Electrodeposition Of Zinc-based Compositionally Modulated Multilayer Films And The Investigations On Their Corrosion Behaviors

A new type of coating systems so-called compositionally modulated multilayerflims (CMMF) has been gradually gaining interest amongst researchers, because theselayered-structure coatings possess improved properties or novel phenomenon such asincreased mechanical strength, micro-hardness, giant magnetoresistance and corrosionresistance. CMMF consists of a large number of thin laminar deposits of metal or alloylayers, and each layer has its own distinctive role in achieving preferred performances.During last decades, the electrodeposition of zinc-based CMMF coatings for protectionof steel substrate from corrosion has been briefly investigated. To date, however,relatively few reports have given any evidences to "back-up" the enhanced corrosionresistance afforded by CMMF coatings. Therefore, further investigations are required toexamine the efficacy of this type of coatings on steel substrates and more convenientmethod are needed for the electrodeposition of zinc alloy CMMF coatings by usingsingle bath technique. The aim of the present work was to investigate the possibility ofelectrodepositing zinc-based CMMFs on steel substrate from the optimized electrolytesusing dual bath technique (DBT) and single bath technique (SBT), assess their corrosionperformance using mainly scanning electron microscopy (SEM), salt spray test,corrosion potential measurement and anodic polarization methods, and examine thecorrosion mechanism. The main results of the dissertation are as follows:1) Zinc and nickel CMMF coatings were produced by successive deposition from arevised zinc sulphate bath and a new developed nickel bath containing nickel-ammoniacomplexed electrolyte. The nickel bath has been particularly effective for the productionof zinc and nickel CMMF due to the low stress in the nickel deposits and less possibilityof a displacement reaction taking place with zinc surface due to the use of weaklyalkaline nickel bath. The surface and cross-sectional morphologies of zinc-nickelCMMF samples were examined using scanning electron microscopy (SEM).Zinc-nickel CMM coatings with uniform surface appearance could be obtained from these dual baths. A silvery grey colour was obtained with zinc as top layer and dullfinish with nickel as the top layer. Cross-sectional morphologies showed the layeredstructure was clear and no micro-cracks could be seen in the nickel sublayers. Corrosionresistance evaluated by means of neutral salt spray test showed that zinc-nickel CMMFcoatings were more corrosion-resistant than the monolithic coating of zinc or nickel ofsimilar thickness. Compared to the Zn/Ni CMMF coatings, Ni/Zn CMMF coatings withnickel sublayer adjacent to the steel substrate were more corrosion resistant, becausethere still was some nickel deposits remaining on the surface of substrate after corrosiontest, which would provide, to some extent, protection for steel. Results obtained fromcorrosion potential measurement and anodic polarisation suggest that pores and perhapsmicro-cracks existed in nickel sublayer which played an important role in achievingimproved corrosion resistance for zinc-nickel CMM coatings. Based on results fromelectrochemical measurement and the micrographic characteristics of zinc-nickelCMMF coatings after corrosion testing, a probable corrosion mechanism of zinc-nickelCMM coatings was proposed, the protection efficiency of zinc-nickel CMMF coatingsmay be thought to depend on the barrier effect of nickel sublayers and the sacrificialeffect of zinc sublayers. In reality, it may rely on producing the optimum protectionproperties of nickel sublayers.2) Prior to the production of Zinc and Zn-Ni alloy CMMF, the Zn-Ni alloy platingwas investigated to understand the effect of constituent and plating parameters of sulfateelectrolyte on the nickel content and surface morphologies of Zn-Ni alloy coatings. Thezinc and Zn-Ni alloy CMMF was electrodeposited from a revised zinc bath and theoptimized Zn-Ni alloy bath. Coated samples were evaluated in terms of surfaceappearance, surface and cross-sectional morphologies, as well as corrosion resistance.Microstructural characteristics examined using field emission gun scanning electronmicroscopy (FEGSEM), confirmed the layered structure, grain refinement of the zincand Zn-Ni alloy CMMF coatings, and revealed the existence of micro-cracks resultingfrom the internal stress in the thick Zn-Ni alloy sublayers. The internal stress of Zn-Nisublayer could be reduced to some extent by the electrodeposition of zinc sublayer. Thecorrosion resistance evaluated by means of salt spray test showed that the zinc and Zn-Ni alloy CMMF comings were more corrosion-resistant than the monolithic coatingsof zinc or Zn-Ni alloy alone of the same total thickness, and the Zn-Ni/Zn CMMFcoating system was more corrosion resistant than the Zn/Zn-Ni alloy system with asimilar configuration. Based on the analysis on the micrographic features of zinc andZn-Ni alloy CMMF coatings after corrosion test, the remaining coating material ofZn-Ni alloy deposit scattered on the steel substrate was the possible reasons why theZn-Ni/Zn CMMF coatings have a better protective performance.3) The electrodeposition of Zn-Co alloy was carried out using direct current (DC)electrodeposition method. Primary investigations indicated that it is difficult to getZn-Co alloy with high cobalt content because of the anomalous codeposition propertiesof Zn-Co alloy. The application of pulse current (PC) instead of DC was a suitableapproach to achieve Zn-Co alloys with a very wide range of alloy compositions andproperties by simply varying the applied pulse parameters. The results obtained fromthe investigation on the effect of pulse parameters on the cobalt content in Zn-Co alloydeposits showed that the cobalt content in the deposits depends mainly on the averagecurrent density and the value of the reverse pulse fraction. It is possible to electrodepositZn-Co alloy coatings with a very wide cobalt content range of 10-90 wt% bymodulating pulse parameters. The surface morphologies of Zn-Co alloy deposits wereexamined using scanning electron microscopy (SEM), and an attendant energydispersive X-ray analyser (EDA) was used to analyse the composition of Zn-Co alloydeposits. Grain size, surface appearance and internal stress in the deposits were alsoimproved significantly by introducing reverse current. Fine grain, more compact crystalstructures and crack-free Zn-Co alloy CMM coatings could be obtained by adjustingpulse plating parameters. By referring to the relationship of cobalt content and pulseparameters, a series of Zn-Co alloy CMMF coatings were produced using acomputer-aided pulse plater unit. The surface appearance, surface and cross-sectionalmorphologies of the Zn-Co alloy CMM coatings examined using FEGSEM, showedthat the production of Zn-Co CMM coatings is possible from a single bath bymodulating pulse current.4) The corrosion mechanism of CMMF was investigated using neutral salt spray corrosion test, corrosion potential measurement, anodic polarisation measurement, andmicrograph examination method. The probable reasons for the zinc-based CMMFcoatings to have much better protective performance were attributed to the sacrificialeffect of zinc sublayers and the barrier effect of noble metal sublayers, as well ascorporatively beneficial interaction.