Agitator shaft/impeller coating

[nbog]

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?
Thanks,
Nick

 

[kenvlach]

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.

 

[nbog]

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.

 

[kenvlach]

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 K_sp 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.