Letter 23991

Chromate reaction with deionized water [Illinois] 

+++

My company manufactures avionics equipment that uses chromated aluminum heat sinks and hardware that is mounted to printed circuit boards. I have been trying to determine the sequence for fabrication for our circuits and have seen that the yellow chromate finish appears duller following a deionized wash cycle to clean flux from the circuits. My questions are: What is happening to the chromate material? and if "washing off", can the material plate on electronics or fiberglass of the pcb? Is the aluminum still protected?

Steve Kleinhardt
avionics - Chicago, IL, United States

First, I do not know, but from several years of applying the chromate, I would expect some of it to dissolve in DI water as it is a wonderful solvent. You might also be getting some reaction of the chromate with the dissolved flux. I will guess that you are OK if you still see a uniform yellow. Salt spray or some other accelerated corrosion testing will be a better indicator of it being OK or not. This also is "relative" and means virtually nothing until the corrosion testing is referenced or benchmarked against your real life use. I would assume that this process is a relative standard in your industry. I also assume that you are letting the chromate set for 24 hours before working with it. This allows the bulk of the moisture in the gel film to evaporate and "set".

James Watts
- FL

Sounds like the "deI water" contains chlorides which is a frequent occurence when the resins are not properly washed. The film that wipes off will not plate out onto you other electronics or plastics. The aluminum is still protected but not as much as before the deI wash. Incidentally, you mentioned "heat sink", well, guess what, the chromate deteriorates above 140 F.

Robert H Probert Robert H Probert Technical Services Garner, NC, USA Editor's note:        Mr. Probert is the    author of Aluminum How-To

I want to answer or follow-up on this subject publicly (in non-commercial fashion).

 

My company is a supporting advertiser at finishing.com and we want the contact information to reach the inquirer privately.

 

I want to post a new question or inquiry of my own on a different subject.