Corrosion Theory
Humans have most likely been
trying to understand and control corrosion for as long as they have been using
metal objects. The most important periods of prerecorded history are named for
the metals that were used for tools and weapons (Iron Age, Bronze Age). With a
few exceptions, metals are unstable in ordinary aqueous environments. Metals
are usually extracted from ores through the application of a considerable
amount of energy. Learning the processes that take place to make even the
smallest corrosion reactions and how in turn it relates to human health are
fascinating. Certain environments offer opportunities for these metals to
combine chemically with elements to form compounds and return to their lower
energy levels.
Corrosion is the primary means by
which metals deteriorate. Most metals corrode on contact with water (and
moisture in the air), acids, bases, salts, oils, aggressive metal polishes, and
other solid and liquid chemicals. Metals will also corrode when exposed to
gaseous materials like acid vapors, formaldehyde gas, ammonia gas, and sulfur
containing gases. Corrosion specifically refers to any process involving the
deterioration or degradation of metal components. The best known case is that
of the rusting of steel. Corrosion processes are usually electrochemical in
nature, having the essential features of a battery.
When metal atoms are exposed to
an environment containing water molecules they can give up electrons, becoming
themselves positively charged ions, provided an electrical circuit can be
completed. This effect can be concentrated locally to form a pit or, sometimes
a crack, or it can extend across a wide area to produce general wastage.
Localized corrosion that leads to pitting may provide sites for fatigue
initiation and, additionally, corrosive agents like seawater may lead to
greatly enhanced growth of the fatigue crack. Pitting corrosion also occurs
much faster in areas where microstructural changes have occurred due to welding
operations.
Corrosion can be defined as the
degradation of a material due to a reaction with its environment.
Degradation implies deterioration
of physical properties of the material. This can be a weakening of the material
due to a loss of cross-sectional area, it can be the shattering of a metal due
to hydrogen embrittlement, or it can be the cracking of a polymer due to
sunlight exposure.
Materials can be metals, polymers
(plastics, rubbers, etc.), ceramics (concrete, brick, etc.) or
composites-mechanical mixtures of two or more materials with different
properties. Because metals are the most used type of structural materials most
of this web site will be devoted to the corrosion of metals.
DEFINITION OF METAL CORROSION.
Metal Corrosion can be defined as
the destructive attack of a metal through interaction with its environment.
DRIVING FORCE FOR CORROSION.
Most metals used in the
construction of facilities are subject to corrosion. This is due to the high
energy content of the elements in metallic form. In nature, most metals are
found in chemical combination with other elements. These metallic ores are
refined by man and formed into metals and alloys. As the energy content of the
metals and alloys is higher than that of their ores, chemical re-combination of
the metals to form ore like compounds is a natural process.
Metals corrode because we use
them in environments where they are chemically unstable. Only copper and the
precious metals (gold, silver, platinum, etc.) are found in nature in their metallic
state. All other metals, to include iron-the metal most commonly used-are
processed from minerals or ores into metals which are inherently unstable in
their environments.
This golden statue in Bangkok,
Thailand, is made of the only metal which is thermodynamically stable in room
temperature air. All other metals are unstable and have a tendency to revert to
their more stable mineral forms. Some metals form protective ceramic films
(passive films) on their surfaces and these prevent, or slow down, their
corrosion process. The woman in the picture below is wearing anodized titanium
earrings. The thickness of the titanium oxide on the metal surface refracts the
light and causes the rainbow colors on her earrings. Her husband is wearing
stainless steel eyeglasses. The passive film that formed on his eyeglasses is
only about a dozen atoms thick, but this passive film is so protective that his
eyeglasses are protected from corrosion. We can prevent corrosion by using
metals that form naturally protective passive films, but these alloys are
usually expensive, so we have developed other means of corrosion control.
FUNDAMENTAL MECHANISM OF ATTACK.
Corrosion of metals takes place
through the action of electrochemical cells. Although this single mechanism is
responsible, the corrosion can take many forms. Through an understanding of the
electrochemical cell and how it can act to cause the various forms of
corrosion, the natural tendency of metals to suffer corrosion can be overcome
and equipment that is resistant to failure by corrosion can be designed.
The Electrochemical Cell.
As in all chemical reactions,
corrosion reactions occur through an exchange of electrons. In electrochemical
reactions, the electrons are produced by a chemical reaction, the oxidation, in
one area, the anode, travel through a metallic path and are consumed through a
different chemical reaction in another area, the cathode. In some cases, such
as the common dry cell battery, electrochemical reactions can be used to supply
useful amounts of electrical current. In marine corrosion, however, the most
common result is the transformation of complex and expensive equipment to
useless junk.
Components.
In order for electrochemical
reactions to occur, four components must be present and active. These
components are the anode, cathode, electron path, and electrolyte.
Anode.
In an electrochemical cell, the
anode is the site where electrons are produced through the chemical activity of
the metal. The anode is the area where metal loss occurs. The metal loses
electrons and migrates from the metal surface through the environment. The
electrons remain in the metal but are free to move about in response to voltage
gradients.
Cathode.
The cathode in an electrochemical
cell is the site where electrons are consumed. For each electron that is
produced at an anodic site, an electron must be consumed at a cathodic site. No
metal loss occurs at sites that are totally cathodic.
Electron Path.
In order for electrons to flow
from the anodic sites to cathodic sites, the electrons migrate through a
metallic path. This migration occurs due to a voltage difference between the
anodic and cathodic reactions. Electrons can move easily only through metals
and some non-metals such as graphite. Electrons from electrochemical reactions
cannot move through insulating materials such as most plastics nor can they
directly enter water or air. In some cases, the electron path is the corroding
metal itself, in other cases, the electron path is through an external
electrical path.
Electrolyte.
Electrolytes are solutions that
can conduct electrical currents through the movement of charged chemical
constituents called ions. Positive and negative ions are present in equal
amounts. Positive ions tend to migrate away from anodic areas and toward
cathodic areas. Negative ions tend to migrate away from cathodic areas and
towards anodic areas.
Anodic Reactions.
Metal loss at anodic sites in an
electrochemical cell occurs when the metal atoms give up one or more electrons
and move into the electrolyte as positively charged ions.
Typical Reactions.
The generic chemical formula for
this metal loss at anodic sites is:
M ---> M+ + e-
where:
M = uncharged metal atom at the
metal surface
M+ = positively charged metal ion
in the electrolyte
e- = electron that remains in the
metal
This type of chemical reaction is
called oxidation even though it does not directly involve oxygen but only
results in an increase in positive charge on the atom undergoing oxidation.
More than one electron can be
lost in the reaction as in the case for iron where the most common anodic
reaction is:
Fe ---> Fe+ + 2e-
where:
Fe = metallic iron
Fe+ = ferrous ion that carries a
double negative charge
Correlation Between Current Flow
and Weight Loss.
For each specific anodic reaction
a characteristic number of electrons are produced in the reaction of one metal
ions. Thus, all other things being equal, the metal loss is proportional to the
number of electrons that are produced. As the electrons produced migrate to
cathodic areas through the electron path, the metal loss is proportional to the
current flow. In cases where more positively charged ions are produced, more
electrons flow for a given number of corroding metal atoms but the current flow
remains proportional to the metal loss.
Cathodic Reactions.
The electrons that are produced
at anodic sites are consumed at cathodic sites. The type of chemical reactions
that consume electrons are called reduction and have the generic chemical formula:
R+ + e- --> R
where:
R+ = a positive ion in solution
e- = an electron in the metal
Ro = the reduced chemical
In reduction, the chemical being
reduced gains electrons and its charge is made more negative. In some cases, the
where the ion in solution has a multiple positive charge, the total positive
charge on the ion may not be neutralized. In other cases, the chemical which is
reduced may not be a positive ion but is a neutral chemical which then becomes
a negatively charged ion in solution in a reaction such as:
R + e- --> R-
What is Rust?
Rust is a generic term used to
describe different iron hydroxides and oxides, Fe(OH)2, Fe(OH)3, FeO(OH),
Fe2O3.H2O that form when iron corrodes.
The common form of Rust is a Red
products Fe2O3 called hematite.
What causes Rust to form is the
reaction between iron and water; for iron exposed to the atmosphere it could be
either water condensing from air or rain.
The oxygen in the air dissolves
in the water and causes rust to form.
There are always two distinct
chemical reactions in a corrosion process, the basic chemistry of corrosionis:
1) Anodic Dissolution of Metal
(Iron) that goes into solution (water)
Fe ----->
Fe2+ + 2e-
2) Cathodic Reduction of Oxygen
dissolved in water
O2 + 2H2O + 4e- ----> 4OH-
The final reaction is:
Fe2+ + 2OH- ----->
Fe(OH)2
This iron hydroxide will then
further reacts with oxygen to give the final red product (Rust):
Fe2O3.H2O
Corrosion Types - Corrosion Forms
Although there is only one
fundamental mechanism of corrosion, the electrochemical cell, there are several
Corrosion forms or Corrosion Types that
can occur. Each form of attack has a specific arrangement of anodes and
cathodes and the corrosion which occurs has a specific location and pattern.
Each form of corrosion can be effectively controlled during design if it is
anticipated. By understanding the various forms of corrosion, the conditions
under which they occur, and how they are quantified, they can each be addressed
and controlled. The most important Types of Corrosion are:
Uniform Corrosion
Concentration Cell Corrosion.
