Under mildly oxidizing conditions, some metals exhibit the property of
passivity. Passivity is a state of electrochemical inactivity which is probably
caused by a layer of metal oxide or adsorbed oxygen on the metal surface.
Examples of metals displaying active-passive transitions are iron ,chromium ,nickel
and titanium.
In this paper the shape of the anodic polarization curve for an active passive
metal is discussed, and the importance of the Flade potential and the
critical passivating current density is illustrated.
Since passivity provides metals with a natural corrosion resistance, methods
for achieving passivity have been investigated. The following methods have been
discussed:
a- Alloying Iron with chromium, nickel and molybdenum to produce the
stainless steels.
b- Addition of passivators to liquids in contact with Iron.
c- Noble-metal alloying for improving the corrosion resistance of titanium
and chromium in deaerated acid solutions.
In addition, the breakdown of passivity by chloride Ions has been briefly
described.
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