Sciencemadness Discussion Board

Removing rust from lab equipment with phosphoric acid

mycotheologist - 1-5-2012 at 08:53

My lab stands and clamps have gotten so badly rusted in my lab that theres literally piles of iron oxide powder building up around them. I got some 80% phosphoric acid from a hydroponics shop and from what I've gathered, H3PO4 removes rust then leaves the metal with a rust proof coating. I'm wondering how I should go about this though. I'm thinking I should let the equipment soak in a bath of the acid but how much do I need to dilute the phosphoric down to and how long should I leave it soak? Will the rust just fall off by itself or will I need to scrub it off? The most important part is the threaded parts where you screw the poles in, is there much risk of damaging those parts?

Hexavalent - 1-5-2012 at 09:03

I have a similar problem - my tripods for my Bunsen burners are now more rust than metal on the surface, particularly around where the legs screw into the actual triangle itself. My advice would be looking for a company that sells the phosphoric acid, perhaps in a hardware shop, and look at the instructions on the back of their bottle as well as the concentration of the acid itself they list. You could also look on their website, if they have one.

Another, more experimental way, is to take a small rusted sample of iron not from your stands and test it with the product yourself. If you need to scrub/agitate it, then do so when actually cleaning your labware. If not on the trial sample, then it would appear the same on your tripods.

inspector071 - 1-5-2012 at 09:20

Naval jelly is commonly used to remove rust. It's a somewhat thick gel, containing around 70% phosphoric acid. I would use a wire brush to get most of the flaky bits off, then let the stuff soak in a solution of phosphoric acid. If it's similar to the Parkerizing process (a conversion coating used on firearms, knives, and other stuff) the process should be done when it stops bubbling. You should be left with a dark grey/black coating. As long as you havent lost much material to rust, the phosphatizing will leave the threads alone. After you're all done, it wouldn't hurt to coat all your iron and steel parts with some silicone spray lube as further rust preventative.

Endimion17 - 1-5-2012 at 11:22

Use fairly concentrated solution of phosphoric acid (40-50%) and pure methanol mixed in 1:1 ratio. I use it every time I need to remove some rust in my lab.
The rust removing stuff people buy in stores is essentially phosphoric acid, methanol and sometimes something to make it jelly-like. Don't throw the solution away, it's fine for several rust removing jobs.

Soak the stuff in the solution for a few hours, and then, using gloves, scrape the stubborn rust with a steel wool. Soak until everything has a nice coating. Then wipe off the excess black coating (it tends to build up too much).
Rinse it with distilled water, dry it with paper towel and hair dryer immediately. That leaves a porous, matte surface. Spray it with WD-40 which will enter the pores and keep the water out. Every few months spray it again, and try to keep rust prone equipment in plastic bags.
I did this with my clamps. Perfect job, no rust after several years.

bahamuth - 1-5-2012 at 14:08

Soaking in phosphoric acid takes to much time for me, so I just boil the rusted parts in it, as hot as I can get it without foaming over.
That is 85% phosporic acid at around 200-250 degrees.

It usually don't do much to borosilicate glass but ceramic tabletops and sodium silicate glass is eaten away very very fast..

Couldn't find any pictures of what it looks like afterwards as I probably didn't take any, but assure you that if the acid and the part gets hot enough, the steel will be completly shiney, and will not rust on exposure to water for some time, almost like some greasy surface layer forms..

Additionally it seems as hot phosphoric acid don't chew through un-rusted steel very fast so no excessive pitting or corrosion is apparent even after 10 min+ at full boil as hot as I got it at my ceramic tabletop hot plate.


In the process called phosphatizing (Parkerizing) one needs phosphoric acid and a transition metal phosphate salt, which I think the rust combining with the acid provides and thus one gets a nice protective layer.

rivetboi - 2-5-2012 at 14:29

A couple points concerning this topic:

1) There are rust converters and there are rust removers. Both are phosphoric acid based but the first one usually has a zinc compound in it to give you a nice zinc phosphate coating that resists rust quite well. Both will leave you with a protective layer but the latter's iron phosphate is rather delicate and tends to flake off.

