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Corrosion Prevention: Alodine and Zinc Chromate: folow ups,

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messydeer(at)yahoo.com
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Posted: Tue Feb 07, 2006 7:13 am Post subject: Corrosion Prevention: Alodine and Zinc Chromate: folow ups,

Thanks everybody for being thankful. I've taken so much from this group over the past several months, so it's nice to add something.

   

  I'll be asking a few follow up questions to these guys in a little while, so please post any you have. I'll ask about the stain 'shadow' that remains or recurrs. I was told it's fine to paint over, but I will double check to make sure it would be nothing but a minor cosmetic defect.

   

  I hit the aluminum with a Blendes drum and also with Alumiprep and scotchbrite for the first time last night. The Alumiprep didn't work so-so. I used a GREEN scotchbrite and maybe 20# hand pressure after mixing the Alumiprep 1:2, the highest suggested mix ratio. Did that after a 6-8 minute dwell time. 

   

  The Blendex drum on an angle grinder by itself worked much better. It looked like that stain shadow was left after an initial pass using 10-20# pressure on the grinder. It was much shinier and the shadow was only noticeable at certain reflective angles. I'm not sure this is what Dr. Dean talked about or not. I was able to get rid of it with a second pass and twice as much pressure. If this will be the interior side, I might not even have to do that. One of my questions I'll ask.

   

  I emailed a few pics of this to the list. I'm told it may take a few days to post and the group will be notified.

   

  Finally, it seems there's a debate on the grade of scotchbrite to be used. I'm using green and have heard others say maroon is better. Is that a finer or coarser grade? 

   

  Cheers,

  Dan



		

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messydeer

Joined: 13 Feb 2006
Posts: 214
Location: Bellingham, WA

Posted: Mon Feb 13, 2006 4:16 pm Post subject: Corrosion Removal

Here's some correspondence I've had with Sheldon Dean regarding a couple other quetions I had. It reads top to bottom.



Question



Sheldon:



We talked for a bit last Monday about the water stains I have on my 6061 here in Bellingham, WA. 



I used a Blendex drum (like scotchbrite) to sand off the corroded areas. One or two light passes makes it nice and smooth. I can’t feel any difference between this and the non-corroded areas. But there is a dark stain remaining that can be easily seen from across the room. Sanding it again with a little more pressure makes it visible only at certain angles of reflection from a couple feet away. More and more pressure is required to remove it completely. 



I’m hoping you will say this dark stain that is present after one or two abrasive passes and which appears after removing the whitish corrosion will have no physical effect at all, only cosmetic. Aluminum with this dark stain is safe to conversion coat and/or prime. Is this the hydrogen in the aluminum you talked about that is hard to get rid of but effects only the appearance?



I also may not have mentioned cardboard. There’s a few sheets of aluminum that have a very light corrugated stain on them from rain soaked cardboard. It comes off real easy. Not near as tenacious as the water stain. Any concern here?



Thanks again for your time and knowledge.



Dan Morehouse



Reply



Dan,



The white to gray powdery water stains are severe stains.  The black color you see under the gray white powder is a collection of tiny (smaller than the wavelength of light) metallic particles.  These are usually embedded in an oxide layer that makes them hard to remove.  The black color occurs whenever small metallic particles are present and are commonly seen when you rub an aluminum surface with a wet cloth.  I suggest changing the orientation of your drum and see if that helps to remove them.  Ultimately they will disappear as you have discovered.  The dark stain is an integral part of the stain.  Actually, when a water stain first appears it looks black.  As the process continues the white powdery material deposits on top of the black layer which is why you see it after you remove the powdery material.  The black material increases in thickness as the stain process continues which is why it is more difficult to remove. The corrosion process that causes the stain does put tiny quantities on hydrogen into the metal surface, and this hydrogen can affect the way the metal responds to chemical etching processes.  However, it does not affect the mechanical properties significantly.  It would not affect the development of a conversion coating such as Alodine, nor does it bother primers.  The stain that occurred with the cardboard present is different because the cardboard allows oxygen to permeate through it, and that maintains the passivity of the aluminum. If you remove it mechanically it is gone.  I suspect that there would not be any hydrogen associated with it.



Good luck,

Sheldon 



Question



Sheldon:



I'll try orienting the drum to rotate in different directions if I want to get more of it off. But it sounds like taking off the stain only until 'smooth' would be okay for either an interior surface that has Alodine and/or primer for a finish. If I left that for the exterior, I'd prime and paint over it, or spend more time with the drum and remove it completely. Fortunately, most of the sheets do not have water stains on both sides. So I may be able to still get a polished bare aluminum finish. 



Does this sound okay?



Thanks again for your help,

Dan



Reply

Dan,



Sounds OK to me.



Sheldon

(End of correspondence)



I've just found out you can post using the forum format, as I'm doing now.  I thought I was supposed to be notified when my emailed pics were posted. But since I haven't heard, I assume they have not or won't be posted. When I get more time, I'll see about posting them via the forum.



Ciao!

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