Corrosion, which is the degradation of material due to reactions with the environment, is usually electrochemical in nature. For this reason, an understanding of basic electrochemistry is necessary to the understanding of corrosion. Robert Heidersbach, author of "Metallurgy and Corrosion Control in Oil and Gas Production," will take you on this journey. This presentation is a sample of chapter 2 of his book, which can be found on Google Books.

1. CHEMISTRYOF
CORROSION:
PARTTWO
B Y R O B E R T H E I D E R S B A C H

2. CONTINUED FROM
A PREVIOUS
PRESENTATION...

3. At low electrode current densities, the change
in potential can be plotted. These plots of
potential versus logarithm current are often
termed Evans diagrams, after Professor U.R>
Evans of Cambridge University, who
popularized their use.
As stated above, most oilfield corrosion rates
are controlled by the low concentrations of
reducible species in the environment. These
specify must migrate, or diffuse, to the metal
surface in order to react. The rate of this
diffusion is controlled by the concentration of
the diffusing species in the environment, the
thickness of the boundary layer where this
diffusion is occurring (largely determined by
fluid flow or the lack thereof), temperature,
and other considerations. The resulting
concentration polarization can be written as:
ELECTRODE
POTENTIALS AND
CURRENT

4. ELECTRODE
POTENTIALS AND
CURRENT

5. The other terms are the same as described as
above in discussions in the Nernst equation
and activation polarization (Tafel slope)
behavior.
In corrosion, the limited concentrations of
reducible species produce concentration
polarization only at cathodes. At low current
densities, the concentration polarisation is
negligible, and, as the reduction current
density approaches the limiting current, the
slope quickly becomes a vertical downward
line.
ELECTRODE
POTENTIALS AND
CURRENT

6. ELECTRODE
POTENTIALS AND
CURRENT
The total polarization of an electrode is the
sum of both the activation and concentration
polarization. The combined polarization for a
reduction reaction on a cathode is:
As stated earlier, most oilfield corrosion rates
are determined by the concentration of the
reducible chemicals in the environment.
For surface equipment, most corrosion rates
are determined by the concentration of
dissolved oxygen in whatever water is
available.

7. The importance of potential in determining
corrosion rates is apparent from the above
discussions. Academic chemistry reports tend
to describe potentials relative to the standard
hydrogen electrode, which had been
arbitrarily set to a potential of zero. In field
applications, it is common to use other
reference electrodes. The most common
reference electrodes used in oilfield work are
the saturated copper-copper sulfate
electrode (CSE), used in onshore applications,
and the silver-silver chloride electrode used
for offshore measurements, where
contamination of the CSE electrode would
produce variable readings.
ELECTRODE
POTENTIALS AND
CURRENT

8. Marcel Pourbaix developed a means of
explaining the thermodynamics of corrosion
systems by plotting regions of
thermodynamic sustainability of metals and
their reaction plots on potential versus pH
plots. The regions of a Pourbaix diagram can
be described as:
Immunity: The metal cannot oxidize or
corrode (although it may still be subject to
hydrogen embrittlement)
Corrosion: Ions of the metal are
thermodynamically stable and the metal
will corrode.
Passivity: Compounds of the metal and
chemicals from the environment are
thermodynamically stable, and the metal
may be protected from corrosion if the
passive film is adherent and protective.
POTENTIAL-PH
(POURBAIX)
DIAGRAMS

9. Many users of Pourbaix diagrams miss the
final point above. Thermodynamics alone
cannot predict if passive films will be
protective or not.
There are a number of important points useful
for oilfield corrosion control:
POTENTIAL-PH
(POURBAIX)
DIAGRAMS
Water is only stable over a potential range
of slightly more than one volt. This is very
important in cathodic protection.
Iron (carbon steel) is covered with iron
oxides (passive films) in most aqueous
environments. Unfortunately, these
passive films are not protective and other
means of corrosion are necessary.
The potentials at which iron (carbon steel)
is protected from corrosion do not coincide
with the immunity regions on the Pourbaix
diagram.

10. The diagrams for zinc, aluminum, and
cadmium, commonly referred to as the
amphoteric coating metals, have passive
regions in neutral environments These metals
also have low corrosion rates in neutral
environments and higher corrosion rates in
both acids and bases.
Pourbaix diagrams have limitations in
addition to the inability of thermodynamics to
predict the protectiveness of passive films.
These include the idea that they can only be
calculated for alloys, although experimental
Pourbaix diagrams have been reported,
POTENTIAL-PH
(POURBAIX)
DIAGRAMS

11. The following ideas have been discussed in
detail this chapter:
Electrode potentials are determined by:
Corrosion is electrochemical in nature
Most metal surfaces have both oxidation
and reduction occurring simultaneously
If the predominant reaction is oxidation,
the metal will corrode
The most important reduction reaction is in
oxygen reduction for many oilfield
systems. If no oxygen is available, the
corrosion rate will often be very low.
Meta chemistry
chemicals in the environment
Temperature
SUMMARY

12. These potentials are usually measured
against either copper-copper sulfate or
silver-silver chloride electrodes, depending
on the environment.
Corrosion rates are often expressed by the
average depth of penetration, and this can be
misleading because most oilfield corrosion is
localized in nature.
The pH of the environment has a major effect
on corrosivity.
Passive films may limit corrosion in many
environments, but carbon steel, the most
common oilfield metal, seldom forms
adequately protective passive films, and
other means of corrosion are often necessary.
SUMMARY

13. Checkoutthepreviewofmybook,
"MetallurgyandCorrosionControlin
OilandGasProduction"onGoogle
Booksnow!
B Y R O B E R T H E I D E R S B A C H