Gold passivate/Zinc chromate vs. clear/blue passivate 2

[twizel]

Hi All

I have an application using small disposable CO2 Cylinders that are screwed into a brass regulator housing. Initially the brass was Dezincifaction Resistant Brass but was changed to 385 due to supply problems. The CO2 cylinders are steel with either a Gold or clear passivate depending on the supplier. We have found that some of the Clear passivate CO2 cylinders are rusting in the brass housings around the thread in a relatively short time. I would set up a test to see if the gold passivate cylinders ( which are now hard to come by) corrodes but had hoped someone might me able to point me in the right direction.

Is the corrosion being caused by the lower quality clear passivate or is it because of the 385 brass. Would the use of dezincifacation resistant brass help this problem?

Thank you for your help,

Regards,

 

[TVP]

My guess is that the clear passivate/chromate conversion coating is not as corrosion resistant as the gold passivate/chromate conversion coating. However, there are certainly other possibilities such as defects in the coating, clear parts have thinner zinc layer, etc. I don't believe that the brass alloy has any influence. Clear chromates generally are very thin, while gold chromates are somewhat thicker. If these are hexavalent based chromates, the darker color means more Cr6+, and hence more corrosion resistance.