Does the chromium in stainless steel contain 'chrome 6' (Cr6+ ) and is this a potential health hazard
Valency states of chromium
The valency, (oxidation state), of chromium metal as an alloying constituent of stainless steels is 0, (zero).
Chromium atoms are present in stainless steels in ‘substitutional’ lattice positions, replacing iron atoms. This is the same as other ‘large’ atoms from elements such nickel. The atoms are held together in the lattice structure by the ‘metallic bond’. This involves the sharing of electrons between atoms with no loss or gain of electrons from atom to atom. The valency state is therefore taken as 0, (zero).
The chromium in solid stainless steels should not be regarded as a health hazard.
In contrast ionic bonding in compounds, such as sodium chloride, (common salt), involves the exchange of electrons between atoms and hence valency states of 1, 2, 3 etc. depending on how many electrons the element has lost or gained. It is compounds involving chromium ‘ions’ with a valency state of 6, (which includes chromates), that have been identified as a cause for health concerns. This valency state is also referred to as ‘chromium 6’, ‘hexavalent chromium’, or ‘Cr6+‘
Release of chromium if stainless steels corrodes
If stainless steels are subject to corrosion metal ions are released from the alloy into the surrounding environment. Under these conditions, chromium ions are usually in the trivalent state, (Cr3+), which like the chromium in the un-corroded steel, is not considered to be a health hazard. There is one very specific example where corrosion may produce very small quantities of hexavalent chromium at ambient temperature. This is where some strong oxidisers with a pH of 10-14 are in prolonged contact with stainless steel and cause corrosion over time.
Chromium in stainless steel welding fumes
Fumes from welding stainless steels may contain hexavalent chromium ions, depending on the process and any fluxes used.
This is described in more detail, separately; fume associated with welding stainless steel
Efficient local exhaust ventilation systems should normally be suitable for maintaining exposure limits below the 0.05 mg/m3 limit for hexavalent chromium ions.
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