ارزيابي الكتروشيميايي رفتار خوردگي و تريبوخوردگي پوشش هاي آمورف و نانو كريستال كبالت- تنگستن توليد شده به روش رسوبدهي الكتروشيميايي

چكيده انگليسي :

Electrochemical evaluation of corrosion and tribocorrosion behaviors of amorphous and nanocrystalline Co W electrodeposited coatings Navid Fathollahzade Masoule n fathollahzade@ma iut ac ir Date of Submission Sep 10 2013 Department of Materials Engineering Isfahan University of Technology Isfahan 84156 83111 Iran Degree M Sc Language Farsi Supervisors K Raeissi Associate Professor k raeissi@cc iut ac ir A Saatchi Emeritus Professor saatchi@cc iut ac ir Abstract Cobal tungsten coatings were electrodeposited on copper substrates using a citrate ammonia bath The structure of resultant coatings were totally amorphous with 48 wt W totally nanocrystalline with 35 wt W and mixed with 40 wt W respectively All coatings had a nodular surface morphology but a microcrack network was also observed in totally amorphous and mixed coatings The cyclic voltammograms of both amorphous and nanocrystalline coatings revealed a low current plateau around the open circuit potential exhibiting a passive behavior In each cycle the redox features of Co were seen but the copper features were only observed for totally amorphous coating Mott Schottky analysis exhibited an n type semiconductivity for both amorphous and nanocrystalline coatings By donor density calculation using the slopes of Mott Schottky curves it was revealed that the passive film of amorphous coating had higher crystal defects Potentiodynamic polarization and electrochemical impedance spectroscopy analyses showed that the amorphous coating had the highest corrosion resistance before and after tribocorrosion experiments Moreover at anodic potential of 0 05 V vs Ag AgCl where a diffusion controlled dissolution process surly occurs the amorphous coating was the most corrosion resistant respect to other coatings The plugging of microcracks by corrosion products were observed at SEM images of the amorphous coating after EIS measurements This could prevent the build up of galvanic microcells and thus eliminate the negative effects of microcracks on corrosion behavior of amorphous coatings In tribocorrosion test at OCP and over the course of sliding the potential of totally amorphous coating shifted to more active potentials due to widening of wear scar and thus increasing in active surface areas The responsible of this observation was the lowest hardness of totally amorphous coating compared to the other ones However during sliding of totally nanocrystalline coatings there was no significant alteration in OCP values On the other hand during sliding of mixed coating OCP values shifted to nobler values owing to the transition of coating structure from amorphous to nanocrystalline caused by exfoliation and approaching to the substrate surface in wear scar In tribocorrosion tests under anodic polarization at the start of sliding a sharp increase in current density readings occurred due to the effect of mass transport MT and coating depassivation phenomena In tribocorrosion tests under cathodic polarization it was found that the ball motion could eliminate the concentration polarization and enhance the rate of hydrogen evolution or water reduction reactions on the surface of coating In all states of tribocorrosion experiments the proportion of MT in the evolution of OCP and current density readings was higher than the proportion of wear accelerated corrosion WAC Hence the process of coating dissolution was considered to be mainly MT controlled process In OCP condition the volume loss of totally amorphous coating was the highest probably due to the lowest hardness A surface fatigue wear mechanism was observed for all coatings due to the formation of superficial microcracks in the vicinity of wear scars Keywords Co W coatings Amorphous Nanocrystalline Corrosion Tribocorrosion Cyclic voltammetry Mott Schottky EIS PDF created with pdfFactory trial version www pdffactory com

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ارزيابي الكتروشيميايي رفتار خوردگي و تريبوخوردگي پوشش هاي آمورف و نانو كريستال كبالت- تنگستن توليد شده به روش رسوبدهي الكتروشيميايي

پوشش كبالت - تنگستن , ولتامتري چرخه اي , موت-شاتكي , طيف نگاري امپدانس الكتروشيميايي