Stainless Pipe Tempering Discoloration

Why is my stainless stovepipe changing color?

From time to time, folks will contact us asking whether they've damaged their stovepipe.  It looks burned or something.  Does this mean I had a chimney fire?  Is the pipe's structural integrity compromised?

Probably not.


Tempering on our legacy stainless single-wall pipe

When you heat stainless steel in the presence of oxygen, a thin layer on its surface will change color.  The color will correspond to the highest temperature achieved.  This is a phenomenon called tempering.  Because solid fuel stovepipe is designed to withstand temperatures of around 2,100 degrees F, it's possible to get the whole rainbow of colors without structurally damaging the pipe.

Our earliest stovepipe customers (around 2016) received stainless steel single-wall pipe, which makes the tempering effect more obvious.  We have since switched to black painted single-wall pipe, which provides a more consistent look.  However, single-wall clamps are still susceptible to tempering, as well as a small exposed part of the single-to-double adapter.  The outer wall of the insulated pipe is unlikely to get hot enough to change colors in normal use.

Since the effect is only on the surface of the pipe, it is possible to remove the color.  You could return the pipe to its original condition with an abrasive polishing compound.  However, it's probably not worth the trouble.  As soon as the stovepipe gets back up to temperature, the fresh steel will take on the tempered color again.

Operating Temperatures

Tempering on our current stainless fittings

If you have a probe style thermometer mounted 12-24" above your stove, normal flue operating temperatures are around 300 to 450 degrees F.  450 degrees creates a medium brown color.  However, it's common to briefly exceed the target temperature from time to time, so purple or blue colors appearing on the lower flue parts are common.  The stove body can also get hotter than the target flue temperatures from time to time, so purple or blue colors are more likely to appear closer to the flue flange.

It's generally best to avoid over-firing your stove to reduce the risk of damage to the stove, but achieving flue temps in the 500-700 degree F range is unlikely to cause any damage to the stovepipe itself.  If you ever see any part of your stove glowing (around 900 degrees F and above), you should dampen down the air controls to prevent damage to your stove.

Tempering of your stovepipe is a good reminder of how hot your flue can get, and how important it is to properly maintain your flue system to prevent a chimney fire.  We recommend regularly inspecting your flue system and mechanically sweeping it any time creosote buildup is more than 1/8".  Our rotary chimney cleaning system is a fantastic tool for cleaning small diameter chimneys thoroughly and easily.

4 thoughts on “Stainless Pipe Tempering Discoloration”

  1. Ellie Greenfield

    Hi, I am looking for a single layer steel pipe chimney to sit on top of our wood stove, to cover the flu. I am seeking to achieve the discoloured rainbow effect. Would really appreciate your advise, if you no longer sell them, if you could name a supplier or advise me on how to get hold of one. Many thanks,

    1. Hi Ellie-

      We don’t currently offer single-wall stovepipe in stainless, but Dickinson Marine has some in 3″, 4″, and 5″. If you’re looking for stainless single-wall stovepipe in 6″ or 8″ sizes, your local hearth supply should be able to source it for you, or a search for “DuraVent DuraBlack Stainless” will return quite a few vendors online.

  2. My stainless double wall insulated pipe caught fire last year on an outdoor wood burner. When I inspected and cleaned it for the upcoming winter I noticed a few places inside that are bulged out a little bit kinda like wrinkles. Nothing appears cracked or split. There’s no discoloration on the outside wall. Should I be concerned? I use a creosote treatment regularly so I was surprised to see it catch fire.

    1. Gary-

      Good question. If you ever have a chimney fire, it’s worth having a professional inspect the chimney before you use it again. In this case, since there’s a visible change in the structure of the pipe, I’m certain that the professional inspecting would find the pipe compromised and recommend replacement. The cost of a bit of new pipe is well worth the peace of mind that if you have another chimney fire, then the pipe will do as good of a job of containing it as it should.

      It’s also worth mentioning that creosote treatment is helpful, but not a substitute for regular mechanical sweeping. We’re a big fan of the rotary cleaning kit that attaches to a cordless drill. It’s a snap to use and gives your chimney a thorough scrubbing in just a few minutes, so there’s no reason not to do it regularly.

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