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[Ken]

A few months ago I began to wonder if it was possible to make my own rust converter, the price of shop bought being a bit steep these days.
After some research and trial I have made my own and trialled it in comparison with kurust. The result is much the same, rust is converted into a dark blue/ black finish. I’m happy that it works.
For anyone wanting to try it the formula is as follows.
100g tannic acid
900 ml de ionised water
50 ml ethanol
approx 2 ml of phosphoric acid

The ethanol brakes the ‘ surface tension’ of the solution allowing it to spread.
The Ph should be around 2 to 2.5 ( I used litmus paper to test)

Tannic acid ( which is a powder and regarded as harmless) I bought from apc pure
The phosphoric and ethanol from shop-hdchemicals.co.uk

[Mycroft]

Cheers for posting Ken. I go through Kurust like water and am tempted to try this. Especially if its any improvement.

You can't do one for Metal Filler can you? Big Boy has more than doubled in price in recent months 

[Peter Holden]

gareth, I have used plain phosphoric acid as rust treatment but I am interested in breaking the surface tension

Peter

[Wittsend]

A few drops of washing up liquid should do it.

 

[alchad]

Interesting point about using ethanol to break the surface tension.

Personally I’m a massive fan of “Milkstone Remover” which is basically dilute phosphoric acid, just Google for suppliers, roughy £25 for 5 litres which should last car DIY folks years.

Alchad

[Clifford Pope]

I've always assumed that rust "converter" is doing something more than simply dissolving rust and leaving a black stain of ferric phosphate. Does it adhere permanently, does it actually form a barrier to further rusting?

What properties does ferric phosphate possess better than paint, eg zinc paint?

[alchad]

That’s how I understand it - it forms a barrier

From the horses mouth

https://jenolite.com/rust-remover-vs-rust-converter/

[Wittsend]

I've always assumed that rust "converter" is doing something more than simply dissolving rust and leaving a black stain of ferric phosphate. Does it adhere permanently, does it actually form a barrier to further rusting?

What properties does ferric phosphate possess better than paint, eg zinc paint?

You can Google for the chemistry.

The iron phosphate forms an almost insoluble barrier that keeps oxygen and moisture (in the air) from reacting with the iron (or steel).
Problem is that the phosphate barrier can be worn or scratched - letting air and water into the "wound".

The phosphate barrier acts much like etch primer on aluminium alloys and is an excellent base for paint.
Ideally, a primer, then undercoat, finally topcoat.

You can put the work piece in an electrolytic phosphoric acid bath and get a more impenetrable barrier, suitable for small items, powered by a battery charger.
 

 

[Ken]

I tried phosphoric acid, immersing the component. It left a powdery pale yellow coating. I was unsure how well paint would adhere to it plus the fine dust when rubbed bothered me. The tannic acid seems pretty much as the commercial rust converters.

[2286]

That mention of yellow suggests chromate?

[diffwhine]

I tried phosphoric acid, immersing the component. It left a powdery pale yellow coating. I was unsure how well paint would adhere to it plus the fine dust when rubbed bothered me. The tannic acid seems pretty much as the commercial rust converters.

Every bit of steel I dip in dilute phosphoric acid comes out with a dark coating on the rusted areas. I'm intrigued if your ones come out with a yellow powdery coat because that's certainly not my experience. Stupid question possibly, but is it definitely phosphoric acid, and are you definitely dipping ordinary steel?

[Ken]

Let me check again in case I ‘miss spoke’ !!!

[Ken]

Yes, my mistake the yellow powder finish was from trying oxalic acid which had been recommended. I tried using the phosphoric acid alone and found that the rust turned dark brown but that may be due to the phosphoric I have being 45%.
The formula based on tannic acid is correct and works as described. Apologies for the confusion.

[rusty66]

2ml of phosphoric acid sounds awfully little. And you used 45% acid? I’ll try your recipe, but the phosphoric acid I have and that I used in the past is 85%.

I dilute with water but far stronger than the 2ml. I have never really measured quantities.

[alchad]

I was just about to echo Rusty66’s comment about just 2ml of phosphoric acid - had a good quip about it being almost homeopathic 😀- then I Googled tannic acid.

Tannic acids are substances of natural origin, extracted using particular solvents obtained from different plant species. Tannic acid is an essential element in a rust converter. It reacts with iron oxides converting them into iron tannate, a bluish-black stable compound.

So for Ken’s mix it sounds like the tannic acid is doing most of the work? Question for Ken - did you come across this formulation online? I’m curious as to what extra the small amount of phosphoric acid is supposed to achieve.

Alchad