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The zincate immersion process for plating aluminium: A kinetic, electrochemical and morphological study

Robertson, Sherryl, G. (1995) The zincate immersion process for plating aluminium: A kinetic, electrochemical and morphological study. PhD thesis, Murdoch University.

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The zincate immersion process is used commercially to treat aluminium prior to electroplating. The aluminium is immersed into an alkaline zincate solution where the oxide layer is dissolved. An intermediate zinc coating is then formed on the surface through displacement. Despite having been used commercially for many decades, the process still experiences a number of problems. The reaction has been studied previously, but no one has yet taken advantage of the fact that the zincate immersion process is a cementation reaction, and a large body of knowledge exists on how to determine the mechanism of these types of reactions. This thesis presents such a study.

In the first part of this thesis, the results from a detailed investigation of the kinetics of the cementation of zinc by aluminium from an alkaline zincate solution are presented. The concentration of zincate ions was found to have a substantial effect on the nature of the reaction. At low zincate concentrations (0.01 M) the reaction was diffusion controlled, while at high concentrations (0.5 M) the reaction was chemically controlled. The rotation rate, sodium hydroxide concentration and temperature were also found to be of significance. The effect of changing the experimental parameters on the reaction could be interpreted using appropriate Evans’ diagrams (i.e. superimposed polarization diagrams for the reduction of zincate and the anodic oxidation of aluminium). Mixed potentials, recorded as a function of time under the various experimental conditions, provided complementary information. The morphology of the zinc deposits formed ranged from being spongy, non-adhesive and non-cohesive to being thin, flat, adhesive and cohesive. Again, the changes in morphology could be rationalized through the use of Evans’ diagrams.

The final part of this thesis presents a detailed study of one of the more common modifications to the zincate immersion process used industrially. This is the addition of ferric chloride, complexed with potassium sodium tartrate, to the zincate solution. The effect of this addition was investigated using the same techniques as above. It was concluded that the ferric chloride acts by oxidizing the edges of the zinc crystals, thereby inhibiting their formation and growth and resulting in the deposition of a more compact coating. Tartrate was found to have no purpose other than as a complexing agent for Fe(III). Alternative additives were explored.

Item Type: Thesis (PhD)
Murdoch Affiliation: School of Physical Sciences, Engineering and Technology
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Ritchie, Ian
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