Intergranular Corrosion
Atmospheric Corrosion
Erosion Corrosion
Fretting Corrosion
Dealloiyng
Formicary Corrosion
Microbiological Corrosion - MIC
Galvanic Corrosion or Bimetallic
Corrosion
Pitting Corrosion - Crevice
Corrosion
Stress Corrosion Cracking
Cavitation Corrosion
Hydrogen Embrittlement
Immunity
Uniform Corrosion.
If a metal is not immune to
attack and corrosion cannot be completely eliminated, uniform corrosion is
considered the form of corrosion that can be tolerated in marine structures and
equipment. It is also relatively easy to control uniform corrosion to
acceptable levels through judicious selection of materials, the application of
corrosion control measures, and to allow for any corrosion which does occur.
Definition.
Uniform corrosion is the attack
of a metal at essentially the same at all exposed areas of its surface. At no
point is the penetration of the metal by corrosion twice as great as the
average rate.
Mechanism.
Uniform corrosion occurs when
there are local anodic and cathodic sites on the surface of the metal (see
Galvanic Corrosion). Due to polarization effects, these locations shift from
time to time and a given area on a metal will be act as both an anode and as a
cathode over any extended period of time. The averaging effect of these
shifting local action cells results in a rather uniform attack and general loss
of material and roughening of the surface.
Examples.
Rusting steel in the atmosphere
and the corrosion of copper alloys in seawater are common examples where
uniform corrosion is usually encountered. Steel submerged in seawater also
suffer uniform corrosion but can also suffer from non-uniform attack under some
circumstances.
Appearance.
In uniform corrosion, the metal
loss occurs at essentially the same rate over the entire metal surface. Smooth
surfaces are usually roughened during uniform corrosion. This form of corrosion
is characterized by the lack of any significant non-uniform attack such
aspitting or crevice corrosion.
Corrosion products commonly
remain on uniformly corroding surfaces but these can be removed by velocity, by
mechanical action or by other mechanisms.
Significant Measurements.
Weight loss is the most commonly
used method of measuring the corrosion rate of metals when uniform corrosion
occurs. In this method, a test sample is cleaned, weighed, and its surface area
is measured. It is then exposed for a specific period of time, re-cleaned and
re-weighed.
The amount of metal loss as
measured by the weight loss is used to calculate the loss in thickness of the
metal assuming that the corrosion was absolutely uniform. In some cases this is
further verified by thickness measurements.
These results are commonly
expressed in “Mils perYear” or “Microns per Year.” It must be remembered that
these rates are usually calculated from weight loss rather than thickness loss
and are only valid if the attack was uniform. The maximum error of this measurement
is theoretically a factor of two if the rule that attack can be no greater than
twice the average rate at any given point is properly applied.
Concentration Cell Corrosion.
A difference in environment
between sites on a single metal can also result in increased electrochemical
activity. This difference in environment can be due to non-uniform deposits or
fouling on the surface, or, more commonly, built in features which create
significant difference in environment.
Crevices at joints are the most
common cause of these built in environmental differences. This form of
concentration cell corrosion called “crevice corrosion” is often the most
difficult form of corrosion to avoid in design and also is one of the most
common causes of failure of marine equipment.
Definition.
Concentration cell corrosion is
corrosion that is accelerated by differences in environment between separated
areas on a single metal.
Mechanism.
Any situation that creates a
difference in environment between areas on a single metal can cause
concentration cell attack. The basic mechanism is essentially the same as in
galvanic corrosion but in the case of concentration cell corrosion the driving
force is the difference in potential between a single metal exposed to
different environments rather than the difference in potential between two
different metals exposed to a single environment. The rates of attack
experienced in concentration cell corrosion are affected by relative
anode/cathode areas in the same manner as in galvanic corrosion. In crevice
corrosion, the resistance of the electrolyte to the flow of ions can also be a
significant factor in limiting attack in deep tight crevices.
Oxygen Concentration Cells.
Dissolved oxygen has a
significant effect on the corrosion of many metals. This is particularly true
for alloys such as stainless steelswhere the corrosion resistance of the alloy
is dependent upon abundant oxygen for the stability and self-repair of
protective films. Oxygen is also an active participant in the most predominant
cathodic reaction in many environments. The oxygen content of the electrolyte
inside a crevice is usually low as oxygen is consumed by both corrosion and
biological activity and replacement of oxygen inside the crevice is limited.
The crevice can be formed by metal-to-metal contact, by contact of a metal with
a nonmetal or under deposits of debris or fouling. For a metal with a passive
film, the metal tends to become active within the crevice where the lack of
oxygen causes the passive film to be less stable and less easily repaired. The
resulting active/passive cell has substantial driving potential as noted on the
galvanic series. The anodic area within the crevice is normally small with
respect to the cathodic area outside the crevice and with this adverse area
ratio, the corrosion inside the crevice can be very rapid. Once initiated,
crevice corrosion can also be accelerated by the formation of aggressive
chemical compounds within the crevice which further accelerates the attack
within the crevice. In the case of stainless steels, the chromium and nickel
chlorides which are formed are very acidic and crevice corrosion can be very
rapid once initiated.
Oxygen concentration cell
corrosion can also occur on metals which do not have passive films. In this
case, the difference in oxygen content makes the area with low oxygen content
predominantly anodic with respect to more highly oxygenated areas. The reason
for this is due to the effect of the law of mass action on the predominant
cathodic reaction in neutral and alkaline environments.
This reaction is:
2 H2O + O2 + 4 e- ----> 4 OH-
The law of mass action indicates
that, where the oxygen content is high, the cathodic reaction will occur more
readily than when the oxygen content is low. Thus areas where the oxygen
content is low will not be as effective a cathode, anodic reactions will
predominate and the area will act as an anode. Where the oxygen content is
high, the cathodic reactions will predominate and the area will act as a
cathode. Thus, the area inside a crevice will be anodic with respect to the
area outside and the same process as described above for active/passive oxygen
concentration cells will occur. Oxygen concentration cell crevice corrosion is
particularly insidious. First, many material are susceptible to this form of
attack that have otherwise excellent performance in marine environments.
Second, the attack often occurs deep inside crevices in sealed areas, in
joints, and in fasteners where a very small amount of corrosion can result in a
serious failure. Third, as it occurs deep within the crevice, it is difficult
to detect. Crevice corrosion of this type often remains hidden until revealed
by failure.
Intergranular Corrosion
Just as most engineering metals
are mixtures of one or more metals, they consist of large numbers of individual
metal crystals called grains that are joined together at their surfaces or
grain boundaries. As there can be differences in composition at or adjacent to
these grain boundaries, selective corrosion can occur at these sites.
Definition.
Intergranular corrosion is a
selective attack of a metal at or adjacent to grain boundaries.
Mechanism.
There are three mechanisms that
have been identified as causing intergranular corrosion in various situations.
1. The first mechanism is the
selective attack of grain boundary material due to its high energy content.
Metal crystals form in an ordered arrangement of atoms because this ordered
arrangement has a lower energy content than a disordered arrangement. Grain
boundaries are highly disordered as they are at the boundaries of crystals which,
although they are internally ordered, have random orientation with respect to
each other. The disordered grain boundary is often 10 to 100 atoms wide and
these atoms have a higher energy than the surrounding atoms. Higher energy
material can be more chemically active than lower energy material and thus, the
grain boundary material can be anodic with respect to the surrounding grains.
When this occurs, the anodic area is small and the cathodic area is large,
thus, rapid attack can occur. The result is that the individual grains are no
longer joined with the strong grain boundary “glue” and disintegrate leaving a
powdery residue and rough grainy surface.
2. A second mechanism is
selective attack of grain boundary material that has a different composition
from the surrounding grains. When metals crystallize from the molten state, the
crystals tend to be more pure than the molten material. This is because the
pure metal crystals are more ordered and have a lower energy content than if
they contained large amounts of impurities. In some cases, most of the
impurities are concentrated at the grain boundaries. When the composition of
this impure material causes it to be more anodic than the surrounding grains,
rapid attack can occur with results similar to those described above. When the
composition of the impure grain boundary material causes it to be more cathodic
than the surrounding grains, the favorable anode/cathode area ratio makes this
situation relatively innocuous. Contamination of grain boundaries can sometimes
also occur after manufacture. Mercury on aluminum can penetrate and contaminate
the grain boundaries and cause subsequent intergranular attack. This is why
mercury and mercury compounds are prohibited aboard aluminum ships or on
aircraft.
