Electrodeposition and metal finishing

Metals are used for many purposes, but they are often susceptible to corrosion (Chapter 16). Protection against corrosion26 brings huge economic benefits. Often protection is done by electrodeposition of a layer of another metal, more inert (and more expensive) on the substrate27, for example on iron and steels. Because of the importance of efficient protection there has been much laboratory investigation into electrodeposition mechanisms, but there are still empirical factors to be explained satisfactorily. 

The mechanism of electrodeposition or electrocrystallization28 29 involves, as a first step, the reduction of a cation on the substrate surface (aided by an applied potential or current) to form an adatom, and its migration over the surface to an energetically favourable site. Other atoms of the electrodeposit aggregate with the first, forming the nucleus of a new phase. The nucleus grows parallel and/or perpendicular to the surface. Clearly, a number of nuclei can form and grow on the surface. When all the electrode surface is covered with at least a monolayer, deposition is on the same metal rather than on a different metal substrate. As is to be expected, the formation of the first layers determines the structure and adhesion of the electrodeposit. 

Qualitatively the process is very similar to the formation of a precipitate in homogeneous solution. The difference is in the structure of the precipitate30, as well as its formation, in homogeneous solution being affected by the degree of supersaturation and in electrodeposition by the overpotential. 

We now consider briefly how nucleus formation on an electrode surface, electrocrystallization, can be studied. Nucleation normally follows a first-order rate law 

We are assuming an equal nucleation energy for all nucleation sites. In reality, the energy is less where there are breaks in structure such as grain boundaries, dislocations, etc. 

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