Do I Need to neutralise phosphoric acid?

[boggie]

Hi All,

 

I have treated the rust on the doors with an overnight dip in a 20% Phosphoric Acid bath and it has worked well. Do I need to neutralise the acid prior to painting? I could wash them with water first but I imagine this could potentially set the rust process off once more because I have added water into the equasion. I did a trial run on an old rusty bolt and after the acid treatment I painted it with Rust-Not primer. It all still looks OK after 1 week so I was wondering if it was OK to simply prime the doors without pre-washing to remove the dried phosphoric acid away. Is this OK?

 

Thanks,

Ian

Edited June 25, 2010 by boggie

[TR Man]

Well done, using phosphoric acid is the best way to get rid of rust. I have used phosphoric acid a lot on my TR3A restoration. I agree with you about the water, but maybe if you rinsed the door with water and then immediately dried it with a hair drier or a heat gun it would probably be ok. I must say all I have ever done, having used phosphoric acid, was to clean it off with a dry cloth immediately after the reaction took place. It never caused any problems with paint afterwards. You should apply anti-rust paint immediately.

 

If you are using phosphoric acid again just paint it on with a brush and it can be agitated with some stiff bristles over the bad bits. You can wipe it off and then re-apply over the now smaller more resistant areas of rust and agitate again until all the rust has been neutralised and lifted. I have never dipped car parts in phosphoric acid and I don't think it is necessary to leave them soaking that long (i.e. over night) though I am certain it won't do any harm and I am sure you will have no rust remaining!

 

Regards,

 

Martin

Edited June 25, 2010 by TR Man

[Menno van Rij]

Don't know about the p/acid stuff, but I can relate to your question about using water on an unpainted surface. I had the same doubts when watching a guy (professional) in a paint shop using wet-'n'-dry sanding paper! Talking about using water on a unpainted surface...

 

The answer to the question 'how to get rid of the water and sanding dust?' Answers your (or better: our!) initial questions also, I think.

 

Menno

[John Morrison]

Ian,

 

where did you by the P Acid, quantities and price?

 

John.

[TR 2100]

Do I need to neutralise the acid prior to painting?

 

You need to remove it, that's for sure.

On a smooth surface, you may get away with dry cleaning although

I would probably have used at least a rag damp with alkali.

 

Be extra careful with edges, especially double skins spot-welded

together. The acid will penetrate and you'll never get it out.

 

Any reaction is likely to be some time in the future so I hope

you don't get caught out with the priming you've done already.

 

AlanR

[john.r.davies]

Phosphoric acid reacts with rust, iron oxides, to produce ferric phosphate, a relatively insoluble compound, unlike, say, iron chlorate or sulphate. Thus, the acid does not continue to attack the workpiece as those acids would, and the layer of phosphate on the surface dries to a stable coating that is a good base for further paint layers.

 

However, a coat of black phosphate may hide more rust beneath, and vigourous brushing even wire brushing is recommended to remove the phosphate layer until all oxide has gone. If after soaking and drying with no wash-off with water there is a white coat of acid crystals on the work, then this is a sign that more brushing is needed.

 

Phosphoric acid is used by dairy farmers to clean their milking machines, and is sold by farmers' supply stores in 5 litre cans as "Milkstone Remover" for about £10. That is a LOT cheaper than proprietary, phosphoric acid based, products though some of them have the advantage of being a thixotropic jelly. It is also a LOT safer than mineral acids - phophoric acid is used in foods, to give that sharp taste familar in CocaCola. It gives Coke its reputation as a rust remover, but you would have to buy a lot of Coke to de-rust a car!

 

John

[TR Man]

Phosphoric acid reacts with rust, iron oxides, to produce ferric phosphate, a relatively insoluble compound, unlike, say, iron chlorate or sulphate. Thus, the acid does not continue to attack the workpiece as those acids would, and the layer of phosphate on the surface dries to a stable coating that is a good base for further paint layers.

 

However, a coat of black phosphate may hide more rust beneath, and vigourous brushing even wire brushing is recommended to remove the phosphate layer until all oxide has gone. If after soaking and drying with no wash-off with water there is a white coat of acid crystals on the work, then this is a sign that more brushing is needed.

 

Phosphoric acid is used by dairy farmers to clean their milking machines, and is sold by farmers' supply stores in 5 litre cans as "Milkstone Remover" for about £10. That is a LOT cheaper than proprietary, phosphoric acid based, products though some of them have the advantage of being a thixotropic jelly. It is also a LOT safer than mineral acids - phophoric acid is used in foods, to give that sharp taste familar in CocaCola. It gives Coke its reputation as a rust remover, but you would have to buy a lot of Coke to de-rust a car!

 

John

 

 

Go by what John says above - he is correct.

 

Regards,

 

Martin

Edited June 27, 2010 by TR Man

[RogerH]

Ian,

primer coats need a good dry/clean surface to take hold.

Etch primer contains phosphoric acid to make a key in the surface and attack any microscopic rusty stuffs.