3. A third mechanism is selective
attack adjacent to the grain boundaries due to the local depletion of an
alloying element. This form of attack can occur in many stainless steels. It is
called sensitization. Many stainless steels rely on a combination of nickel and
chromium for their corrosion resistance. As both nickel and chromium are
expensive, they are added only in amounts necessary to obtain the necessary
corrosion resistance. Another element, which is commonly present in ail steels,
is carbon. In stainless steels, carbon atoms tend to concentrate at the grain
boundaries as an impurity during solidification. Chromium carbides can form
adjacent to the grain boundaries during welding and heat treatment. When these
compounds form, the chromium is removed from the alloy adjacent to the grain
boundaries and the resulting alloy does not have enough chromium content to
remain passive. Again, there is a very unfavourable anode/cathode area ratio
and rapid attack can occur. Three different methods are used to avoid this type
of attack in stainless steels during welding or other heating.
a. The first method to avoid
sensitization is through heat treatment. At high temperatures (above 1,800°F),
chromium carbides are unstable and will redissolve if they have formed. At low temperatures,
(below 1,000°F) the chromium and carbon atoms cannot move and formation of
chromium carbides is prevented. Formation of the chromium carbides is a problem
primarily in the ranges of 1,100 to 1,600°F. When welding stainless steel, some area adjacent
to the weld is likely to reach this temperature range long enough to form
amounts of chromium carbides. When this occurs, or when the alloy is otherwise
sensitized, it should be heated to temperatures above 1,800°F to redissolve the
carbides, then rapidly cooled to below 1,000°F to avoid carbide formation.
b. The second method used to
avoid sensitization in stainless steels is to reduce the carbon content of the
alloy to very low levels. These low carbon grades (such as 304 L and 316 L; L
stands for low carbon) do not have enough carbon to form carbides and is thus
resistant to sensitization during welding. Care must be taken, however, to not
introduce additional carbon during welding from contamination, such as can be
caused by oil or grease.
c. The third method used to avoid
sensitization in the stainless steels is to intentionally add an element that
will combine with the carbon but is not required for passivity of the alloy.
Titanium and niobium have a greater affinity for carbon than chromium. They are
added to the alloy during manufacture in amounts to combine with all of the
carbon present in the alloy and thus inhibit sensitization. Type 321 stainless
steel contains titanium and Type 347 stainless steel contains niobium. These
alloys, or the low carbon grades, should be used when welding without heat
treatment is required.
Examples.
Aluminum alloys are susceptible
to intergranular attack, usually the type that is caused by segregation of
impurities at the grain boundaries. In addition to the stainless steels, some
nickel alloys are also subject to sensitization and subsequent intergranular
attack.
Appearance.
Intergranular attack caused by
high grain boundary energies or impurities at the grain boundaries results in
attack with a grainy residue and rough surface. Under high magnification, the
individual grains are often visible. Intergranular attack of aluminum alloys is
associated with pitting or other localized attack. Sensitization in stainless
steels has a similar grainy appearance. When caused by welding it is often
localized in narrow bands adjacent to the weld and is sometimes called “knife
line attack.”
Significant Measurements.
Microscopic examination of
sectioned samples is often required to verify that intergranular attack has
occurred. There are several standardized methods for determining the resistance
of stainless steels to sensitization.
ATMOSPHERIC CORROSION
The atmospheric environment
varies drastically with regards to corrosivity depending on the geographical
location.
Electrolyte.
In atmospheric corrosion, the
electrolyte is moisture from precipitation, fog or dew, sea spray, or other
sources.
The three factors that have the
most influence on the corrosivity of the atmosphere at a given site are:
(1) the amount of time that
exposed surfaces remain wet at the site,
(2) the amount of chloride from
the sea that reaches the surfaces
(3) the amount of industrial
pollutants (mainly acids) that reach the surfaces.
In all atmospheric environments
there is an excess of oxygen, thus the corrosion of most metals in atmospheric
environments is not limited by the amount of oxygen present and can proceed
rapidly when the electrolyte is present.
The corrosivity of the atmosphere
varies drastically.
The corrosion rate of steel, for
example, can vary by a factor of 100.
In general, the least corrosive
atmospheres are found in dry inland (desert) sites and the most corrosive sites
are industrial or industrialmarine sites.
Moist tropical locations are very
corrosive due both to the time of wetness and the high temperatures
experienced.
However, local conditions and
features of design have an influence on corrosive attack that often exceeds the
differences experienced due to geographical conditions.
Thus, generalizations regarding
specific site corrosivity based on the corrosion of a single metal at a single
location at a given site can be misleading.
It is not prudent to ignore the
possibility of corrosion at a dry inland site nor to consider corrosion
inevitable at a marine industrial site.
The rates of attack can vary at
different sites, but the mechanisms of attack, features that cause accelerated
attack and corrective measures for corrosion prevention and corrosion protection
that can reduce the attack are similar.
Erosion-Corrosion
Erosion-corrosion is a general
term that refers to a corrosion process enhanced by the action of flowing
fluids. The process can also be characterized by whether the fluid contains
solid particles, is in the form of impinging droplets, or is undergoing
cavitation. Cavitation is the formation and sudden collapse of vapor bubbles in
a liquid.
Manifestation and Quantification
Erosion-corrosion can result in
general corrosion that occurs at a higher rate than would be expected under
stagnant conditions. In this case the measure of erosion-corrosion would be the
rate of surface waste or the rate of penetration. The more usual effect of
erosion-corrosion, however, is localized attack, which can appear as grooves,
gullies, waves, rounded holes, etc., and usually exhibits a directional pattern
that correlates with the direction of flow.
The appearance of cavitation
damage is similar to pitting. However, the pitted areas are closely spaced and
the surface is usually considerably roughened. Severe cavitation damage can
completely remove sections of metal.
Cavitation damage that results
from fluid movement relative to the metal surface can usually be correlated
with the direction of flow A condition conducive to erosion corrosion is the
flow of corrosive fluids (gas or liquid) relative to a metal surface.
The rate of corrosion depends
upon the flow rate of the fluid. Turbulent flow results in much higher
corrosion rates than Iaminar flow. Hard particles entrained in the flowing
fluid can enhance erosion corrosion.
Cavitation damage is caused by
flow discontinuities that result in the formation and subsequent coUapse of
vapor bubbles on a metal surface. High-velocity drop impingement, such as
raindrops on a helicopter rotor, can also result in rapid surface damage.
Mechanism
Erosion-corrosion implies that
the fluid medium is potentially corrosive to the metal. Erosion facilitates the
corrosion process. This fact distinguishes erosion-corrosion from pure erosion
or mechanical wear. Erosion corrosion influences the rate of corrosion by
changing the conditions of local cell action. The corrosion process is
accelerated if the fluid speed is sufficient to remove weakly adhered corrosion
products from the surface.
Removal of these products reduces
their polarizing or inhibitive effect.
At the breakaway speed the fluid
begins to remove the corrosion film and the corrosion rate increases. A steady
corrosion rate is achieved at the speed at which the fdm is completely removed.
Fluid flow also maintains a uniform concentration of corrodent at the metal
surfaces.
Impingement of suspended hard
particles can accelerate the damage to the protective film. and can cause
mechanical damage to the underlying metal.
Application Constraints and
Protection Approaches
Several methods for preventing or
minimizing damage resulting from erosion-corrosion are available.
They include
1. Material selection. Select
materials with better resistance to erosion-corrosion.
2. Design considerations.
Streamline the flow, avoid designs that create turbulence. Minimize abrupt
changes in flow direction. Introduce smooth aerodynamic or hydrodynamic flow
channels; avoid roughly textured surfaces. Carefully align pipe sections. Avoid
flow obstructions in design or obstructions that can arise under operations,
increase the thickness of material in vunerable areas, install renewable impingement
plates or baffles, and design for easy repair by using interchangeable parts.
3. Aherarion of environment.
Decrease fluid stream speed to achieve laminar flow, regulate the concentration
of dissolved oxygen in the environment to achieve optimal film-forming
characteristics, provide falters for removal of suspended solids, and provide
condensed moisture traps in vapor lines.
4. Specification of suitable
coatings or linings. Use of hard-facing may be helpful in some situations and
resilient barriers may be helpful in others, e.g., cavitation.
5. Cathodic protection. Provide
cathodic protection whenever possible.
When surfaces move in relation to
each other, this relative motion can result in abrasion. This abrasion can
increase the attack at these fraying surfaces.
Erosion corrosion is the result
of a combination of an aggressive chemical environment and high fluid-surface
velocities. This can be the result of fast fluid flow past a stationary object,
such as the case with the oil-field check valve shown on the left below, or it
can result from the quick motion of an object in a stationary fluid, such as
happens when a ship's propeller churns the ocean.
Surfaces which have undergone
erosion corrosion are generally fairly clean, unlike the surfaces from many
other forms of corrosion.
Erosion corrosion can be
controlled by the use of harder alloys (including flame-sprayed or welded hard
facings) or by using a more corrosion resistant alloy. Alterations in fluid
velocity and changes in flow patterns can also reduce the effects of erosion
corrosion.
Erosion corrosion is often the
result of the wearing away of a protective scale or coating on the metal
surface. The oil field production tubing shown above on the right corroded when
the pressure on the well became low enough to cause multiphase fluid flow. The
impact of collapsing gas bubbles caused the damage at joints where the tubing
was connected and turbulence was greater.
Many people assume that erosion
corrosion is associated with turbulent flow. This is true, because all
practical piping systems require turbulent flow-the fluid would not flow fast
enough if lamellar (nonturbulent) flow were maintained. Most, if not all,
erosion corrosion can be attributed to multiphase fluid flow. The check valve
on the left above failed due to sand and other particles in an otherwise
noncorrosive fluid. The tubing on the right failed due to the pressure
differences caused when gas bubbles collapsed against the pipe wall and
destroyed the protective mineral scale that was limiting corrosion.
Fretting Corrosion
Definition.
Fretting corrosion is an attack
that is accelerated by the relative motion of contacting surfaces.
Mechanism.
Fretting corrosion is usually a
combination of corrosion and abrasive wear. The motion between the surfaces
removes protective films and results in accelerated attack. Also, most
corrosion products are abrasive and their presence increases the removal of
protective films and in direct abrasion of the metal.
Examples.
Fretting was common in riveted
joints on ships and other riveted structures where cyclic loads were
experienced, but this has largely been eliminated through welded construction.
