Corrosion of Constantan or Copel at elevated temperatures 1

[Sodboa]

I am trying to develop a device that would use a Copel/Chromel thermocouple in a application that might see temperatures up to 1500°F (815°C). I understand that the Copel or Constantan material would be at or exceeding it's upper limits, so I am looking for an alloy that may extend the life of the thermocouple at elevated temperatures. I see there are a few variations of Constantan, but not sure which one would resist corrosion.
Constantan - 44Ni 54Cu
Copel - 43Ni .05Cr 1.75MN 55Cu
Copel X - 45Ni .05Cr .05 Mn 55Cu
Copel XM - 43Ni .05Cr .7Mn 55Cu
Copel XS - 44Ni .9Mn 53 Cu

I can guess that the Ni and Cr are there for corrosion resistance, but such small variation in Mn have to be there for a reason.

The Chromel should be good up to 1100°C at least from what I have found.

 

[EdStainless]

While the metals may survive 1500F you do know that the junction will be worthless.
People like 'E' because it has a high mV/deg output, and this makes it good for use below RT.
However the output curve is very non-linear (9th order) and they are very sensitive to contamination.
Any exposure in dry hydrogen, vacuum, or any sulfur (even small traces) will accelerate the drift of output.

I should add that the Chromel will undergo significant changes as well. These have been documented starting around 1,000F.

The Cr and Mn are there to manage the properties. You can't afford to make ultra-high purity wire for TC use.

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P.E. Metallurgy, Plymouth Tube

 

[Sodboa]

I'm not actually worried about temperature measurement. I'm using the EMF as a power source, think like the thermocouple on your water heater that keeps the pilot lit. I just want to make sure I have material that won't corrode away. The high mV/deg is precisely why I want to use it for a power source.

 

[EdStainless]

You need to look up 'thermopile' This is how power is generated on satellites using radiation sources, and I have seen tractors with no alternator, just T-P on the exhaust for electricity.

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P.E. Metallurgy, Plymouth Tube

 

[Sodboa]

Thermopile is basically a series of Thermocouples in series. I'm really interested in getting to an alloy that will give me the best corrosion resistance at high temps, so that's why I kept this in the material area.

 

[EdStainless]

Many use Ni vs Ni-Cr to do the job.
Many in flame detection service use 30-60 junctions to give 500mV-1V output.

http://www.aqpl43.dsl.pipex.com/MUSEUM/POWER/thermoelectric/thermoelectric.htm

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P.E. Metallurgy, Plymouth Tube

 

[Compositepro]

You can use a jacket or sheath around the thermopile for corrosion resistance so it becomes a non-issue.

 

[Sodboa]

I am putting potting the thermocouple into a stainless steel can, but the Constantan is going to be exposed to this elevated temperature. If air (oxygen) can get to it in anyway, the copper is going to oxidize. I suppose some form of ceramic coating could help, but if the material is resistant, that is much more preferable.