Flash rust after treatment with phosphoric acid

[Micha]

I soaked the fuel tank in a solution of lemon salt for a few days, then rinsed it well and soaked for two and a half days in phosphoric acid mixed with water.
The metal got a nice shade of gray, but after a few hours the whole inside was covered in flash rust.
Why is this happening?
What did I do wrong?
How do I handle that now?

[schrader7032]

I googled "flash rust" and got quite a few hits.

[niall4473]

I presume that you mean after you removed it from the phosphoric acid solution?
Firstly did you dry the tank inside, was it humid where you were, and did you do anything to exclude oxygen from getting to the bare exposed steel?
I would probably repeat the treatment, flush out, and do the above, the last by coating with an ethanol resistant resin or paint as soon as the tank is properly dry inside.
Good Luck!

[Daves79x]

You've got to get the inside of the tank very dry, very quickly. Rinsing with extremely hot water helps, then compressed air out in the very hot sun. Rinsing with alcohol will remove trace moisture. Let it sit upside-down and pat any puddled water from around the filler neck with paper towels. Then coat the inside with a light oil if you are storing it, or fill with gas if not.

It's a sucky job, no doubt about it.


Dave

[Micha]

Thank you all.
I also found a lot of links on the subject, but did not understand why this is happening, what I did wrong and how to handle the tank now, after flash rust has already formed.

I understood the explanations, but I do not understand this: why do you need to paint or oil the tank after etching?
That is, if the walls were etched with acid, why does rust continue to form?

[schrader7032]

Just about all things steel, expect for stainless steel, will rust over time when exposed to air. It's just a natural process. Thus the need to coat the inside of the tank to keep the air/oxygen away from the steel.

[weh8127]

Two reasons. The surface of the steel is now chemically clean so nothing comes between it and air which is 20% free oxygen which is a hugely active element and wants to bond with everything it can, in this case the iron in the steel. Secondly, etching the surface does the same thing for the oxygen as it does for the material you will use to coat the interior of the tank; it increases the surface area for everything to cling to. I'm no chemist; I just watch a lot of PBS.

[scottiesharpe]

where the water, iron and air meet, an electrochemical reaction pulls oxygen from the air, forming hydroxide ions in the water. Where metal is covered in water, iron atoms lose electrons, which causes the metal to slowly disintegrate; ionized iron dissolves into the water. In the water, the dissolved iron reacts with the hydroxide ions to form rust. --sciencing.com

Acid-etched metals must be "rinsed" with solvent. The solvent will remove moisture from the surface. Good solvents are Acetone, lacquer thinner, MEK, Isobutyl Acetate, etc. but ensure that the solvent you use is compatible with the next coating process.

[weh8127]

Yeah, what he said.

[Rodolfo850]

you could dry with an electric heat gun or propane torch, also can be protected with RUSTLICK 631 (similar to WD40) or RUSTLICK B (soluble rust inhibitor if used on a Vapor blast cabinet), it will avoid flash rust..