Fretting is, however, still encountered in bolted joints and flanges where
there is not enough bolt tension to eliminate movement in the joint. Thermal
expansion with frequent cycling can also result in fretting attack. Any
combination of corrosion and wear will almost always be worse than the action
of either one separately.
Appearance.
Fretting corrosion usually
results in scuffed surfaces in joints or at other wear sites. If inspected soon
after the relative motion ceases, the surfaces will often be bright and have
corrosion products attached to the surfaces.
Significant Measurements.
There are no standard tests for
fretting corrosion. When encountered, it is addressed through mechanical design
rather than material selection. Where it cannot be eliminated it can sometimes
be reduced by using inhibitive caulking compounds in the joints.
Dealloying Corrosion
Dealloying Corrosion
Most of the commonly used
metallic material are alloys formed from mixing two or more metals. Pure metals
are usually too soft and weak to be used structurally. In this form of
corrosion, dealloying, corrosion selectively
attacks one or more constituent of the alloy mixture.
Definition of Dealloying.
Dealloying is the selective
corrosive attack of one or more constituent of a metallic alloy.
Mechanism of Dealloying
Corrosion.
As can be seen from the galvanic
series, constituents of many common alloys have widely separated positions on
the galvanic series. In the case of brass, the main constituents are zinc and
copper. In the case of cast iron, the main constituents are iron and graphite.
When the surface of such alloys is exposed to an electrolyte, galvanic action
proceeds with the more anodic material being selectively attacked. In many
cases, the cathodic material remains behind and is bound into its original
shape by a residue of remaining anodic material and corrosion products. The
strength of the remaining material is, however, greatly reduced and will often
fail duringnormal handling. Single phase material, where the alloy constituents
are well mixed, are often less susceptible to this form of attack than alloys
where phases of largely different composition are present. In many alloys, heat
treatments have been developed specifically to make the alloy more homogeneous
and less susceptible to dealloying.
Examples of Dealloying Corrosion
The dezincification of brass and
the graphitization of cast iron are common examples of dealloying.
Appearance of Dealloying
Corrosion.
In dealloying, the size and shape
of the original component is often retained. The remaining constituent is often
a different color than the original alloy and the depth and location of attack
can be easily identified by this color change. Dealloying can either occur over
the entire surface (layering) or localized in pits (plug type).
Significant Measurements.
Weight loss is not a significant
measurement of the impact of dealloying. The depth of attack must be measured
by sectioning and microscopic examination. The impact of dealloying on the
strength of the material can be assessed through mechanical testing. In many
cases, the depth of attack is self-limiting, particularly in the plug type of
attack but the limiting depth is significant, often in the order of 1/4 inch.
The fact that there is a limiting depth is significant only for very thick
walled sections.
Microbial Corrosion
Microbial corrosion (also called
microbiologically-influenced corrosion or MIC) is corrosion that is caused by
the presence and activities of microbes. This corrosion can take many forms and
can be controlled by biocides or by conventional corrosion control methods.
There are a number of mechanisms
associated with this form of corrosion, and detailed explanations are available
at the web sites listed at the bottom of this section. Most MIC takes the form
of pits that form underneath colonies of living organic matter and mineral and
biodeposits. This biofilm creates a protective environment where conditions can
become quite corrosive and corrosion is accelerated.
The picture below shows a biofilm
on a metallic condenser surface. These biofilms can allow
(Courtesy of www.asm.org)
corrosive chemicals to collect
within and under the films. Thus the corrosive conditions under a biofilm can
be very aggressive, even in locations where the bulk environment is
noncorrosive.
(Courtesy of www.micscan.com)
MIC can be a serious problem in
stagnant water systems such as the fire-protection system that produced the
pits shown above. (see Pitting Corrosion). The use of biocides and mechanical
cleaning methods can reduce MIC, but anywhere where stagnant water is likely to
collect is a location where MIC can occur.
Corrosion (oxidation of metal)
can only occur if some other chemical is present to be reduced. In most
environments, the chemical that is reduced is either dissolved oxygen or
hydrogen ions in acids. In anaerobic conditions (no oxygen or air present), some
bacteria (anaerobic bacteria) can thrive. These bacteria can provide the
reducible chemicals that allow corrosion to occur. That's how the limited
corrosion that was found on the hull of the Titanic occurred. The picture below
shows a "rusticle" removed from the hull of Titanic. This combination
of rust and organic debris clearly shows the location of rivet holes and where
two steel plates overlapped.
(Couresy of www.dbi.sk.ca)
Much microbial corrosion involves
anaerobic or stagnant conditions, but it can also be found on structures
exposed to air. The pictures below show a spillway
(Courtesy of www.meic.com)
gate from a hydroelectric dam on
the Columbia River. The stress corrosion cracks were caused by pigeon droppings
which produced ammonia-a chemical that causes stress corrosion cracking on
copper alloys like the washers used on this structure. Since it's impossible to
potty train pigeons, a new alloy resistant to ammonia was necessary.
In addition to the use of
corrosion resistant alloys, control of MIC involves the use of biocides and
cleaning methods that remove deposits from metal surfaces. Bacteria are very
small, and it is often very difficult to get a metal system smooth enough and
clean enough to prevent MIC.
Galvanic Corrosion Bimetallic
Corrosion
Galvanic corrosion is a localised
corrosion mechanism by which metals can be preferentially corroded.
This type or form of corrosion
has the potential to attack junctions of metals, or regions where one
construction metal is changed to another. Frequently this condition arises
because different metals are more easily fabricated into certain forms; an
example might be a door frame manufactured from aluminium extrusions (aluminium
extrudes extremely well into architectural sections), but with a door handle
fabricated from stainless steel tube to exploit its higher strength and
abrasion resistance.
Galvanic corrosion is well known
to most designers, specifiers and fabricators, but often the only rule in force
is "don't mix metals".
What Conditions are Needed
For galvanic corrosion to occur
there are three conditions which must be met:
Condition 1. Metals must be far
apart on the galvanic series
The galvanic or electrochemical
series ranks metals according to their potential, generally measured with
respect to the Standard Calomel Electrode (S.C.E.). The results are often
viewed as a galvanic corrosion chart or galvanic corrosion table similar to
that on the third page.
This chart says that the
"anodic" or "less noble" metals at the negative end of the
series - at the right of this diagram, such as magnesium, zinc and aluminium -
are more likely to be attacked than those at the "cathodic" or
"noble" end of the series such as gold and graphite.
The critical point is the
difference in potential of the two materials being considered as a joined pair.
A difference of hundreds of millivolts is likely to result in galvanic
corrosion, but only a few tens of millivolts is unlikely to be a problem.
Condition 2. The metals must be
in electrical contact
The two different metals must be
in electrical contact with each other. This is of course very common. The two
metals can be bolted, welded or clamped together, or even just resting against
each other.
Condition 3. The metal junction
must be bridged by an electrolyte
An electrolyte is simply an
electrically conducting fluid. Almost any fluid falls into this category, with
distilled water as an exception. Even rain water is likely to become sufficiently
conducting after contact with common environmental contaminants.
If the conductivity of the liquid
is high (a common example is sea water) the galvanic corrosion of the less
noble metal will be spread over a larger area; in low conductivity liquids the
corrosion will be localised to the part of the less noble metal near to the
junction.
The Area Effect
The relative area of the anode
and cathode has a pronounced effect upon the amount of corrosion that occurs
due to Galvanic Corrosion. A small anode (the less noble metal, such as
aluminium) joined to a large cathode (the more noble metal, such as stainless
steel) will result in a high current density on the aluminium, and hence a high
rate of corrosion.
The corrosion is concentrated by
the area difference. Conversely if the area of the anode is large compared to
that of the cathode this dilutes the corrosive effect, in most cases to the
extent that no problem occurs. It is common practice to use stainless steel
fasteners to fix aluminium sheeting or signs, but if aluminium screws were used
to fix stainless steel the screws may rapidly corrode.
An apparent contradiction of the
area effect on Galvanic Corrosion occurs when the component comprised of the
two metals is only partly wetted. Consider for instance a stainless steel bolt
in an aluminium plate; if water collects in the corner at the edge of the bolt
but the remainder of the plate remains dry, the effective area of the less
noble aluminium is only the wetted region, which may be only a similar size to
that section of the bolt that is wetted .... thus it is quite possible for the
aluminium plate to be galvanically attacked in the region immediately
surrounding the bolt. Only the wet “area” counts.
Crevices & Stagnant
Conditions
As shown in the electrochemical
series chart there are two different
potentials associated with each stainless steel grade. The less noble value
shown in outlined boxes is that which applies if a crevice is formed between
the two dissimilar metals or such as beneath bio-fouling.
Such a crevice could be from the
design or fabrication of the component, and formation of biological films is
more likely in stagnant or slow-flowing sea water. The result of these stagnant
conditions is oxygen depletion and the less noble potential which can make the
stainless steel susceptible to corrosion in conditions that might otherwise be
considered non-corrosive.
Passive Surface Films and effect
on Galvanic Corrosion
Stainless steels naturally form
passive surface films this is what makes them “stainless”. This film also
reduces the amount of current available for corrosion, so slows the corrosion
rate down compared to some other galvanic pairs.
Avoidance of Galvanic Corrosion
The methods for avoidance of
galvanic corrosion are in general suggested by the above descriptions of the
conditions necessary for its occurrence.
