Rust Removal With Phosphoric Acid

[astjp2]

Phosphoric acid converts the iron oxide to Iron phosphate, it is also used as a type of Parkerizing on rifles. If you want the best quality, get an even coat of rust on what you want to soak, new parts, dip in water, let them sit wet for a short period of time and you will have enough rust to get the gray finish when you use the acid. I did some bolts one time, the acid sat overnight and they became brittle, I was snapping off a 1" diameter bolt with a 3/8" drive ratchet.

 

[British Steel]

I did some bolts one time, the acid sat overnight and they became brittle, I was snapping off a 1" diameter bolt with a 3/8" drive ratchet.

Yep, hydrogen embrittlement, can occur with plating and acid treatments, but it's mainly a problem with high-carbon alloy steels - not a worry with cast iron.
Electrolysis, I'm told, doesn't cause it due to the polarity of the current - I always use electrolysis on steel anyway, I don't like the loss of sound metal you get with acid dips.

Dave H. (the other one)

 

[John Hasler]

Electrolysis, I'm told, doesn't cause it due to the polarity of the current - I always use electrolysis on steel anyway, I don't like the loss of sound metal you get with acid dips.

Phosphoric acid is too weak to cause any loss of metal.

 

[Grahammon]

Do you use the phosphoric acid at 100% strength or dilute it with water?

Guys please be careful, phosphic acid comes in various concentrations. 80% and 20% and in whatever trade concentration the commercial removal brand names make it up
You can buy it from Lab supplies and all of the places you mention.
Just google it, but in its 80% form the manufacturers data sheet have it as highly caustic with the triangular hazard symbol of a test tube dissolving flesh. You'll have seen the tankers on the highway with the symbol.
In its 20% form the data sheet has it is classed as an irritant. And below 20% I believe you can handle it without harm.
I am no chemist and take what ever I've said with due caution as any one who writes on a forum page. But there is to my mind a very different full strength between Concentrated phosphoric acid in its 80% form like concentrated hydrochloric or sulphuric acid to one that that is already been dissolved in water.
Check when you buy it what it's concentration is.
I ve been a lurker on the site for a while and was looking for different ways of rust removal, saw a youtube with some one using a US product called evaporust or something like that, which seems very impressive. the guy was using bare hands to handle it. Which indicates its concentration is relatively low
in the UK that product costs around £50+£45! The currency conversion is just scary around $150
so was looking for a way of copying it that wasn't too dangerous.
I've tried the electrolysis method and it works but is so messy, knew of the phosphoric acid method and was looking at the cheapest ways of buying it. But just didn't like the thought of leaving a litre (quart) bottle of concentrated acid in the garage.
I'll throw in another material into the discussion and that is Sulphamic acid, bought as a white powder, is dissolved in water.
Seems a miraculous material used for descaling kettles, rust removal and 100s of other uses rather like phosphoric acid. Both are used in Food production at very low concentrations . Not tried it yet but aim to purchase a small amount and see how I get on.

The site won't let me paste any URLs so I can't link you to the safety data sheets.
The chemical supplier was a UK one mistralni.co.uk that sells online in the description shows the industrial use for the product you are looking at. I'm sure your locality will have similarly suppliers.
apologies for trolling my intention was to help.

 

[British Steel]

Hi Graham, as I said I always use electrolysis, but I've found The Secret of non-messy tanks: Carbon electrodes instead of iron / steel, I use arc gouging rods (£15 for 50, enough for a pretty big tank or a couple of small ones, also good for anodising) with the copper stripped (either ferric chloride PCB etchant, "spirits of salts"/"brick acid" (hydrochloric acid) or battery (sulphuric) acid) leaving and inch to solder wires to - these fit into holes drilled through a frame made of plastic overflow pipe and pushfit elbows to keep them upright around the periphery of the tank.
The carbon rods don't grow the "barnacles" I used to get on steel sacrificial electrodes, the electrolyte doesn't get as disgusting (still goes black from the magnetite particles the rust turns into), I don't have to scrape/brush the electrodes clean - works for me!

Dave H. (the other one)

 

[Nammar]

Gents, I have been using phosphoric acid as a rust remover for more than 20 years and find it to be an excellent way to restore parts, no matter how rusted they are. Phosphoric acid will not eat steel or paint, even though paint will deteriorate if left in a weak solution for a long time. There are two ways that I use phosphoric acid, the first been in its neat form (as you get it from the supplier) or in a diluted form, in an acid bath application. When wanting to de-rust lightly rusted parts, like machine tools or even the bed ways of machines, I use it as it comes from the supplier, with Scotch Bright cleaning pads. After scrubbing the affected areas, I wipe off and oil immediately. When de-rusting heavily rusted parts, I have a plastic drum which I fill using tap water and add one litre of phosphoric acid to every 20 litres in the plastic drum. The amount of tap water used is enough to completely cover the parts to be cleaned. I let the parts lay in the acid for the number of days that is required for the rust to be dissolved. To speed up the process, I remove each part been cleaned on a daily basis and scrub it under running water and scrape off the softer rust that has been loosened using a hand scraper, so that the harder rust comes into contact with the phosphoric acid solution. Once the parts have been completely de-rusted they look like they have been sandblasted. Once cleaned of rust, I do a final clean, dry in the warm sun or oven (when the wife is not looking) and a quick undercoat spray or oiling, to prevent flash rusting from occurring. As mentioned above, boiled linseed oil is also an option to prevent further rusting and is easily removed using paraffin, etc.

