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چكيده انگليسي :

83 Effect of Additives on the Structure and Properties of Electrodeposited Zinc Coatings Shohreh khorsand s khorsand@ma iut ac ir 2011 Department of Materials Engineering Isfahan University of Technology Isfahan 84156 83111 Iran Degree M Sc Language FarsiSupervisors M A Golozar Prof golozar@cc iut ac irK Raeissi Assoc Prof k raeissi@cc iut ac irAbstractZinc coatings were electrodeposited on steel substrate from an acidic sulphate bath The effect of oxalateanions and 1 2 diaminopropane as chelating agents on nucleation and growth mechanism surfacemorphology predominant texture and corrosion behaviour of zinc electrodeposits was investigated Evaluation of nucleation mode of zinc in the presence and absence of oxalate anions showed that theinstantaneous nucleation is predominant in both cases pH 2 From the analysis of the chronoamperometrictransients and the AFM images the nuclei density has been calculated The values obtained from AFMimages are one order of magnitude higher than those obtained by analysis However a less number of nucleiare formed on steel substrate in the presence of oxalate anions Electrochemical impedance spectroscopyrevealed that in the bath containing oxalate anions the growth of deposit is semi infinite diffusion controlled While in the absence of oxalate anions the charge transfer control was governed In the absence of oxalateanions the texture components of zinc coating were mostly basal and low angle pyramidal planes whereas inthe presence of oxalate anions the high angle pyramidal and prism planes were dominant Furthermore oxalate anions changed the zinc surface morphology from packets of hexagonal platelets to a granular nature It was observed that the corrosion resistance of zinc electrodeposits is related to the crystallographic planesexposed to aggressive solution and not only the planes considered as texture components The findingshighlight the role of surface morphology of the coatings on exchange current density of water reduction andits subsequent effect on the corrosion resistance In the presence of oxalate anions the zinc coating producedhad a better corrosion protective property than that deposited from bath without oxalate anions The 1 2diamionpropane greatly affected the zinc deposits and its effects can be attributed to its retarding effect ofnucleation and growth The voltammetric analysis has shown that the presence of 1 2 diaminopropane in thesulphate bath shifts the reduction potential of Zn II to more negative values Analysis ofchronoamperograms obtained in the absence and presence of 1 2 diaminopropane indicates that theprogressive nucleation is predominant in both cases pH 5 Electrochemical impedance spectroscopyrevealed that in the bath containing 1 2 diaminopropane the surface diffusion of zinc adions should beconsidered slower than in the absence of1 2 diaminopropane This additive produces a uniform andhomogenous zinc deposit along with the reduction in zinc crystallite size By inhibiting the growth of 00 2 planes 1 2 diaminopropane affected the texture of zinc deposit too The corrosion resistance of zincdeposits in 5 NaCl solution was improved at the presence of 1 2 diaminopropane in the electrodeposionbath KeywordsZinc electrodeposition Nucleation Growth Oxalate anions AFM 1 2 diaminopropane

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