Don’t Mix Metals. If only one
material is used in a construction the problem is avoided (Condition 1 is not
present – no mixed metals) andGalvanic Corrosion will not take place. Be
particularly aware of zinc plated or galvanised fasters in stainless steel
sheets – a common substitution because of perceived cost savings or better
availability. These less noble fasteners in the galvanic series are likely to
be rapidly attacked.
Prevent Electrical Contact. It is
often practical to prevent electrical contact between the dissimilar metals
(removal of Condition 2). This may be achieved by the use of nonconducting (eg
rubber or plastic) spacers, spool pieces or gaskets, perhaps in conjunction
with sleeves around bolts. For the same reason a gap may be left between
galvanised roofing and a stainless steel down-pipe.
Corrosion potentials in flowing
sea water at ambient temperature. The unshaded symbols show ranges exhibited by
stainless steels in acidic water such as may exist in crevices or in stagnant
or low velocity or poorly aerated water. The more Noble materials at the left
side tend to be cathodic and hence protected; those at the right are less Noble
and tend to be anodic and hence corroded in a galvanic couple.
Prevent the Wetted Junction.
The third Condition can be
removed by ensuring that no electrolyte remains at the intermetallic junction -
this may require extra attention to drainage or to protection from the weather.
A good covering of paint or sealant over the junction can be effective to avoid
Galvanic Corrosion.
Use the Area Effect to avoid
Galvanic Corrosion.
The area effect should also be
considered in avoiding corrosion damage, particularly in selection of fastener
materials.
Stainless steel fasteners can be
used to hold aluminium structures, but the area effect will not apply if the
wetted area shrinks over time due to evaporation. In the situation of 304 or
316 fasteners used in conjunction with other less noble structural materials,
the fastener will be galvanically protected by the surrounding large, less
noble area.
Positively Use Galvanic
Protection to avoid Galvanic Corrosion.
The galvanic effect can also be
used to provide corrosion protection. For example it is prudent to guard
against possible crevices, perhaps associated with marine fouling, or simply
under bolt heads, by specifying slightly more noble bolt materials. An example
is the use of 316 fasteners in conjunction with 304 structural materials – the
minor galvanic protection afforded the fasteners improves their corrosion
resistance.
Pitting & Crevice Corrosion
of Stainless Steel
Stainless Steels are a family of
alloys exhibiting good resistance to attack by many of the environments
encountered in industry and in domestic, commercial and marine exposure. Their
resistance is not perfect, however, and the large number of grades of stainless
steel now available is largely because of this challenge of finding cost-effective
resistance to these various environments.
The corrosion resistance of
stainless steels to some environments can be described by corrosion resistance
tables, as the corrosion which does occur is a fairly uniform metal thinning
over time. This is termed "General Corrosion". "Localised
Corrosion" by contrast results in attack at certain specific sites while
other parts of the metal may remain totally unaffected.
Studies of corrosion failures of
stainless steel have indicated that pitting and crevice corrosion types are
major problems, and together account for perhaps 25% of all corrosion failures.
What is Pitting Corrosion?
Under certain specific
conditions, particularly involving chlorides (such as sodium chloride in sea
water) and exacerbated by elevated temperatures, small pits can form in the
surface of the steel. Dependent upon both the environment and the steel itself
these small pits may continue to grow, and if they do can lead to perforation,
while the majority of the steel surface may still be totally unaffected.
What is Crevice Corrosion?
Crevice Corrosion can be thought
of as a special case of pitting corrosion, but one where the initial
"pit" is provided by an external feature; examples of these features
are sharp re-entrant corners, overlapping metal surfaces, non-metallic gaskets
or incomplete weld penetration. To function as a corrosion site a crevice has
to be of sufficient width to permit entry of the corrodent, but sufficiently
narrow to ensure that the corrodent remains stagnant. Accordingly crevice
corrosion usually occurs in gaps a few micrometres wide, and is not found in
grooves or slots in which circulation of the corrodent is possible.
Environmental Factors
The severity of the environment
is very largely dependent upon two factors - the chloride (Cl-) content and the
temperature - and the resistance of a particular steel to pitting and crevice
corrosion is usually described in terms of what % Cl- (or ppm Cl- ) and °C it
can resist. It should be noted that the most common grade of stainless steel,
Type 304, may be considered susceptible to pitting corrosion in sea water (2%
or 20,000 ppm chloride) above about 10°C, and even in low chloride content
water may be susceptible at only slightly elevated temperatures. A safe
chloride level for warm ambient temperatures is generally about 150ppm
(150mg/l). Grade 316 is more resistant and is commonly used in ambient sea
water, but can be attacked in crevices or if the temperature increases even
slightly.
The velocity of the liquid is
also significant; a stagnant solution is more likely to result in pitting and
crevice attack, particularly if there are particles to settle out of the
liquid. Note that there may also be a problem from stress corrosion cracking if
austenitic stainless steels are used in chloride containing water at
temperatures over about 60°C.
<
>
Which Steels are Susceptible?
All stainless steels grades can
be considered susceptible, but their resistances vary widely. Their resistance
to attack is largely a measure of their content of chromium, molybdenum and
nitrogen. Another factor of importance is the presence of certain metallurgical
phases (in particular the grades 303, 416 and 430F containing inclusions of
manganese sulphide) have very low resistances, and ferrite may be harmful in
austenitic grades in severe environments. A clean and smooth surface finish
improves the resistance to attack. Contamination by mild steel or other
"free iron" greatly accelerates attack initiation.
Measurement of Resistance to
Attack
Laboratory tests have been
developed to measure the resistance of metals to both pitting and crevice
corrosion. This testing has two main aims - firstly to enable ranking of each
alloy in order of resistance, and secondly as a quality control measure, to
ensure that particular batches of steel have been produced not just with
correct composition, but also have been properly rolled and heat treated.
The most commonly used test is
that in ASTM G48, which measures resistance to a solution of 6% ferric
chloride, at a temperature appropriate for the alloy, shown in the graph above.
If an artificial crevice is added to the sample the test measures crevice
corrosion resistance rather than pitting resistance.
The temperature which is just
high enough to cause failure of this test is termed the Critical Pitting
Temperature (CPT) or the Critical Crevice Temperature (CCT). Alternative
laboratory tests can be carried out using electrochemical cells with a variety
of test solutions. The results obtained in laboratory tests are approximate
only, as factors such as surface finish, water velocity, water contaminants and
metallurgical condition of the steel are all important.
Pitting Resistance Equivalent
Number (PRE)
From experience it has been found
that an estimate of resistance to pitting can be made by calculation from the
composition as the Pitting Resistance Equivalent Number:
PRE = % Cr + 3.3 x %Mo + 16 x %N
Various multipliers (up to 30)
for Nitrogen have been used in this equation; with the higher values often used
for the austenitic stainless steel grades; in any case the effect of nitrogen
is very important, hence the requirement by many suppliers (including Atlas)
that the highly resistant grade 2205 have a minimum nitrogen content of 0.14%.
This also explains the trend in extremely high pitting resistant alloys for
even higher nitrogen levels. The super duplex grade UR52N+ (UNS S32520/S32550)
typically contains 0.2% nitrogen, while the super austenitic grade 4565S (UNS
S34565) typically contains 0.45% nitrogen.
Effect of Welding
The welding process results in
metallurgical changes in both fusion zone and heat affected zone. In most alloy
systems some degradation in pitting and crevice corrosion resistance occurs in
welding, but these effects can be minimised if proper materials and practices
are used. Proper materials usually involves over-alloyed consumables and
practices includes proper heat inputs. It is important that correct information
be sought from suppliers. Again looking at the extremely high pitting resistant
alloys it has been found that the high molybdenum alloys are particularly
susceptible to fusion zone micro-segregation, leading to lowered pitting
resistance. Alloys such as 4565S which achieve their pitting resistance by high
nitrogen rather than very high molybdenum levels have been found to be less
affected by weld segregation.
Measures to Reduce Pitting and
Crevice Corrosion
Control the environment to low
chloride content and low temperature if possible.
Fully understand the environment.
Use alloys sufficiently high in
chromium, molybdenum and/or nitrogen to ensure resistance.
Prepare surfaces to best possible
finish. Mirror-finish resists pitting best.
Remove all contaminants,
especially free-iron, by passivation
Design and fabricate to avoid
crevices.
Design and fabricate to avoid
trapped and pooled liquids
Weld with correct consumables and
practices and inspect to check for inadvertent crevices.
Pickle to remove all weld scale
Stress Corrosion Cracking
Metals are useful in engineering
structures because of their strength, ductility, and durability. Ductility is
extremely important as it allows the material to deform in response to loading
thus redistributing the stresses. In some cases, however, chemical interactions
with the environment can reduce the ductility of metals so that they behave
more like brittle materials when subjected to stress.
-Caustic Stress Corrosion
Cracking
-Chloride Stress Corrosion
Cracking - CSCC
Definition.
Stress corrosion cracking is the
intergranular or transgranular cracking of a material due to the combined
action of tensile stress and a specific environment.
Mechanism.
Even after many years of
intensive study, the exact mechanism of stress corrosion cracking remains a
matter of extensive disagreement and study. It is commonly attributed to the
rupture of protective films at the tips of pits or pre-existing cracks due to the
applied stress. In many cases, the materials appear to be totally resistant to
corrosion in a given environment until stresses are applied. They then crack
catastrophically without any sign of other corrosion attack.
