Research On The Process And Mechanism Of Electrodeposition Of Tin–cobalt Alloy

Tin–cobalt alloy has found wide application as alternative to decorative chromium,gun black coatings,connector and lead-free plain bearing,and it has been considered as a promising alloy.Although the electrodeposit ion of tin–cobalt alloy has been achieved from strongly acidic fluoride bath,weakly acidic gluconate bath and alkaline pyrophosphate bath,so far,there is no plating bath that can meet the requirements of both coating performance and future development of environmental protection.From the point of view of environmental protection and satisfying the coating performance,the electrodeposit ion of tin–cobalt alloy from strongly acidic and weakly acidic bath was studied in this thesis,the deposit ion behavior and mechanism of metal ions with respect to the deposition process were further investigated by electrochemical method.Geometry optimization and frequency analysis of the stannous ion and methanesulfonic acid complex were performed by employing the Gaussian 09 sofeware with the B3 LYP method.The interaction between methanesulfonic acid and stannous ions and the bonding characteristics of Sn–O bond were analyzed by calculation with the Multiwfn software together with analysis in the framework of AIM(atoms in molecules),ELF(electron localization function)and charge decomposit ion analysis(CDA).Coordination interaction between methanesulfonic acid and stannous ion was verified.Cyclic voltammetry and Chronoamperometry were employed to investigate the nucleation and growth mechanism of stannous and cobalt ions on glassy carbon electrode from the methanesulfonic acid electrolyte.The results show that stannous ion deposits on glassy carbon electrode via the nucleation and growth process,and it follows the mechanism of progressive nucleation with diffusion controlled growth.Variation of the deposit ion potential,concentration of stannous ion,temperature,p H and additives don’t change the nucleation and growth mechanism of stannous ion.In methanesulfo nic acid electrolyte,the nucleation and growth process of cobalt on glassy carbon electrode is accompanied by the hydrogen evolution side reaction.At high cobalt ion concentration,the nucleation and growth mechanism of cobalt on glassy carbon electrode is hardly affected by p H,temperature and addit ives,and always follows the instantaneous nucleation with kinetic controlled growth mechanism.At low cobalt ion concentration,the mechanism changes with variation of p H: it follows the mechanism of progressive nucleation with diffusion controlled growth at p H=2.0 while it follows the mechanism of instantaneous nucleation with kinetic controlled growth at p H=1.5,and at p H=1.0,it changes to the mechanism of progressive nucleation with kinetic controlled growth.A strongly acidic methanesulfonic acid based tin–cobalt alloy plating bath with EPE2900 as addit ive was developed,uniform and compact tin–cobalt allo y coating could be obtained at p H=2.0.A stable weakly acidic tin–cobalt allo y plating bath with potassium citrate and sodium gluconate as complexing agents was developed,such bath has high stabilit y and stannous ion is hardly to be hydrolyzed and oxidized.Bright tin–cobalt alloy coating can be obtained in the weakly acidic bath,and tin–cobalt alloy plat ing from the bath has advantages of wide current densit y range and stable coating composition.The optimized bath composit ion and operating condit ions were: 0.1 mol/L Sn(CH3SO3)2,0.2 mol/L potassium citrate,0.4 mol/L Co(CH3SO3)2,0.4 mol/L sodium gluconate,0.5 mol/L H3BO3,2 g/L sodium saccharin,p H=5.0,room temperature,no stirring,and current densit y of 1 A/dm2.Cyclic voltammetry was employed to study the voltammetric behavior of metal ions in the weakly acidic plating bath,the results indicate that the deposition of tin and cobalt on Pt electrode has lower overpotetial than on glassy caobon electrode,tin layer deposited on platinum electrode dissolve s completely in the anodic scanning,while the tin layer deposited on glassy carbon electrode cannot be completely dissolved.There is no significant difference in the dissolution behavior of cobalt layer deposited on the two electrodes.Tin–cobalt alloy coating is almost insoluble in anodic scanning,indicating a high chemical stabilit y of the alloy coating.The decomposition of the steady state polarization curve on barss substrate shows that the deposition of stannous ion from the alloy bath is diffusion controlled,and cobalt ion hinders the diffusion of stannous ion from bulk solution to the cathode surface,while convection result ing from hydrogen gas promotes the mass transfer of stannous ion.The deposit ion of cobalt ion from alloy plating bath is hardly affected by stannous ions and hydrogen evolution.Cobalt content in alloy coating is stable at j > 1 A/dm2,this phenomenon results from that hydrogen evolution promotes the deposit ion of tin.The optimal deposition potential was determined to be-1.0 V.In the bath with potassium citrate and sodium gluconate as complexing agent,the nucleation and growth mechanism of stannous ion on glassy carbon electrode is mainly affected by potassium citrate,and sodium gluconate has no significant influence.With the negative shift of deposition potential,the nucleation type of stannous ion changes from progressive nucleation to instantaneous nucleation.The nucleation and growth of cobalt ion on glassy carbon electrode follows the mechanism of progressive nucleation and is hardly affected by deposit ion potential.For the codeposit ion of stannous ion and cobalt ion,their nucleation and growth follows the mechanism of instantaneous nucleation with diffusion controlled growth and is hardly affected by deposition potential.