Environmental Engineering Reference
In-Depth Information
Figure 2.18 (a) Electrochemical behavior of a normal metal corroding with a diffu-
sion-controlled cathodic process showing the effect of velocity. (b) Corrosion rate as a
function of velocity.
sive by an initial exposure to fuming nitric acid. Its subsequent exposure to a
corrosive media, such as dilute sulfuric acid, will bring down the corrosion rate
drastically, by an order of 10 4 to 10 6 . However, this induced passivity is not
stable and is destroyed by vibration or scratching of the surface. Addition of
highly oxidizing chemicals like chromates or nitrites, called passivators , to the
corrosive media helps iron to remain in the passive state. With the addition of
chromium in iron, passivity is achieved in relatively low oxidizing conditions.
Stainless steels, with chromium content above 12%, constitute a class of passive
alloys used as corrosion-resistant materials under oxidizing conditions; they lose
their passivity under reducing conditions.
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