FACTORS INFLUENCING CORROSION
A number of factors influence the stability and breakdown of the passive
film, and include (1) surface finish, (2) metallurgy, (3) stress, (4) heat
treatment, and (5) environment. Polished surfaces generally resist
corrosion initiation better than rough surfaces. Surface roughness tends
to increase the kinetics of the corrosion reaction by increasing the exchange
current density of the HER or the oxygen reduction at the cathodes.
Metallurgical structures and properties often have major effects on
corrosion. Regions of varying composition exist along the surface of
most metals or alloys. These local compositional changes have different
potentials that may initiate local-action cells. Nonmetallic inclusions,
particularly sulfide inclusions, are known to initiate corrosion on carbon
and stainless steels. The size, shape, and distribution of sulfide inclusions
in 304 stainless steel may have a large impact on dissolution
kinetics and pitting susceptibility and growth. Elements such as chromium
and nickel improve the corrosion resistance of carbon steel by
improving the stability of the passive oxide film. Copper (0.2 percent)
added to carbon steel improves the atmospheric corrosion resistance of
weathering steels.
Stresses, particularly tensile stresses, affect corrosion behavior.
These stresses may be either applied or residual. Residual stresses may
either arise from dislocation pileups or stacking faults due to deforming
or cold-working the metal; these may arise from forming, heat-treating,
machining, welding, and fabrication operations. Cold working increases
the stresses applied to the individual grains by distorting the crystals.
Corrosion at cold-worked sites is not increased in natural waters, but
increases severalfold in acidic solutions. Possible segregation of carbon
and nitrogen occurs during cold working. Welding can induce residual
stresses and provide sites that are subject to preferential corrosion and
cracking. The welding of two dissimilar metals with different thermal
expansion coefficients may restrict expansion of one member and induce
applied stresses during service. Thermal fluctuations of fabrication
bends or attachments welded to components may cause applied stresses
to develop that may lead to cracking.
Improper heat treatment or welding can influence the microstructure
of different alloys by causing either precipitation of deleterious phases
at alloy grain boundaries or (as is the case of austenitic stainless steels)
depletion of chromium in grain boundary zones, which decreases the
local corrosion resistance. Welding also can cause phase transformations,
formation of secondary precipitates, and induce stresses in and
around the weld. Rapid quenching of steels from austenizing temperatures
may form martensite, a distorted tetragonal structure, that often
suffers from preferential corrosion.
The nature of the environment can affect the rate and form of corrosion.
Environments include (1) natural and treated waters, (2) the atmosphere,
(3) soil, (4) microbiological organisms, and (5) high temperature.
Corrosivity in freshwater varies with oxygen content, hardness,
chloride and sulfur content, temperature, and velocity. Water contains
colloidal or suspended matter and dissolved solids and gases. All these
constituents may stimulate or suppress corrosion either by affecting the
cathodic or anodic reaction or by forming a protective barrier. Oxygen
is probably the most significant constituent affecting corrosion in neutral
and alkaline solutions; hydrogen ions are more significant in acidic
solutions. Freshwater can be hard or soft. In hard waters, calcium carbonate
often deposits on the metal surface and protects it; pitting may
occur if the calcareous coating is not complete. Soft waters are usually
more corrosive because protective deposits do not form. Several saturation
indices are used to provide scaling tendencies. Nitrates and chlorides
increase aqueous conductivity and reduce the effectiveness of natural
protective films. The chloride/bicarbonate ratio has been observed
to predict the probability of dezincification. Sulfides in polluted waters
tend to cause pitting. Deposits on metal surfaces may lead to local
stagnant conditions which may lead to pitting. High velocities usually
increase corrosion rates by removing corrosion products which otherwise
might suppress the anodic reaction and by stimulating the cathodic
reaction by providing more oxygen. High-purity water, used in nuclear

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and high-pressure power units, decreases corrosion by increasing the
electrical resistance of the fluid. Temperature effects in aqueous systems
are complex, depending on the nature of the cathodic and anodic
reactions.
