Rust Removal using Phosphoric Acid

[38E]

I haven't read this whole thread and I'm not a chemist but I do have over 30 years experience in the OEM automotive finishing industry and a lot more years as a hobby car restorer.

There's a lot more to preparing steel for painting than merely applying phosphoric acid and I don't intend to outline it all ere but suggest that people use the specific products provided by the major paint companies for exactly that purpose.

PPG for instance have always had their metal prep solutions available from body shop suppliers and provided spec sheets outlining their usage for different substrates but now they have produced an excellent manual specifically aimed at restorers and I'd recommend it for anyone seriously interested in the subject.

I'm not sure if PPG products are available in the UK but there are probably domestic equivalents. The manual can be downloaded here as a .pdf:-

http://us.ppgrefinish.com/PPG-Refinish/ ... tion-Guide

[chrisfell]

There's one sure way of ridding your car of rust, and it doesn't involve dipping or painting or plating using any fluids or gels.

1. Scrape away the paint until there is bare metal.
2. If you find rust, any rust, cut it out.
3. Fill the hole with mild steel of a suitable gauge.
4. Smother the whole thing with etch primer.
5. Paint to taste.
6. Drive until rusty and repeat.

[Hugo]

There's one sure way of ridding your car of rust, and it doesn't involve dipping or painting or plating using any fluids or gels.

1. Scrape away the paint until there is bare metal.
2. If you find rust, any rust, cut it out.
3. Fill the hole with mild steel of a suitable gauge.
4. Smother the whole thing with etch primer.
5. Paint to taste.
6. Drive until rusty and repeat.

Great in theory, but not such good advice in pratcice. Practically every bit of steel on the car, even perfectly sound metal, will have a bit of rust on it somewhere or other. If it's bad enough to warrant cutting out and welding, that's easy (it's an easy decidion to make, I should say). The question is how to treat all the other little bits of surface rust etc. that you're going to find all over the place.

[SEJohnson95]

OK, I couldn't ignore this thread any more - I mean, come on... SCIENCE!

(First of all, my university report is now finished. Thank god for that. (8,000 word discussion, 22K in all, 110 pages ) Yes, I am indeed alive. I only seem to surface every now and then at present.

Right - here's my 50 centimes (1/2 Swiss Franc) worth. I've been chatting on the phone to my mate who is an inorganic materials postgraduate. I'm an organic chemist by trade and although know a bit about this stuff, I thought I'd ask someone who knew what they were doing.

Firstly, let's just dig into a bit of formulae - iron oxide, more specifically iron (III) oxide is Fe₂O₃. This exists in a few forms - the classic "rust" colour which we typically observe and despise. The compound also exists in the form of Fe₃O₄. This is the basis of the mineral magnetite and is comprised of a mixture of iron (II) and iron (III). Fe(III) is the most stable and it will

This is the stuff you are most likely observing upon treatment with the acid (I shall return as to why in a bit) but essentially any Fe(II) you form is not going to want to hang around in that form for very long (solid state is different, with crystallography and the like) and will convert back into Fe(III) - and then you're back to square one. Fe₃O₄ will not provide adequate protection and in itself is not a passive layer.

Iron phosphate is also - in its pure form - yellow. The likelihood that you will fully react the phosphoric acid is slim (it's a triprotic acid - three protons rather than the one of HCl), besides which the stuff is quite water soluble so is unlikely to stick around. In theory, it is possible to form iron phosphate but then you risk it being hydrated and reforming iron oxide in either form and then you're back to square one - the ions can substitute. You usually purchase the stuff as its hydrated form. Iron phosphate is also therefore not a good passivant. Recall that your solution is 85% wt so there's 15% water in there, with a bit of extra oxygen knocking around in air which dissolves into water.

Industry tend to use HCl for dissolving "rust" but this can be removed with any mineral acid in principle. The issue with treating a metal with acid it its purity - trace metals or carbon within the steel could also react which is only likely to further compromise the integrity of the metal. You could also form HCl through reactions with the steel and ions in the water too.

Boiling acid in any form is also not a particularly nice thing, not recommended. Highly concentrated tripotic acids are not the nicest things to have knocking about either. There is no substitute for stripping metal back with a grinder or sander, welding new metal in and etch priming. (Richard for example uses the zinc chromate etch primer - bloody evil stuff but it does the job well. You can spot weld through it and you leave primer everywhere). Plus unless you do anything to neutralise the acid I can't say I would be be sure it wouldn't cause problems - paintwork leaching for example.

