Page 43 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

LOCALIZED CORROSION 21

potentiodynamic or potentiostatic methods. The important parameters are (i) critical
current density, i characterizing the active–passive transition; (ii) pitting potential
crit
where stable pits grow, and (iii) the repassivation or protection potential (after reversal
of potential scan direction), below which the already growing pits are repassivated
and the growth stops (18, 29).
Cyclic potentiodynamic polarization used in determining pitting potential con-
sists of scanning the potential to more anodic and protection potentials during the
forward and return scans and compare the behavior at different potentials under iden-
tical conditions. The polarization curve of an alloy (with or without coating showing
active–passive behavior may be obtained in a chosen medium as a function of chloride
concentration). E , E , E ,or E represent pitting potential or breakdown potential,
b
p
pit
bd
while E prot , E rep refer to protection potential and repassivation potential, respectively.
Scan rates of 0.05–0.2 mv/s may be used along with argon/nitrogen bubbling. The
breakdown potential corresponding to considerable increase of anodic current at a
certain scan rate corresponds to the condition for the initiation of localized attack. The
more noble the breakdown potential, the greater is the resistance of the metal/alloy to
pitting or crevice corrosion. The potential at which the hysteresis loop is completed on
reverse polarization scan determines the potential below which there is no localized
attack (30) (ASTM G5) (ASTM G61). The absolute values of pitting or breakdown
potential and the protection potential depend on the scan rate and do not reflect the
induction time for pitting. Allowing too much pitting propagation to occur along with
changes in chemistry can influence the reversal in the scan rate (22).
Some experimental work suggests the convergence of pitting potential (E ) and
pit
protection potential (E ) to a unique pitting potential (31, 32). Later on, the concept
prot
of unique pitting potential corresponding to the most active value of E determined
p
after a long incubation time and the most noble value of E measured following
r
minimal pit growth have been advanced. It has also been suggested that the station-
ary pitting potential corresponds to a value between that of pitting and protection
potentials. A critical pitting temperature has been defined below which a steel in
chloride solution such as FeCl would not pit irrespective of potential and exposure
3
time (16). A good measure of pitting susceptibility is the difference between pitting
and protection potentials. Alloys that are susceptible to pitting tend to exhibit a large
hysteresis. This range of potentials can correspond to metastable pitting correspond-
ing to a region where pits initiate and grow for a limited time before repassivation.
The reasons for stop in the growth of large pits are different. Metastable pits are typi-
cally micrometer in size with a lifetime of seconds or less, but may continue to grow
to form large pits under certain conditions (4, 31).
Pitting tendency increases with increasing temperature. At low temperatures, high
pitting potentials are observed. Temperature dependence of pitting susceptibility of
stainless steels has been used in the ranking of steels with respect to their pitting
resistance as high pitting potentials at low temperatures and low pitting potentials at
high temperatures are observed.
Methods used to study localized corrosion consist of galvanostatic methods and
potentiostatic methods. At constant chosen currents the evolution of potential as a
function of time is noted until the rate of change in potential approaches zero. This