Seawater is roughly equivalent to 31⁄2 percent sodium chloride, but
also contains a number of other major constituents and traces of almost
all naturally occurring elements. Seawater has a higher conductivity
than freshwater, which alone can increase the corrosion of many metals.
The high conductivity permits larger areas to participate in corrosion
reactions. The high chloride content in seawater can increase localized
breakdown of oxide films. The pH of seawater is usually 8.1 to 8.3.
Plant photosynthesis and decomposition of marine organisms can raise
or lower the pH, respectively. Because of the relatively high pH, the
most important cathodic reaction in seawater corrosion processes is
oxygen reduction. Highly aerated waters, such as tidal splash zones, are
usually regions of very high corrosion rates. Barnacles attached to metal
surfaces can cause localized attack if present in discontinuous barriers.
Copper and copper alloys have the natural ability to suppress barnacles
and other microfouling organisms. Sand, salt, and abrasive particles suspended
in seawater can aggravate erosion corrosion. Increased seawater
temperatures generally increase corrosivity (Laque, ‘‘Marine
Corrosion—Causes and Prevention,’’ Wiley-Interscience, New York).
In general, atmospheric corrosion is the result of the conjoint action of
oxygen and water, although contaminants such as sodium chloride and
sulfur dioxide accelerate corrosion rates. In the absence of moisture
(below 60 to 70 percent relative humidity), most metals corrode very
slowly at ambient temperatures. Water is required to provide an electrolyte
for charge transfer. Damp corrosion requires moisture from the
atmosphere; wet corrosion occurs when water pockets or visible water
layers are formed on the metal surface due to salt spray, rain, or dew
formation. The solubility of the corrosion products can affect the corrosion
rate during wet corrosion; soluble corrosion products usually increase
corrosion rates.
Time of wetness is a critical variable that determines the duration of
the electrochemical corrosion processes in atmospheric corrosion.
Temperature, climatic conditions, relative humidity, and surface shape
and conditions that affect time of wetness also influence the corrosion
rate. Metal surfaces that retain moisture generally corrode faster than
surfaces exposed to rain. Atmospheres can be classified as rural, marine,
or industrial. Rural atmospheres tend to have the lowest corrosion rates.
The presence of NaCl near coastal shores increases the aggressiveness
of the atmosphere. Industrial atmospheres are more corrosive than rural
atmospheres because of sulfur compounds. Under humid conditions
SO2 can promote the formation of sulfurous or sulfuric acid. Other
contaminants include nitrogen compounds, H2S, and dust particles.
Dust particles adhere to the metal surface and absorb water, prolonging
the time of wetness; these particles may include chlorides that tend to
break down passive films [Ailor (ed.), ‘‘Atmospheric Corrosion,’’
Wiley-Interscience, New York].
Soil is a complex, dynamic environment that changes continuously,
both chemically and physicially, with the seasons of the year. Characterizing
the corrosivity of soil is difficult at best. Soil resistivity is a measure
of the concentration and mobility of ions required to migrate
through the soil electrolyte to a metal surface for a corrosion reaction to
continue. Soil resistivity is an important parameter in underground
corrosion; high resistivity values often suggest low corrosion rates. The
mineralogical composition and earth type affect the grain size, effective
surface area, and pore size which, in turn, affect soil corrosivity. Further,
soil corrosivity can be strongly influenced by certain chemical
species, microorganisms, and soil acidity or alkalinity. A certain water
content in soil is required for corrosion to occur. Oxygen also is generally
required for corrosion processes, although steel corrosion can occur
under oxygen-free, anaerobic conditions in the presence of sulfatereducing
bacteria (SRB). Soil water can regulate the oxygen supply and
its transport. [Romanoff, Underground Corrosion, NBS Circ., 579,
1957, available from NACE International, Houston; Escalante (ed.),
‘‘Underground Corrosion,’’ STP 741, ASTM, Philadelphia].
