Dissolution Mechanism of Metal Substrate into the Electrodeposited Film

Abstract

In cathodic electrodeposition process, it is said that the dissolution of metal substrate into the electrodeposited film does not occur. However, there are the reports that metal ion of substrate exists in the film, and hypothesis of following cathodic reaction is proposed :
M + 4 OH^-→ [MO₂]^n- + 2 H₂O + (4-n)e
In order to elucidate dissolution mechanism of metal substrate, authors had studied both metal ion contents of the electrodeposited film with the added electric current and the amount of dissolved metal of substrate when it is dipped in the electrodeposition bath without current.
In this study, the authors used the iron and zinc sheet as substrates. Metal ion concentration in the electrodeposited film is measured by the atomic absorption spectrometer.
The amount of metal ion contained in the film was calculated by the following equation :
M=m×c
where M : amount of metal ion
m : metal ion concentration in the electrodeposited film
c : nonvolatile matter of the electrodeposited film
On the other hand, weight change of substrate after electrodeposition is measured by removing the electrodeposited film with solvent, and weight change of substrate after dipping in electrodeposition bath without current is also measured.
As the result of this study, following facts were found.
1. In case of the zinc substrate, the anodic dissolution amount of zinc by anodic electrodeposition reached to one half of electrochemical equivalent of zinc.
On the other hand, zinc is also found in cationic electrodeposited film. It is not dissolved by the added current but dissolved by acidity of cationic electrodeposition bath
2. In case of the iron substrate, iron does not dissolve into the film in both anionic and cationic electrodeposition paint which are maked on the market now.