The Research On The Corrosion Of Plating Bath And The Processes Of Electroless Plating Nickel And Electroplating Zinc On Magnesium Alloy

Magnesium alloys have a number of desirable features, including low density, high strength-to-weight ratio, excellent castability and damping characteristics. So they have good social and economic benefits and they are extensively applied in many fields such as aerospace, automobile, mechanical and electronic components etc. However, poor corrosion resistance has often baffled it from further exploitation. The main cause is that the second phase or impurity of alloys brings to couple corrosion. Moreover, the stable and compact hydrate film can not be formed on the surface of magnesium alloys and corrosion is easy to take place. The processes of elctroless plating nickel or electroplating zinc are investigated to protect the substrate of magnesium alloys in our experiment.In the experiments, the metallographic microscope, SEM and EDX were applied to observe and determine the surface appearance and component of the plating; 3% NaCl solution test was used to asses the corrosion resistance of the plating. The score test and thermal shock test were applied to determine the adhesion of plating and PdCl₂ test was used to determine the stability of plating solution.The key step of pretreatment process— immersion zinc was improved, the corrosion of electroless nickel plating bath was systemically studied and the bath coposition was modified by adding muliti-complexing reagents. At the same time, electroplating zinc on magnesium alloy was also studied. The research results shows that:1. The process of immersion zinc was improved. Double immersiom is better than once immersion zinc and the zinc alloy film is better than pure zinc film.2. There is evident corrosion in the sulfate bath of electroless nickel plating to magnesium alloy at low temperature, SO₄^2- does not derectly corrode MgF₂ film on magnesium alloy. The main cause is that MgF₂ film is incompact and Ni²⁺ can accelerate the corrosion of substrate of magnesium alloy. At the plating temperature, the surface of magnesium alloy will be passivated because of increasing reactivity of F at elevated temperature and the MgF₂ film is more compact.3. The performances of electroless nickel plating bath are evidently improved by applying three complexing reagents, which are complexing reagent I as main complexing reagent, complexing reagent A and complexing reagent B as assistant complexing reagents. The good plating is obtained in the electroless bath. Therefore,...