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Temperature limits of yellow chromate passivation

I am an electrical engineer resonsible for a cables program and have a problem with passivation. I currently have a supplier of zinc cobalt over nickel plated aluminium with a yellow chromate passivation who is baking the parts after passivation to 170 degrees C for 10 minutes. The bake is to cure marking ink. The parts have started to show surface discolouration after limited exposure to humidity/temp. I understand that to bake to this temp may destroy the effectiveness of the passivation. What is the maximum temperature (& time at temp) that this passivation may be exposed to without degradation of corrosion prevention performance?

Paul Stillwell
defence electronics - Leicester, UK
2000

Old standard was 140F for virtually any length of time. With zinc-iron, can you switch to black. It has superior corrosion resistance, will hide nearly any degradation better than yellow and I have been led to believe that it will stand a higher temp, such as your cycle.

James Watts
- Navarre, Florida
2000

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Hexavalent chromate coatings are indeed heat sensitive. Max recommended dry off 10-15 mins at 90-95C. However, I have found that on hot dip galv, even powder cure temps can be withstood (required for degassing), if the coating is fully dried at low temp before racking up the temp. The parts after powder coating still meet BS 6497 which requires 500 hours acetic acid salt spray.

Roger Bridger
- Croydon Surrey UK
2000

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Dear Mr Paul,

What works for us is to first Zinc blue passivate the part and mild dry it at about 70 - 80 ' C. Without aging it too much go directly into the yellow chromate and dry it at a temperature not above 60'C. Age this for two days then Screen print and bake at 170'C without the passivation rupturing.

Try to make your life easier by finding an ink which cures at a lower temperature instead !

Regards,

Khozem Vahaanwala
Saify Ind

Bengaluru, Karnataka, India

2000