On Monday, 13 April 2020 08:58:02 UTC+1, The Natural Philosopher wrote:
On 12/04/2020 21:50, Paul wrote:

https://www.hunker.com/13415944/how-...th-naval-jelly

An experiment I did with this years ago, when you rinse
off the rust, any shiny metal underneath *rusts almost instantly*
after the rinse. It takes a very short time for the surface
rust to come back. This has to do with the naval jelly etching pores
into the metal and increasing the surface area. You cannot
possibly apply a finish fast enough, to stop that from happening.
You would need a finish that can be applied right over un-rinsed
pure naval jelly (which is not going to happen).

I don't know what 'naval jelly' is, but phosphoric acid is not like
that, and phosphoric acid is what the OP was asking about.
IME phosphoric acid immediately converts any iron oxide to iron
phosphate, which is stable, and impervious to air and water.

And black, not shiny.

Once rinsed and dried its a good base for zinc primer.

HCl works the way Paul describes. I sometimes use it but yes, if you rinse it then let it dry it'll already be rusty. Cleaned metal must be coated immediately, eg with zinc or paint.

Zinc coating is best if you want it to last well. Can be done with a dead battery electrode & battery charger. For something like a gate, pickle & hot zinc dip sure is easier/quicker, and the zn coating is thicker and gets in where handheld electro-zinc doesn't.


NT