Phosphoric acid for rust removal

[RLamparter]

I'm going to derust an old vise and I probably will need to immerse it in about 3 gallons of liquid.

I've done the electrolytic method and vinegar with salt on other items before, so I wanted to try phosphoric acid this time. I don't want to buy Evaporust since I'm told it's phosphoric acid with a brand name, which just raises the price. I've read that phosphoric acid can be purchased at the auto parts store as a metal prep, but is one brand or type better than the others, i.e. more concentrated, etc.? Is there a phosphoric acid solution at Lowes or other builders supply that would work as well and be comparably priced?

 

[Rudd]

Others may have other thoughts, but to me phosphoric acid is a rust converter, not a remover. It turns iron oxide into iron phosphate, which is very stable. Ospho is a popular brand you can get at paint stores pretty cheap. DO NOT let the surface get wet with water before the phosporic has done it's job and dried, or you get the dreaded white powder which won't let paint adhere - it has to be mechanically removed - think wire wheel.

Ospho is meant to be brushed or sprayed on, just enough to do the job. I don't think I've ever heard of dunking.

 

[Alpacca Fortyfive]

If you are in a dairy farming area, the farm supplies have it as milk scale remover for milking equipment. Check the label as several other acids such as sulphamic get used for the same purpose.

About 30% is almost a saturated solution.

 

[johnoder]

auto parts store as a metal prep

Dupont's version of this, 5717S, was 20 bucks a gallon ten years ago.

I'll bet there is a cheaper way.

Plus one for convert, not remove.

Rusty steel immersed in this stuff just turns black, which to me means none was removed.

John Oder

 

[14tony]

Evaporust is mostly Citric Acid not Phosphoric Acid. It's non toxic and works great. I have found that the best way to buy it is in powder form in a five gallon bucket. Add water when ready to use.

 

[Lakeside53]

Evaporust is not phosphoric or citric acid... not even remotely close. During use, the ph varies from slightly acidic to slight alkaline as the active ingredients are consumed.


If you use Ospho, you need to scrub off the black sludge before it drys and hardens after the vice is is washed off. Even though they say it can be painted over, you need to remove the lumpier material, and it can be difficult. On heavily rusted items, You'll need to do this a few times and re-coat with acid.


I use all of the common method of rust removal regularly. In this case, I'd de-rust the vice in a tub of lye solution via electrolysis.

 

[S_W_Bausch]

Other members have suggested Behr concrete rust stain remover, at Home Depot

Other members have suggested Behr concrete rust stain remover, at Home Depot

Home Despot (big mean multinational)
Home Deport (doesn't scare the day laborers sitting in the parking lot, though)

 

[RLamparter]

Evaporust is mostly Citric Acid not Phosphoric Acid. It's non toxic and works great. I have found that the best way to buy it is in powder form in a five gallon bucket. Add water when ready to use.

I didn't spot a powder form when visiting the Evaporust web site. Where do you find it in powder form?

 

[rons]

The green solution that is sold in stores in the paint section. The solution is applied to
rusty nail heads before painting. Jasco and the MustForRust are probably what you are
looking for.

But for 3 gallons I would go to the grocery store and get distilled white vinegar.
A stronger and faster method is to use Muriatic acid, but dilute with water.

 

[SouthBendModel34]

It seems that regardless of what method of rust removal is used, it always helps to mechanically remove the worst of the loose flakes before you start. A single-edged razor blade, a utility knofe blade, or the like, followed by a wire brush or wheel, seems be universally helpful.

The Phosphoric Acid at Home Despot is called "Behr Concrete Rust Stain Remover"

You can get a Phosphoric Acid solution from Kano Labs called Ex-Rust which has some sort of helpful surficant added. (Probably something simple like a detergent.) KanoLabs.com

Regarding electrolysis, honestly, unless the object to be de-rusted is VERY greasy, I don't think the clear danger of using Lye as an electrolyte is justified. Washing soda (Sodium Carbonate, which is NOT the same as Baking Soda, which is Sodium Bicarbonate.) works perfectly well on most items and is quite safe.

JRR

 

[alex71]

if you want a cheap version of Evaporust, get yourself some molasses. Same active ingredient, just not as concentrated, so it takes a little longer. Works great though.

I don't think I'd want to buy enough evaporust to dip a vise... If I had to derust a vise I'd probably go with a wire wheel on a grinder.

 

[alex71]

The green solution that is sold in stores in the paint section. The solution is applied to
rusty nail heads before painting. Jasco and the MustForRust are probably what you are
looking for.

But for 3 gallons I would go to the grocery store and get distilled white vinegar.
A stronger and faster method is to use Muriatic acid, but dilute with water.

careful with Muriatic acid. It will eat into the base metal if you leave it in too long. It will also cause any steel in the vicinity to flash rust. Nasty vapors.

