Mill Scale removal with sulfuric acid

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I have read that sulfuric acid works better than hydrochloric/muriatic or phosphoric acids.  Sure enough, three applications of Naval Jelly (which contains phosophoric acid, as I understand it) haven't gotten through the thick scale on a big piece of plate I'm trying to clean up.Sourcing sulfuric acid isn't easy.  I've done some digging, and read that automotive battery electrolyte is about 30% strength sulfuric acid, but that some professional drain de-cloggers are more than 90% strength sulfuric acid.  So I went out and bought a $10 bottle of the drain stuff. It's double-bagged and warns that it's concentrated sulfuric acid.  Okay, great.My question: at what dilution strength should I use this stuff on my steel?I'm going to have good ventilation and baking soda on hand to neutralize it.Jack OlsenMy garage website
Reply:Jack,Nitric acid is probably the best(fastest)  for pickling, but it's also the most dangerous to work with.  The brown fumes generated from a nitric acid bath are extremely toxic.I've never seen straight sulfuric used, but I imagine it would get the job done.  Hmmm, you better do some digging on the net or in your local library and make sure that hydrogen sulfide, aka H2S, isn't a by-product of this reaction.  H2S is extremely toxic, and can render a person unconscious at very low(ppm) concentration levels.For your sulfuric acid I'd start at a 10% solution(by volume).  You can always add more acid to the solution if the reaction isn't working fast enough.  it's hard to take acid back out, and unless you've got extra space in the container, it's tough to dilution back down to a lower concentration.  You can also add some heat if your setup allows for this.  A little heat can dramaticly speed up reaction rates.   I'm talking about a bath temp of 120-140F, not like boiling water.Remember also that the chemical reaction that occurs when you're pickling can also produce heat; sometimes lots of heat.  The soup that forms when acids react with metals can foam up pretty quick if you're also generating a lot of heat and hydrogen gas byproducts.  You can easily boil over and spill acidic foam if you're not careful.The reaction rate will be proportional to the surface area, so if you're pickling a large plate, or lots of small parts, be prepared for lots of action.   If the reaction gets going too quickly always have a means to safely remove the piece of steel from the pickling bath.   Doing so will break the chemical reaction that's producing heat and noxious gases.  A couple more thoughts with regards to safety.  In case no one has told you, always add acid to water.  The dilution of the acid releases heat; lots of heat.  If you add water to acid, you can get instant steam, and this will throw acid back in your face.  You also need to think about the container your using for the solution.  If you're adding acid, do so in small amounts, mix the solution, and pay attention to the temperature of the acid bath.  Nothing worse than dumping a bunch of acid into a solution and having the heat of dilution cause your plastic tub to melt.  Pyrex glass is best for heat and chemical resistance, but easy to break.  Make sure you know what your plastic tube is made of, and ensure that it's resistant to sulfuric acid.There may be other issues I've overlooked here, so do your homework before trying this.  I hope you've got a good set of rubber gloves, an apron, and a full face shield.Benson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector
Reply:great words of wisdomwell spoken   ***SAFETY****FIRST***
Reply:i've done some test pieces with vinegar.  works great.  i put 3 inches of 2"x2"x1/8" in vinegar and took the scale off in a couple of days.i'm sure you've searched and found out that the fumes can also cause rust.  so, dont do this next to your stock pile of metal/garage.the only reason i didnt use the vinegar is because the piece that I was going to pickle was 300lb+ 4 foot square, half inch thick.  and dont have a tank that big... and it was inconvenient to let the plate sit for 2 days on one side (using silicone to build a wall around the plate then put vinegar in the center.), then 2 more days on the other.  i just got it commercially acid dipped.how big is the pieces you're trying to de-scale?  that makes a HUGE DIFFERENCE.
Reply:I think I'd try a sandpaper wheel on a 7" angle grinder first...MM350P/Python/Q300MM175/Q300DialarcHFHTP MIG200PowCon300SMHypertherm380ThermalArc185Purox oaF350CrewCab4x4LoadNGo utilitybedBobcat250XMT304/Optima/SpoolmaticSuitcase12RC/Q300Suitcase8RC/Q400Passport/Q300Smith op
Reply:Thanks.  I'm doing a small test now.  It's too big to dip -- 30"x60" -- 1" thick.  I have good ventilation -- and ammonia and baking soda waiting in the wings.Jack OlsenMy garage website
Reply:Hey Jack, why are you bothering to get the mill scale off? I'm a hobby welder like you and I used to be pretty anal about mill scale. Haven't bothered getting it all off for years now and I haven't had any problems. I just hit it with an abrasive disc to get the loose bits off.(Sorry to offend the purists.)ScottYeah, I carry.House keys, wallet, some change, usually a newspaper, maybe a pen.
Reply:Nitric acid is probably the best(fastest) for pickling, but it's also the most dangerous to work with. The brown fumes generated from a nitric acid bath are extremely toxic.
Reply:Just for general purposes, on rusty steel, phosphoric acid acts only as a rust converter not a remover.
Reply:Why?dis be a Welding table top, no?Ed Conleyhttp://www.screamingbroccoli.com/MM252MM211 (Sold)Passport Plus & Spool gunLincoln SP135 Plus- (Gone to a good home)Klutch 120v Plasma cutterSO 2020 benderBeer in the fridge
Reply:My welding table sits under an awning outside. It gets wet occasionally and usually has a thin film of rust on it as well as the odd bit of paint, grease etc. None of which bothers me. But recalling the condition of Jack's garage from some earlier pics, I suspect he'll have the cleanest welding bench in history.Yeah, I carry.House keys, wallet, some change, usually a newspaper, maybe a pen.
