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CURSO DE FUNDAMENTOS DE CORROSIÓN, DEL DEPARTAMENTO DE DEFENSA DE LOS EEUU
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201A
Corrosion Basics
201A Ch1 Introduction to Corrosion
Section 1: Introduction
IInnttrroodduuccttiioonn
Welcome to the Corrosion 201-A course, which provides details on how to recognize
and treat twelve different forms of corrosion that adversely affects systems,
equipment, and infrastructure throughout industrial, and government communities.
Since the impact of corrosion is particularly devastating to the Department of
Defense, this course focuses on the prevention and treatment of corrosion of
weapon systems, support equipment, and the associated infrastructure. Through
this course, the learner will be exposed to engineering content and concepts.
TThhee CCoorrrroossiioonn PPoolliiccyy aanndd OOvveerrssiigghhtt OOffffiiccee
The DoD Corrosion Policy and Oversight office (CPO) is responsible for
implementing the congressionally-mandated Corrosion Prevention and Mitigation
Program; and has assembled this course to help the defense acquisition community
better understand and implement corrosion-related responsibilities. The CPO's
operation is directed and controlled by Title 10, Section 2228, Instruction
5000.67, and the DoD Corrosion Prevention and Mitigation Strategic Plan.
DDooDD SSttrraatteeggiicc PPllaann
The DoD strategic plan articulates DoD corrosion program policies, strategies and
objectives; prevent, detect and treat corrosion, transcend traditional corrosion
control methods by implementing modern corrosion control techniques throughout
design, fabrication, manufacturing, operation, maintenance and disposal
processes.
201A Ch1 Introduction to Corrosion
Section 2: Terms and Definitions
DDeeffiinniittiioonn ooff TTeerrmmss
Lets begin with some basic definitions.
Corrosion and oxidation are both terms that are used to describe the
deterioration of materials. However, corrosion is often associated with the
deterioration of metals and their alloys while oxidation is more commonly
used to describe the deterioration of non-metallic materials.
Deterioration is the loss or degradation of performance or properties such
as strength, durability, and appearance.
Material is the stuff that we make things out of, such as metals, polymers,
ceramics, composites, nano-technology items, etc.
Reaction is the response of material to the environment.
Environment is the surroundings to which a material is exposed.
Environmental Properties include chemical composition, form of moisture, and
operating temperature. Flow, wet, dry cycles, stress, and abrasion can also
be factors. The environment will affect the type and rate of corrosion as
well as the damage that results.
CCoonnddiittiioonnss oonn CCoorrrroossiioonn SSeevveerriittyy
Materials, particularly metals and metal alloys vary widely in their resistance
to the corrosive effects of the environment. Three conditions determine the
potential severity of corrosion damage.
1. the corrosion resistance of the metal or alloy or the oxidation resistance
of other materials.
2. the length of time the metal or alloy or non-metallic material is exposed to
the corrosive environment, and
3. the severity of the environment
These three conditions must be considered carefully when selecting materials and
designing systems and facilities that will be operated or used in specific
corrosive environments.
201A Ch1 Introduction to Corrosion
Section 2: Terms and Definitions
CCoosstt ooff CCoorrrroossiioonn
The choice and use of materials in corrosive environments is also an economic
decision. In a study funded by the Department of Transportation in the year 2000:
the annual cost of corrosion in the United States was estimated at $276 billion
and that figure continues to rise. More recent DoD studies estimated the annual
cost of corrosion within the military is over $20 billion.
The objective is to reduce this annual cost by at least one-third. Corrosion
reduces scarce resources, system and facility readiness, performance, and safety.
CCoorrrroossiioonn DDeecciissiioonn PPooiinnttss
Therefore, effective management decision-making is needed at all organizational
levels. Complex interactions between multiple disciplines are required for
effective corrosion prevention and mitigation. Decision-makers such as designers,
engineers, logisticians, manufacturers, operators, and maintenance personnel must
consider several items in material selection. They must:
choose alloy classes or other classes of materials that are less prone to
corrosion;
design to avoid high risks or plan mitigation;
specify production and manufacturing practices to avoid detrimental effects
or to mitigate effects;
recognize high risks and use effective tools, techniques, and practices to
mitigate these risks;
perform failure analysis by identifying the relevant forms of corrosion to guide
root cause analysis and corrective action.
201A Ch1 Introduction to Corrosion
Section 3: Additional Resources
CCoorrrroossiioonn IInnffoorrmmaattiioonn RReessoouurrcceess
The DoD has established corrosion technology information sources. At any time,
you may click on the RESOURCES button for additional information within this
course. Other corrosion technology information is available through NACE
International, ASM International, and SSPC (The Society for Protective Coatings)
and others.
