Why not pure phosphoric acid for rust removal?

[Ross365]

Phosphoric acid is used a lot in the dairy industry for milkstone removal and is the active component in a lot of rust remedy stuff - it turns the rust to a dark black coating.

But if you put phosphoric on shiny steel, it leaves an invisible film of metal phosphate (it may slightly dull the metal). Re hydrochloric acid, the chloride is ion is one of the best initiators of corrosion in the world, and once it as adsorbed on metal, is hard to remove. I generally avoid chloride solutions of any kind. The nuclear reactor at Hartlepool once sucked in saltwater water in to it's cooling circuits (early 1980's). Some of my mates bought new cars or paid off large chunks of their mortgages on the overtime they got out of de-chlorinating that.

 

[fizzy]

You have to be VERY careful with hydrochloric - you don't want those fumes in your lungs.

 

[fizzy]

Great test here - hydrochloric by far the best.

 

[decca]

I bought a 80% tub of phosphoric off eBay and am using it on bare surface rust. Do you have to wash it off or can you leave it and paint over it.

 

[Seadog]

it is very good for rust and millscale removal and leaves bright clean steel

For 30 seconds after you've rinsed it off. It then stats to rust in front of your eyes.

 

[fizzy]

For 30 seconds after you've rinsed it off. It then stats to rust in front of your eyes.

Apparently application of wd40 straight away stops that.

 

[Seadog]

Not much use if you're intending painting the surface.

 

[fizzy]

Not much use if you're intending painting the surface.

You could always degrease it later.
When I feel a bit better I will give it a go.

 

[Morrisman]

That sounds more like Muriatic/Hydrochloric acid - that is what is used in brick/concrete cleaner and the fumes are very potent and will rust anything nearby - it is very good for rust and millscale removal and leaves bright clean steel
Phosphoric acid is used a lot in the dairy industry for milkstone removal and is the active component in a lot of rust remedy stuff - it turns the rust to a dark black coating.

Good point. It may well have been muriatic, the name rings a bell.

I’m sure I’ve come across ‘Phossy acid’ at some point though.

 

[fizzy]

Good point. It may well have been muriatic, the name rings a bell.

I’m sure I’ve come across ‘Phossy acid’ at some point though.

You will have drunk it.
Coca Cola and many others.

 

[Ross365]

This thread just seems to confirm what I already thought; there seems to be a general lack of knowledge about how to get the best results from phosphoric. I'm in no doubt at all that phosphoric can do a really good job when used correctly, but incorrect use can be a screw-up.

 

[MetalMonkey]

This thread just seems to confirm what I already thought; there seems to be a general lack of knowledge about how to get the best results from phosphoric. I'm in no doubt at all that phosphoric can do a really good job when used correctly, but incorrect use can be a screw-up.

It is fairly straightforward. If you are buying it concentrated, dilute phosphoric acid down to about a 20% - 30% mix with water.

Apply it to the rust as you would with any other commercially available rust treatment, i.e work it in to any nooks and keep the surfaces wetted for about 10 minutes.
Let it dry, and the rust turns black. Reapply if you missed a bit or it needs further treatment.

For paint prep I then like to rub it down prior to priming - you don't need to remove every trace, because the rust is now neutralised and is actually provides a good key for the primer anyway.
I like to knock off any dusty residue with a scuff pad and panel wipe it over just prior to spraying.

Basically the same steps you use for something like Jenolite, which funnily enough is 30% phosphoric acid but a lot more expensive!

 

[Seadog]

When you dilute it make sure that you add acid to water and not vice versa.

 

[graffian]

I use phosphoric acid.
A ~15% solution poured in a very rusty[thick crusty rust] petrol tank and the next morning the tank will be clean grey steel, the phosporic
acid solution that is drained from the tank is completely clear. If it's not clean grey steel let it soak until it is.

 

[charlysays]

I use phosphoric acid when I'm in a rush on outer panels (I use deox gel otherwise). I use it mostly under a car where perfect prep is not required and I don't want the mess and rinsing/ flash rust issues with deox gel.

It can get very technical as to the chemistry... Some say there will be salts welded to the substrate which will spoil your day, others say the acid keeps attacking stuff forever.

I've not had any issues with using phosphoric acid 40% to remove and neutralise rust in craters. I brush it on, leave it 10 mins, then keep it moving with a fine wire brush. I wipe clean with a rag to see how it's progressing. When I'm happy at the level of rust removal I wipe it clean, allow to dry.
It leaves a protective coating which seems to stop corrosion for a good while indoors.

When I'm ready to paint I hit it with a wire wheel, then 60 grit, then degrease with IPA and epoxy mastic.
Not had any adhesion issues with any test panels I did this way either.

Rinsing under a car isn't a particularly good idea if any seams are open.

I tend to use TM chemicals scale-x which wets out better than pure phosphoric acid.

 

[Ross365]

It can get very technical as to the chemistry... Some say there will be salts welded to the substrate which will spoil your day, others say the acid keeps attacking stuff forever.

I tend to use TM chemicals scale-x which wets out better than pure phosphoric acid.

I don't think that there is a problem with phosphoric "attacking stuff forever", but it may well be the case that chloride ions can get involved in "chain reactions" which persist and why I never use hydrochloric.

As for wetting out, you might try adding some IPA to the mix as a thinning / wetting agent. I don't think it will get involved in the chemistry of what goes on, but might make the phosphoric easier to apply.

 

[Farside]

There's an online chemical supplier that sells phosophoric acid, 85% for £3/l, compared to £12/l for the stuff sold as rust remover (which is probably also much weaker). Any reason not to use the cheaper pharma grade generic acid? This is for submerging rusty parts.

And is it enough to rinse parts or do they need to be neutralised?

I often use 20 or 30% dilute phosphoric acid, painted on, left to dry overnight, then hose off next day. I had good results with that for years.
As an experiment I didn't hose it off and painted over it - that was a couple of years ago and nothing's broken through the paint yet, on a bracket that's exposed to the worst of the weather 24/7.

 

[mtt.tr]

I remember my boss left a pot of hydrochloric acid open a warm workshop most of the machine tops went rusty

 

[Morris]

I use the full strength phosphoric acid for rust removal, works great. I do a final wash at 15ish % and dry, never had a problem. If you dry from the strong dip you can get a black sticky mess appear from crevices.

 

[qwakers]

I use phosphoric acid diluted and paint over it a couple of hours later after it’s all turned white. Never had any paint issues, never wash it off either.