What Caused This Change?

Lots of heat?

Agreed!

Is there a prize?

The color change could be caused by oxidation due to heat (~ 400ºC), but without knowing the composition of the metal, it's hard to say...It could just as easily be some chemical accumulation or reaction...or it could be a combination of the two...

316 ss

Well it looks like a temp sensor insert for a boiler, so I would say much lower heat for a longer period of time....The high chromium content of the 316 would resist oxidation for a significant period of time...

It definitely looks like heat effects. We make exhausts from 304 and 316 and they go this colour after seeing around 400degC.

Hi guys!

My assumption would be that only the lower 1 3/4" of the device is directly subjected to the heat/working fluid. The threaded portion and head would be dissipating heat into the mating threads and air, and stay somewhat cooler.

How about dissimilar metals between the plug and the flange entry, and dissimilar cooling coefficients between them and the fluid, whatever that is, allowing it to seep into the threads as we see in the pic. The stuff on the hex-head is just crap as far as I can tell. The change in shape, HMMM. Go back to supply and check the P/N!

IMHO!

I was on board with most of the others on this forum. My theory was heat. However, I still needed to confirm my guess. So, I had the bolt cold cut and I received the results this morning.

It turns out rcapper called it with little effort. Maybe it was a guess, but he stated it rather directly. I think he was able to call it.

Thank you rcapper. Your answer pointed to my OP being wrong. It happens. Evidently my belief that the discoloration was due to heat was in the majority. We were all wrong. Thank you.

As to some other respondents. . . love you too!

-A-

Cool but now the real questions to see how to prevent it.

What was the installation? What is the service? What are the service conditions? Any environmental elements that could have come in contact with this (other than 90% humidity)?

Hey! I thought you were gone!!

You see, there is still hope for this band of misfits. Someone had a helpful answer. . . . mostly. . . with no data. . . but he was right!

There are experienced engineers on here who can be helpful.

As to all of your questions, you (of all people) KNOW I can't answer any of them. Proprietary technology and all that. Suffice it to say that we use equipment to make stuff. Just like everyone else.

-A-

Is there some copper compound in the water? Iron is more electropositive than copper (or less, can't remember which way round it is), so if you dip mild steel into copper sulphate solution it soon gets a coating of copper. Maybe stainless does the same, the colour looks about right.

I've noticed similar colours on sump plugs. i.e. in a cars oil pan/sump.