I hope that clears a few things up - happy to try and explain stuff myself.

Disclaimer: This is not from somebody with experience in the motor industry but rather works with inorganic materials on a regular basis, and also an organic chemist .

[mgcjag]

Hi Simon....thanks for posting....lots of info...but to be hosest it went over my head a bit......in the last paragraph you mention (unless you nutralise it).....is water good enough......or are there salts still left on the surface.....can you scrub them off.....or does this damage the converted layer......my interest is in box sections on chassis where the outside can be mechanically cleaned till bright steel.....but want to do something for the light/rust inside..thanks.. Steve

[paydase]

Very interesting thread, issue addressed many times but still not clearly answered.
I dare chiming in, chemistry background too, though university is long time ago...
Simon made a good point wrt the fact that water is always around and that phosphates love being hydrated. Others already addressed the various issues concerning the density of the layer, the need to rinse, dry and wipe it or not.
All of these salts are polycristalline structures of chemical compounds that are usually not perfectly covering (cracks, microscopic pathways) nor stoechiometric, meaning evolution during time (through hydration, diffusion of oxygen and hydrogen leading to oxidation underneath, transformation towards other salts or oxides, etc.).
Additionally, traces of acid or reactive compounds contained in the phosphoric bath may indeed remain trapped on the surface or within the covering layer after treatment, even after rinsing, wiping off and/or drying.
A posssible approach to address this would be to neutralise the remaining reactive elements after phosphoric treatment using an adequate chemical buffer or chelating media (maybe some ammonia based solution, dunno, would need some investigation not to annihilate the beneficial effect of the phosphatation). Some of the treatments sold by brands may include this kind of secondary treatment.
If the treatment solely consists of phosphoric acid, I think that it is important to thoroughly rinse (to remove most of the reamining acid) and dry (to remove most of the trapped water) the surface afterwards, then to softly brush the excess phosphate (to leave only the minimum thickness that is passivating the steel; more thickness is prone to possible cracks or flaking), then to apply the usual primer asap.
Meaning also that it is not adviseable to treat areas that are more or less hidden or closed.
Btw, avoid chlorhydric acid at all, it would guarantee rust coming back even more!

[Series1 Stu]

Brilliant!
Now we're getting somewhere.

I suggest the process is:-

1 Abrade rust away best as possible.

2 Phosphoric acid dip for time appropriate to strength of solution and severity of rust.

3 Rinse with lots of water.

4 Pat dry

5 Hot air dry

6 Etch prime x 3

7 Paint

8 Pray and hope for the best

9 Repeat in 20 years time

[38E]

Etch primer is old tech. There have been much better methods available for years. And you should never apply etch primer over a phosphate treatment in any case.

There is an awful lot of nonsense on car forums and even from professional bodyshop people regarding painting. If you want to listen to those folks, ask yourself how long the average collision repair is expected to last.

If you want your car to look the same in 25 years as it did the day it was painted, why not follow the procedures and use the products specifically designed for that purpose like those mentioned in my previous post, designed by the same industrial chemists who specify what industry uses to paint your new car and who know what works and what doesn't.

A premium paint job would consist of properly cleaned steel which is then treated with a zinc phosphate conversion coat, properly rinsed and then coated with a catalyzed epoxy primer. Not an etch primer. Then the hi-build and colour coats can be applied.

The zinc phosphate treatment converts the steel surface to a lattice-like crystalline structure ideal for paint adhesion and provides maximum corrosion resistance. Practically every industrial or commercial paint line will use some form of phosphate treatment before any paint goes on.

All this can be done in the home workshop using the products mentioned, basically cleaning solution, phosphating solution and clean water, Scotchbrite pads and elbow grease.

[Hugo]

With respect, you're talking about new cars and we're discussing old cars. There is a big difference.

[38E]

No, I'm talking about restoring old cars.

The PPG restoration manual I linked to is aimed at the serious hobby restorer. I am, or was a hobby restorer who also happened to work on the design of industrial paint systems and have managed to successfully apply some of that knowledge to my restorations.

