Increased emissivity in stainless steel

[isoca]

I am working in a project where I have to increase the thermal emissivity of a tube made of SS 316L. One option is to produce an oxidized surface treating the material at 350ºC in an air atmosphere in order to have a blueish surface.
Does anyone have any experience in this type of process? I mean, to oxidize stainless steel to increase its thermal emsivity. My client needs an emissivity value greater than 0.4
Thanks!

 

[IRstuff]

https://www.design1st.com/Design-Resource-Library/engineering_data/ThermalEmissivityValues.pdf

https://www.engineeringtoolbox.com/emissivity-coefficients-d_447.html

https://www.omega.co.uk/literature/transactions/volume1/emissivitya.html

Sanding blasting, plus some sort of anodizing/oxidizing

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[EdStainless]

Roughen the surface, either pickle or blast and then oxidize.
I would go hotter than 350C. You may not want a thick soft oxide, but something in between. You will want hot enough that you lose the metallic luster. It is hard to predict a temperature to use because it will vary by surface condition and atmosphere (esp humidity).
We used to do this for Ni alloy tubes used in solar absorbers, they were oxidized at 1065C

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[isoca]

Hi, Thanks for your prompt answers.
The issue now is how can I measure the achieved emissivity in an easy way..

 

[TugboatEng]

The cheap handheld IR thermometers don't compensate for emissivity. Comparing a surface temp measurement to a treated vs. untreated surface should give you enough information to determine the difference in emissivity. I believe the cheap IR thermometers assume an emissivity of 0.8

 

[EdStainless]

Hire a good optics lab to do it. This is a physics problem that requires sensitive instruments.
Or you could do it comparatively, but it is difficult.

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[isoca]

Thank you for your answers.
The idea of doing it easily, is to be able to measure it in situ during the development stage. Of course final control should be carried out in an device specific for emissivity measurement.

 

[IRstuff]

You can actually do a pretty decent job of testing, even with cheap instruments. Arrange your IR pyrometer at a 45 degree angle to the normal of the surface; move a very cold object, say a cup of LN2, to the specular reflection angle. If the apparent temperature changes by a lot, then the reflectivity is too high

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[EdStainless]

Emissivity is a measure of how much the absorbance matches black body absorbance. The absorptivity + reflectivity has to equal 1 (since the material has no transmissivity).
You do have to be careful because the properties will vary depending on the wavelength and angle of incidence. This is an issue with tubes because you will be looking a range of angles at once, so this can muddy the results. And it will vary with temperature also.
Has your customer specified a temp (or range) and wavelength of interest?
If you have a simple single wavelength narrow band IR sensor with adjustable emissivity you could heat the tube and then adjust your sensor emissivity to try to match the temp. Of course you have to do this in the dark. But it could give you a rough idea.

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