Carbon steel, the most widely used engineering material,
accounts for approximately 85%, of the annual steel production worldwide.
Despite its relatively limited corrosion resistance, carbon steel is used
in large tonnages in marine applications, nuclear power and fossil fuel
power plants, transportation, chemical processing, petroleum production
and refining, pipelines, mining, construction and metal-processing equipment.
The cost of metallic corrosion to the total economy must be measured
in hundreds of millions of dollars (or euros) per year. Because carbon
steels represent the largest single class of alloys in use, both in
terms of tonnage and total cost, it is easy to understand that the
corrosion of carbon steels is a problem of enormous practical importance.
This is the reason for the existence of entire industries devoted to
providing protective systems for irons and steel.
Carbon steels are by their nature of limited alloy content, usually
less than 2% by weight for total of additions. Unfortunately, these
levels of addition do not generally produce any remarkable changes
in general corrosion behavior. One possible exception to this
statement would be weathering steels, in small additions of
copper, chromium, nickel and phosphorus produce significant
reduction in corrosion rate in certain environments.
Because corrosion is such a multifaceted phenomenon, it is generally
useful to attempt to categorize the various types. This is usually
done on environmental basis. In this article, atmospheric corrosion,
aqueous corrosion and some other corrosion types of interest, such as
corrosion in soils, concrete and boilers and heating plants will be
addressed.
Atmospheric corrosion
Atmospheres are often classified as being rural, industrial or
marine in nature. Two decidedly rural environments can differ
widely in average yearly temperature and rainfall patterns,
mean temperature, and perhaps acid rain, can make extrapolations
from past behavior less reliable.
The corrosion of carbon steel in the atmosphere and in many aqueous
environments is best understood from a film formation and brake down
standpoint. It is an inescapable fact that iron in the presence of
oxygen and water is thermodynamically unstable with respect to its
oxides. Because atmospheric corrosion is an electrolytic process,
the presence of an electrolyte is required. This should not be
taken to mean that the steel surface must be awash in water; a
very thin adsorbed film of water is all that is required.
During the actual exposure, the metal spends some portion of the
time awash with water because of rain or splashing and a portion
of the time covered with a thin adsorbed water film. The portion
of time spent covered with the thin water film depends quite
strongly on relative humidity at the exposure site. This fact
has led many corrosion scientists to investigate the influence
of the time of wetness on the corrosion rate.
Rusting of iron depends on relative humidity and time of exposure in
atmosphere containing 0.01% SO2. The increase in corrosion rate
produced by the addition of SO2 is substantial. Oxides of nitrogen
in the atmosphere would also exhibit an accelerating effect on the
corrosion of steel. Indeed, any gaseous atmospheric constituent
capable of strong electrolytic activity should be suspected as
being capable of increasing the corrosion rate of steel.
Because carbon steels are not very highly alloyed, it is not surprising
that most grades do not exhibit large differences in atmospheric-corrosion
rate. Nevertheless, alloying can make changes in the atmospheric-corrosion
rate of carbon steel. The elements generally found to be most beneficial
in this regard are copper, nickel, silicon, chromium and phosphorus.
Of these, the most striking example is that of copper, increases from
0.01-0.05%, decrease the corrosion rate by a factor of two to three.
Additions of the above elements in combination are generally more
effective than when added singly, although the effects are not additive.
Aqueous Corrosion
Carbon steel pipes and vessels are often required to transport water
or are submerged in water to some extent during service. This exposure
can be under conditions varying temperature, flow rate, pH, and other
factors, all of which can alter the rate of corrosion. The relative
acidity of the solution is probably the most important factor to be
considered. At low pH the evolution of hydrogen tends to eliminate
the possibility of protective film formation so that steel continues
to corrode but in alkaline solutions, the formation of protective
films greatly reduces the corrosion rate. The greater alkalinity,
the slower the rate of attack becomes. In neutral solutions, other
factors such as aeration, became determining so that generalization
becomes more difficult.