[Hugo]

Sorry, I was going by this; "designed by the same industrial chemists who specify what industry uses to paint your new car and who know what works and what doesn't.".
Is there an argument against using etch primer? I use it simply because everybody I know does so. I'm told it will resist rust because it is acid-based. I have no clue if this is correct or not - I am as ignorant as the next man about this subject.
In fact, have we even reached a consensus after all this debate?

[mgcjag]

Hi Clive...i did download the PPG guide....the only ref to rust removal was do as much as possible by mechanical means..
For maximum corrosion protection, use both SX579 and SX520.
SX579 METAL CLEANER
Cleans metal and stops rust.
Will remove slight surface rust.
Leaves a rust-colored phosphate coating BUT it is
NOT rust!
SX520 METAL CONDITIONER
Conversion coating
Leaves a blue/green-colored zinc phosphate coating with
added corrosion resistance.
REMINDER: Bare steel starts to rust in 30 minutes a

So very similar to other products offered in the UK...but still dosnt deal with the problem of rust in awkward to get to areas, box sections, seams etc....... So but dont think its what some of us are after...maybee we want too much.but thats what we are trying to find out....... My ideal would be ... for example a piece of box channel, say 3ft long would be to soak it in something to disolve all the rust.....take it neutralise with water dry then prime....this sounds simple but everything i read point to being able to neutralise but not getting rid of all the surface salts

[38E]

Is there an argument against using etch primer? I use it simply because everybody I know does so. I'm told it will resist rust because it is acid-based. I have no clue if this is correct or not - I am as ignorant as the next man about this subject.
In fact, have we even reached a consensus after all this debate?

Hugo,
The phosphate treatment is much better because it provides a key for the paint to "bite" into and adhere. It also aids corrosion resistance such as that from a stone chip because it prevents any rust from spreading.

[38E]

Is there an argument against using etch primer? I use it simply because everybody I know does so. I'm told it will resist rust because it is acid-based. I have no clue if this is correct or not - I am as ignorant as the next man about this subject.
In fact, have we even reached a consensus after all this debate?

Hugo,
The phosphate treatment is much better because it provides a key for the paint to "bite" into and adhere. It also aids corrosion resistance such as that from a stone chip because it prevents any rust from spreading.

Check out "zinc phosphate conversion coat".

[38E]

Hi Clive...i did download the PPG guide.......but still dosnt deal with the problem of rust in awkward to get to areas, box sections, seams etc....... So but dont think its what some of us are after...maybee we want too much.but thats what we are trying to find out....... My ideal would be ... for example a piece of box channel, say 3ft long would be to soak it in something to disolve all the rust.....take it neutralise with water dry then prime....this sounds simple but everything i read point to being able to neutralise but not getting rid of all the surface salts

Steve,
I agree that is a problem and the procedures are mostly aimed at the exterior panels. I don't personally like using chemical solutions inside places where they cannot be rinsed out properly. I did have a few items dip-stripped and it was a nightmare getting out all the residue.

What I did mostly was to use a light sandblaster to clean out as many of the inaccessible box sections as possible. This is a lot easier where you have panels opened up so you can get inside. There are also usually plenty of access holes that can be used. However, it's also a lot of work cleaning it all out later with a vacuum cleaner with skinny extensions. Getting in behind the dash for instance was quite a challenge.

A sandblast is also an excellent key for epoxy and by using a small Binks type spray gun, I sprayed 2-part epoxy into all these places as much as possible. Later, I spayed all the box sections again with LPS3, a heavy bodied mil-spec rust inhibitor.

Here's my car 20-odd years ago. When I sold it recently, there wasn't a speck of rust anywhere and no paint defects showed up.

[SEJohnson95]

Hi Simon....thanks for posting....lots of info...but to be hosest it went over my head a bit......in the last paragraph you mention (unless you neutralise it).....is water good enough......or are there salts still left on the surface.....can you scrub them off.....or does this damage the converted layer......my interest is in box sections on chassis where the outside can be mechanically cleaned till bright steel.....but want to do something for the light/rust inside..thanks.. Steve

The water will not neutralise the excess acid - merely dilute it (which if you're adding water to an acid would result in a bit of heat too). Excess acid would need neutralising with a corresponding amount of base (alkaline solution perhaps) which would effectively form "salts" -as that is a product of an acid and base reaction. I suppose rinsing would remove these inorganic salts but then you're throwing water into it again.

