[Ren]

Hello everyone. I'm looking for an answer regarding odd behavior of chemical process I'm generally familiar with. When restoring metal parts I often use weak solution of phosphoric acid, which reacts with oxides of iron, leaving steel surfaces gray, mat and slightly dusted. Such surface is passivized and ready to be painted over.
Recently I tried to use this method to clean inner surface of a tank, to prepare it for an epoxid paint. Immediately after the application the surface looked as expected, consistently silvery gray, shiny, with the characteristic smell of steel. What was my surprise some 24h later, after curing and drying. The whole surface seems to be smooth and painted-like with red-brownish glossy paint! The first idea was of some major impurity coming into the process but repeated procedures with a different bottles of acid resulted in the same outcome. Any guesses what is happening there? What is that surface, and how stable is that? I'm not much into the risk of painting over that without a knowledge of what happened, for one of reasons is the epoxid paint is nearly impossible to be completely removed even when not having a proper bond with an underlying material. I'm attaching pictures before and after the process, unfortunately I didn't make one freshly after applying the acid.

Before first try:The result:

[chenbeier]

Is this really steel. Probably  chromium or nickel plated iron.

[Ren]

Well, not that easy to tell if it is nickel plated inside but not from outside, considering normal reaction of the acid on the testing spot. I wouldn't bet on that, but I could try to strip it clean with something stronger, let's say 15% HCl. I usually use it during nickel plating, even though not in such large scale, the tank in question has quite a surface area.
I've been also warned somewhere, because it is a gas tank, that longtime exposure to petrol products can affect the reaction too, even though I'm not sure how true is that. The tank was empty for years, recently thoroughly rinsed by water and acetone, followed by multiple phosphoric acid tries I mentioned before.
edit: I forgot to ask, IF it would be nickel plating, what is the surface now and how stable could it be?

[chenbeier]

Probably a copper layer.

[Ren]

So, I made some pictures of the whole process to show how much it is NOT a copper layer. I even cleaned the surface with 15% HCl before all this.

Let's start with our usual rusty steel:


After bathing in the phosphoric acid we can see and smell nice, clean steal, exactly as expected:


But now, as the surface slowly dries, weird rusty-like patches start to appear:


After 24h of curing is the surface left like this, smooth like painted, brown and odd:


It seems the process can be repeated any number of times. I do use those phosphoric acid-based products very often but never saw anything like that till now, usual surface is gray and flat, almost dusted like here.