium alloy corroded severely in nickel preplating solution, additive A and additive B can decrease corrosion of magnesium alloys.4. The electrocrystallization of nickel on magnesium alloy is described by the mechanism of continuous nucleation. With the increase of deposition overpotential,the crystal nucleus dots will be raised rapidly. Immersion zinc layer can increase the adhesion between nickel layer and substrate of magnesium alloy. It is also protected by magnesium alloy from corrosion and reduces the speed of replacing reaction of nickel ion in the solution. Cathodic deposition potential of nickel should be controlled to less than corrosion potential, preferably from -1.0V to -1.3V.5. Saccharin, Sodiumbenzenesulfnate and p-toluene sulfonamide can be used as microlite agents in nickel electroplating. More exiguous grains of nickel will be obtained through synergistic reaction of several additives.