Non-Cyanide Silver Electroplating Using DMH As Complexing Agent And Investigation On Electrodeposition Behaviors

Silver is a metal electroplated for wide applications in both decorative and electronic engineering fields. A cyanide plating bath is most often used, but the cyanide compounds have strong toxicity and a large amount of cost is required for securing safe working conditions and waste treatment. With the enhancement of human environmental consciousness day by day, it is urgent to develop alternative non-cyanide silver solutions for the cyanide silver bath.In the present work, 5,5-dimethyl hydantoin non-cyanide silver electrodeposition baths was choosed as the research baths by comparative study on several non-cyanide silver electrodeposition baths. And potassium carbonate was choosed as conductive salt, potassium pyrophosphate as anodic passivation inhibitor and hit903 as additive. Based on the single factor study, the optimal composition formula was obtained by an orthogonal method. The optimal electrolyte composition is: silver nitrate 30g/L, 5,5-dimethyl hydantoin 100g/L, potassium carbonate 80g/L, potassium pyrophosphate 40g/L, hit903 additive 10ml/L. The influences of operation conditions including electrolyte composition, temperature and pH on current efficiency, limited current density, and coating quality were investigated. Higher the plating temperature and silver nitrate content, higher the limited current density. The coatings obtained in optimum plating conditions possess higher Vickers microhardness, better corrosion resistance and welding properties. The adhesion strength between the coated layer and the substrate is very high. The qualities of coatings and bath obtained with DHM techniques are equal to those obtained with cyanide bath.To improve the qualities of silver coatings, pulse plating process was investigated. Based on the optimal composition and technique conditions by the orthogonal test, the preferred pulse plating parameters were determined and they are: the pulse frequency 40%,the duty cycle 3ms,the pulse average current density 0.6A/dm2. The effects of pulse parameters on the surface morphologies and crystal texture of silver coatings were also studied. SEM analysis indicated that bright and fine silver coatings could be obtained at current density 0.6A /dm2. The increases of pulse on time and pulse current density resulted in more compact deposits, however, the deposits are coarse when pulse on time and pulse current density increased further. Excessive shorter pulse off time leads to the concentration of ions in inner pulse diffusion layer decreasing significantly and concentration difference polarization, as a result, the deposits qualities become poor. Excessive longer pulse off time results in the recrystallization of silver deposits and the crystallite size of the coating becomes larger. XRD analysis indicats that pulse parameters have strong influence on the crystal texture of silver deposits. The silver deposits prepared by shorter pulse on time and low pulse current density exhibit growth orientation of crystal face (111) and (220), and the silver coatings prepared by longer pulse on time and high pulse current density exhibit growth orientation of crystal face (111) and (311). And pulse off time has no distinct influence on crystal texture of silver coatings. The quality of silver coating obtained under optimized pulse parameters is better than that of silver coating fabricated with cyanide bath.The mechanism of the reduction of complex ions was put forward and the kinetics parameters of cathodic process were calculated through the investigation of electrodeposition process. The results indicats that the DMH is stable in the range of measurement region and no additional reaction in the reduction process of complex ions. The cathodic deposition process of complex ions is a diffusion-controlled and irreversible process and silver electrodeposition process is also a three-dimensional nucleation process. The addition of potassium pyrophosphate dose not change the cathodic deposition process, but decreases the anodic polarization. The stable species in the bath is AgY2- by calculation from thermodynamical data and the result of electrochemical experiment and a two-step mechanism involving AgY as intermediary was also determined. The kinetics parameters of cathodic process were measured by Tafel and their vales are: i⁰=1.53×10^-2mA/cm²,α=0.3, K=6.72×10^-8cm/s.The influence of the additive on deposition kinetics and mechanism of electrocrystallization process was discussed. The results of Tafel and CV curves showes that the additive decreases the exchange current density values and electrode reaction rate constant, reduces the cathodic current density in the region of the peak and moves the nucleation loop to the negative direction. Chronoamperometric results indicates that the electrodeposition process of silver at GC electrode belongs to a three-dimensional progressive nucleation mechanism and the additive does not change the nucleation mechanism, but inhibits the crystal epitaxial growth. The analysis of nucleation mechanism showes that the additive increases the nucleation rate constant per site and the saturation number density of nuclei, but does not change the number of atoms in the critical nucleus, and the number of atoms in the critical nucleus is 1.