Rust Removal using Phosphoric Acid

[paydase]

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

What I would suggest for closed areas (where wiping off the phosphate layer and/or removing traces of acid would not be possible ), once the transformation of rust using phosphoric acid has been completed is :
- rinsing with clean water to get rid of as much remaining acid as possible through dilution (don't forget that substantial acid may remain trapped in very closed areas such as seams and welds and that the acidity (pH) is a log scale);
- then spraying a phosphate buffer which has a neutral ph of 7, that is an equimolar solution of e.g. sodium monophosphate and diphosphate (avoiding other more alkaline media such as baking soda which has an alkaline ph of 9); see for example:
https://www.thoughtco.com/make-a-phosph ... ion-603665
The big advantage of such a solution to neutralise phosphoric acid is that not only it has a neutral pH but also that phosphates are quite soluble in it; which means that remnants of phosphoric acid and undesirable thicker aggregates/layers of iron phosphates could be dissolved and washed out. Be careful however to use a diluted buffering solution and to do it swiftly (trial tests are recommended), not to attack the freshly formed passivating phosphate layer;
- then rinsing again with clean water;
- finally drying with compressed air.
The following step being the spraying of usual primer/paint and/or protecting wax

Btw, I suspect that the two-step packages of phosphoric treatments sold by industries include a second solution (they never say what it is) that may well be such a buffer with various addtives...

[64etype]

A chelation product such as Evaporust is the only way to get at rust inside seams or voids. But it does take some ingenuity to keep the target surfaces wetted. This photo illustrates how rust literally dissolves and washes away. I also used homemade wands with various spray patterns inside sills and voids. Very time consuming and works best in warm temperatures. The sump pump is sitting in a catch pan, which is in turn sitting on a heating pad, for example.

[SEJohnson95]

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? .

Yes I believe we are referring to zinc phosphate in this case, the phosphate anion is the common thing here but every source I have seen talks about using zinc phosphate. The yellow evil stuff is indeed zinc chromate etch primer.

"Red lead primer" is quite a commonly used coating too, this is a mixture of lead tetroxide (which gives the red colour) and calcium plumbate.

A quick check on the MSDS (Material Safety Data Sheet) for Fosroc Galvafroid reveals the main composition to be zinc powder and zinc oxide, along with various aromatic hydrocarbon solvents (naphtha, alkylated benzenes, cumenes, xylenes, that sort of thing) - So Galvafroid is a "zinc-rich" zinc oxide primer.

It seems difficult to find any concrete standard guidelines on phosphate content of drinking water in the UK. I did find an interesting paper on why phosphates are added to water though

"In many developed countries, legacy lead piping is a major source of lead contamination in drinking water, which has been associated with reduced cognitive development in young children and an increased risk of coronary heart disease or stroke because of increased blood pressure. Public water utilities in the U.K. and parts of Europe and North America routinely dose drinking water supplies with phosphate to prevent
pipe corrosion and the dissolution of lead. For example, in the U.K., phosphate dosing is currently the only viable approach to achieve the drinking water standard of <10 μg L−1 lead in the taps of customers.7 Inorganic phosphate (commonly phosphoric acid or monosodium phosphate) is dosed to drinking water supplies, leading to the formation of lead phosphate or calcium phosphate precipitates on the inside of service lines and household plumbing. These precipitates have lower solubility than lead corrosion products (primarily lead carbonates) that otherwise line the inside of drinking water supply pipes, thereby reducing
the concentration of lead in solution alongside the concentration of other solutes derived from pipe corrosion products, including copper."

It gets complicated quickly again as they are trying to track phosphates with labelling and all sorts.

Paper:
http://pubs.acs.org/doi/pdf/10.1021/acs.est.5b01137
DOI: 10.1021/acs.est.5b01137
Environ. Sci. Technol. 2015, 49, 9020−9028

[Hugo]

So "red primer" and "red lead primer" are two completely different things? Hammerite sell a "red primer" which is "red oxide" (iron oxide), and red lead primer is ... what you said!
I just get more and more confused - glad I'm not a chemist! ;)

[mgcjag]

Hi All....think im backing out of this one now.....one thing i know for sure is that im glad that my E type is already fully sorted .....will steer clear of acids and stick with mechanical removal (i have a grit/soda blaster) and carry on useing proprietry products....and cavity wax......all the best.... Steve

[SEJohnson95]

Yes, Hammerite red oxide primer and red lead primer are different things. MSDS for hammerite red oxide primer shows a very similar composition to the latter (c.f. naphtha,xylenes, mesitylenes and so on) with the key ingredient being "zinc phosphate pigment" and "talc, magnesium silicate" which I presume acts as a drying agent. I do not know about how effective H.R.O primer is, but then I have little experience with metal prep and painting and the like.

Being a chemist is indeed very confusing. Still, at least it's not physical chemistry - that stuff goes right over my head!

