This document discusses corrosion and passivity of metals, specifically: 1) It defines passivity as the formation of a thin surface film under oxidizing conditions that provides corrosion resistance to some metals and alloys. 2) It describes potentiostatic polarization as a technique to control metal polarization in electrolytes to observe corrosion behaviors. 3) It lists applications such as corrosion product analysis, alloy selection, and localized corrosion analysis. 4) It discusses concepts related to passivity including passive current density, primary passivation potential, and critical current density.

1. Saad Bin Hasan
B.Sc. In Chemical Engineering, JUST

2.  Potentiostatic polarization curve of active-
passive metal (Fe).
 Fade potential of Fe.

3.  Passivity is defined as a condition of
corrosion resistance due to formation of thin
surface film under oxidizing conditions ,
some metals and alloys having simple barrier
films with reduced corrosion of active
potential .
 Passive metals are susceptible to local
breakdown and accelerated localized attack.

4.  Potentiostatic is a polarization technique that
allows for the controlled polarization of
metal surfaces in electrolytes, in order to
observe cathodic and anodic behaviours.

5.  Corrosion product identification and
analysis
 Industrial and chemical plant corrosion and
chemical attack
 Alloy selection
 Crevice corrosion analysis
 Electronics corrosion
 Local corrosion analysis

6.  ipass- Passive current density
 Epp - primary passivation potential
 icrit - Critical current density

7.  For an active-passive metal exposed to a cathodic reaction,
the corrosion rate will increase upto certain velocity levels,
beyond which the corrosion rate decreases rapidly to a very
low value on the onset of passivity and would remain at
passive state for still higher velocities.
 Corrosion rate of an active-passive alloy initially increases
with oxidizer concentration (while in its active state).
 As soon as passive state is reached, the corrosion rate
steeply decreases to a very low value and remains at this
low corrosion passive level.
 With still further increase in oxidizer concentration,
corrosion rate further increases due to transpassive
behavior.

8.  Flade Potential (EF) the
potential at which a
metal changes from a
passive state to an
active state
Fig: Decay of passivity showing Flade potential

9.  If the potential as a function of time is monitored
after interrupting the applied current, the potential
value first changes to a value more noble on the
hydrogen scale.
 Then slowly changes and finally rapidly decays
towards the normal active value.
 The noble potential reached just before rapid decay.

10.  EF = E0
F – 0.059 pH (for Fe, Ni, Cr and alloys of Fe).
 Stability of passivity is related to EF
 The lower the E0
F, the easier it becomes for
passivation and higher film stability.