Page 46 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

24 INTRODUCTION AND FORMS OF CORROSION

to note that replicates of exposed and blank unexposed samples are recommended.
It is useful to note that factors such as edge effects, direction of rolling, and surface
conditions must be considered (22, 35).

1.4.12 Electrochemical Techniques for the Study of Localized Corrosion

Electrochemical noise (EN) has been found to be useful in metastable pitting by
recording the galvanic current between two nominally identical electrodes at the cor-
rosion potential of a single electrode (18, 35). The spatial separation of the anodic
processes in the pit and the cathodic processes in the surrounding surfaces necessitates
the passage of current that gives rise to the noise signals. As the localized corrosion
sites are small and of the order of 100 μm in diameter or less, current densities inside
2
the cavities can amount to the order of 1 A/cm .
EN studies can be performed under open-circuit potential and very close to the
natural conditions of pitting. EN measurements in chloride medium have been stud-
ied extensively. It has been suggested that the current transients observed below the
pitting potential may be attributed to the formation of pit embryos (18). EN mea-
surements are widely used in the studies of metastable pits. There are many factors
involved in the interpretation of EN data (26). Some relevant factors are sample size,
sampling time, and system noise. It appears that current noise gives a clearer picture
than potential noise (18, 36).
EN gives instantaneous data that can show the relative corrosion resistance of
different alloys in a particular medium over a long period. However, agitation or con-
vection can mask the noise signals, making it difficult to simulate the operational
conditions in certain situations. The ZRA can measure the current flowing in a circuit
without introducing the additional voltage drop associated with the standard ammeter.
As is the case with the potentiostat, the main functional component is an operational
electronic amplifier that supplies the current necessary at its output to maintain zero
potential difference between the two input potentials so that no current flows into or
out of its input terminals (31). EN data obtained from microelectrochemical studies
of stainless steels under potentiostatic conditions showed the current noise expressed
as a standard deviation of the passive current increases linearly with the size of
i
the exposed area whereas the pitting potential decreases (37). However, to complete
the electrochemical studies and distinguish between repassivating superficial pits and
penetrating pits, microscopic studies are highly desirable. The scanning reference
electrode (SRET) should prove to be a suitable complementary technique (18).

1.4.13 Electrochemical Impedance and Localized Corrosion
Electrochemical impedance spectroscopy for studying localized corrosion involving
pitting has been the subject of interest in recent times. The statistical variation of pit
nucleation and the absence of steady states prevent long-term measurements in the
low-frequency region. In addition, in the pitting region, a complicated Nyquist plot
is obtained and difficult to interpret. However, Mansfield et al. (39) demonstrated
that characteristic changes occur in the low-frequency region. It is useful to note that