Research On The Mechanism Of Bath Reactions And Interface Reactions Between Electroless Nickel Bath And Magnesium Alloys

Magnesium alloys have many desirable features, including low density, highthermal conductivity, high strength to weight, good electromagnetic shieldingcharacteristics, etc., which make it valuable in a number of structural applicationsincluding aerospace, electronics, computer parts and automobile fields. However, therelatively poor abrasion and corrosion resistance of magnesium alloy limit itsextensive utilization. Electroless nickel （EN） deposition is a viable option to improvethe wear and corrosion characteristics of Mg alloy, but challenges exist in thedeposition process due to the corrosion of Mg in the EN bath. Moreover, mostresearchers are committed to improve the corrosion resistance of the coatings whichdeposited in a once-plating bath only, rather than in cycles. On one hand, this paperstudied the characteristics of coatings, interfacial reactions between the magnesiumalloys and the EN bath; on the other hand, a more systematic work about the changesof plating bath in the plating processes was also studied, and thus an EN bath formulawith high stability and good buffering capacity was obtained.By using scanning electron microscopy （SEM）, energy dispersive X-rayspectroscopy （EDX）, X-ray diffraction （XRD）, atomic force microscopy （AFM）, et al,the surface morphologies and microstructure of the coatings obtained by variouspretreatments and in different plating baths were characterized. The deposition rate,deposition potential （Edep） and deposition current （idep） in different pH value,temperature, and concentration ranges of compositions of EN bath were determinedby electrochemical techniques. The main research results are as follows:1. Influences of three different pickling and activation processes on direct ENplating on Mg alloys have been studied in order to obtain an environment-friendlyprocess of pretreatment. A chromium-free pretreatment has been finally establishedby the procedure of pickling with （HNO₃+H₃PO₄）, twice activations with K₄P₂O₇andNH₄HF₂. It is found that a coarse surface and proper F/O ratio were produced on theMg substrate via the new pickling-activation process. The coatings coated by this newprocess exhibit excellent adhesion and corrosion resistance. The results also foundthat two important types of chemical reactions in the initial deposition on Mgsubstrate were occurred. They are:（1） replacement reaction between the magnesiumsubstrate and nickel ions;（2） autocatalytic deposition reaction of nickel itself. 2. The effects of ammonium sulfate （AS） in bath on the deposition rate,efficiency of hypophosphite, bath stability, and characteristics of EN deposits werealso studied. The results show that the rate of EN can be improved by adding AS whenits concentration is lower than12g·dm^-3. The bath stability test indicates thatstability of the EN bath can be significantly reduced by a low concentration of AS,whereas little impact on bath stability was observed with an increased ASconcentration. The results also indicate that a low accumulation of AS in the platingbath did not adversely affect the adhesion and corrosion resistance of the coatings. Anoptimum concentration of AS can decrease grain size, refine microstructure, andimprove corrosion resistance. However, a very concentrated AS (>16g dm^-3) platingbath may increase stress, hence creating cracked cross-section morphologies.3. The effects of complexing agents on the bath life, plating rate and coatingquality were studied in detail. The results show that the changes of pH value not onlychanged the nature of the bath solution, but also had a strong impact on the P content,microstructure and electrochemical properties of the EN coatings. The bufferingcapacity of the EN bath can be improved obviously with the addition of optimumconcentration of acetate ammonium, and thus the pH value and bath stability wouldnot be changing quickly in MTO cycles, the operational life can be extended from4MTO to9MTO.4. In order for obtaining more kinetics information of the EN plating processes,the polarization behavior of EN plating in different baths was investigated withpolarization curves. Influence rules of every process parameters on the polarizationcurves and EN deposition in a complete bath were also described. From thesevariations in the deposition potential （Edep） and current density （idep）, a kineticexpression employing the Butler-Volmer equation and mixed potential theory was setup and was in good agreement with the experimental findings. It was confirmed thatthe EN deposition process in the present researches was under a mixed control.Electrochemical impedance spectroscopy （EIS） was proposed to measuring the agingability of the EN bath. With prolonged aging time, the charge transfer resistance （Rd）of samples increased obviously, the deposition rate decreased, and the surfacemorphology of the coatings observed became coarser. These results show that theelectrochemical measurement may be effectively in monitoring EN deposition rateand characteristics of EN bath in the future.5. Lastly, the dynamical equation of EN plating on Mg alloy in an acidic bath wasderived by studying the effects of concentrations of metal salt, reducing agent, pH value, and temperature on the deposition rate. The equation shows that the depositionrate increased with increasing temperature, concentration of H－2PO2, pH value （4⁶）,and decreased with increasing concentration of complexing agents. For nickel ions,the rate increased gradually with increasing concentration (x, g·dm^-3) when x<4.69.However, it decreased when the concentration exceeded4.69. The apparent activationenergy （Ea） calculated from the Arrhenius equation is approximately38.03kJ·mol^-1,this could explain the reason why the deposition reaction did take place readily.The innovations in the dissertation as bellow:（1） An environmentally friendlypickling and activation process with chromimu-free and only low hydrogen fluoridewas exploited;（2） The effects of accumulation of sulfate ion on the plating bath anddeposits were studied by simulate method;（3） The influences of pH value changes inthe deposition process on the characteristics of the coatings were analyzed, and aplating bath with an optimum concentration of pH value buffering agent wasascertained;（4）Electrochemical behavior of EN plating in different baths was studiedby polarization curves and EIS methods. A monitoring technique may be developedand applied to industrial production lines based on this new measurement;（5） Anempirical deposition rate equation for EN on Mg alloys in an acidic bath was obtained.This is favorable for better control over the quality of EN coatings by varying thetemperature, pH value and composition concentrations of the plating bath in thedeposition process.