Sound technical content, curated with aloha by
Ted Mooney, P.E. RET
Pine Beach, NJ
The authoritative public forum
for Metal Finishing 1989-2024
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Emissivity of Coatings? Alochrom, Black Plating, etc.
Q. High Thermal Emissivity for Aluminum Electronics Boxes
My company is searching for an inexpensive way to increase the Radiation Thermal Emissivity (Black Body) ability of our Spacecraft Electronic Enclosures.
Simply put, our Electronics Boxes are TOO HOT, and we need that extra "cooling margin" potentially available by using a Black coating or paint.
A "perfect" Black Body has an Emissivity of 1.0000
Internet research shows us that Clear Anodized Aluminum has an Emissivity of 0.83, whereas a Black Anodized Aluminum surface is about 0.86.
That's not good enough.
Krylon has an "Ultra Flat Black" spray paint with a reported Radiation Emissivity of about 0.96.
Do YOU have any recommendations?
Intergalactic Spaceworx LLC- Fort Wayne
March 19, 2024
⇩ Related postings, oldest first ⇩
Q. I am looking for any information on "emissivity" or heat radiation properties of anodized aluminum. What is it and how can it be kept constant from load to load. A potential customer has told me that a difference of 0.2 can occur from load to load and this is too much of a difference. She needs assurance that this will not happen, but I can't do this if I'm not even familiar with the term.
Erin Lewisanodizing - Calgary, Alberta, Canada
2000
A. Emissivity, generally indicated by Greek letter epsilon, and ranging from 0 to 1, is the ability of a body to receive or transmit heat power as infrared radiant energy.
The radiant power emitted by a body is proportional to its temperature to the fourth. An ideal "black" body has an emissivity = 1. For the real bodies the emissivity depends to a large extent on the surface finishing. For specular surfaces, emissivity can be as low as 0.03, for rough surfaces over 0.8.
Typically, emissivity of black anodised aluminum is 0.85.
Emissivity can be measured by means of suitable instrumentation.
Basics on the topic of power radiation can be found in every book of heat transmission; a lot of data can be found in many handbooks, e.g., edited by NASA or ESA.
pasquale cirese- campi bisenzio - Italy
A. Just a correction in the term "transmit" used by Pasquale. Transmit means a wave, e.g., IR, passing through without absorption, like in light transmitting through a glass. Then there is reflectivity. Most relatively smooth metal surfaces tend to reflect a substantial portion of the IR radiation. The ideal Black Body with emissivity of 1 absorbs all heat received and emits all of it. No transmission and no reflection (unless my fundamentals are questionable).
Mandar Sunthankar- Fort Collins, Colorado
Q. I'm trying to track down emissivity data for some finishes. My most urgent need is to find the emissivity for chromate finish aluminum MIL-C-5541 class 1A. Does anybody know it? Does anyone know a good reference for properties of other coatings ?
Peter Kostka- Madison, Wisconsin, USA
2000
Ed. note: See some numbers offered by Bob Knapp, below.
A. Peter, Here is a procedure to determine the emissivity of any substrate... You will need an infrared temperature gun that has adjustable emissivity and a touch type temperature sensor.
Take a chromated piece and check the temperature with both the touch gage and the IR gage. Adjust the emissivity on the IR gage until the temperature reads the same as the touch gage.
When the same temperature is reached, make note of the emissivity. This procedure assumes that the touch gage is correct. Hope this helps. Kelly
Kelly Draper- West Plains, Missouri
Emissivity of chromate conversion coated aluminum
Q. Mathematical modeling of equipment enclosures is fast becoming a necessity if the professional Engineer wants to investigate Thermal Capacity of Equipment Enclosures. My particular question relates to the EMISSIVITY of ALOCROM. Various coatings can effect the amount of radiated heat from a housing. Various publications give figures as indicated below :
Polished Aluminium - 0.06,
Iridited Aluminium - 0.07,
Anodized Aluminium 0.81,
Aluminium Oxide - 0.33, etc.
However, I have been unable to find any figures relating to ALOCROM.
Can you please help?
BOB KNAPPBAE SYSTEMS - ESSEX, U.K.
2001
Q. Have you managed to find this data? I am looking for the same information.
Richard BennettUK Astronomy Technology Centre - Edinburgh, U.K.
September 11, 2009
A. Hi. Alocrom is a Henkel trade name for chromate conversion coating of aluminum per MIL-DTL-5541 [on DLA] (to my understanding, they use the Alocrom name in Europe whereas they use the Alodine name here). Iridite is a Macdermid trade name for an equivalent chromate conversion coating of aluminum. So, they are on the same approved products lists, are for the same purpose, and should have equivalent emissivity.
But a small complication is that Alocrom, Alodine and Iridite are trade names for a range of such processes, not a single process. For precision I think it will prove necessary to specify a specific Alocrom like Alocrom 1200. Henkel may have this data.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Q. Given a blackbody radiant cure zone for a paint finishing oven, if there is an area with clear metal (i.e., high "e" coating has been removed) will the (heated) surface / skin temperature be greater than surrounding area?
Cindy Dilworthindustrial / process air handling - Detroit, Michigan
2005
A. Hi Cindy. Because most metals are highly conductive, I don't think a significant temperature difference would persist for more than a few seconds -- so I doubt that it's a real issue. But if it is, it involves quite a bit of calculation based on the absorptivity of the bare & coated areas at the temperature of the infrared source, the emissivity of the bare & coated surfaces at the baking temperature, and the thickness & thermal conductivity of the metal in question :-)
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Black Plating to give an Emissivity of .80 or higher
We are currently using a stainless steel pipe section in a machine and would like for the finish to be black. I'm not a plating guru as I usually specify the basics like black oxide, nickel, zinc, hard anodizing, etc. and am looking for help to determine what the best solution is to our situation.