So far as box sections are concerned I don't know what else to suggest other than the usual waxoyl treatment. POR-15 or similar products work by forming a non-permeable coating over the rust which prevents any further moisture getting to it to continue the oxidation process.

I really don't know what else to suggest, I'm a bit stuck. I don't wish to put too big a downer on the technique but I'm not too sure what's going on myself yet.

From my understanding of this paper on phosphate coating (http://phys.mech.nw.ru/e-journals/RAMS/ ... ayanan.pdf) then the bath contains a mixture of phosphoric acid and phosphate salts (zinc phosphate I think) which results in the deposition of a layer of insoluble crystalline phosphates. This is what stops the oxidation process. (compare with POR-15 at this point) Unsure on the longevity of the coating for use in box sections. Simply washing with phosphoric acid is not going to achieve the same effect as you are not allowing for the build up of this passive layer of phosphates. These baths also contain "chemical" additives (often chromium compounds) which speed up the process as often phosphate coating is a slow process - highlighting again how simply painting acid onto the surface will not achieve the desired effect.

You make it sound like salts are harmful but I believe that this phosphate salts are the very reason that a crystalline layer will form to protect metal.

From the paper:

"Acid cleaning or pickling using acids such as HCl, H2SO4 and H3PO4 is a very effective method for the removal of rust and mill scale. Dilute solutions (5-10% by weight) of H2SO4 and HCl are used in presence of inhibitors to remove the inorganic contaminants by converting them into their ferrous salts. Pickling in H2SO4 is usually performed at high temperatures (about 60 °C). H3PO4 is an excellent time-tested cleaning agent which not only removes organic and inorganic solids present on the metal but also causes chemical etching of the surface by reacting with it to produce a mechanically and chemically receptive surface for subsequent coating formation."

The paper seems to suggest the opposite to what my friend is suggesting as he doubts the total conversion to form iron phosphate. One thing I am fairly certain is that simply treating with phosphoric acid is not going to have the same effect as the "phosphate coating" process referred to here. It is also in agreement with what Clive said about chemical etching for keying in for paintwork.

Zinc chromate primer is evil stuff there's no doubting it (cancer in a tin effectively) but it does provide a good etch surface for paintwork and then like. Keys in nicely and will prevent oxidation of iron. Only going on what I have learned from Richard, who has been using the stuff on cars for the past 20 years or so.

Certainly finding this whole thing very confusing...

[Hugo]

It's got to you, hasn't it? Your last sentence is definitely confused - I think you mean you would not be a fool to admit you're confused ;)
Anyway, back to business; My understanding is that "phosphate" is merely an adjective. Are we talking about zinc phosphate? And that evil yellow 2K stuff that I call etch primer is actually zinc chromate, is it not? So what about zinc oxide? And "red" oxide - what is that? Google tells me it is iron oxide. I thought that's what we were trying to get rid of. My friends the welders swear by Galvafroid, which is a "zinc-rich primer". Is that zinc or is it zinc oxide? I have a tin of 1k etch primer, but's it's all in Greek so I have no idea what's in it.
I, too, would be a fool not to admit I'm confused - probably more so than when I came in.

[Hugo]

[quote="Hugo"]It's got to you, hasn't it? Your last sentence is definitely confused - I think you mean you would not be a fool to admit you're confused ;)

Nooooo! That's not what I meant to say!!! In fact I'm damn sure it's not even what I wrote! I give up!

[Series1 Stu]

The problem with internet forums is that people speak with apparent authority when they don't really know much about the subject.

I'm not convinced that any of us know what we're talking about with any degree of certainty. Hence my fact finding mission.

So I'm wrong about etch primer? Ok, maybe I am but I'm here to learn. So instead of berating people for their lack of knowledge why not offer alternatives in way of product names etc.

As far as neutralising the phosphoric acid is concerned, surely the water rinsing process will ultimately​ dilute the acid to a negligible level of concentration. There is probably a higher level of phosphates in mineral water than in tap water after it's been used to neutralise the phosphoric acid.

Please, let's keep it civil.

[Hugo]

Do we have anything approaching a consensus on any aspects of this topic? I've lost the plot completely! (Mind you, I didn't know much to start with!).
One thing I will say is that I wouldn't bother trying to 'neutralise' any rust inside box sections, sill etc. I would just flood it all with cavity wax - something that stays soft and will creep into all the nooks and crannies and stay there.