Serious Education ... plus the most fun you can have in metal finishing.
Hexavalent vs. Trivalent chromate toxicity(2003)
I am wondering about toxicity of hexavalent chromate coatings. Hypothetically, what if someone swallowed a very small piece of metal coated in such a way (like the size of a small BB), would that be a fatal dose, or cause sickness, or would that not be enough to cause any harm at all as a one time event. Secondly, are trivalent chromate coatings considered completely safe and non-toxic, or are they just a little 'less' toxic than regular hexavalent chromate. In other words, if the event described above took place with a trivalent chromate coating, would there be any toxicity concerns? (I am using ingestion for a worst case scenario).
student - Duluth, Minnesota, USA
Nobody wants financial liability for answering such a medical question. Hexavalent chromium is both an acute and chronic health hazard at even very low levels; I know of no "safe" dose. While a trace of trivalent chromium is essential to health (a vitamin/mineral tablet may contain 120 micrograms) larger amounts are toxic. How much is on your BB-sized metal, and how much is already in your food and water?
The government similarly waffles on drinking water standards to avoid lawsuits for water failing to meet standards and to minimize treatment costs. The U. S. EPA has set both the Maximum Contaminant Level Goal and the legal Maximum Contaminant Level (MCL) at 0.1 ppm for total chromium in drinking water. However, many toxicologists prefer a much lower level for hexavalent chromium. California is adopting a lower MCL for hexavalent chromium on ++++, which is likely to be 0.050 ppm (while public health is the primary concern, the law requires the MCL value to be technically and economically feasible).
Form your own opinion on the relative toxicities of hexavalent and trivalent chromium. Search for and read MSDS's for chromic acid [hexavalent] and chromium (III) oxide.
By the way, a portion of the hexavalent chromium is reduced to trivalent during formation of chromate coatings, so an actual comparison would be less clear than initially presented.Ken Vlach
- Goleta, California
Finishing.com honored Ken for his countless helpful,
carefully researched responses. This is his obituary.
Rest in peace, Ken. Thank you for your hard work
which the world continues to benefit from.