For cooling (heat transfer) purposes we would like to make this stainless steel pipe a "black body". Our analysts say we need a material with an emissivity of above .80 or preferably .85 to perform this task. Any ideas? We're looking for low cost as this section is very long and if it works we could be producing thousands of this component.
We've also considered switching the pipe material to carbon steel, but the plating would have to protect the pipe from a corrosive atmosphere. Any low cost plating options for carbon steel would be appreciated as well.
Thanks
Government - Oak Ridge, Tennessee, USA
2007
A. Black chrome and nickel plating is the first choice for low emissivity, so plating is probably unsuited for what you want. Black oxide on stainless steel is probably "good", but perhaps not 0.8 or higher. Rough surfaces are much better than polished surfaces. How about paint?
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
A. Perhaps, black zinc chromate? I'm a bit out of my area of expertise, but I've seen a lot of it done, and the coating looks pretty 'flat' to me.
Dave Wichern
Consultant - The Bronx, New York
Q. Has anyone tested for the emissivity of black oxide or passivation of stainless steel? I have one data point from a local plating company which stated an emissivity of .72, which is less than desired. However, I have a text book which states .87 for "stable" oxidized stainless steel ... Does anyone else have input?
Thanks
- Oak Ridge, Tennessee, USA
2007
A. You can measure emissivity to a fairly high level of accuracy with an infrared camera such as those manufactured by FLIR. Search for the term "emissivity" on Youtube to learn more.
David Browninfrared training - Billerica, Massachusetts
January 22, 2010
Q. Does anyone have an information on emissivity of E-coat?
Zin Dolgonosov
- Fremont, California, USA
January 3, 2013
Heat transfer by radiation
Q. Which finish is better for heat emissivity in electronic housings, Powder coating or anodizing? What are their particular emissivity values?
Kuljeet SinghEngineering design - India
February 7, 2020
A. Hi Kuljeet. Anodized aluminum is pretty high: 0.80-0.86 per this page and other threads on finishing.com. The emissivity of most powder coatings is probably a little higher, with some approaching 1, but others as low as 0.5 when so designed.
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Q. Very interesting thread with lots of good responses, but a couple of queries from my end, please:
(1) per Ted's "rough surfaces are much better than polished surfaces". Is this based on surface area thinking? If not, I am curious why rough powder coat surface would be inherently more likely to eject photons below 100 °C than a gloss one, unless it was affected by surface leveling additives? In that case, and this is interesting, when space is limited would a matte textured powder coat be any help?
(2) I am very interested in an answer to Zin's question. Particularly e-coat vs powder coat emissivity below 100 °C?
ACRONYMS FOR NEWBIES:
SOC = system on chip
PCBA = printed circuit board assembly
NPI = new product introduction
As PPG define e-coating thus "E-coating, also known as electrodeposition coating, is a method of painting that uses electrical current to deposit paint on a surface." Generic e-coat seems to be an epoxy resin so I suspect it might be very similar to generic polyester powder coat. If they answer my query I shall post it here.
The application is finishing req'd for a small enclosure for a 40 mm diameter SOC PCBA in a production NPI. We are now doing thermal testing on prototypes. The radiative surface is a cylinder 50 mm dia. with exposed walls 10 mm high and neither top nor bottom are useful for dissipation.
Physical design for electronics - Vancouver [BC]
August 7, 2021
A. Hi Nigel. I am neither a theoretician nor an emissivity expert, and am probably not the guy to answer your questions. But I have looked at a number of emissivity charts, and a good one for your question seems to be at
https://www.thermoworks.com/emissivity-table
because it lists numerous paints as well as other materials, and seems to pretty conclusively show (slightly) lower emissivities for gloss paints.
It is not so much the case that e-coat is epoxy, but rather that most e-coat is used as a primer and epoxy is a good thing to use for that. Polyester e-coats are used as decorative final finishes for mild exposure situations where a thin single coat will suffice. There may be acrylics as well but I'm not familiar with them.
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Thanks Ted. Very much appreciate your reply. For those interested, here is another table:
https://www.design1st.com/Design-Resource-Library/engineering_data/ThermalEmissivityValues.pdf
Unfortunately neither this nor the one you linked provide the all-important temperature (that is wavelength) at which these values are applicable. But will assume that it is <100 °C in both.
As with many such tables, apart from the fundamental raw materials where everyone references the same original data, one finds numerous +/- variations in the more random/obscure (and therefore ultimately more useful!) materials.
My typical working assumption has been, that if one is in the range 0.8+ to 0.9+ for one's finish, the differences in functional thermal radiation loss is in the range of variations, or noise, inherent in a typical commercial electronics housing assembly.
Anyways, I think we've about worn out this chat, so thanks, stay safe, and regards, until next time.
- Vancouver, BC Canada
A. Hi. Some such charts imply that they are for 20 °C. You are clearly right about "noise" -- I viewed but did not really study a PhD thesis about the difficulty in getting good numbers in many applications because of the interference from conduction and convection. You have to either actually conduct the experiment in a vacuum chamber, and still make allowances, or else go through endless experiments and statistical analyses. It may be a 'chat', but hundreds of people will read it, so thanks!
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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