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Chemical conversion coating degradation under heat and water

 

I am a Producibility Engineer working on a US Defense contract to produce a Self Propelled Howitzer (light armored tank). I am working with electronic enclosures that are to use spray cooling technology. An inert fluid will be sprayed directly onto components to keep them at an operating range of 65 degrees F ± 10 degrees. A proposal has been made by the enclosure vendor to use a class 3 Chemical Conversion coating on the inside of the electronics enclosure. We are looking for a low electrical resistance. I have been told that Chemfilm breaks down when exposed to water vapor and 50 degrees F temperatures. At 70 degrees you obtain total degradation. The vendor says NO!

The box will be under 12 PSI internally. The cooling system will utilize a 2 fluid process. One fluid will be distributed to all boxes to a cooling plate where it will dissipate the heat from fluid inside the electronics box which has an internal pump and sucks the liquid off the bottom of the box and sprays it directly onto the components. Thus reusing the fluid in the box. There are chemical and particle filters in both internal and external loops. The box however will be maintained and repaired in the field to the component level. This inevitably will introduce all sorts of foreign debris, including water vapor. Is there research findings on whether degradation of Conversion coatings (under these and other conditions)is a factor or not? The box will not lend itself well to Nickel plating. I have also been told that Anodizing will not allow us the low electrical resistance we require. Are there similar EPA friendly alternatives?

For the record, as a Producibility Engineer we prefer to use Chem-film whenever possible due to it's low cost and relative application process.

Jon Boshea
- Fridley, Minnesota, USA

I have done parts for similar applications. While more expensive than chem-film electroless nickel will work without any ifs, ands or buts. Electroless nickel will plate complex shaped objects, be electrically conductive and withstand a wide range of temperatures and environments.

Todd Osmolski
- Charlotte, North Carolina, USA
 

I would have bet a cup of coffee that it would not work over the long term, because you use hot water to bleach chem-film for a poor mans clear. It might work, I would try a test for several weeks on say 3 units, checking daily.

James Watts
- Navarre, Florida
 

Update to my original question....First I want to correct myself, it is Celsius not Farenheit we are dealing with. So the literature I have (Mil-Std 5541e paragraph 6.11) indicates degradation at 65 °C. Secondly, it is not only water but also the lack of water that can have adverse effects on the chromate. We are running a box at 65 C, spraying an inert 3M fluid (water content is I think around 7 ppm). 5541e says the most common problem with the heat is dehydration of the film. I have been told that running a box at 65C for 96 hours will totally degrade the coating. We know that electroless nickel is an option but I believe it can be very costly compared to Conversion coating. I have also been told that these boxes may not be good candidates for nickel because of all the "nooks and crannies". Weight is also a major factor. The lighter the better.

Jon Boshea
- Fridley, Minnesota
 

Do not guess or think. Test! it is the only way that you will know what this SPECIFIC application will do.

James Watts
- Navarre, Florida