My most recent de-rusting job was the top covers of four heavily rusted Briggs and Stratton engine steel cowlings. This took 4 days to clean in the very weak solution and all traces of rusting, which was very hard and about 2 mm thick to start with was gone. Of course, the steel looks pitted after such a clean-up, but the rusting has been stopped in its tracks. From experience, if a part been cleaned is not completely submerged a sulphate build-up occurs at the air solution interface, which is hard to remove from the part. It also makes for uneven cleaning of the parts. Also, if you forget about the acid bath and leave the parts in for too long, (more than a week) they get heavily sulphated, as the sulphuric acid forms a white sulphate layer on the parts been cleaned, which is a nightmare to remove by wire wheel or by sand paper.

A few years ago I cleaned some corroded brass and copper plumbing fittings in my acid bath, which worked very well, but after that, the steel parts that I put in the acid bath took on a copper colour hue, which actually looked as if you had coper plated the parts. This stayed like this until I renewed the acid solution in the acid bath. You can use phosphoric acid to clean rusted low grade stainless steel, but it does etch the shiny finish and make it look grey or makes a blotchy finish. My dear better half poured some kettle de-scaler down the kitchen sink and it took me ages to polish out the grey blotches etched into the shiny stainless steel surface using Scotch Bright pads and elbow grease. If you look closely, you can still see the staining in the stainless steel.

A word of warning, phosphoric acid eats cast iron and aluminium. I don't recommend using the acid bath method for these materials. (Been there and done it!) You will end up with unusable parts if you do. In some cases, completely dissolved into the solution. As always, please use PPE when working with chemicals and have your acid bath out of the way of children or animals. I put a wire mesh ontop of my acid bath, which I have in the back yard, so that birds or cats don't end up falling in. I discard the spent weak acid solution into the municipal sewerage system, which helps to keep the pipes clear of calcium or fat build-up, if anything.

Something else to consider, any acid or any alkaline mixed with mineral oil or vegetable oil makes soap. If you want to clean off oil from cement, just pour some old battery acid onto the affected floor area, leave it to stand overnight and the next day wash it off using tap water and a scrubbing brush and you won't even see any more traces of the spilt oil. The acid would have converted the oil to soap and the soap will clean the cement when you scrub the floor. In the acid bath method of cleaning, if your parts do have a light layer of oil or grease on them, this should be removed on the first day of laying in the acid bath.

The above is my experience of using phosphoric acid. I hope that you have great results from this method of cleaning, as I have. Geoffrey.

 

[John Hasler]

From experience, if a part been cleaned is not completely submerged a sulphate build-up occurs at the air solution interface, which is hard to remove from the part. It also makes for uneven cleaning of the parts. Also, if you forget about the acid bath and leave the parts in for too long, (more than a week) they get heavily sulphated, as the sulphuric acid forms a white sulphate layer on the parts been cleaned, which is a nightmare to remove by wire wheel or by sand paper.

While a crust will form if the part is not fully submerged, it isn't sulphate. There is no sulfur in phosphoric acid.

A few years ago I cleaned some corroded brass and copper plumbing fittings in my acid bath, which worked very well, but after that, the steel parts that I put in the acid bath took on a copper colour hue, which actually looked as if you had copper plated the parts. This stayed like this until I renewed the acid solution in the acid bath.

That's interesting. I'll have to experiment with it.

A word of warning, phosphoric acid eats cast iron and aluminium.

I use phosphoric acid on cast iron frequently with no problems. Gives a very nice finish. Chemically cast iron should react with phosphoric pretty much like steel, and in my experience it does. Just about any acid will attack aluminum, of course.

Heating the solution to near boiling is said to speed the process enourmously (as it should) and also improve the finish. I have not tried this. Soaking in hot phosphoric acid with manganese dioxide added is basically what Parkerizing is.

 

[BGHansen]

Yep, hydrogen embrittlement, can occur with plating and acid treatments, but it's mainly a problem with high-carbon alloy steels - not a worry with cast iron.
Electrolysis, I'm told, doesn't cause it due to the polarity of the current - I always use electrolysis on steel anyway, I don't like the loss of sound metal you get with acid dips.

Dave H. (the other one)

You can boil the trapped hydrogen out of the steel by heating it to around 400F for an hour or so. Do a little googling for the exact formula. It's what platers do if steel bolts are acid washed before plating. If you can't boil the hydrogen out for some reason, you can prequalify fastener by torquing them up and letting them set for 24 hours. If they don't break after 24 hours, you won't have a HE problem.

Bruce

 

[Nammar]

Sorry, where I mentioned sulphate above, I meant Phosphate. My mistake. I should have put the brain in gear first.

What happened to me when I put a cast iron part into my phosphoric acid bath for a long soak, it came out with all of the surfaces soft, rendering the casting to the scrap heap. The cast iron looked as if the carbon had been leached from the steel up to 2 mm deep. I don't know what type of cast iron it was. You can of course use neat phosphoric acid to clean off machined surfaces of cast iron manually, but I won't be putting any cast iron castings into my acid bath again. Geoffrey.

 

[grepper]

+ 1 for electrolysis with carbon gouging rods. Inexpensive, noncorrosive and works great even with badly rusted parts. Use A&H Washing Soda for electrolyte, and the only thing to dispose of is soapy water.