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Molybdenum Phosphate Conversions... on Cadmium?

I'm currently in the process of finishing design of a steel (4130) nut for aerospace use. However, I need to match finishing on another previously designed part - which I only know to be cadmium plated (~.0002" thick), with evidence of a molybdenum/phosphate conversion.

Most cadmium specifications I'm familiar with (e.g. AMSQQP416, AMSC8837) allow chromate or phosphorous conversions (the latter usually to aid to paint adhesion), and I'm also aware of some moly/phos conversions for zinc plated parts.

Any experience or knowledge of using molybdenum and phosphorous conversion over cad plating? AMS2426 (Vacuum Deposited Cadmium Plating) allows specification of a conversion coating, but I wouldn't say any suppliers or material "experts" I've asked have instilled any degree of confidence in me!

Jeff Schneider
Buyer - Titusville, Florida
July 23, 2008

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July 24, 2008

I am not aware of a molybdenum phosphate. I think that you mean Manganese phosphate.

Molybdenum di sulfide is a coating much like a paint. Normally it needs to be baked, but some do not. It is for lubrication more than corrosion prevention.

Manganese phosphate is a VERY heavy and coarse conversion coating (black) that is intended to hold oil or grease for lubrication or to serve as a break in lubricant.

Phosphates do not offer much corrosion protection on their own, some but not much. They function as a paint base or an oil holder or a matte dark coating for cosmetic uses.

For corrosion protection, I would go with a chromate. Yellow is good and olive drab is better. A few companies offer a forest green that is good also.

The OD, if baked, will urn a very nice black but will loose a lot of its protective properties. I have seen this done on turbine engine compressor blades that were diffused nickel cadmium plated.

James Watts
- Navarre, Florida