1. Chromium Fe-based solid solution so that the electrode potential to improve
2. Chromium electronic absorption of iron so that iron-passivated
Anodic passivation is due to be prevented from arising from reaction of metal and alloy corrosion resistance phenomenon can be improved. Passivation of metals and alloys constitute the theory of many major film theory, deals with the electronic order of adsorption.
Valency states of chromium
The valency (oxidation state) of chromium metal as an alloying constituent of stainless steel is 0 (zero).
Chromium atoms are present in stainless steel 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 steel 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+’
If stainless steel are subject to corrosion metal ions are released from the alloy into the surrounding environment. Under these conditions, chromium ions should be in the trivalent state (Cr3+), which like the chromium in the un-corroded steel, should not be a health hazard.
Fumes from welding stainless steel may contain hexavalent chromium ions, depending on the process and any fluxes used
Efficient local exhaust ventilation systems should normally be suitable for maintaining exposure limits below the 0.05 mg/m3 limit for hexavalent chromium ions.
Source: Zhejiang Yaang Pipe Industry Co., Limited (www.yaang.com)