Examples.
Many materials, particularly high
strength materials, are susceptible to stress corrosion cracking when exposed
to a specific environment. For example, cold worked brass, which is found in
ammunition cartridges, is susceptible to stress corrosion cracking when exposed
to an environment containing ammonia. In chloride containing environments,
titanium alloys, aluminum alloys, and high strength stainless steels are
susceptible and specific alloys, which are resistant to stress corrosion
cracking, should be used. The stresses required to initiate and propagate
cracking are often low and many failures occur due to residual stresses rather
than applied stress.
Caustic stress corrosion
Caustic stress corrosion, or
caustic embrittlement, is another form of intergranular corrosion cracking.
The mechanism is similar to that
of chloride stress corrosion.
Mild steels (steels with low
carbon and low alloy content) and stainless steels will crack if they are
exposed to concentrated caustic (high pH) environments with the metal under a
tensile stress. In stress cracking that is induced by a caustic environment,
the presence of dissolved oxygen is not necessary for the cracking to occur.
Caustic stress corrosion cracking was first encountered in the operation of
riveted steam boilers.
These boilers were found to fail
on occasion along riveted seams. Failure was attributed to caustic-induced
cracking at the highly stressed regions near and under the rivets.
Boiler water could easily flow
into the crevices which existed under the rivets. Radiative heating would cause
the water in the crevices to boil. As steam was formed, it would escape from
the crevice.
More boiler water would then flow
into the crevice, boil, and pass from the crevice as steam. The net result of
this continuing process was concentration of caustic under the rivet.
The combination of high stress
and high caustic concentrations eventually led to destructive cracking of the
boiler vessel.
Where the rate of steam
generation (boiling) is high, it is more difficult to eliminate the problem of
solute concentration in regions of the boiler. Caustic stress corrosion may
concentrate in such regions as the water evaporates rapidly, but sufficient
concentration of caustic by such a mechanism to induce stress corrosion
cracking is considered unlikely.
Available data indicates that
caustic concentrations greater than 10,000 ppm, and probably up to 50,000 ppm,
are required to induce caustic stress cracking (40,000 ppm NaOH is equivalent
to 40 grams per liter or 1 mole per liter).
The pH of such a solution is on
the order of 14. An alkaline environment is produced and controlled by use of a
solution having some properties of a buffer, that is, one that tends to retard
or slow a reaction or tends to force it in one direction or the other.
Chloride Stress Corrosion
Cracking - CSCC
Chloride Stress Corrosion
Cracking is a localized corrosion mechanisms like pitting and crevice
corrosion. The three conditions that must be present for chloride stress
corrosion to occur are as follows.
- Chloride ions are present in
the environment
- Dissolved oxygen is present in
the environment
- Metal is under tensile stress
Austenitic stainless steel is a
non-magnetic stainless steel grades consisting of iron, chromium, and nickel,
with a low carbon content. This alloy is highly corrosion resistant and has
desirable mechanical properties. One type of corrosion which can attack
austenitic stainless steel is chloride stress corrosion.
Chloride stress corrosion is a
type of intergranular corrosion. Chloride stress corrosion involves selective
attack of the metal along grain boundaries.
In the formation of the steel, a
chromium-rich carbide precipitates at the grain boundaries leaving these areas
low in protective chromium, and thereby, susceptible to attack.
It has been found that this is
closely associated with certain heat treatments resulting from welding. This
can be minimized considerably by proper annealing processes.
This form of corrosion is
controlled by maintaining low chloride ion and oxygen content in the
environment and the use of low carbon steels. Environments containing dissolved
oxygen and chloride ions can readily be created in auxiliary water systems.
Chloride ions can enter these
systems via leaks in condensers or at other locations where auxiliary systems
associated with the nuclear facility are cooled by unpurified cooling water.
Dissolved oxygen can readily enter these systems with feed and makeup water.
Thus, chloride stress corrosion cracking is of concern, and controls must be used
to prevent its occurrence.
Figure 1 Intergranular Corrosion
Cracking
Figure 1 illustrates
intergranular stress corrosion cracking. The pressure of a tensile stress opens
up intergranular cracks and accelerates further corrosion.
Chloride stress corrosion is a
particularly significant problem in the operation of nuclear facilities because
of the wide use of austenitic stainless steel, and the inherent presence of
high tensile stresses associated with pressurization.
Chloride stress corrosion cracks have
been known to propagate in austenitic stainless steel at stresses of about
one-fifth yield strength with chloride concentrations of less than 50 ppm.
Tests show that the 18-8 stainless steels
grade are susceptible to chloride stress attack when both the chloride ion
concentration and dissolved oxygen concentration are above certain values.
The region of susceptibility for
austenitic stainless steel is illustrated in Figure 2.
Figure 2 Austenitic Stainless
Steel Chloride Stress Corrosion Cracking
Note that when dissolved oxygen
is present at about 1 ppm, chloride stress corrosion cracking can be initiated
at chloride ion concentrations near 1 ppm.
However, when the concentration
of dissolved oxygen is very low, susceptibility to chloride stress corrosion
cracking is reduced.
High temperature tends to
decrease the time required for chloride-induced cracking to occur, but there
appears to be no practical temperature limit below which cracking will not
occur, given sufficient time and severe conditions.
The curve in Figure 2 is valid
for temperatures in the range 470°F to 500°F.
Cavitation Corrosion.
Under high velocity flow
conditions, particularly when the flow is turbulent, areas of high and low
pressure will be induced. In areas of low pressure, gas and vapor bubbles will
be produced. When these bubbles move to an area of higher pressure, they
collapse and their implosion creates a pressure wave that can remove protective
films and cause increased corrosion.
Definition.
Cavitation corrosion is corrosion
that is enhanced through the formation and collapse of gas or vapor bubbles at
or near the metal surface.
Mechanism.
As described above, the formation
and collapse of gas or vapor bubbles in a liquid can cause localized damage to
the films responsible for limiting corrosion. Once this localized corrosion is
established, the local roughening can often serve as a new site for further
turbulence and more cavitation attack.
Examples.
Cavitation is commonly
encountered in pumps and in high speed propellers. It is enhanced when
entrained air is present in systems with high velocity flow. Cavitation can
also occur on or near high intensity sound generators. Stainless steels, some
nickel alloys, and titanium alloys are highly resistant to cavitation damage
but even these will be attacked under severe conditions.
Appearance.
Cavitation corrosion is similar
to erosion corrosion and pitting is usually encountered. Cavitation can often
be verified by a hydrodynamic analysis that can be used to locate and minimize
bubble formation or move the area of bubble collapse to an area where the
attack will have a minimal effect.
Significant Measurements.
There are no standard tests for
cavitation attack. Only through actual full scale tests or from experience can
failure due to cavitation be avoided. High velocity flow should be avoided in
the design of all systems.
Appearance.
Stress corrosion cracking must be
evaluated using microscopic examination of the cracked sections. The cracking
is often branched. Stress corrosion cracking can occur in the presence of other
forms of corrosion attack or without the presence of other visible attack.
Significant Measurements.
In general, alloys known to be
susceptible to stress corrosion cracking should be avoided. In some cases,
special heat treatments can minimize the susceptibility to stress corrosion
cracking. Many tests have been developed to test the susceptibility of metals
to stress corrosion cracking. All of these combine mechanical loading, often in
the presence of a pre-existing crack, and exposure to the specific environment
of interest. For alloys with limited sensitivity to stress corrosion cracking,
critical stresses can be defined below which stress corrosion will not occur.
The structural analysis and manufacturing processes required to accommodate
these critical stress criteria are often very complex and the use of highly
resistant materials is recommended.
Hydrogen Embrittlement.
Hydrogen can enter most metals.
Due to the small size of the hydrogen atom, it can migrate through the metal
structure and cause a loss of ductility similar to that experienced in stress
corrosion cracking.
Definition.
Hydrogen embrittlement is the
severe loss of ductility of a metal when hydrogen has been introduced into the
metal structure.
Mechanism.
Hydrogen atoms can enter a metal
either from hydrogen gas, usually at elevated temperatures, or from atomic
hydrogen that is electrolytically formed on its surface. This hydrogen can
either reduce the energy required for forming cracks under stress or can
accumulate at areas of high stress, such as crack tips, and cause pressure,
which directly assists crack propagation. High strength materials in general
are the most susceptible to hydrogen embrittlement. Hydrogen can be formed
electrolytically during electroplating, during welding when hydrogen is present
in the electrode material, in the electrode coating, in the shielding gas, or
simply as moisture on the metal surface, or when excessive cathodic protection
is applied (potentials more negative than minus 1.2 volts are normally required
for significant hydrogen formation by cathodic protection.)
Examples.
Ferritic and martensitic
(magnetic) steels, particularly those with a yield strength in excess of 130
ksi, are particularly prone to hydrogen embrittlement. Austenitic
(non-magnetic) stainless steels are less susceptible. When hydrogen pickup is
suspected, such as in electroplating or welding, the hydrogen can be removed by
baking at 200 to 300°F. Hydrogen pickup during welding is normally prevented by
using low hydrogen electrodes and mild preheating to remove water from the
surfaces being welded. Appearance. Other than catastrophic failure by cracking,
there is often no visible evidence of hydrogen embrittlement. In extreme cases,
where hydrogen gas bubbles are formed inside the metal, shinny internal
blisters are visible at the fracture surface.
Significant Measurements.