[Simon P]

Still, at least it's not physical chemistry - that stuff goes right over my head!

That's only because you're a little......mmmpppffff......rusty...

[SEJohnson95]

That's only because you're a little......mmmpppffff......rusty...

Of all the people to come up with that, I could have put money on it being you, Mr Pimblett. It also doesn't help that I despise physical chemistry either

[ralphr1780]

Yes, Hammerite red oxide primer and red lead primer are different things. MSDS for hammerite red oxide primer shows a very similar composition to the latter (c.f. naphtha,xylenes, mesitylenes and so on) with the key ingredient being "zinc phosphate pigment" and "talc, magnesium silicate" which I presume acts as a drying agent. I do not know about how effective H.R.O primer is, but then I have little experience with metal prep and painting and the like.

Being a chemist is indeed very confusing. Still, at least it's not physical chemistry - that stuff goes right over my head!

Zinc phosphate is an anticorrosive pigment, talc is a filler/extender, magnesium silicate is a clay thickener.

[Series1 Stu]

I don't know about consensus but I do know how I'm going to continue, and that is exactly as I have done so far. I will continue to use my sand blaster to remove the bulk of the rust and then immerse in phosphoric acid solution to treat the rest. I might even repeat that part of the process until I'm happy that the rust has been dealt with.

Once I'm happy that the rust is taken care of, I will apply a good quality primer and always user relatively new stock to ensure its as effective as it can be. I think what is important at this stage is to complete encapsulate the part in paint to prevent corrosive materials reaching the substrate.

Better still is to apply a suitable protective coating of zinc (for steel, at least) before the painting process. It's surprising what zinc can do to improve the life of your steel parts.

After that it's down to care, maintenance, the climate and luck.

Regards

[johnetype]

I think the consensus is that if you use just Phosphoric acid to treat rust you should wash it off afterwards so as to leave no unreacted acid or salts behind but that is a very difficult thing to do completely in practice.

The rust treatment products that do not need washing off afterwards such as POR15 Metal Prep contain both Phosphoric acid and Zinc Phosphate and it is the addition of the Phosphate that removes the need to wash it off.

Whichever rust treatment product you use, if any, the metal then needs to be encapsulated in a high quality primer to prevent further oxygen from reaching the metal. For this, like others, I've found POR15 rust preventative paint to be the best (but expensive).

[mgcjag]

Hi John....the instructions for POR 15 metal ready are to rinse off with water....from the instructions....If Metal Ready is not rinsed off you will get a large accumulation of zinc phosphate on the surface which will
affect proper adhesion with POR-15

Steve

[johnetype]

Steve, sorry, yes you're right, the point I should have made was that you could use it inside a box section - referencing back to one of the earlier posts - provided you can wash it out afterwards, whereas with pure Phosphoric acid, you'll never be able to wash it all away.

[paydase]

Steve, sorry, yes you're right, the point I should have made was that you could use it inside a box section - referencing back to one of the earlier posts - provided you can wash it out afterwards, whereas with pure Phosphoric acid, you'll never be able to wash it all away.

John, I'm not sure POR15 is recommended for application in closed, difficult to access, areas.
My understanding is that it's a good sticky rust converter and sealing material if you apply (paint) it thoroughly on a surface so as to make a thick and well covering material on all and recessed areas, stopping penetration of oxygen and moisture from outside down to the metal.
If areas remain uncovered (or badly covered), then rust will propagate. And I don't think you can easily spray POR15...

[Hugo]

That is my understanding also.

[johnetype]

Serge, with respect to difficult to access areas I was referring to POR15 metal prep and other similar Phosphoric acid/Zinc Phosphate rust convertors, not POR15 the primer paint.

Wherever you've used a rust convertor you should apply a primer or wax afterwards to seal the metal from further oxygen and water and that coverage needs to be continuous. I agree that in a box section, getting continuous coverage using a paint would be difficult and personally, I'd use a wax like Dinitrol or Waxoyl, because unlike primer paints, waxes will creep over time into/over any small missed areas.

"POR15" is not a rust convertor, just a high quality primer paint that prevents further moisture and oxygen reaching the metal it's applied to. It does imply on the tin that you can spray it; whether that's an effective way to apply a very expensive product depends on what you're trying to apply it too and I don't know what health precautions you would need to take. Even with a brush it goes on very thinly and a little goes a long way.

John

[angelw]

John wrote:

"POR15" is not a rust convertor, just a high quality primer paint that prevents further moisture and oxygen reaching the metal it's applied to. It does imply on the tin that you can spray it; whether that's an effective way to apply a very expensive product depends on what you're trying to apply it too and I don't know what health precautions you would need to take. Even with a brush it goes on very thinly and a little goes a long way.

Hello John,
It can be sprayed, but you can throw the spray gun away if it's not cleaned thoroughly and its a difficult material to clean from the gun. Let it dry on a brush and you can drill and tap, or drive nails through the resulting dried mass.

Regards,

Bill