Electroless oxidation of metals

While the metal or alloy electroless deposition reactions can be considered as cathodic processes, formation of oxides at the metallic surfaces without an external current source can be analyzed as anodic processes. This type of deposition can be illustrated in the example of chemical oxidation of aluminum in chromic acid solutions.9 

In this oxido-reduction process, aluminum metal is oxidized and the anodic reactions can be represented as  

The standard Gibbs energies and the standard electrode potentials for the reactions (22) and (23) were calculated according to the thermodynamic data available in the literature.10 The cathodic reactions include the reduction of chromium Cr(VI) to Cr(III) and hydrogen evolution, as presented in the following equations  

Based on the reactions (24) and (25), the overall process of oxidation of aluminum with chromic acid solutions can be written as  

4A1+8H++Cr203 - Al203+2A13++Cr203+3H2+H20 (26) with AG° = -2,546.378kJ/mol. 

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