A few questions about anodization

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In summary, anodization of aluminium involves using an aluminium object as the anode and dilute sulfuric acid as the electrolyte, with a carbon rod/pipe/plate as the cathode. When a voltage of 12 volts DC is applied, anodization occurs and forms a layer of aluminum oxide on the surface and within the metal. This layer can be stripped off with caustic soda and the process must be done correctly the first time for the coat to last indefinitely. The order of oxidation is determined by comparing standard potentials, but in real solutions, formal potentials may be different and can result in a reversal of the oxidation/reduction order.
  • #1
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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!
 
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  • #2
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.
 
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  • #3
but how about the priority of discharging.i mean why hydroxide ions is discharged first,instead of Al
Thank you!
 
  • #4
Are you deciding on the order of the oxidation by comparing standard potentials?
 
  • #5
Borek said:
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.
 
  • #6
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.
 
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1. What is anodization?

Anodization is an electrochemical process that forms a protective oxide layer on the surface of a metal, typically aluminum. This layer provides increased corrosion and wear resistance, as well as aesthetic benefits.

2. How is anodization achieved?

Anodization is achieved by immersing the metal in an electrolyte solution and passing an electric current through it. This causes oxygen ions to bond with the metal, forming a layer of aluminum oxide on the surface.

3. What are the benefits of anodization?

Anodization provides several benefits, including increased corrosion and wear resistance, improved durability, and enhanced decorative options through the ability to dye the oxide layer. It also maintains the conductivity of the metal.

4. What are the different types of anodization?

There are three main types of anodization: Type I, Type II, and Type III. Type I is a thin anodized layer used for decorative purposes. Type II creates a thicker layer and is commonly used for corrosion and wear resistance. Type III, also known as hard anodizing, produces an even thicker and harder layer for increased durability.

5. What are the limitations of anodization?

Anodization is only effective on certain metals, with aluminum being the most common. It is also limited in terms of the colors that can be achieved, as well as the size and shape of the object being anodized. Additionally, anodized layers can be scratched or damaged if not properly cared for.

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