Phosphoric acid ( flash rust )

[Ross365]

Many thanks for that. I have been cleaning the majority of rust through either electrolysis or grinding to bare steel where possible. The reason for the phosphoric is to try to neutralise the remaining rust and hopefully less likely to return after painting.

What do you mean by neutralise? I'm guessing that it might be the conversion of iron oxide/hydroxide to phosphate. But, how well is this bonded to the metal that is beneath it? It it's not well bonded, then neither is the paint that goes on to it.

For some primers, like zinc phosphate, it's essential that the inhibitor chemicals go directly on to bare metal so that electrochemical reactions can occur to provide corrosion resistance. If there are deposits of phosphates on the surface, that may impede those reactions.

Products for "dealing with rust" based on phosphoric acid have been around a very long time, and I used to use them. It seems to me that there is confusion now about when it's appropriate to use them. If the people who make your paint don't also make a phosphating agent, that may be an indication of what their assessment is.

 

[fractured]

What do you mean by neutralise? I'm guessing that it might be the conversion of iron oxide/hydroxide to phosphate. But, how well is this bonded to the metal that is beneath it? It it's not well bonded, then neither is the paint that goes on to it.

Yes, basically to convert the iron oxide that may not have been reached with electrolysis. I think I get most of it as I give it plenty of time to work. After electrolysis I pressure wash, wire brush, pressure wash.

The items I have done so far have generally been to go back under the land rover so have been given two coats of Bonda rust primer and a top coat. They are then getting treated with a lanolin based corrosion preventer, so I am not too worried about the finish. If I do get corrosion it won't be the end of the world, however if the items were on show I would probably look for the complete compatible package.

 

[charlysays]

I watched some tests on YouTube regarding this.
The guy tested adhesion of epoxy primer on various bits of steel including one which had been treated with phosphoric acid then wet on wetted filler over the top of the Epoxy to act as a lever- chipping at the filler was a good way to test adhesion to the substrate.

The upshot was when the epoxy was applied over a phosphoric acid treated surface adhesion was significantly compromised and the filler ripped the primer off. In all other situations the filler chipped away without any debonding.

The best compromise in my opinion is use phosphoric acid at around 20% strength, work it around thoroughly, let it react for up to a day, wipe it over with damp cloths and use a panel heater to dry it pronto, then go over it again with some 80g papers, solvent degrease and then paint. That way you'll have phosphate coating or even no rust in edges/ seams/ craters but the best adhesion possible overall having used phosphoric.

However, if your surface is st3 already then I only use phosphoric in seams etc.

 

[Dcal]

Have you a link to the test?

Sounds interesting and maybe another couple of options to try out if the forum paint test ever gets off the ground.
Citric acid would be worth trying as well because if the sample is emersed for long enough it leaves a nice fine etch.

 

[charlysays]

Have you a link to the test?

Sounds interesting and maybe another couple of options to try out if the forum paint test ever gets off the ground.
Citric acid would be worth trying as well because if the sample is emersed for long enough it leaves a nice fine etch.

Found it

Interesting stuff! Basically from what I gather , phosphoric or citric under epoxy is ok so long as the somewhat powdery residue is removed. The stable coating will still remain in the craters etc, it's just a way of getting some extra protection in those pockets.

 

[fractured]

I watched some tests on YouTube regarding this.
The guy tested adhesion of epoxy primer on various bits of steel including one which had been treated with phosphoric acid then wet on wetted filler over the top of the Epoxy to act as a lever- chipping at the filler was a good way to test adhesion to the substrate.

The upshot was when the epoxy was applied over a phosphoric acid treated surface adhesion was significantly compromised and the filler ripped the primer off. In all other situations the filler chipped away without any debonding.

The best compromise in my opinion is use phosphoric acid at around 20% strength, work it around thoroughly, let it react for up to a day, wipe it over with damp cloths and use a panel heater to dry it pronto, then go over it again with some 80g papers, solvent degrease and then paint. That way you'll have phosphate coating or even no rust in edges/ seams/ craters but the best adhesion possible overall having used phosphoric.

However, if your surface is st3 already then I only use phosphoric in seams etc.

I have used a 40% phosphoric acid but I don't know if that is better or worse than 20%.

I don't use the acid to remove rust, just the bits I can't remove via flap disc or wire brush - this tends to be in corners etc where they won't reach. The rust tends to turn black and the bare steel goes a dull grey. I then wash off before it dries, dry the water off, scotchbright then IPA or acetone and wiped with clean tissue.

I hope this works as it is a bit of effort to do.

What I can say is I have not had much luck with the rust convertors. It does not work if the rust is too deep or if there is not enough rust. That can be difficult when using wire brushing or grinding to prepare the surface.

 

[Ross365]

Somebody, somewhere must have a set of rules on exactly how/when/where to use phosphoric. Used right, I'm sure it can be useful, but used wrong, it can be detrimental. But as I've never found that set of rules, I've used it less and less over the years.

 

[charlysays]

Somebody, somewhere must have a set of rules on exactly how/when/where to use phosphoric. Used right, I'm sure it can be useful, but used wrong, it can be detrimental. But as I've never found that set of rules, I've used it less and less over the years.

i think there's always going to be some kind of "at your own risk" with use of acids really.

The big no-no seems to be leaving it dry and trying to paint right over the residue as it is, see above video- very poor adhesion, and the other one is using hydrochloric acid which is horrendous stuff. Most of us are invested long term in these sorts of projects so it's definitely worthwhile doing adhesion tests to see what works and what doesn't.

I think I'll do a couple of test pieces with jotamastic and see how good the adhesion is with no phosphoric vs a "no rinse" phosphoric use but with some wiping up of excess and then mechanical cleaning once dry.

With acids being able to work in areas where mechanical means and sometimes even blasting can't and the fact most of us have seen that a phosphated previously rusted surface seems to resist further corrosion for a long time even without any other coating, It's got to be worth trying to find a way to include phosphoric acid as means of prepping rusty surfaces.

 

[Ross365]

I moved in to my house almost 30 years ago. There were two hand valves on the tank for the heating oil; brass bodies with steel wheels on top and they were rusted as badly as they could possibly be. I cleaned them up as best I could, getting all the visible rust off, then immersed them in phosphoric and then immersed in IPA and then oven-dried. Painted with Humbrol enamel, they still looked really good when the tank was replaced 28 years later. Immersing in IPA is only going be possible with smaller stuff. If you wipe with IPA, I wonder if that can be bad as IPA absorbs water vapour?