Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close