Originally Posted by HuFlungDung  Interesting, if not a little alarming.
What would the equation be for release of Cr+6 ions into the air from cutting metal, either with heat or with tools? Just because 'chromium' is present doesn't mean there is any provable risk if the sequence that produces it is highly improbable under ordinary conditions.
Indeed, is chrome as an alloy in a hexavalent state within the mix? |
In an alloy chromium is present as chromium the metal, or element if you want to talk chemically. I highlighted your comment which is pertinent; the situation is that if you expose molten chromium to atmospheric oxygen it may oxidize and one of the oxidation states is Cr+6. However, naturally occurring chromium exists almost exclusively in its ionic form as the Cr3+ ion, which is known as ‘trivalent chromium’. Now the whole point is that it cannot exist as an independent ion with 3 or 6 positive charges; it has to latch onto one or more negative ions that balance the charges to be electrically neutral. I have no idea what these negative ions could be but possibly they could arise from any flux being used or simple contaminants on the metal surface. I think it is improbable that much, if any, chromium six is produced in welding, laser or plasma cutting; normally in any chemical reaction the most stable product is formed and that is the trivalent form. But nobody can prove that which is why I made the comment about pointy headed types.
The real hazard with chromium six is when compounds based on Cr+6 are used; notice these are pigments. Many stable pigments are what chemists call coordination complexes; a positively charged metal ion surrounded by negative ions which are very often derived from coal tar or by synthesis from petroleum products; sometimes they are derived from plant materials, in these cases they are organic anionic compounds where organic simply means they contain carbon atoms. These combinations of organic anions and metal cations can stabilize an oxidation state for the metal that does not occur naturally, and are often highly coloured. If you know anyone who dyes fabric and uses a two part dye with the coloring operation being done with a pigment which is then fixed using a mordant solution, they are creating coordination complexes embedded in the dyed fabric from the reaction between the pigment and metal salts in the mordant. Sometimes there is a dramatic color change during the fixing, or mordanting. Many times pigments based on organic compounds are not stable and they will bleach particularly when exposed to high temperatures or UV radiation, i.e. bright sunlight. The anionic compounds do not have to be derived from compounds that contain carbon, they can be inorganic which just means they don't contain carbon atoms. (Chemists are not very original when it comes to naming things.
) Many times these inorganic coordination complexes are very stable even at high temperatures.
The reason these compounds can be hazardous to humans either as cancer causing agent (carcinogenic), genetic material damaging agents (mutagenic) or by causing birth defects (teratogenic) is because our metabolic chemistry uses all sorts of biochemical compounds that are chemically similar or even identical to coordination complexes. Actually the oxygen transport protein, hemoglobin, contains what is essentially a coordination complex the 'heme' which actually holds the oxygen; and is a brilliant color. Many enzymes in the body, especially the ones involved in oxidation reactions which utilize antioxidants from the diet, and correct DNA damage, among other things, also contain 'coordination complexes' and chromium in its trivalent form is essential at a very low level in the human diet.
I have no idea what the precise chemical or biochemical mechanism would be that allows chromium six to cause problems; I doubt that anyone knows. But it is certain that exposure to it can cause an increased incidence of certain cancers; so it is a good idea to avoid it if possible. The point is do not be paranoid; when welding or cutting any metals a proper respirator should be worn because there are a lot of potentially nasty things produced. And in the case of cutting it is the metal particles; but these, and any chromium compounds, will be filtered out by a correctly fitting mask.
And I hope I haven't bored anyone.
I know a lot of people skim over the need for safety equipment, etc; I don not. I mention above the system we have for removing the coolant mist; what I haven't mentioned is the system we using for th welding room and sanding/deburring room. The extractor system for the machines is 3500 cubic feet a minute but on the two rooms for welding and sanding, which have a total floor area of about 240 square feet, we have a 7500 cfm unit. The rooms have a ceiling height of 8 feet so you can calculate how frequently the air is changed. The filters on this unit are three stages with the final bags taking out submicron particles.