[11echo]

OK I KNOW this subject has been beat to death! BUT hear me out, and we can go from there.
I just purchased a small old knee mill (WWII vintage). I have taken it all apart and had all the major parts hot tanked. This has removed all the paint and grease/oil from all the places I couldn’t have hoped to reach. BUT it has allowed a surface rust to form on all the parts. SO I need to remove this rust and re-paint. To do this I have decided to try the muriatic acid method! Now before everybody started telling me they have some better way, let me state my goals and the obvious problems! This mill is broken down to about 6 LARGE parts …i.e. they are large, and being cast iron weight a ton! So I can’t move them around with easy nor do I have a large “tank” to submerge them. ALSO I would like to get this mill back together in short order, so I can’t wait days for some rust removal process to work. And lastly I’ve spent most of my money get this mill, so coughing up $$ for some semi-exotic rust removal system just isn’t going to happen! I have reviewed most (I can’t say all) the common rust removal receipts, and have determined that the use of muriatic acid “appears” to be the fastest/cheapest way! It does have some “limiting” factors, but nothing that can’t be worked through. AND that is why I’m posting here now, although I’ve searched the internet for a way to use muriatic acid to remove rust on metal, I haven’t found a totally clear picture as how to do this! SO with you guy’s input I would like to run that down. I’ll state what I know and the steps I plan to take, and if you can add anything where you feel it is appropriate, please feel free!!!
1. In a well ventilated area, using rubber gloves and eye protection, I use muriatic acid reduced 10 to 1 (acid to water) in a spray bottle.
2. I spray the part and with a non-steel brush lightly brush the wetted parts. Rise with water and re-do if rust remains.
3. Once the part is rust free, neutralize remaining acid on the part with baking soda and water.
4. Towel dry and heat part to drive out remaining moisture.
5. Finally prime and paint.

…Now obviously I can do this to only one or two smaller parts at a time. I don’t plan to do this around any of my shop tools, and when this is over I plan to give the remaining muriatic acid to my neighbor for his pool! …SO can anybody WITH experience add anymore to my procedure? …OR a totally different way that’s faster/cheaper? THX! …Mark

[oldred]

Obviously you are determined to do this but just as with body work if you do you will in all likelyhood regret it. The problem is that although it will remove the rust you see (actually what you see is NOT rust, it is the already oxidized byproduct of the rusting process which you can not see) it will set in motion a process that will lead to more damage than you have now. Once the acid contaminates the iron (ESPECIALLY iron castings which tend to be somewhat porous) the chemical reaction that starts will be nearly impossible to stop. Some will say you simply need to neutralize the acid, much easier said than done, and while steps to do this will slow the damage it hardly ever stops it completely. The rust solutions both dissolve the oxide on the surface and convert it to a phosphate that tends to stop the rusting process, the Hydrochloric (Muriatic) acid on the other hand will dissolve the oxides but as far as stopping the rusting process it will be like throwing gasoline on a fire! I have battled this for years on mining equipment that was exposed to Sulfuric acid and I can tell you that once the metal is contaminated it can really make a mess. It is not as simple as removing the iron oxides that you can see you must take into account the chemical process that you are about to start, and this is one that you don't want any part of! Do yourself a favor and use Phosphoric acid because it will remove the oxides also but it will start the right chemical reaction that will leave the phosphate coating that will inhibit rust instead of accelerating it. The Phosphoric acid may take a little longer but it is definitely the better way to go, usually the fastest way out of something just leads right back in!

[11echo]

oldred ...From what I read phosphoric acid IS the way to go "IF" you can submerge the part. I can't do that! But if your telling me all I need to do is wet the surface of the part with phosphoric acid, like muriatic acid, then I'll be happy to try. But as I indicated what the controlling factor are, WHAT would you suggest!??? ...I"m here to learn!

[oldred]

You are right in that submerging the part would be best but actually you really just need to keep it moist, cover the part with plastic maybe? I don't know if there are any economical sources for large quantities of a gel like Navel jelly but that would work if you can find something. Can these parts be sandblasted?

[11echo]

Yes these parts could be sand blasted (I'd have to tape up the machined surfaces). But now your getting into cost and mobility is an issue.
...SO how long would these surfaces need to be wetted with phosphoric acid solution?

