Environmental Engineering Reference
In-Depth Information
Figure 2.17
Effect of oxidizer addition on the corrosion behavior of a metal in acid
solution.
In the situations where the reduction reaction is diffusion-controlled, the an-
odic polarization curve may intersect the cathodic polarization curve in the
i
L
range (Fig. 2.18), the value of which increases with increasing velocity.
The corrosion rate increases as the velocity or
i
L
increases, but it assumes a
steady value where
i
L
i
a
and the reaction becomes activation-controlled.
The kinetic considerations and graphical representations will be taken up fur-
ther in the discussion on passivity (Section 2.4) and galvanic corrosion (Sec-
tion 3.3).
2.4 PASSIVITY
Passivity refers to the phenomenon of loss of chemical reactivity of a metal or
an alloy in an environment where thermodynamically the reaction ought to have
occurred. It results from the formation of a thin, oxidized, protective film on the
surface of a metal. Many metals active in the emf series, including important
structural metals like aluminium, iron, nickel, chromium, titanium, and their
alloys, can be passivated simply by exposure to strong oxidizing media or by
anodic polarization or both. Other metals that show passivity include silicon,
tantalum, niobium, molybdenum, and zirconium.
Usual corrosion conditions are not sufficiently oxidizing to induce passivity
in iron, but they do passivate aluminium and titanium. Iron can be rendered pas-