However the presence of a layer of phosphate may not be the best surface, and as John so very well pointed out, may be hiding some renegade rust.

You could wipe the surface down with petrol (usual H&S rules) whilst the P/acid is still wet/damp however as it is water soluable then water and a thorough dry should sort it.

 

Entrapped acid will extinguish itself after is has eaten enough rust - otherwise you would only need a teaspoonful to do a whole car.

 

Roger

[boggie]

Thanks Chaps,

 

What if I were to wire brush / sand the area them prime it in rust-not primer?

 

Ian

[88V8]

I'm blowed if I can find an agricultural supplies company in the area, so haven't managed to buy any Milkstone Remover yet. I need some for the winter campaign on The Shed. The chassis needs doing inside and out while the TR is away with Stuart.

 

On any rusty bits of the TR I've used Jenolite, two coats. That includes the whole of the door skins, and I didn't wash or wipe it off, but I did let it dry thoroughly. Then zinc primer. You could also use Jenoseel.

http://www.dgprotect...CFZeY2AodRj3q-g

 

If failing to wash is going to bite me in the a*se one day, it's waited more than two years on the door skins, they were a bit rusty and are fine so far.

 

Ivor

[john.r.davies]

OoAaargh, Ivuuuuuuuurrrrrrrr!

U bain't be goin' to foind no farrrrrrrmerrrrrrr's supply in Shepperrrrton.

'coz they sheep don't give no milk!

har, har, har, har, har, har, har, har, har, har, har ahr ,ahra, ahr ,ahra ahr ,ahra (cough, spit)

 

But there are several outfits on't 'net that will oblige by mail order; De Laval, Evans Anodine, Farm & Country Supply.

Some sell it in 60 litre containers - you could sell it on!

 

Hope you can get some.

 

John

[Rodbr]

Hi John,

I got it through Tarff Valley up in Dumfries , I expect that Mathers or West Cumberland Farmers do it cheaper. PM me if you have trouble and I can pick you some up and drop it off. It works a treat and left over night in a bucket for really thick stuff,you need to wire brush really well to open up the remaining stuff with additional soaking. 50 year old items come up like new.

Cost was £23 per 5 litres.

For long or odd shape items i.e TR2 turret cross brace, use pre-pasted wallpaper tray (Homebase), or cheapo plastic window box the long narrow type.

 

Question for the chemists amoung us, does the acid deplete or can you re-use after straining through coffee filter as you can with dry acid pickle used in pre-plating procedure.

Also does it work better if you warm it up using a Bain marie process. For that read, bucket inside another of hot water. It seemed to work well with the acid pickle.

 

I found the dry acid mix tended to form back to crystal when left, but the supplier advised me that it could be reused after straining the **** out.

 

Incidently I used a GP1 Alkaline cleaner which removes remaining grease and organic material, followed by a fresh water wash, and then into the acide pickle. Again warm conditions speed process up.

Check this out http://www.gaterosplating.co.uk/index.php.

The guy could not be more helpful.

Rgds

Rod

Edited July 5, 2010 by Rodbr

[Tapadas]

Does anyone know what happens when Phosphoric acid after it turns rust to Fe3 and then gets hot?  

I had a rusty iron heater that I treated with Phosphoric Acid (PA) and then put to work with a wonderful fire going.  The fumes were terrible.  Are they fatal?  Do I just continue to burn the fire and keep out of the way or is it a more pernicious problem? 

Do I need to do something to get rid of the PA on my heater  or will the effect wear off in time?  

[john.r.davies]

As I said above, iron phosphate is almost insoluble, but the acid is very soluble.  So, wash the part in lots of water. 

John

[Charlie D]

Many years ago I had a Rover P4 (90). For some reason I painted the rusty cast iron exhaust manifold with phosphoric acid, just to see what would happen.

When I started the engine and the manifold got hot (really hot) smoke and choking fumes filled the garage. I went outside and made sure the garage was well ventilated before I returned

To my surprise the manifold had turned snow white. I was a work of art and the finish was as smooth as could be (as smooth as cast iron can be.)

I thought about turning my discovery in to a money making idea. Selling little bottles of this "Magic exhaust  paint" at classic car shows, without revealing what it was.

Then my accountant said that it was possible someone might buy it, use it, and die, so I thought better of it.

However once the initial smoke and fumes were over there was no further reaction and I drove the car for many years more without ill effect.

Charlie.

Edited January 29 by Charlie D

[john.r.davies]

Using phosphoric acid on heated engine parts seems unwise:  IRON PHOSPHATE.pdf  Quote,  "In combustion emits toxic fumes."  Specifically Phosphorus oxides.

JOhn

 

[Trev Good]

What about panel wipe?

[john.r.davies]

"Panel wipe" is naptha, petroleum distillate, highly inflammable and insoluble in water.    Why on Earth would you want to wipe down after phosphoric acid treatment with that?

Let alone spray it on a hot engine?!