[techron]

your procedure with muriatic acid right on, i use it full strength on small parts and to disolve rust inside antique motorcycle gas tanks i restore. you have to neutralize it with baking soda. but i'm with oldred, muriatic acid is nasty stuff, if not COMPLETELY neutralized, the rust will come back almost right away. try phosphoric acid or electrolisis, muriatic acid on big parts is impracticle. oldred knows his stuff.

[nissan]

I wipped down my bare steel 37 chevy body with picklex 2 years ago and it hasn't rusted yet.they use it in the powder coating industry.the company is out of arkansas and sent me a quart for free. it makes the metal easier to mig weld and can be painted over without removal because it is an etching chemical.

[OneMoreTime]

I have used Rust Mort with a good success on things of that nature..Just follow instructions when using it..Then get some primer on as soon as practicable..I like the epoxy's as they hold up a lot better and seal against further rust..

Just my 2 cents

Sam

[F&J]

Don't use muriatic on cast iron...period. If you soak a cast iron part for a while, it will look like the moon...only the craters will be deeper

[oldred]

Originally Posted by F&J

Don't use muriatic on cast iron...period. If you soak a cast iron part for a while, it will look like the moon...only the craters will be deeper

That is what I meant earlier when I said "especially cast iron" because it is slightly porous and will retain some of the acid. I guess technically it may not actually be porous but the irregular micro-grainy type surface will be just as bad.

[grouch]

Found some information that might be relevant here:

MSDS for Picklex [PDF] Less than 10% phosphoric acid

MSDS for Rust Mort (Leads to a PDF) phosphoric acid 25 - 50%

I'm kinda partial to 19th century writing, so I dug up an old book:


A Dictionary of Chemistry, On the Basis of Mr. Nicholson's; The Principles of the Science are Investigated Anew, and Its Applications to the Phenomena of Nature, Medicine, Mineralogy, Agricultures, and Manufactures, Detailed. By Andrew Ure, M.D., 1821

"Diluted muriatic acid rapidly dissolves iron at the same time that a large quantity of hydrogen is disengaged, and the mixture becomes hot."

"Phosphoric acid unites with iron, but very slowly."

I don't think basic inorganic chemistry has changed all that much since 1821, so you have a choice of acid that dissolves or acid that unites.

BTW, keep in mind that one of the things produced from putting muriatic acid on iron is ferrous chloride -- used for etching -- so it's an accelerating process. Leave it on too long (how long is that?) and you could end up with holes or hydrogen embrittlement. Leave phosphoric acid on too long and you get iron phosphate -- a rust inhibitor.

[oldred]

I think when someone is contemplating using Hydrochloric/Muriatic or even battery acid for rust removal, and this has come up a bunch of times here, they tend to look at acids in the wrong way. There seems to be a misconception that "acid" is a substance in itself instead of being just one of the properties of different chemicals that happen to be acidic, thus the mistaken idea that acid is acid just some acids are stronger than others. Phosphoric acid and Muriatic acid are two different chemicals that affect iron in drastically different ways but because Phosphoric acid removes rust then some get the idea that a stronger acid would work even better. Also that book makes a good point in that the way different acids affect iron has been well known for a heck of a long time and if Muriatic acid or Sulfuric acid (sulfuric acid for rust removal has come up a couple of times) were a good choice for rust removal they would be popular for that use, they are not.

[Kruzin Karl]

Just come on down here to sunny South Florida and look at all of the rotted out pool trucks.

This subject has been beaten to death, and I still can not comprehend how anyone would even begin to consider using muriatic acid on any type of metal.

[bondo]

BUT it has allowed a surface rust to form on all the parts. SO I need to remove this rust and re-paint. This mill is broken down to about 6 LARGE parts …i.e. they are large, and being cast iron weight a ton!

Ayuh,....

So wirebrush it off,+ Slap a coat or 2 of Rustolium on it........
No Muss, No Fuss,.... If it chips, touch it up......

You'll be a Pile of Dust, long before those Cast Iron Parts are.....

I still can not comprehend how anyone would even begin to consider using muriatic acid on any type of metal.

Ayuh,.... I use it on steel sculptures,......

It "Antiques" them,....... Really Quickly............

[Kruzin Karl]

And I suppose, it keeps on "Antiquing" them, Real Quickly, right?