Ask the Expert Question-and-Answer Archive (Wastewater Treatment)
by Mike McGinness, EcoShield Environmental Systems, Inc.
Hexavalent Chromium on Parts
Is there a formula to calculate the amount of hexavalent chromium present on a part (fastener) that is zinc plated and has a chromate conversion coating?
None that I know of, and I can think of several good reasons that any formula you found would be highly suspect. The problem would the variability in the conversion coatings concentration, reactivity, substrates, bath temperature, rinsing and residence time in the chromate conversion bath could all affect the results. Also I am not sure there is a reliable, agreed upon "standard" test method to determine the hexavalent chromium content of the part. I think I have seen some discussion group or ATE-NMFRC questions recently regarding people looking for a test method for this purpose due to a new European directive for cars and ultimate recycling requirements for them but that is all I have seem so far.
Personally my feeling is that a good "clean" DI final rinse would remove any water soluble residual hex chromium (since hexavalent chromium is water soluble) and that anything else would be a solid part of fastener. If they are looking for the water soluble Hexavalent chromium then a DI leach test method would work. If someone comes up with a standard test method (like the solid waste TCLP procedure) then you could use that test on a statistically random sampling of your fasteners and make your own formula based on your parts and conversion coating process which would take into account any variations in your conversion coating process.