Fittings for gas pipes got veneer of rust during pressure test

Installed new black pipe for gas. Plumber will hook it up to meter and appliances. Pumped it up to 15psi for pressure test. Was fairly shocked a few hours later to see a veneer of corrosion on the fittings, like they had been there for years. There is a slow leak that I need to find, pressure is going down over a period of hours. I am guessing that as it depressurizes, the water vapor in the compressed air is condensing. Still should I be worried about that corrosion appearing on the outside? The metal is from the local plumbing supply store, US/Canadian not Chinese.

I am kinda wishing now I had installed galvanized fittings.

Edit: The rust has a distinctly localized look to it. E.g. on the elbows, it follows the weld line along the spine of the elbow.

Sounds like surface rust from condensation due to temperature differential. Forget about it.

Black pipe was born to rust.

The surface rust protects from deeper rusting. Elbows are cast as far as I know and do not have a weld line. That's a die line.

ballvalve:
>>>Elbows are cast as far as I know and do not have a weld line. That's a die line.<<<

You can't use cast on a gas line!

I was not discussing "cast iron" .....You can cast steel just as you cast cast iron. Elbows are not fabricated with welds.

Elbows are black steel, not CI, thanks.