2) It is far better to use a copper or brass brush to scrub away the loose red rust before using any solution on rusty metal. This gets rid of the structurally unsound corrosion and exposes the adherent rust to your bath solution. This minimizes solutions waste as well as shortens the process time.

3) All processes should be followed with a sealant coating. I like the suggestion of silicone grease. But i dislike WD40. WD40 is the cannibalizing lubricant that evaporates away. Its great for short term lubrication but the Stoddard's solvent in it tends to take all lubricant properties with it as it evaporates. Oh, and most any vegetable or petrochem oil will work as a good sealant. Or you can try a polymerized oil for better durability.

4) I am a big fan of electroplating my rust vulnerable equipment if I can. And a big thank you for posting a question which I was able to answer. I might be a beginning hobby chemist but IRL i'm a mechanical engineer with a specialty in coatings.

Arthur Dent - 3-5-2012 at 03:43

I've used Phosphoric acid to remove rust from a set of lab jacks that had the central screw rusted and also a few badly rusted ring clamps.

It's efficient, although the "lime buster" solution I use is only 5% solution. A more concentrated solution would definitely be better.

One big factor that's common to many home labs on this forum is that their lab site is often a damp, cold, unheated room or shed, an environment that is somewhat problematic for delicate equipment like complex glassware pieces or electronic stirplates. Add to that the chemical mist that deposits on equipment and the dampness that keeps this mist more active on metallic parts and it's hard to keep your gear in good shape.

So the best thing to do after getting rid completely of the rust is to spray it with WD40 occasionally, just to keep a thin mineral oil film on it. Even better is stuff like McLube ( http://mclube.com/products/81 ) which is Molybdenum grease in aerosol form. I used to spray all screw threads with this stuff and it was awesome, but sadly, my last can was used up long ago and I never found a local source for this stuff again.

But I reflect the posts above mine. Keep your metallic stuff slightly lubricated, and store away the stuff you don't use often in a dry place, maybe in ziploc bags with a bit of machine oil on 'em.

Robert


Neil - 3-5-2012 at 14:52

WD40 = Water Dispersant 40

It removes heavy long lasting oils but is excellent for de-watering surfaces. Used near bearings it will thin and wash out the thick lubricating oils and will in the long run cause more corrosion and more friction. On metal surfaces, in my experience, it washes out waxes and heavy oils and offers protection for a while but not for very long, once it is gone the surfaces are essentially bare and rust up.

There are a number of waxy oils sold for rubbing onto metal surfaces, like table saws.

They keep the saws machined surfaces smooth, dry and completely rust free even in damp basements, they work even better on things that are not constantly having wood dragged over them.

Endimion17 - 4-5-2012 at 07:12

^^then how is my equipment still rust free, after several years?
WD40 won't just wash away the grease and then evaporate, leaving iron naked. I have completely different experience with it.
Yes, I do apply it two times a year. The process is quick and neat, you get no thick grease everywhere and it smells quite nice.

tolerable humidity in the lab

chemrox - 6-5-2012 at 20:57

What is the maximum tolerable humidity for a lab? I'm running ventilation and replacing with system air in violation of local HVAC codes. I have a dehumdifier that is keeping the lab down to 35-40% without running water. I have a separate thermostat for the lab too and that helps when its half way balanced.

[Edited on 7-5-2012 by chemrox]

mycotheologist - 22-5-2012 at 09:17

I finally got around to doing this so I poured the 250mL of H3PO4 into a pyrex baking dish and am bathing the rusted metal rods in it. Its working. I notice there is a strong, metallic smell iron smell coming from it. I doubt FePO4 is volatile enough to give a strong odour, any idea whats causing that smell?

Arthur Dent - 22-5-2012 at 11:29

Quote: Originally posted by mycotheologist  
any idea whats causing that smell?


