Determining if you have hexavalent or trivalent zinc chromate coating

A. Hi MP. We've appended your inquiry to a thread on the subject of chromate testing, wherein you'll be able to see a good discussion on the subject of spot testing for the presence of chromates.

Regarding spot testing for the presence of zinc plating, I think you'll find topic 20173, "Spot test to Verify Zinc Plating. ASTM A239 / Preece Test" quite interesting.

Luck & Regards,

A. As a general rule, hex chrome finish will be yellow (also green & black are common). A tri chrome finish will be bluish. There are some vendors working on color variations for the tri-blue, but this should start you in the right direction.

A. Correct me if I'm wrong, and believe me I'm no expert, but just because a chromate coating is blue-ish does NOT mean that it is of the Trivalent variety! Many of the newer formulations of chromate coatings are indeed Trivalent due to the new restrictions and regulations, but not all of them.

There are also blue chromate coatings that are the Hexavalent kind. Sometimes these are referred to as Clear chromate, Blue chromate, or even sometimes White chromate. So do not assume it is Trivalent just because it's blue.

As far as a chemical test that you can do - that's not my area. Maybe someone of more extensive knowledge can answer that part for ya, Chief.

A. There is a chemical, 1,5 diphenyl carbizide, that in the presence of hex-chrome will turn pink-violet in color. One needs to handle this chemical carefully, its a suspected carcinogen.

Recipe is as follows... dissolve 0.5 grams of the 1,5 DPC in 50 ml acetone ⇦this on eBay or Amazon [affil links] Warning! highly Flammable! . Dilute slowly while stirring with 50 mls DI water. Store in refrigerator.

To test there are different thoughts and processes. You can dropper directly onto surface of part (put white towel or similar underneath to be able to see the color change). Leach the parts in small qty of boiling DI water for 5 minutes and then dropper the 1,5 DPC into the leachate... color will turn pink-violet (depending on Cr+6 conc.)

There are also a couple of ways to quantify the Cr+6 concentrations. EPA/ MIBK extraction with flame AA or spectrophotometrically.

Good Luck,

A. RoHS compliance can be tested by following a procedure set out in GM spec 3044W. You can probably buy the test kit from a local company, Kocour. Be advised, that using this test some products can give a false positive. The more accurate method is to send a sample of the batch to a lab for testing using an ICP. Generally, most end users rely on a certification from the plater stating that the finish is RoHS, WEE or ELV compliant. The alternative statement the finisher might make is that the part does not contain hexavalent chromium.

A. Another option is to try portable x-ray fluorescence (XRF) for your fasteners that are out in the shop. But keep in mind that neither XRF nor ICP (in the lab) can distinguish between trivalent and hexavalent Cr coatings. If you know what else is in the Cr coating you might be able to look for that species, but that is an indirect way of doing things.

A. Cr 6+ can reduce sodium thiosulphate ⇦this on eBay or Amazon [affil links] solution (iodometery).But Cr3+ cannot reduce above solution.

A. The only method which can tell the difference between Cr6 or Cr3, is by an indicator named diphenyl carbazide. The point is that it has to be freshly made up.

A. (1) Greater than the maximum permissible under RoHS, WEEE, ELV, etc.
(2) ISO 3613 and General Motors GMW 3034 are test specifications for quantitatively determining the hex chromium content of such films
(3) It is impossible to visually ascertain whether a film contains hexavalent or trivalent chromium.

A. There IS a spot test using diphenyl carabazide. It is not known to give false positives, but it can (in my personal experience) give false negatives. That's why the ISO 3613 method (and its' derivatives) is probably the best available at the current time. It's an easy test to run. Keep in mind, though, that trivalent passivates protect zinc surfaces by generating hexavalent chromium during the corrosion process; thus, if the parts have started to corrode, they may very well have hexavalent chromium on the surface. So a positive test doesn't necessarily mean that the plater did not perform the process correctly, or that his supplier supplied a product or process that is not ELV-RoHS-compliant; it's just the nature of the coating.