Page 159 - Corrosion Engineering Principles and Practice

134 C h a p t e r 5 C o r r o s i o n K i n e t i c s a n d A p p l i c a t i o n s o f E l e c t r o c h e m i s t r y 135

• Salt-saturated aqueous solutions
• Concentrated chloride solutions
• Concrete
• Soil
• Low-conductivity drinking water
• Process streams of chemical plants at elevated temperatures
• Coatings
When CMAS are used in a relatively corrosive environment,
produced data may be reduced to a single parameter so that the probe
can be conveniently used for real-time and online monitoring
purposes. The most anodic current has been used as a one-parameter
signal for the CMAS probes. Because the anodic electrodes in a CMAS
probe simulate the anodic sites on a metal surface, the most anodic
current may be considered as the corrosion current from the most
corroding site on the metal.
In a less corrosive environment or with a more corrosion-resistant
alloy, the most anodic electrode may not be fully covered by anodic
sites until the electrode is fully corroded. Therefore, the most anodic
electrode may still have cathodic sites available, and the electrons
from the anodic sites may flow internally to the cathodic sites within
the same electrode. The total anodic corrosion current, I , and the
corr
ex
measured anodic current, I may be related by Eq. (5.28).
a
I ex = I e corr (5.28)
a
where e is a current distribution factor that represents the fraction of
electrons resulting from corrosion that flows through the external
circuit. The value of e may vary between 0 and 1, depending on
parameters such as surface heterogeneities on the metal, the
environment, the electrode size, and the number of sensing electrodes.
If an electrode is severely corroded and significantly more anodic
than the other electrodes in the probe, the e value for this corroding
electrode would be close to 1, and the measured external current
would be equal to the localized corrosion current.

5.6.3 Cathodic Protection
Cathodic protection is widely used on small to extremely large
structures to protect metals and particularly steel against corrosion.
This can often be accomplished by using a protective current that is
either generated by a power supply in what is called impressed
current cathodic protection (ICCP) or by using another metal that
corrodes more readily than the metal being protected and therefore is
sacrificed is the process. Chapter 13 provides a much detailed
discussion of this important technique.