 

[9100]

What you need is called "inhibited hydrochloric acid". The inhibitor stops it from attacking good iron but allows it to attack the oxide. You definitely don't want to use normal hydrochloric acid (muriatic is another name for technical grade hydrochloric). I once used it to derust an Indian motorcycle speedometer drive and when I got done the parts no longer screwed together, but just fell in. The last time I bought inhibited hydrochloric acid, I got it from a chemical supply house. I don't know the current rules about it.

Bill

 

[Newman109]

I buy phosphoric acid in 70% solution. It's $20 a gallon from a local chemical supply house. It's used in so many things, including food and drinks that the price stays low.

I dilute it (pouring acid slowly into water) to around 20% solution. If I had a vise such as the OP, dropping it into a gallon or more of dilute solution overnight would leave the rust as a nice patina which can be wiped or wire-brushed off.

I've been using the stuff for years. The used solution can be filtered back into a plastic jug through a clean rag and used over and over again.

It's expecially nice for motorcycle fuel tanks to remove rust and scale. Whether it's a rust remover or converter is a semantic issue. It works, that's all that I know.

It's also nice when you have something that is clean, bare steel that you don't wish to paint, powdercoat or plate. The patina left after an overnight bath in phosphoric acid will prevent rust for a long, long time.

I stay away from so-called muriatic acid. It's nothing more nor less than hydrochloric acid and, although it's widely used in industry to pickle metal, if you leave it unattended too long, it will eat your metal. Unlke phosphoric acid, which is not too terribly bad if you happen to get incidental contact on your skin, muriatic acid will cause real problems much more quickly.

The benefit of phosphoric acid is that it's readily available, inexpensive and safer than other types of chemical agents for rust.

 

[Forestgnome]

Naval Jelly-Sticky phosphoric acid...works on vertical surfaces.

 

[Clive603]

My experience is that decently effective rust removers and rust converters containing phosphoric acid aren't the same suggesting some careful chemistry. Ages back I "obtained" two one gallon bottles of same brand industrial rated remover and converter from the firms stores. Looked the same, just as painful if you got it on your hands and just as aggressive on concrete but very different actions. The remover really did remove the rust from immersed objects leaving heavy deposits on the base of the container. The converter didn't do much if stuff was immersed but turned light rust blue black if rubbed on as per instructions. Intended for use after the remover had done its stuff it was very effective.

Interestingly both became pretty much inactive on rust after about 5 years storage. Still had a taste for people and concrete tho' clearly remaining very acidic.

Both much stronger than any consumer nostrums available in the UK at that time. That said my best result was around 1980 when I used Kingston Krust converter to fix localised rust patches on a PortalDoor up and out type garage door which was around 25 years old at the time. Killed the rust dead with no sign of recurrence to date. The door is 10 years younger than me and wearing better. it should see me out.

Clive

 

[SAG 180]

I bought straight 80% phosphoric from the local cleaning chemical shop cheaper than the 30% strength rust converter to use on a 1970's vintage sheet steel spark plug blaster. At about 30% strength it worked it's way under the hammertone paint and lifted it, then converted the rust in half a day or so and after a full day the rust was gone completely leaving a grey bare metal surface. I was under the impression it only converted rust and didn't remove it but either the higher strength or the steel type used for the stampings (leaded?) had some effect on the rust removal.

 

[Newman109]

I bought straight 80% phosphoric from the local cleaning chemical shop cheaper than the 30% strength rust converter to use on a 1970's vintage sheet steel spark plug blaster. At about 30% strength it worked it's way under the hammertone paint and lifted it, then converted the rust in half a day or so and after a full day the rust was gone completely leaving a grey bare metal surface. I was under the impression it only converted rust and didn't remove it but either the higher strength or the steel type used for the stampings (leaded?) had some effect on the rust removal.

As you say, it's not necessary to use phosphoric acid full strength. A dilute solution (20-30%) will work just as well with a longer exposure.

 

[JST]

I use the concrete etch phosphoric, or the rust remover phosphoric, whichever I see at the store I end up at when I want some.

Here is a before and after on an HSS cutter, with about 25 or 30 minutes "in", IIRC. I had to go somewhere, so I terminated the test. I did this one for an example, positioning the part the same for both shots.

I dilute teh store solution about 5 to 1 or a bit more

 

[captf71]

I used to get a product called "Clean-N-Etch" concrete cleaner which was about 40% phosphoric acid and I used it on steel items that I didn't want to wait for electrolysis to work on. It worked quite well, fast and was about $9.00 for a 1/2 gallon.

A few weeks ago a friend wanted some so I took him to the big box lumber store and found that there is now a new version of this product that no longer lists phosphoric acid as the active ingredient but is now made with "Urea Monohydrochloride" whatever that is.

We checked the 20+ bottles that were there and found one that still had phosphoric acid listed as the active ingredient so he lucked out. If you can find an older version of this product that is the phosphoric acid solution it works quite well.

I'm told that swimming pool supply stores carry phosphoric acid but I haven't stopped in to verify that yet as I have about 2 gallons of the older product that I've had for a while and it still works fine.