CORROSIVE ENVIRONMENTS 15
Carbon steel, the most common metal used in oilfi eld
systems, corrodes at unacceptable rates in many aqueous
environments, and pH adjustment is a common means
of controlling corrosion.
Most readers are familiar with the idea that saltwater
is more corrosive than freshwater. The combined effects
of dissolved oxygen and salt concentration on the cor-
rosivity of water are shown in Figure 3.6 . As increasing
amounts of salt are added to water, the electrical con-
ductivity of the electrolyte increases and so does the
corrosion rate. At the same time, the oxygen solubility
decreases continuously with additional concentrations
of salt, and this limits the corrosion rate because oxygen
reduction is the rate - controlling chemical (reduction)
reaction.
5
The same phenomenon happens with all
other salts. The maximum corrosion rate is at approxi-
mately 3% salt — the exact concentration depends on
temperature and the salt involved.
8
This explains why
highly concentrated brines, such as those used in packer
fl uids, are noncorrosive, provided they are properly pH -
adjusted and have little or no dissolved oxygen.
Figure 3.6 explains why freshwater, low in salt, is less
corrosive than saltwater, but the most important point
to be learned from this picture is that, even at its most
corrosive, only about one - third of the corrosion in salt-
water is due to salt — most of the corrosion would occur
anyway due to the presence of oxygen.
Many reports on corrosion ascribe corrosion damage
to the presence of chlorides, the most common anions
found in seawater and often found in freshwater as well.
This dates back to analytical chemistry practices in the
early twentieth century, when qualitative analysis tech-
niques (methods of determining the presence of various
chemicals in the environment) were relatively new. The
fi eld methods for identifying chloride were relatively
easy, and many authors started blaming chlorides for
damage caused by salts. It was unnecessary to identify
the other components of salt, as there will always be
cations (positively charged ions) present to balance the
charge of the negatively charged anions. This practice
Figure 3.5 Simplifi ed diagram showing the effect of relative
humidity and pollution on the corrosion of carbon steel.
6
Corrosion loss
Relative humidity (%)
50 60 70 80 90 100
Pure air
Air
polluted
with SO
2
Air polluted
with SO
2
and
solid particles
Figure 3.6 The corrosion rate of iron in air - exposed fresh-
water at varying salt (sodium chloride) concentrations.
6
Conc. NaCl (wt %)
Relative Corrosion Rate
2
1
0
035 10 20 302515
shutdown, when acidic moisture can condense on rough
surfaces and cause corrosion similar to that on automo-
tive muffl ers during times when the system is cold
enough for condensation.
It would seem logical that atmospheric corrosion
would not occur until the relative humidity is 100%, but
this is not the case. Research dating back to the 1920s
has shown that corrosion can occur once the humidity
reaches a “ critical humidity ” of approximately 60 –
70%.
6 – 9
Many structures, especially on the away - from -
the - sun side (the north side in northern latitudes), stay
above this critical humidity virtually all the time, at least
whenever the temperature is above freezing.
8
It is
important to realize that heat sinks; for example, large
structural members on offshore structures can remain
below the critical humidity long after the sun comes up
and corrosion has diminished elsewhere on the same
structure. The presence of deliquescent salts means that
many surfaces remain wetted even in sunlight. This is a
very important consideration when painting structures,
because “ fl ash rusting ” due to surface moisture can
quickly form and severely degrade the adherence of
primary coatings to painted structures (Figure 3.5 ).
Most metal exposed to atmospheric corrosion is
carbon steel , and the most common method of corro-
sion control is by the use of protective coatings (paint-
ing). Some process equipment, storage tanks, and
electronic control systems are protected by the use of
inerting gases, heaters, deliquescing agents, or vapor
phase inhibitors. Control lines, conduit, and similar
tubing are often stainless steel on offshore structures.
Water as a Corrosive Environment
The effect of pH on corrosion of carbon steel was dis-
cussed in Chapter 2 , Chemistry of Corrosion, fi gure 2.9.
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