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Agitator shaft/impeller coating

Agitator shaft/impeller coating

Agitator shaft/impeller coating

I have an agitator with 304 SS shaft/impeller. It is used to dissolve metal nitrate which becomes acidic as it dissolves so I have HNO3 generated in-situ. I noticed that the resulting solution, instead of being transparent, turns into darkish, although still transparent liquid. I am thinking about coating the impeller/shaft with PE or something else would be better?

And how would you do it?

RE: Agitator shaft/impeller coating

It's better to use a more corrosion resistant impeller -- mixing in a solid that maybe doesn't dissolve quickly will wear off a coating.  Plastics (PE can be applied by powder coating) don't stick very well to 3xx SS, and will peel as soon as the first hole or tear develops.  
HNO3 itself isn't too corrosive toward 304 SS (which is used for 55-gallon drums of nitric) but the metals & impurities in the nitrate may contribute to a bad interaction.
What metal nitrate, quality of water, what color is formed, have you analyzed the raw materials & solution for Fe, Cr & Ni?

Consider using a 316 (CF8M if cast) SS impeller. Also, check any setscrews if used; they are usually a harder but less corrosion resistant SS.

If you have a high speed mixer, maybe erosion is the problem. A slower but larger diameter mixer may help, then.

RE: Agitator shaft/impeller coating

Actually, I run the mixer at lower speed (let's say 100-150 rpm). Solution becomes grayish when it's finished. Doing the same thing in a lab (magnetic stirrer/beaker) results in a clean fluid).
Isn't there some kind of hard, dense PE formulation, if I remember I ran onto it somewhere (trade magazine)?

I will definitely have to do analysis on metal ions in the product.

RE: Agitator shaft/impeller coating

You can do a somewhat crude analysis for [Fe+3] by a simple pH titration.  Slowly add 1 N NaOH while stirring and observe the pH at which reddish-brown Fe(OH)3 precipitates.  Then either use Ksp or a solubility curve to get [Fe+3].
Solubility curves vs. pH for various metals (only a partial one for Fe+3): http://www.finishing.com/Library/metalpH.html

Check both the lab prepared sample & the bulk.  A faint color in a bulk tank usually appears colorless in a beaker.

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