Heresite, MIL-C-18467A — Material applied by a proprietary- 

 process developed particularly for fin coils Industry pre- 

 ferred coating. 



Chromate Conversion Coating, MIL-C-5541-B (Material), MIL-C-81706 

 (Application) -- Chemically produced, chromate conversion coatings 

 can be applied uniformly on irregular surfaces such as completely 

 assembled condenser coils. This coating has good thermal con- 

 ductivity and, in itself, provides a high level of corrosion 

 protection for aluminum„ It is inexpensive and is also an 

 excellent base for further paint coating „ Possibly, it is not 

 of sufficient abrasion resistance to be used for pre-coating 

 fin stock. 



Electrostatic Polyester Dip Coating -- Uniform dip coatings are 

 difficult to obtain on fin coils with some conventional paints. 

 The evaporation of solvents from the coil interior can reduce the 

 coating thickness on exterior portions of the fins. Electrostatically 

 deposited coatings such as Sherwin-Williams "Power Clad" enamel will 

 produce a uniform low thermal resistance coating of less than 

 1 mil thickness. 



Other Coatings 



Anodized coatings for aluminum provide good corrosion resistance and 

 uniform thickness on irregular surfaces. However, the coating is porous, 

 brittle, and has high thermal resistance. In addition, it is relatively 

 expensive and therefore appears to be less promising than other alternatives, 

 Coatings currently under development for application to solar collectors 

 may have promise eventually, but this technology is not sufficiently 

 advanced at the present time to warrant testing in an air-cooled con- 

 denser program. 



Coating Selection Considerations 



Coil coatings are subject continuously to a higher service temperature 

 than most painted metal surfaces except boiler smokestacks. For coatings 

 like Heresite, which are baked at temperatures up to 450°F, a 150°F 

 maximum coil operating temperature is no decomposition problem. Nor should 

 such temperature be a factor in the service life of a baked coating. 

 Chromate conversion coatings for aluminum are considered limited to 150°F 

 because heating causes dehydration and insolubility of hexavalent chromium 

 compounds. However, they are being used satisfactorily on automotive 

 transmission and engine oil coolers at temperatures up to 300°F. 



In a letter to CEL dated 28 Jan 1969 commenting on this Laboratory's 

 planning for its condenser materials research, the Officer in Charge of 

 Construction (OICC) at Mid-Pac cites two experiences with an Air Force 

 equipment specification. Both were involved in litigation. He stated: 



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