Letter 21063

Reactions in anodizing bath [Bosnia and Herezgovina] 

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I am a tehnologist on anodizing plant. I have worked for six months. I started work with no experience in this area. We have many problems but we have not enough information about all possible reactions in bath specially in anodizing bath (reaction at anode and cathode) please if it is possible send us your answer about questions

Best regards,

Zorka Majstorovic
- Vlasenica, Bosnia and Herezgovina, Republic of Srpska

This is from Electrochemistry Encyclopedia: Summary of anodizing and sealing reactions

The overall reaction that takes place during anodization is:
2Al + 3H2O ==> Al2O3 + 3H2

This is the sum of the separate reactions at each electrode. The reactions at the anode occur at the metal/oxide and oxide/ electrolyte interfaces. The ions that make up the oxide are mobile under the high field conditions.

At the metal/oxide interface the inward moving oxygen anions react with the metal:
2Al + 3O2- ==> Al2O3 + 6e-

At the oxide/electrolyte interface outward moving aluminum cations react with water:
2Al3+ + 3H2O ==> Al2O3 + 6H+
(In case of aluminum dissolution into the electrolyte during porous film formation, the anodic reaction is: 2Al ==> 2Al3+ + 6e-)

The reaction at the cathode is hydrogen gas evolution:
6H+ + 6e- ==> 3H2

The sealing reaction can be written as:
Al2O3 + 3H2O ==> 2AlOOH*H2O

Dado Macapagal
- Toronto

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