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Author Topic: Acid corrosion of epoxy coating  (Read 3708 times)

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hugh111111

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Acid corrosion of epoxy coating
« on: January 16, 2011, 10:49:05 AM »

Hi,

I would like to identify which property or combination of properties of acids are most aggressive to an epoxy coating.

So my question is this, what properties of an acid do you consider significant in its ability to attack an epoxy coating. The following are the factors I think are important, if anybody can think of any more please add your suggestion....

Dissociation constant (pKa) - The strength of the acid.
Concentration - It makes sense that a 90% solution will do more damage than a 10% solution.
Temperature - Acids at higher temperatures cause more damage
Conjugate base - Is the conjugate aromatic, aliphatic or mineral.
Mobility - smaller molecules are better at penetrating the cross-linked epoxy
Functionality - How many acid groups are on the molecule, monoprotic, diprotic etc. (different pKa for each)
Physical state - Solid, liquid or gas


If you can think of any more factors I would appreciate your contribution.

Thanks
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Enthalpy

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Re: Acid corrosion of epoxy coating
« Reply #1 on: January 18, 2011, 05:14:10 PM »

How strong an oxidizer!

How good a solvent... Even if it happens to be an acid.

As for concentration: would have made sense... But molecules don't understand our logic. Mild steel resists nitric acid only if concentrated.
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hugh111111

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Re: Acid corrosion of epoxy coating
« Reply #2 on: January 19, 2011, 12:30:33 PM »

Thanks for your contribution Enthalpy. This is exactly the kind of ideas I’m looking for.

Many of our customers come to us with enquiries on how resistant our coatings are to a particular acid. We have a lot of data on the more common acids but often we can only make educated guesses when it comes to the more unusual situations.

If we could compile all the parameters of an acid and test our coating with acids that represent different combinations of these parameters then perhaps we could build a regression model to predict the performance of the epoxy coating. Unfortunately I think this may be too ambitious.

Your point on ‘how strong an oxidiser’ illustrates how an acid may have several modes of attack on the epoxy. We have certainly seen several modes of epoxy failure. Some acids will soften the epoxy other acids will make the epoxy hard and brittle (is acid cure occurring?) sometimes the film is corroded away while another acid may cause blistering but not weight loss. I think it is fair to say that acid corrosion is a very complex phenomenon.

I also agree with your point that the acid can act as a solvent. We had already noticed that organic acids are much more aggressive to the epoxy than inorganic acids – even though organic acids are weaker acids. If I do go ahead and try to build a regression model I will include solubility parameter.

If anybody has anything else to add to the list or if you have any opinions or general observations I would very much like to hear. Is anybody aware of a papers published on a similar approach to this problem? Thanks for your help.
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