[Dolphinsiu]

I get stucked in these two questions.

1. Explain the protective action of the anodized coating (on aluminium)?

My attempted answer:

The two major protective actions are corrosion-resistant and abrasion-resistant.

1) Corrosion-resistant
Aluminium is a very base metal and form oxide easily from atmospheric air. However, this form of oxide contribute to only very little extent to corrosion resistance. As the thinkness of anodized coating indicate the degree of anti-corrosion, the thickess of the coating can be enhanced and become more resistant to corrosion by anodization. The thicker the coating, the more resistant to corrosion is the coating as this coating is an insoluble solid and acts as a physical barrier as the interface of air and aluminium to protect the aluminium from further corrosion from H2O and O2.

2) Abrasion-resistant
Aluminium oxide is hard, adhesive and impermeable. As strong electrostatic force is involved in aluminium oxide, high melting point and boiling point is resulted. Thus, it is not easy to be broken and more energy is required for removal of anodized coating.

Do my concept is correct? or need modification? Do anyone have one's own idea?

2. If a drop of copper sulphate solution is left on the surface of a piece of anodized and unanodized aluminium plate speculate the possible outcomes, if any. Explain your speculation.

I cannot answer this question. Too difficult. um...Is it related to the displacement reaction 3Cu2+ + 2Al --> 3Cu + 2Al3+ ? Do anyone have an idea on it? Thank you!

[ARGOS++]

Dear Dolphinsiu,

You are absolutely right; it’s very easy to get stuck with such!
And there is a cause for:
Too many people know too less of this quite wide field of science.
(I’m - still too!)

I will only talking about Aluminium!:
Why is it a wide field?  There are nearly as much different kinds of coatings, - of course all made of Aluminium oxide -, as there are different recipes of the electrolyte in use!
Nearly all of these different coatings have also different features/qualities too!

As they have anyway some basics in common, a lot of your “answers” are correct but some “violate” other parts of your answers.

Correct is, that the natural (unanodized) coating is only of several nanometres, but its enough to protect for a whole life if the coating gets not injured. In case of mechanical injuries, the Aluminium repairs itself. But by mechanic-chemical attacks (Chlorides, Mercury for Example) there is mostly no real help possible anymore.

Some people tell you, that the anodized coating will be amorphous and porous (Wiki) and that such will give you the possibility to colour the coatings later.
As you may know, if it would be true, that the Aluminium would immediately “try” to repair such defects.
More correct is that the coating will follow “Epitaxy” rules, and that such is the cause why there is a strong “bonding” between metal and its oxide.
The “Pores” are in real channels inside the crystal structure and depends very strong with its diameters by the Electrolyte you use. These channels you can later fill with Pigments and close it by an additional process.
Quintessence:   Corrosion-resistant is not direct correlated to the thickness of the coating, but this parameter makes it more robust to mechanical injuries.
The coating has anyway a Mohs hardness of 8-9, that’s why it is very abrasion-resistant (with the epitaxy)! That the oxide is a very good insulator with a high melting point is well known.

Finally, as both coatings are NOT porous and copper sulphate is not a strong etching reagent, nothing should happen with your drop, expect that it may dries up with time.

I hope it may be of help to you!

Good Luck!
                    ARGOS⁺⁺

P.S.: If you prefer I can post a published model of a coating made in weak Sulphuric acid.
.

[Dolphinsiu]

Thank you! But I still have questions.

Porous films occurs if sulphuric acid is used as electrolyte.
But will the answer for Q.2 is different if the anodized aluminium is porous ?

Also, what is 'Epitaxy' rules? (Wiki only mentions there are two types of epitaxy: homoepitaxy and heteroepitaxy, but Wiki seems not mentions what the rule is.)

um..I would you to post the model of coating in weak sulphuric acid as I also will do this experiment on Tuesday.

[ARGOS++]

Dear Dolphinsiu,

You’re welcome!

“Epitaxy” in simple words is, if the underlying substrate (“Wirtskörper”) is defining the rules for who and which parameter the growing of the crystal structure has to follow.
So the prior boundary layer of the Aluminium surface is defining the absolutely regular crystal structure of the growing oxide coating.

That’s why there are REAL, REGULAR FORMAD Channels and not Pores as you can see in the picture! Of course, that the channels are “closed” on their end facing the Al!
Now you will know that the “Formation”/Coating process is not of such simple nature, as many people think, but it’s easy to realize!
That the boundaries of the oxide seen crystal structure will correspond with the identical boundaries of the underlying Aluminium, I have not to tell you.

Even in the presence of the Channels the coating still remains “impermeable” as you told, so your answers are absolutely correct.

In case you use as Electrolyte a 5% g/g Boric Acid instead of weak Sulphuric Acid you can form the coating WITHOUT Channels, as it is required for the of electronically capacitors and their special features/qualities! (I did it myself several times.).
In this case you are not able to go above a thickness for the given “Formation Voltage”.

In the German Picture (“from” a Swiss Aluminium Company “AluSuisse”) the term “GrenzSchicht” means  “boundary layer” containing metal AND oxide simultaneously.
The Diameters are given in Angstroms (= 0.1 Nanometer).

I hope to have been of help to you!

Good Luck!
                    ARGOS⁺⁺

P.S.: If you need a higher resolution picture for your work, please tell me.
.

[Dolphinsiu]

I understand a lot. Thank you very much!
um... I want higher resolution one for work.

You are very clever. Are you a professor in University?

[ARGOS++]

Dear Dolphinsiu,

Thank you very much for so much flowers!,  -
But I’m not a Professor and I’m not at an University.
I’m in Chemical & Software & Consulting Business.

Good Luck!
                    ARGOS⁺⁺

[Dolphinsiu]

Thank you!