UBC Theses and Dissertations

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UBC Theses and Dissertations

Electrodeposition and corrosion study of nanocrystalline cobalt and cobalt-iron alloy coatings Nik Mohd Masdek, Nik Rozlin


Nanocrystalline materials with grain sizes less than 100 nm, have attracted considerable attention due to their enhanced properties as compared to their polycrystalline counterparts. However, investigation on the corrosion resistance of these materials is still lacking. In order to further expand their future applications, their corrosion behaviour is of great importance. Hence, in this study, nanocrystalline Co and CoFe alloy coatings were prepared through the electrodeposition process and their electrochemical corrosion behaviour were investigated. Depending on the environment, the effect of the nanocrystaline grain sizes as well as Fe alloying resulted in different corrosion responses. A decrease in grain size was observed with an increase in iron concentration that also leads to a change in crystal structure from HCP to BCC phase structure. Meanwhile, microhardness was first seen to increase gradually with the increase in iron content before it decreases with higher iron concentration of more than 18 wt%Fe. The corrosion properties of these electrodeposited nanocrystalline Co and CoFe alloy coatings were also studied in solutions ranging from acidic to alkaline. Nanocrystalline Co and CoFe exhibit higher corrosion rates in 0.1 M H₂SO₄ than in an alkaline environment. In deaerated acidic solutions, both nanocrystalline Co and CoFe alloy coatings showed only active anodic dissolution without any transition to passivation. Meanwhile, two stages of passivation were present in the alkaline solution which is reported to be due to the formation of a duplex passive film. The presence and absence of saccharin as an additive in the electrolyte both showed detrimental and beneficial effect on the corrosion behaviour of these nanostructured deposited coatings. By employing the electrochemical quartz crystal microbalance (EQCM), it was observed that the mass decrease was significant with an increase in saccharin concentration indicating an active dissolution. However, saccharin was observed to hinder the formation of a protective passive film on CoFe alloy coatings in alkaline solution. The presence of the sulphide ion also significantly decreased the corrosion resistance of deposited CoFe alloy coatings. The presence of sulphide accelerates markedly the anodic reaction. The dissolution rate was two times faster when S²− was added.

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