Properties of Zinc-Dichromate

[jafarr]

I have been searching the web and cannot find a clear listing of the properties, namely mechanical, of yellow Zinc-Dichromate.

My main concern is that it is not good to use it above 140 degrees F. That's all anybody been able to tell me.

If some one can point me in the right direction of a resource or some chart or something, it would help a lot.


Thanks,

Jamie

 

[unclesyd]

This is referring to an electroplated zinc part with a chromate conversion coat. It is a common callout in the fastener industry.
I’ve have never seen that low of a temperature limit on the coating. I know it’s used at a higher temperature quite regularly.
I’ll check or maybe some one can jump in with the temperature limit.

 

[jafarr]

It is actually a mild carbon steel washer with a yellow zinc-dichromate plating. For our application, we "need" to degrease the parts so the primer will stick. Our degreasing operation uses trichloroethyene at an elevated temp.

From past we have experienced a dehydrating of the plating and premature failure due to seperation between the plating and the metal.

 

[israelkk]

What jafarr refering to is the checmical conversion coating for aluminum alloys (see MIL-C-5541),this treatment will not hold if exposed over 60 Celsius degrees (140F).

 

[kenvlach]

Chromate conversion coatings are commonly applied to Al, Cd, Mg and Zn. Different specifications apply according to substrate, but all of these chromates are hydrated gel coats that degrade (become powdery, lose some corrosion resistance) when heated to dehydration temperatures. For chromated zinc, ASTM B201 states that this degradation becomes rapid above 65^oC. The same limit applies to chromated cadmium, ASTM B766. A limit of 60^oC applies to chromated aluminum, ASTM B449 and MIL-C-5541E. MIL-M-3171 allows much higher temperatures, e.g., 450^oF for chromated magnesium.

For properties of chromated zinc, see ASTM B201 and B633. Thickness, corrosion resistance, and abrasion resistance [not much -- a soft gum eraser at 10 psi is used to test] are specified. I would like to add that chromating is available for all types of zinc, including galvanized (hot dip), mechanically plated and zinc diecastings, not just electroplated zinc.

Also, the temperature limits mentioned above do not apply to [properly] painted surfaces, MIL-C-5541E, paragraph 6.11. The paint seals in the moisture of hydration. I have seen many tons of chromated parts powdered coated at furnace temperatures of ~400^oF. I’m not a powder coating expert, but the heating rate and the selection of the proper powder coat resin are important when coating chromated parts in order to prevent outgassing defects, etc. This has been discussed in the Powder Coating Clinic in Products Finishing magazine; go to

http://www.pfonline.com/powder/clinics.html

and click on Cleaning/Pretreatment.

jafarr, when you refer to separation between the plating and the metal, I interpret this as the dehydrated chromate (not normally referred to as ‘plating’) detaching from the zinc plating. Is this correct?

Finally, the hot trichloroethylene is certainly detrimental to both the chromate and the zinc, especially if the trichloroethylene becomes acidic after extended use. Try to chromate after degreasing.