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FINISH: LIGHT CHROMATE CONVERT PER MIL-C-5541 CLASS 3

2006

Hello, my company design and have manufactured a range of telecommunications product from aluminium. Some products such as sheet metal chassis just need to remain conductive for grounding/earthing purposes. Other machined products are designed to conduct radio waves along an internal machined cavity. Similar to waveguide. We have specified "CHROMATE CONVERT PER MIL-C-5541 CLASS 1A" for sheet metal and "LIGHT CHROMATE CONVERT PER MIL-C-5541 CLASS 3" for radio wave conducting parts. CLASS 3 gives a light chromate which dose not reduce the conductivity of the aluminium 6061-T6 surface as much as a heavy chromate.

Our sheet metal worker has just informed me that they will be closing their yellow Hexavalent chromating line and installing a trivalent line using "Clear Chemmetal 9812". This is for ROHs. I feel this will be fine for our indoor sheet metal products. Only significant change will be color.

Now our machinists are talking about a similar move. They have not decided what chemical to use yet. This is much more critical as a change in surface conductivity of the aluminium wall will upset the radio wave insertion loss of our machined products.

Will "Chemmetal 9812" be suitable? Will the thickness affect conductivity?
Is there some other MIL standard I can specify to cover TRI chromate coatings/thickness on aluminium?
Is Alochrome 1000(Silver) Tri or Hex Valent
Is Alochrome 1200(Gold) Tri or Hex Valent

Any suggestions would be appreciated

Regards,

Darren Wass

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I have found that any surface finish inside waveguide is a bad thing. The losses are very bad with dark Alochrom 1200.
It also causes RF leaks due to bad conductivity at joints.

Chris Burton

2007