| Wide applications of magnesium alloys in automobile, aerospace, electroniccommunication and other fields are due to their high strength-to-weight ratio, lowdensity, high damping, good castability and machinability. However, high chemicalreactivity, poor corrosion resistance and abrasion resistance of magnesium alloy limittheir development and applications. So far, all magnesium alloys must chooseappropriate method of surface treatment in order to improve their properties. Varioustypes of method can be applied to increase corrosion resistance of coatigns, butelectroless nickel (EN) coatings are superior. EN coatings show their excellentperformance increasingly mainly owing to the advantages such as wear and corrosionresistance, adaptability for uniform coatings on complex shapes. The Ni-P alloycoatings obtained from bath using sodium hypophosphite as reducing agent have beenwidely used in electronics, machinery, automobile, aerospace and valve industriesbecause of their high hardness, coatings uniformity, as well as high wear andcorrosion resistance. Currently, electroless nickel plating of poor bath life andstability are the main problems they face. Therefore, how to improve bath life andstability has become an urgent issue.The deposition efficiency of nickel and reduction efficiency of sodiumhypophosphite are important specifications for measuring bath properties and play animportant role in bath life and stability. However, the entire deposition efficiency ofnickel and full reduction efficiency of sodium hypophosphite in the plating bath isimpossible due to the side reactions. The higher deposition efficiency of nickel andreduction efficiency of sodium hypophosphite, the higher the bath stability it is. Thusit can improve bath life and stability, avoid massive emissions of harmful ions in thebath, protect the environment and conserve resources.In this paper, we focus on effect of deposition efficiency of nickel and reductionefficiency of sodium hypophosphite in the plating bath in order to provide data foreffective control on bath properties and coatings quality. The experimental results areas follows:1. When the concentration of sodium hypophosphite in the bath is at25-30g·dm3, plating rate, deposition efficiency of nickel, reduction efficiency of sodiumhypophosphite, coating phosphorus content are high and coating coverage, adhesionand corrosion resistance are good. 2. Only when the temperature exceeds343K, the nickel starts deposition on thesubstrates. Normal plating temperature of electroless nickel plating is at353-363K.Adding ammonium acetate can maintain bath pH stable. pH of acid bath is controlledat4.5-5.5.3. Plating rate, deposition efficiency of nickel, reduction efficiency of sodiumhypophosphite and bath pH decline with MTO increasing. Accumulation of HPO32,Na+and SO42ions in electroless nickel plating process are the main factors leading tothe failure of the bath. Adding the amount of components and complexing agents,filtration on time and adjusting bath pH can increase MTO. |
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