Unusual surface rusting

[Jon mackay]

This system was installed in a dry environment and had some light surface rusting on the burner feed. No other rusting was noted.

I was not able to identify any issues with the system but wanted to see if anyone has run into this before.

The system is 9 years old.

[Markus Keller]

I've seen that on some units when conditions are basically dry. I think it is the type of steel they used for the supply pipe. Is that a Rheem unit? I asked a supplier about it a few years ago don't remember the answer though.
I tell clients to have the hvac tech clean it off during the seasonal clean & check.

[Jon mackay]

It's a Weatherking which I believe is manufactured by Rheem /RUUD.

[Bob Harper]

Probably just plain steel instead of aluminized steel as seen in the cabinet around it. It does speak to the humidity levels in that home during summer.

[David Bell]

It is a seam welded formed steel pipe that when manufactured has a light oil on it, once the oil has dissipated the steel will rust.