Trace amounts of phosphine. It's toxic in large volumes, but in trace amounts, it just smells like a cross between garlic and burning sulphur. Keep your baking dish in a well-ventilated area.

Robert

mycotheologist - 22-5-2012 at 14:30

Quote: Originally posted by Arthur Dent  
Quote: Originally posted by mycotheologist  
any idea whats causing that smell?

Trace amounts of phosphine. It's toxic in large volumes, but in trace amounts, it just smells like a cross between garlic and burning sulphur. Keep your baking dish in a well-ventilated area.

Ah, its a good thing I took it outside then. Initially, I had it in my house but once I got that smell I had a feeling it might be dangerous. Its not a very pleasant smell, seeing the rust must have infuenced my mind into associating it with iron because it smells like rust to me. After bathing the metal for a few hours in 80% H3PO4, I've never seen them so shiny before. They are less tarnished than when I got them. My ring stand has taken on a greenish colour for some reason, I think it was made of a different alloy to the rods.

I agree with Endimion17, WD40 leaves a thin, invisible coating after the volatile solvent evaporates and provides long term protection against rust. I had my door fixed recently and the lock smith recommended spraying all the iron metal hinges and door parts with WD40 to protect them against rusting and friction.

[Edited on 22-5-2012 by mycotheologist]

Arthur Dent - 23-5-2012 at 06:32

80%? :o

Wow, that's pretty concentrated! To think that the only stuff I have available is the 5% solution. Where do you get your Phosphoric acid?

Robert

mycotheologist - 23-5-2012 at 08:53

Quote: Originally posted by Arthur Dent  
80%? :o

Wow, that's pretty concentrated! To think that the only stuff I have available is the 5% solution. Where do you get your Phosphoric acid?

Robert


Ordered it from a hydroponics site. I went to my local hydroponics shop years ago looking for nitric acid and while they didn't have HNO2, they had concentrated H3PO4. pH decreasers for hydroponics will either be HNO2 or H3PO4. If I'd known more dilute acid works, I would have diluted it way more because there wasn't enough volume of acid in the bath.

When I take the metal out of the acid bath, they are still covered in acid. Initially I was wiping it off with a cloth before spraying the metal with WD40 but I'm wondering if the WD40 will just displace the acid, eliminating the need to wipe it off with a cloth. I know it displaces water and H3PO4 is probably more polar than water so I'm guessing it does.

Also, heres the problem I'm having: I have a few metal poles that are so badly rusted, they got jammed inside the screw holes of the lab stands so I can't soak them in a shallow acid bath. I tried soaking a cloth in acid then wrapping it around the metal pole and leaving it there but it didn't work. Is there any alternative to actually bathing the metal in the acid?

Endimion17 - 23-5-2012 at 14:22

Quote: Originally posted by mycotheologist  
...

Also, heres the problem I'm having: I have a few metal poles that are so badly rusted, they got jammed inside the screw holes of the lab stands so I can't soak them in a shallow acid bath. I tried soaking a cloth in acid then wrapping it around the metal pole and leaving it there but it didn't work. Is there any alternative to actually bathing the metal in the acid?


I don't know your exact setup, but you might create a wax mould around it, by dripping from a candle or somehow...

EssOilChem - 28-5-2012 at 05:15

Phosphoric acid can be had cheap at Farm/Tractor supply stores by the gallon (close to 50%, which is nice considering the "rust kutter" brand is only like 20% at 5x the price). Most of it is just mixed with surfactants, which shouldn't be a problem in most cases. If you go take a look around there you can find other products with higher concentrations too (I think i've seen up to 80% there but can't remember the name of the stuff, look around the cow and horse chemicals), but i've found anything around 50% or higher has worked very well for rust for me (and is cheaper too), but then again my applications may differ from yours.

I was using the name brand rust stuff till I saw that, in fact I bought some the other day and it has worked well to clean off my iron ladle after melts (especially if I leave it a little warm afterwards, not much is needed). Farmers use it for alot of different things - as well as many other items of interest.