A few questions about anodization

[asdff529]

In anodization of aluminium,the aluminium object to be oxidized is the anode,the electrolyte is dilute sulfuric acid the and the aluminium is the cathode
I was taught that the hydroxide ions will preferentially discharge,then the aluminium object will be oxidized by the oxygen formed(?),but i was also taught that according to the electrochemical series,Al should be oxidized first,when does it happen?
Also,if the object is already fully covered by an oxide layer,how can the oxide layer further increase?I mean the inner aluminium part doesn't have and contact with the solution or whatever,how can a reaction occur?
Thank you!

 

[dacarls]

When a voltage of about 12 volts DC of course and a bunch of amps (25 A) is applied to raw aluminum suspended in 15% sulfuric acid bath, with a large carbon rod/pipe/plate as the cathode, anodization occurs. Aluminum oxide forms at the surface and continues down into the metal, forming crystalline aluminum oxide around pores that contain the sulfuric acid, and continue to conduct electricity to the metal. These can grow down into the metal for quite a way, until the hard layer is quite think (in microns).
When the aluminum is removed and washed, you can't anodize it again without stripping off the existing oxide coat, usually physically or with caustic soda (NaOH). This is messy. Do it right the first time and your aluminum anodized coat will last indefinitely.

 

[asdff529]

but how about the priority of discharging.i mean why hydroxide ions is discharged first,instead of Al
Thank you!

 

[Borek]

Are you deciding on the order of the oxidation by comparing standard potentials?

 

[asdff529]

Are you deciding on the order of the oxidation by comparing standard potentials?

um...i think so because i haven't been taught about the standard potential.i was just told to memorize the order of the oxidation,without understanding it.

 

[Borek]

I am afraid that's not enough. Standard potentials reflect what is happening in standard conditions, that is, when activities of all involved substances equal 1. In the real solution we deal with so called formal potentials - they can be calculated from the Nernst equation (with standard potential being just a starting point), and they are often substantially different from the standard ones, to the point when the order of oxidation/reduction is reversed.