Coloration of SA312 TP316L pipe

[edleit]

While preparing to remove a component of our machine, the insulation (2" thick mineral wool w/ diplag fiberglass coating) was peeled back from one of the gas reformer piping runs to reveal a 6" pipe. This particular run is a 6" NPS (6.625 diam) stainless section that is socket welded into a mating carbon steel (A105) coupling. The pipe material is specified on our drawings as A312 TP316L.

The curious aspect of this is the golden coloration of the 6" pipe. I have never seen goldtone stainless and would like to know what might cause it to appear as such. The operating temperature of the reformate carried in this segmennt is approximately 1200 degrees F.

If this was localized to a region within a few inches of the coupling, I might attribute it to the weld process but this looks to have effected the entire pipe. Could this be surface oxidation caused by the trapped water left behind during the activation of the dip-lag insulation coating?

Any other thoughts as to cause and/or ramifications?

 

[EdStainless]

You have it right, it is very light surface oxidation.
It may be that the pipe was not real bright to start with, and that oxide layer grew until it showed tint.
I would expect lines that are running at 1200F to eventually all show some discoloration, bare ones don't because the outer surface is only 800F. You don't need water to do this, normal air will do it fine.

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