Phosphoric acid question

wrndln

Well-known member
I have been trying to separate some rusted together sheetmetal panels from the rear part of my 29 CCPU. I have been soaking the panel in a trough with EvapoRust in it for about 3 weeks. The EvapoRust is gets rust colored after changing it three times, but appears not to be doing much good - i.e. dissolving the hard rust. I was thinking of using phosphoric acid in the trough to dissolve the rust holding the pieces together. My question is: Will phosphoric acid only attack the rust or will it also go after the good metal?
Thanks for any help.
Rusty Nelson
 
Both. It's just that it takes much longer to dissolve the "good metal". If you can warm your phosphoric acid solution to 150 degrees +/-, it increases the effectiveness immensely.
 
Yes, but it will take longer. Works well to convert rust, at least that is my experience. It can be used as a etcher before primer also.
 
I use muractic acid from hardware store but you have to careful..it is strong and works fast but it will eat most everything but eats rust first,phosphoric is a lot weaker and slower.
 
muractic acid is some serious stuff. I have used it to clean out hoppers on hit/miss engines that have calcium and rust build up. 31% muractic acid mixed 10:1 with water (10 parts water / 1 part acid) will boil out everything. Definitely don't leave it for very long if you use it and neutralize it immediately. Wear gloves rated for acid, a face shield, and a very good respirator. You can actually see the vapors in the air when you open the bottle. Add acid to the water slowing, don't add the water to the acid as it will react and splatter.










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I agree that Muriatic acid is NOT what you want to use as it is WAY too difficult to neutralize afterwards. If you don't get it thoroughly neutralized, it is very possible to have it continue to eat the metal. A few years ago we received a gas tank where the painted tank's metal was soft just like pushing on paper. The tank had been cleaned a year or so earlier with Muriatic and then sealed and painted. It just dissolved under the paint.
 
agree muriatic causes more trouble than it is worth. proper neutralization is nearly impossible. It is the same as Hydrochloric acid, one of the strongest acids known

wish i had a better visualization of just what you are doing. Pix possible? There are no fasteners involved? just 2 pieces of tin rusted together? Warmed Phosphoric, then pull pieces out and play a mapp torch on there carefully, with intermittent spritzing of phosphoric. Not familiar with EvapoRust...it may even have phosphoric in it.
 
I have not used it but have seen amazing results using molasis. Takes a little while but seems to work very well and you can dump it out in your yard without ill effects.

Also dont leave mariatic acid in your garage. The fumes will attack metal, even with the opened lid closed. As i have been told and i tend to listen.... Sometimes.
 
Sounds like you can get the part submerged so why not try electrolysis? It has worked well on several projects for me. Do a search here for more info.
 
agree muriatic causes more trouble than it is worth. proper neutralization is nearly impossible. It is the same as Hydrochloric acid, one of the strongest acids known

wish i had a better visualization of just what you are doing. Pix possible? There are no fasteners involved? just 2 pieces of tin rusted together? Warmed Phosphoric, then pull pieces out and play a mapp torch on there carefully, with intermittent spritzing of phosphoric. Not familiar with EvapoRust...it may even have phosphoric in it.

Just a minor point Tbird, muriatic acid and hydrochloric acid are different names for the same chemical.
 
I tried electrolysis on the cowl assembly and both doors on my pickup. I used a 150 plastic gallon stock tank with washing soda and a 10 amp battery charger. I didn't get very good results. After a week each in the tank, cleaning the anodes (4 of them - old lawnmower blades) 3 or 4 times, I still had some paint left on the doors and a fair amount of rust remaining on all 3 items I did.
Rusty Nelson
 
I would use electrolytic method - and the rinse afterwards with Prep & Etch which can be found for small money at Home Depot.

The P&E is primarily phosphoric acid, which leaves a very good base to start painting on.

Warning though on electrolytic method. It works VERY well at converting red oxide to black oxide - which can be removed with a wire wheel, or even painted over as it stands.