Analysis of the metal for
untrained hydrogen can be used to verify hydrogen embrittlement if heating
subsequent to failure has not driven off the hydrogen gas. Due to the
difficulty in verifying this form of attack, it is often blamed for other forms
of cracking failure, often when simple overload is the actual cause of failure.
Corrosion Type Immunity.
The first form of corrosion
described is the lack of attack, or immunity. This can result from the action
of two basic mechanism. Corrosion test measurements that are used to measure
very low corrosion rates must be used to validate that corrosion activity is
completely absent.
Definition.
Immunity is the lack of
measurable attack on a metal when exposed to operational environments.
Mechanism.
Immunity can result from two
basic mechanisms. In the first case, the energy content of the metal is lower
(more stable) than any of the corrosion products that could possibly form. Such
metals are commonly found in nature as metals that indicates the stability of
the metallic state for these elements. Corrosion of such metals where an
increase of energy is required will not take place naturally just as a ball
will not roll uphill unless pushed. In the second case, there is an energy
tendency for the metal to suffer corrosion, but corrosion activity is prevented
by the presence of a highly stable passive film. Not only must this film be
stable in the environment, but it must be able to repair itself by reaction
with the environment if it is damaged.
Examples.
Gold and platinum are examples of
metals that are normally immune in a wide variety of environments due to the low
energy content of their metallic state. Titanium and some highly alloyed
materials, such as the complex nickelchromium- molybdenum alloy Hastelloy
C-276, have an extremely stable passive film that will repair itself when
damaged.
Appearance.
A metal that is immune to
corrosion in a given environment will not show any change due to corrosion
after exposure.
Significant Measurements.
Metals that do not suffer
corrosion are unchanged by exposure to their environment. Their strength,
weight, size, shape, and surface finish are unaffected by exposure. To measure
very low rates of corrosion, weight loss and other material property
measurements may not be sufficiently sensitive. In these cases, the metal is
exposed to a small quantity of the electrolyte and the electrolyte is analyzed
for the presence of metal atoms from the corrosion process.
Corrosion Fatigue.
Many materials will exhibit a
substantial reduction in fatigue life when exposed to a corrosive environment.
In some cases, the reduction is severe, in other cases it is less dramatic, but
only a very few materials show a fatigue resistance in a corrosive environments
as great as that in dry air.
Definition.
Corrosion fatigue is the reduced
ability of a metal to withstand repeated stress when exposed to the combined
action of stress and a corrosive environment as compared to the effects of
stress alone.
Mechanism of Corrosion Fatigue.
Fatigue resistance can be reduced
by corrosion activity in many ways. In materials that are susceptible to stress
corrosion, fatigue resistance is probably lowered by the rapid propagation of
fatigue cracks after they reach the size required for stress corrosion
cracking. In materials not susceptible to stress corrosion cracking, corrosion
probably enhances crack propagation through direct attack at the crack tips, or
by the formation of stress risers such as pits. Corrosion fatigue is usually
4-20 more severe at low cycling frequency where the longer time to failure
allows more corrosion activity to occur.
Examples of Corrosion Fatigue.
High strength steels are
susceptible to substantial reduction in fatigue resistance in many
environments. The endurance limit (stress below which fatigue failure will not
occur) is often reduced by a factor of ten from that measured in air. Cathodic
protection can increase the resistance of steels to corrosion fatigue, but care
must be taken not to overprotect them ashydrogen embrittlement would then
occur. Titanium alloys, which are not subject to stress corrosion cracking, are
particularly resistant to corrosion fatigue as are some of the more corrosion
resistance nickel alloys, such as Inconel 625 and Inconel 718.Copper alloys and
stainless steels are also susceptible to corrosion fatigue with a reduction of
one-half in their endurance limit being common.
Appearance of Corrosion Fatigue.
Corrosion fatigue gives a
fracture surface similar to ordinary fatigue except that in some cases,
corrosion products are present in the outer sections of the cracks.
Significant Measurements.
In the simplest corrosion fatigue
test, the electrolyte is simply dripped over the surface of a rotating beam
fatigue test specimen. In more sophisticated tests, flat specimens are stressed
as cantilever beams and only tensile stresses are induced on the surface
exposed to the corrosive environment. When cyclic loading is a factor in
design, fatigue data from tests that include the corrosive environment must be
used.
Corrosion Control
There are a number of means of
controlling corrosion. The choice of a means of corrosion control depends on
economics, safety requirements, and a number of technical considerations. The
NASA Kennedy Space Center has a group of trained corrosion professionals who
can provide guidance on corrosion control.
Design
Materials Selection
Protective Coatings
Inhibitors and Other Means of
Environmental Alteration
Corrosion Allowances
Cathodic Protection
Design
Engineering design is a
complicated process that includes design for purpose, manufacturability,
inspection, and maintenance. One of the
considerations often overlooked in designing manufactured products is
drainage. The corrosion of the
automobile side panel above could have been minimized by providing drainage to
allow any water and debris to fall off of the car instead of collecting and
causing corrosion from the far side of the panel.
All of the other methods of
corrosion control should be considered in the design process.
Materials Selection
Carbon Steel
Stainless Steel
Aluminum
Copper Alloys
Titanium
Carbon Steel
Most large metal structures are
made from carbon steel-the world's most useful structural material. Carbon
steel is inexpensive, readily available in a variety of forms, and can be
machined, welded, and formed into many shapes.
This large statue by Pablo
Picasso in front of the Chicago city hall is made from a special form of carbon
steel known as weathering steel. Weathering steel does not need painting in
many boldly exposed environments. Unfortunately, weathering steel has been
misused in many circumstances where it could not drain and form a protective
rust film. This has given the alloy a mixed reputation in the construction
industry.
Where other means of corrosion
control are not practical, other alloys can be substituted for carbon steel.
This normally doubles or more the material cost for a structure, and other
corrosion control methods must be considered before deciding on the use of more
expensive alternates to carbon steel.
Some forms of carbon steel are
subject to special types of corrosion such as hydrogen embrittlement, etc. It
is common practice to limit the allowable strength levels of carbon steel to
avoid brittle behavior in environments where environmental cracking may occur.
High strength bolts cannot be galvanized for the same reason-a concern that
they may hydrogen embrittle due to corrosion on the surface.
Protective coatings, cathodic
protection, and corrosion inhibitors are all extensively used to prolong the
life of carbon steel structures and to allow their use in environments such as
the Kennedy Space Center where the environment would otherwise be too corrosive
for their use.
Stainless Steels
The stainless steel body on this
sports car is one example of how stainless steels can be used. The stainless
steel is virtually immune to corrosion in this application-at least in
comparison to the corrosion that would be experienced by conventional carbon
steel or aluminum auto bodies.
Stainless steels are a common
alternative to carbon steels. There are many kinds of stainless steels, but the
most common austenitic stainless steels (300-series stainless steels) are based
on the general formula of iron with approximately 18% chromium and 8% nickel.
These austenitic stainless steels are frequently immune to general corrosion,
but they may experience pitting and crevice corrosion and undergo stress
corrosion cracking in some environments.
Aluminum
Aluminum alloys are widely used
in aerospace applications where their favorable strength-to-weight ratios make
them the structural metal of choice. They can have excellent atmospheric
corrosion capabilities. Unfortunately, the protective properties of the
aluminum oxide films that form on these alloys can break down locally and allow
extensive corrosion. This is discussed further in the section on intergranular
corrosion.
The highway guardrail shown on
the right is located near the ocean in Florida. The aluminum alloy maintains a
silvery shine except in locations where the passive film has suffered
mechanical damage. The wear caused by the rail touching the wooden post at this
location destroyed the passive film on the edges of the rail and
allowedintergranular corrosion to proceed and cause the exfoliation corrosion
shown above. While the corrosion above is very interesting and makes for an interesting
web site, it is important to note that the railing is decades old and would
have never lasted as long in this location if it were made of carbon steel.
Intergranular corrosion is a
major problem on airplanes and other structures made from aluminum alloys. It
frequently occurs at bolt and rivet holes or at cutouts where the small grain
boundaries perpendicular to the metal surface are exposed.
Copper Alloys
Brasses and bronzes are commonly
used piping materials, and they are also used for valves and fittings. They are
subject to stress corrosion cracking in the presence of ammonia compounds. They
also suffer from dealloying and can cause galvanic corrosion when coupled with
steel and other structural metals. Most copper alloys are relatively soft and
subject to erosion corrosion.
The dezincification shown above
could have been controlled by using inhibited brasses which have been
commercially available since the 1930's.
Titanium
Titanium is one of the more
common metals in nature, but its limited use means that small-scale production
operations result in a relatively expensive metal. In the United States it
finds extensive use in the aerospace industry. The Japanese make extensive use
of titanium in the chemical process industries.
There are two general types of
titanium alloys-aerospace alloys and corrosion resistant alloys. The crevice
corrosion of an aerospace alloy flange in a saltwater application is a classic
example of how titanium gets misused.
Preventing Corrosion in New
Mexico Oilfields
Corrosion control methods
include:
Cathodic Protection
Protective Coatings
Chemical Inhibitors
Plastic or cement liners
Use of special alloys
Solids removal
Removal of corrosive gases
Dehydration
In addition, monitoring corrosion
and scale formation is an important part of the prevention process, allowing
producers to better understand the environment and to be able to predict
problems. Field experience indicates that a systematic, integrated process of
monitoring and failure analysis will, over time, reduce failure rates, and
effectively lower production costs.
