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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 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
更详细的描述在另一篇文章：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.
Fume associated with welding and cutting stainless steels
Fume associated with welding stainless steel
Fume is generated by arc welding processes used for welding stainless steels, both the flux-shielded processes (manual metal arc and flux cored arc) and the gas-shielded processes (tungsten inert gas and metal inert/active gas). Plasma arc cutting also produces fume.
Fume can be defined as the airborne particles and gases arising during welding or cutting by vaporisation and reaction. Flux-shielded processes form particles of complex composition. Ozone is created by the action of ultraviolet radiation on the atmosphere in the gas-shielded processes. The tungsten inert gas process produces little particulate fume but the metal inert/active gas processes generate both particles and ozone.
There have been concerns that fume from welding of stainless steels, particularly the particles containing hexavalent chromium formed in flux-shielded processes, is a cause of cancer. Extensive studies over a long period have not supported this view but have nevertheless shown that there is a slight excess of lung cancer among all welders, whether of mild steel or alloy steels. It is therefore sensible to take action to limit contact with welding fume by process selection and/or engineering controls, such as ventilation and extraction. Local fume extraction is more effective than general ventilation. While personal protective equipment such as an air-fed helmet may also safeguard the wearer, it should only be considered for situations such as confined spaces, if only because ancillary workers are otherwise left unprotected.
There is a statutory requirement to control exposure to welding fume, expressed as a general exposure limit for welding fume and OES (occupational exposure standards) and MEL (maximum exposure limits), for individual elements, as specified by the Health and Safety Executive. Sampling to determine exposure is expensive and should only be considered when control measures have been taken and a residual risk has been identified. There are also methods of calculating exposure levels from consumable manufacturers’ data.
A typical safety data sheet relating to stainless steel can be downloaded from here.
■Welding and other types of “hot work” on stainless steel and other metals that contain chromium
■Use of pigments, spray paints and coatings
■Operating chrome plating baths