The corrosion of steels in aerated seawater is about the same overall
as in aerated freshwater, but this is somewhat misleading because the
improved electrical conductivity of seawater can lead to increased
pitting. The concentration cells can operate over long distance,
and this leads to a more nonuniform attack than in fresh water.
Alternate cycling through immersion and exposure to air produces
more pitting attack than continuous immersion. The effect of
various alloying addition and exposure conditions on the
corrosion behavior is shown in Table 1.
Table 1. Comparison of results under different type of exposure
|
Effects of alloy selection, chemical composition and alloy additions
|
Sea air
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Freshwater
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Alternately wet with seawater or Spray and dry
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Continuously wet with seawater
|
|
Ferrous alloys
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Pockmarked
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Vermiform on cleaned bars
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Pitting, particularly on bars with scale
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Pitting, particularly on bars with scale
|
|
Wrought iron versus carbon steel
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Steel superior to wrought and ingot irons
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Iron and steel equal in low-moor areas
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Low-moor iron superior to carbon steel
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Low-moor iron superior to carbon steel
|
|
Sulfur and phosphorus content
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Best results when S and P are low
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Best results when S and P are low
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Best results when S and P are low
|
Apparently little influence
|
|
Addition of copper
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Beneficial: Effect increasing with copper content
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Beneficial: 0.635% Cu almost as good as 2.185% Cu
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Beneficial: 0.635% and 2.185% Cu much the same
|
0.635% Cu slightly beneficial: 2.185% Cu somewhat less so
|
|
Addition of nickel
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3.75% Ni superior even to 2% Cu; 36% Ni almost perfect after 15-year exposure
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3.75%Ni superior even to 2%Cu; 36%Ni excellent resistance
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3.75%Ni beneficial usually more so than Cu: 36%Ni the best metal in the set
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3.75% Ni slightly beneficial and slightly superior to Cu: 36% Ni the best metal in the set
|
|
Addition of 13.5% Cr
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Excellent resistance to corrosion: cold blast metal perfect after 15-year exposure: equal to 36% Ni steel
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Excellent resistance to corrosion: equal to 36% Ni steel
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Subject to severe localized corrosion that virtually destroys the metal
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Subject to severe localized corrosion that virtually destroys the metal
|
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Behavior of cast irons
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Excellent resistance to corrosion: cold blast metal superior to hot: no graphitic corrosion
|
Undergoes graphitic corrosion
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Undergoes graphitic corrosion
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Undergoes graphitic corrosion
|
Interestingly, the corrosion rates of specimens completely immersed
in seawater do not appear to depend on the geographical location of
the test site; therefore, by inference, the mean temperature does
not appear to play an important role.
This constancy of the corrosion rate in seawater has been attributed
to the more rapid fouling of the exposed steel by marine organisms,
such as barnacles and algae, in warmer seas. It is further speculated
that this fouling offsets that increases expected from the temperature
rise.
Soil Corrosion
The response of carbon steel to soil corrosion depends primarily on
the nature of the soil and certain other environmental factors, such
as the availability to moisture and oxygen. These factors can lead to
extreme variations in the rate of the attack. For example, under the
worst condition a buried vessel may perforate in less than one year,
although archeological digs in arid desert regions have uncovered iron
tools that are hundreds of years old.
Some general rules can be formulated. Soils with high moisture content,
high electrical conductivity, high acidity, and high dissolved salts
will be most corrosive. The effect of aeration on soils is somewhat
different from the effect of aeration in water because poorly aerated
conditions in water can lead to accelerated attack by sulfate-reducing
anaerobic bacteria.
The effect of low levels of alloying additions on the soil corrosion
of carbon steels is modest. Some data seems to show a small benefit
of 1%Cu and 2.5% Ni on plain carbon steel.
The weight loss and maximum pit depth in soil corrosion can be
represented by an equation of the form:
Z = a·tm
Where:
Z - either the weight of loss of maximum pit depth
T - time of exposure
a and m - constants that depend on the specific soil corrosion situation.
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