Cathodic Protection
In general, cathodic protection
is an approach where the metal surface to be protected is made into the cathode
of a corrosion cell. Since corrosion and material loss occurs at the anode,
this approach protects the metal.
There are two types of cathodic
protection, the sacrificial anode and the impressed-current method. The
sacrificial anode method is the simpler method, and utilizes galvanic
corrosion. Sacrificial anodes are pieces of metal usually electrically
connected by a wire or steel strap to the structure to be protected. The metals
used must be less noble than steel (the common oil-field material), such as
magnesium, zinc, or aluminum. The sacrificial anodes are preferentially
corroded, protecting the (cathodic) steel from corrosion. Magnesium and zinc
are usually used in soils, and zinc can also be used in brine environments.
Sacrificial anodes are most often used when current requirements are relatively
low, electric power is not readily available, and when system life is short,
which calls for a low capital investment.
Impressed-current method uses an
external energy source to produce an electric current that is sent to the impressed-current
anodes, which can be composed of graphite, high-silicon cast iron, lead-silver
alloy, platinum, or even scrap steel rails. Impressed-current cathodic
protection is used when current requirements are high, electrolyte conductivity
is high, fluctuations in current requirements will occur, and when electrical
power is readily available.
Cathodic protection interferes
with the natural action of the electrochemical cells that are responsible for
corrosion. Cathodic protection can be effectively applied to control corrosion
of surfaces that are immersed in water or exposed to soil.
Cathodic protection in its
classical form cannot be used to protect surfaces exposed to the atmosphere.
The use of anodic metallic coatings such as zinc on steel (galvanizing) is,
however, a form of cathodic protection, which is effective in the atmosphere.
There are two basic methods of supplying the electrical currents required to
interfere with the electrochemical cell action.
The first method, cathodic
protection with galvanic anodes, uses the corrosion of an active metal, such as
magnesium or zinc, to provide the required electrical current. In this method,
called sacrificial or galvanic anode cathodic protection, the active metal is
consumed in the process of protecting the surfaces where corrosion is
controlled and the anodes must be periodically replaced.
In the second method, impressed
current cathodic protection, an alternative source of direct electrical
current, usually a rectifier that converts alternating current to direct
current, is used to provide the required electrical current. In this system, the
electrical circuit is completed through an inert anode material that is not
consumed in the process.
Coatings
Protective coatings can be used
to protect tubing, downhole equipment, wellhead components, Christmas trees,
and various flowlines and pressure vessels. Coatings work by reducing the
cathodic area available for corrosion reaction. Since no coat can be 100%
holiday-free (without pinholes or defects), coatings are often used in
conjunction with cathodic protection or chemical inhibition.
Quality control parameters for
coating include surface finish/preparation, application techniques, coating
thickness, holiday detection, joint condition, and inspection. Coated equipment
must be carefully handled after coating to prevent defects in the coat.
Corrosion Inhibitors
There is a wide variety of
corrosion inhibitor formulations available that can be selected to handle most
environments in oil and gas production systems except those where oxygen is
present. The application technique must match the systems mechanical and
process considerations in order to assure that the inhibitor will reach the
metal surface where needed. Most corrosion inhibitors, which are typically
organic amine-based compounds, function by establishing a film that protects
metal from corrosive fluids.
The choice of a specific
inhibition program is a combination of technical and economic considerations.
The programs should be modified or adjusted periodically to optimize the
program for cost effectiveness. Additionally, programs should be monitored and
periodically reviewed because systems are continually changing.
Types of inhibitors
There are many types of corrosion
inhibitors for various applications. Generally they can be grouped into two
broad categories, organic and inorganic. Inorganic inhibitors are most often
used in cooling tower water, heating/cooling mechanisms, dehydration glycol,
and sweetening amine solutions. Organic film formers are used in oil, gas, and
water wells, water and gas systems, and flowlines.
Inorganic corrosion inhibitors
are usually metal salts, which act to passivate the metal surface. They have
limited use because they require constant concentrations, are often pH
sensitive, and usually don't work if there are chlorides present.
Organic corrosion inhibitors are
the most common corrosion inhibitors in use in oilfield systems. The majority
of these are "organic film forming inhibitors". These are organic
chemicals with a polar ("water loving") head and a long hydrocarbon
("oil loving") tail. When applied, these compounds align with the
polar head towards the metal and the tail towards the outside, effectively
establishing an oil-wet film on the metal surface. This inhibitor film breaks
the corrosion cell by separating the metal surface and the electrolyte containing
water.
Inhibitor selection
The first step in selecting an
inhibitor is to review the system, its physical layout, mechanical
considerations, and fluids being handled, locating any special or unique
factors.
The second step is to select the
application method(s) that assure the inhibitor gets to where it will be
effective, i.e., the metal surface. This step is very important - large numbers
of "inhibitor failures" were actually application failures.
Common application methods for
wells with packered annuli include formation squeeze, tubing displacement,
partial tubing displacement and yo-yo, and treating strings. Less common
application methods include weighted liquids, dump bailers, wash bailers,
inhibitor sticks, chemical injector valves, and gas lift gas addition.
Application methods for wells
with open annuli include annular batch - operator applied or by treater truck,
and continuous, with a chemical injector pump.
The third step is to review the
properties required for the application technique. Inhibitor properties that
should be taken into consideration during selection include solubility and
dispersability, both in the carrier and in produced fluids (water, oil, gas);
emulsification properties, viscosity, freeze point, thermal stability, corrosiveness,
foaming properties, partitioning (between oil and water), compatibility with
other chemicals, mobility of individual components, compatibility with
downstream process, and environmental concerns.
40 வயது மாத்யூ கிறீன் இருதயப் பிரச்சனையால் உயிருக்கு போராடிக் கொண்டிருந்தார். அவருக்கு கேம்ப்ரிட்ஜ் ஷயரில் உள்ள பாப் வொர்த் மருத்துவமனையில் ஓபரேஷன் செய்யப்பட்டது.
“Just as within the earth is found every kind of seed and within the sky live all the stars, Tulsidas knows that Shri Rama’s holy name is the reservoir of all dharma.” (Dohavali, 29)
To understand the distinctions more clearly, we can look to the first two examples given in the above quoted verse from the Dohavali, or collection of couplets, of Goswami 
Understanding how just one plant grows serves as a substantial foundational base, but then we might also notice other plants. Again, we’ll ask our dad, “Father, wherefrom did these other plants come? Do they grow in the same way as the seeds that produced the tomatoes?” This time we will witness the growth cycles again, note down the results of the experiment in our mind, and then increase our appreciation for the seeds. Gradually, as we start to add to our knowledge base, we may start to appreciate more and more plants and the overall miracle of life.
In both instances, full appreciation is eventually reached, but through a step-by-step process. Eventually, if we study all the plants that grow from the ground and all the stars situated in the sky, we’ll see that the earth and the sky are the abodes of both miracles of material existence. If we would have first studied the earth and the entire sky, then we would have automatically learned about the different plants and stars. The earth and sky are actually more important because they are reservoirs, the holding places, for the individual miracles that were noticed.
Why is it important to know that Rama holds the many systems of religion within Him? Similar to how our appreciation increased when we learned more about the earth and the sky, if we know a little bit more about the object of all religious sacrifice, penance, austerity, charity and dedication, we will be able to understand and appreciate the individual aspects of spiritual life more fully. Rama is not a sectarian figure; He is the Supreme Lord for all of humanity. Though there are various non-different forms of the same original person, Tulsidas especially worships God in His form of Shri Ramachandra, who carries a bow and arrow in His hands and promises to always protect the surrendered souls.
There is an easier method, however, for understanding everything. At its root meaning, dharma is the essential characteristic of something, and in the religious sense it is meant to apply to the soul, which is the individual functioning unit within every living being. To maintain the essential characteristic within, to bring it to the forefront of consciousness and activity, a set of rules and regulations is required, a recommended way of life aimed at evoking the primary properties of the soul. This set of principles can also be described using the word “dharma”; hence the common English translation for dharma being “
Tulsidas declares that in Rama’s name can be found all dharma, or systems of religiosity. This is indeed true, as the name of Rama automatically brings His audience, thus allowing the conditioned souls to directly approach God right away. 
All aspects of religion are similarly just tools meant to lead to bhakti. Let’s say that we take bhakti to be just a method of self-realization aimed at removing one’s attachment to matter and the senses. When the state of Brahman realization, or brahma-bhutah, is attained, then what? The soul is inclined towards activity, so even when it no longer desires association with material life and deference to karma and sense demands, it still must have something to do. Therefore even brahma-bhutah represents an imperfect state, a platform which must lead to something better.
Indeed, devotees can explain every aspect of life, material or spiritual, through its relation to God. This is how the famous acharyas and saints have produced seemingly endless volumes of literature glorifying the Lord and devotion to Him. The name of Rama opens our transcendental eyes and allows us to have a clear vision. Moreover, the bhakti platform is an eternal one, bringing an attitude of undying love and affection aimed at everyone’s dearmost friend, the Supreme Lord. Just as the name of Rama safely holds the different systems of dharma known to man, in the devotee of Rama can be found the fruits and rewards of all religious practice, namely every beneficial quality and attribute one can acquire. Therefore the dust of the lotus feet of a Vaishnava, the devotee of God’s personal forms, is the greatest benediction in life, as the dedication to chanting exhibited by the bhakta serves as the best example to follow.