Almost all commercial alloys are affected by microbiological influenced
corrosion (MIC). Most MIC involves localized corrosion. Biological
organisms are present in virtually all natural aqueous environments
(freshwater, brackish water, seawater, or industrial water) and in
some soils. In these environments, the tendency is for the microorganisms
to attach to and grow as a biofilm on the surface of structural
materials. Environmental variables (pH, velocity, oxidizing power,
temperature, electrode polarization, and concentration) under a biofilm
can be vastly different from those in the bulk environment. MIC can
occur under aerobic or anaerobic conditions. Biofilms may cause corrosion
under conditions that otherwise would not cause dissolution in the
environment, may change the mode of corrosion, may increase or decrease
the corrosion rate, or may not influence corrosion at all. MIC
may be active or passive. Active MIC directly accelerates or establishes
new electrochemical corrosion reactions. In passive MIC, the biomass
acts as any dirt or deposition accumulation where concentration cells
can initiate and propagate. Several forms of bacteria are linked to accelerated
corrosion by MIC: (1) SRB, (2) sulfur or sulfide-oxidizing bacteria,
(3) acid-producing bacteria and fungi, (4) iron-oxidizing bacteria,
(5) manganese-fixing bacteria, (6) acetate-oxidizing bacteria, (7) acetate-
producing bacteria, and (8) slime formers [Dexter (ed.), ‘‘Biological
Induced Corrosion,’’ NACE-8, NACE International, Houston; Kobrin
(ed.), ‘‘A Practical Manual on Microbiological Influenced
Corrosion,’’ NACE International Houston; Borenstein, ‘‘Microbiological
Influenced Corrosion Handbook,’’ Industrial Press, New York].
High-temperature service ($100°C) is especially damaging to many
metals and alloys because of the exponential increase in the reaction
rate with temperature. Hot gases, steam, molten or fused salts, molten
metals, or refractories, ceramics, and glasses can affect metals and
alloys at high temperatures. The most common reactant is oxygen;
therefore all gas-metal reactions are usually referred to as oxidations,
regardless of whether the reaction involves oxygen, steam, hydrogen
sulfide, or combustion gases. High-temperature oxidation reactions are
generally not electrochemical; diffusion is a fundamental property involved
in oxidation reactions. The corrosion rate can be influenced
considerably by the presence of contaminants, particularly if they are
adsorbed on the metal surface. Pressure generally has little effect on the
corrosion rate unless the dissociation pressure of the oxide or scale
constituent lies within the pressure range involved. Stress may be important
when attack is intergranular. Differential mechanical properties
between a metal and its scale may cause periodic scale cracking which
leads to accelerated oxidation. Thermal cycling can lead to cracking and
flaking of scale layers. A variety of molten salt baths are used in industrial
processes. Salt mixtures of nitrates, carbonates, or halides of alkaline
or alkaline-earth metals may adsorb on a metal surface and cause
beneficial or deleterious effects.
There are three major types of cells that transpire in corrosion reactions.
The first is the dissimilar electrode cell. This is derived from potential
differences that exist between a metal containing separate electrically
conductive impurity phases on the surface and the metal itself,
different metals or alloys connected to one another, cold-worked metal
in contact with the same metal annealed, a new iron pipe in contact with
an old iron pipe, etc.
The second cell type, the concentration cell, involves having identical
electrodes each in contact with an environment of differing composition.
One kind of concentration cell involves solutions of differing salt
levels. Anodic dissolution or corrosion will tend to occur in the more
dilute salt or lower-pH solution, while the cathodic reaction will tend to
occur in the more concentrated salt or higher-pH solution. Identical
pipes may corrode in soils of one composition (clay) while little corrosion
occurs in sandy soil. This is due, in part, to differences in soil
composition and, in part, to differential aeration effects. Corrosion will
be experienced in regions low in oxygen; cathodes will exist in highoxygen
areas.
The third type of cell, the thermogalvanic cell, involves electrodes of
the same metal that are exposed, initially, in electrolytes of the same
composition but at different temperatures. Electrode potentials change
with temperature, but temperature also may affect the kinetics of dissolution.
Depending on other aspects of the environment, thermogalvanic cells may accelerate or slow the rate of corrosion. Denickelification of
copper-nickel alloys may occur in hot areas of a heat exchanger in
brackish water, but are unaffected in colder regions.