BUT - and here is the but of the argument - electrolytic methods leave a metal surface which is PRIMO WAITING FOR OXYGEN to return the black steel/iron back to the red version. Think a perfect surface for a chemical bond with oxygen. All those Fe chemical bonds just waiting to be filled with the greedy grabby oxygen bonds.

I usually use a wire wheel or steel wool soaked in P&E all over quickly to get the really loose black off. Rinse with pure deoxygenated HOT water afterwards to remove the excess P&E (don't touch until it dries to a grey film.)

Hence the prep & etch afterwards. It disrupts all those empty chemical bonds, fills them electrochemically, and prevents black rust backsliding back to red rust.

Hope this helps. Good luck!

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Joe K
 
Submerge the part in Molasses (at 6 parts of water to 1 part of molasses) for 3 weeks. It works surprisingly well, is non-toxic, very cheap, has no nasty fumes, and is easy to dispose of afterwards. And as a bonus the treated part is very slow to form rust afterwards. I don't know why so many people use electrolysis, phosphoric acid and other nasty substances when you can use good old tried and proven molasses.
 
I used some white vinegar to derust and free up a heat riser. Took awhile but it did derust it and free it up. Vinegar is cheap depending on how big the piece is that you are trying to derust. It was just an experiment to see if it would work and it did.
 
Joe K: what is de oxygenated water? If you take the O out of H2O, what's left?

Heh. I wondered if my statement would bring a query - I should have elaborated.

Deionized water is water that has had all the chemical contaminants removed: the calcium, magnesium, other chemical impurities to water. Water is SUCH a good solvent that it picks up chemicals from WHATEVER it touches.

Deionized water is good water for use in a boiler since it leaves behind little or none of these contaminants as it boils away. (There is not really a perfectly "pure" water since water is such a good solvent.)

Likewise, deoxygenated water is good water to use in a boiler since it provides little oxygen to promote boiler corrosion. And natural (atmospheric) water is oxygen rich. Water by its nature picks up dissolved oxygen from the air until a balance of sorts is reached.

Heating water changes this balance point: by heating water to near it's boiling point you reduce dramatically how much entrained dissolved oxygen is contained in the water.

Similarly to deionized water, there is not really a totally deoxygenated water - there is always a balance and some oxygen remaining. Plus, on exposure to the air and at room temperature water is quick to re-establish a new higher balance.

In commercial boiler systems, the "deaerator" normally runs from 15 to perhaps 85 psi and is part of the water supply system for the boiler (feedwater system) It's primary role is to provide a place for any dissolved oxygen to "percolate off" and escape back to the atmosphere. This why most power plants and district heating plants will have a single pipe, usually fairly small, which seems to have a wisp of steam issuing forth - not much steam but more this is where the air makes it's way after leaving the water contained in the deaerator.

For us, we really don't have access to a deaerator. Instead we may bring water nearly to the boiling point in an open vessel. This is about the best you can do to "deaerate" water at atmospheric pressure.

There are chemicals (oxygen scavengers) that can reduce the entrained oxygen further, but mostly these are not too safe for home use (hydrazine - a commonly used chemical - has been found to cause cancer in rats) These also scavenge oxygen from your skin causing chemical burns.

So if you wish to do as I recommend, I would simply bring a large pot of water to a boil, let it do so for perhaps 10 minutes, and then bring the water to your phos-coated part and "ladle" (not pour) the water onto your part to rinse it off while it is still nearly at the boiling point. Pouring and splashing aids the pickup of atmospheric oxygen by the hot water - pouring slow without an air gap as in the use of the ladle is probably the best you can do.

Or you can dip the part into the boiling water and swish it around for mechanical agitation - if it will fit in the pot.

The heat of the hot water will help your part dry quickly after it has been rinsed.

Hope this helps - a lot of explanation to explain a simple concept? Sorry for that.

An engineering mind CAN be a social liability.

Joe K
 
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Joe K just explained why my phosphoric acid treatments have failed to prevent future rust breakouts. I am going to keep a pot of water on the boiler for the next episode of derusting parts. Thanks for telling us the rest of the story!
 
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