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Molybdenum Phosphate Conversions... on Cadmium? [Florida] 

July 23, 2008

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. AMS-QQ-P-416 [link is to Techstreet], AMS-C-8837 [link is to spec at TechStreet]) 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? AMS-C-8837 [link is to spec at TechStreet] (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, FL

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
- FL