Reply:why would you pharque around with acid when you can grind?
Reply:This was a thick layer of mill scale.  It would have meant hours of grinding, and going through a lot of grinding wheels, flap-disks and sandpaper.  Full strength sulfuric acid for one hour made a dent in it.  But the kicker was 50% strength sulfuric acid overnight -- for about nine hours -- covered with a layer of Saran Wrap.Needless to say, acid is a pain in the neck in terms of safety gear (special gloves, all skin covered, eye protection and a face shield).  The fumes are annoying, and neutralizing it took six boxes of baking soda and about a quart of ammonia.  But it was also a chance to learn something.Here it is cleaned up:Jack OlsenMy garage website
Reply:60"x30"x1".  510 pounds.  I'm not a guy who can lift 510 pounds.  I had to move it from my Jeep to this cabinet, then rotate it 180 degrees, then flip it like a pancake.  Jack OlsenMy garage website
Reply:Good score jack, but now you'll be chasing Rust on itNot bad in LA but you have removed the Protective Mill Scale layer- any spot on my table that has the Mill scale removed-from grinding- gets rusty- once again not bad since we live inna desert but you'll see.Ed Conleyhttp://www.screamingbroccoli.com/MM252MM211 (Sold)Passport Plus & Spool gunLincoln SP135 Plus- (Gone to a good home)Klutch 120v Plasma cutterSO 2020 benderBeer in the fridge
Reply:Spray it with some anti splatter or pam to keep it shiny and clean and wipe it down once you get a heavy layer of the anti splatter on it.  This will keep a nice burnish going.Esab Migmaster 250Lincoln SA 200Lincoln Ranger 8Smith Oxy Fuel setupEverlast PowerPlasma 80Everlast Power iMIG 160Everlast Power iMIG 205 Everlast Power iMIG 140EEverlast PowerARC 300Everlast PowerARC 140STEverlast PowerTIG 255EXT
Reply:Yeah, even after thoroughly neutralizing everything, you can see the rust forming.  But I rubbed some wax into it and that stopped it.  I think a periodic spray/wipe with wax or oil should probably keep it clean.  We'll see.Jack OlsenMy garage website
Reply:I can picture you giving your tabletop a clean and wax everytime you do your car.Yeah, I carry.House keys, wallet, some change, usually a newspaper, maybe a pen.
Reply:Hey, here's the way to get rid of mill scale with no acids at all.  And the plate won't rust afterwards, unless you spill water on it:web site:  SCS Removes mill scaleWeb page: video of SCS cleaning rust & scale off plate
Reply:What about sandblasting? That is if you had the equipment and the place to do it.Rodney                                   The blessing of the Lord makes one rich, And He adds no sorrow with it.'  Proverbs 10:22 NKJV
Reply:from WIKI answers siteAlways add the acid to the water. Otherwise the acid can splash you and get on your skin and also if it is a concentrated acid, as you dilute it, a lot of heat is generated, and you want the larger volume of water to be able to absorb that heat. As I recall students that added water to acid were immediatly thrown out of chemistry class as being nitwits. The WIKI answer seems mild but I remember the instructor almost going nuts as though doing it wrong would blow down the school.
Reply:Originally Posted by ClockwelderWhat about sandblasting? That is if you had the equipment and the place to do it.
Reply:i prefer machine striping , to chemicals anytime......if u have rust scale, than it needs to be lifted anyways to get to the new layer underneath.........a stack of 5/8 or 7/8 arbor , 5"-8" Resin Fiber Disks........stack about 5 of them together ,to be about the thickness your stone wheel would be on your grinder......they flex to the surface and strip anything like mad . Then just go over that with a D/A and 40grit. The surface looks like tha T-slot Table on those Expensive BridgePorts or other high dollar machinery.i've seen people spend days....i strip the paint on an entire/complete car, in 3 hours this way , using 50 grit resin fiber disks.hope this helps somebody        thermal arc 252i  -  millermatic 350P -   miller XMT, cp300ts, 30a 22a feeders, buttload of other millers, handfull of lincolns, couple of esabs  -   Hypertherm 1250 G3
Reply:hydro-blasting is a good way to go. northern tool sells the attachment for your pressure washer for about $40.00.the pressure washer makes the sand a lot more aggressive than an air sandblaster and you get less bounce-back. we used to use talcum powder on castiron tables to stop rust,dont know how it would work on steel.for the woodworkers,talc works great on saw tables.
Reply:update--it's been a few years since i bought mine,price is now $55.00. here's a link  http://www.northerntool.com/webapp/w...3933_200413933Originally Posted by Knotboredfrom WIKI answers siteAlways add the acid to the water. Otherwise the acid can splash you and get on your skin and also if it is a concentrated acid, as you dilute it, a lot of heat is generated, and you want the larger volume of water to be able to absorb that heat. As I recall students that added water to acid were immediatly thrown out of chemistry class as being nitwits. The WIKI answer seems mild but I remember the instructor almost going nuts as though doing it wrong would blow down the school.
Reply:If it's just plain water, and pure acid with no metals or other compounds involved, when the water is poured into the concentrated acid, lots of heat is suddenly generated which causes the water/now-somewhat diluted acid to boil explosively, throwing the acid all over and for a considerable distance if much is involved. I can't think of anything right now that would cause gases like hydrogen or oxygen to occur, but I won't say it couldn't; it really doesn't matter because it's already bad enough. I knew of a young lady who broke a gallon jug of conc. Sulfuric Acid she was carrying or about to pour from; the acid only got on her lower body, not into her eyes, but it was a major catastrophe for her; she was carried to the men''s locker room/shower (just down the hall) immediately and still got decent burns, IIRC. I wasn't there at the time, so missed the details.