201A Ch1 Introduction to Corrosion
Section 4: 12 Types of Corrosion
1122 TTyyppeess ooff CCoorrrroossiioonn
There are multiple forms (or types) of corrosion, depending on how they are
categorized.
For the purpose of this course, we will discuss 12 types. Each of which is
described in terms of its physical appearance, form, and structure of the damage
resulting from corrosion attack.
201A Ch1 Introduction to Corrosion
Section 5: Identifying the Types
GGeenneerraall CCoorrrroossiioonn OOvveerrvviieeww
General or uniform corrosion is the most readily observed and detected of the
forms of corrosion. It occurs over most of the entire surface of the metal or
alloy thats exposed to the environment as shown here.
It is the type of corrosion for which we have the most knowledge and means to
prevent or control. Because steel is prone to general corrosion, it accounts for
the greatest amount of alloy loss.
PPiittttiinngg CCoorrrroossiioonn OOvveerrvviieeww
Here is an example of severe pitting corrosion.
Pitting is found on metals in areas where the passive film has broken down. Where
the passive film remains stable, there is little or no pitting. Anodes form where
the local breakdown occurs. Significant, serious corrosion is evident at those
anodic sites. The surrounding area becomes a cathode. The corrosion growth
process is autocatalytic. That means that once a pit starts to grow, the solution
becomes more corrosive and the pit becomes self-propagating. The environment,
within the pit, becomes progressively more corrosive and corrosion cells self-
propagate more quickly.
It is more likely for corrosion to continue in the pit rather than for corrosion
to initiate in another location, even if the process is interrupted and
reactivated.
201A Ch1 Introduction to Corrosion
Section 5: Identifying the Types
CCrreevviiccee CCoorrrroossiioonn OOvveerrvviieeww
Crevice corrosion can be the result of design or manufacturing flaws, and it is
often associated with the use of flanges and bolts, tight contact between two
metals, or tape wrapped around a material.
Crevice corrosion occurs on passive metals. Crevices form underneath deposits on
a material. In such restricted conditions, the environment becomes much more
corrosive. If the passive film breaks down, anodes form in the restricted area
beneath the crevice. The outer areas in the freely exposed solution become the
cathode. The result is a localized autocatalytic attack that can accelerate very
rapidly and cause severe damage.
Crevice corrosion also occurs under protective coatings applied to a metal
surface to isolate it from a corrosive environment. Corrosion is induced at flaws
in coatings bonded to the metal surface. The environment penetrates the flaw and
initiates accelerated corrosion in a large area beneath the coating.
CCoorrrroossiioonn FFaattiigguuee OOvveerrvviieeww
If a metal is subject to pitting corrosion, the pits can act as stress risers
that increase the effective level of stress at the bottom of the pit.
Intergranular corrosion can result in the formation of stress risers that have a
similar effect. The alternating, or cycling stresses, further weaken the corroded
metal and corrosion fatigue results.
DDeeaallllooyyiinngg OOvveerrvviieeww
Most metals used in engineering applications are not pure metals but are alloys,
consisting of two or more elements designed to achieve specific properties. For
example, brass is a mixture of copper and zinc.
201A Ch1 Introduction to Corrosion
Section 5: Identifying the Types
Dealloying is the corrosion of one or more of the component metals of an alloy.
The remaining material may retain the original size and shape of the alloy, but
has greatly reduced strength and ductility.
EErroossiioonn CCoorrrroossiioonn OOvveerrvviieeww
Flow-assisted corrosion, or erosion corrosion, occurs or is accelerated when
there is a relative movement between a metal and its environment such as the flow
of fluids through pipes. Abrasive particles in the environment exacerbate erosion
corrosion.
FFrreettttiinngg CCoorrrroossiioonn OOvveerrvviieeww
Fretting is direct wear that is due to small relative motions between two
surfaces that are under load and move relative to each other. The relative
movement is usually cyclic and occurs at a fairly rapid rate. Fretting of either
surface accelerates and increases the effects of corrosion.
201A Ch1 Introduction to Corrosion
Section 5: Identifying the Types
GGaallvvaanniicc CCoorrrroossiioonn OOvveerrvviieeww
The next type of corrosion we will look at is Galvanic corrosion. As its name
implies, it is corrosion due to the galvanic action between two or more
dissimilar metals, alloys, or electrically conductive non-metals.
This is one of the more common forms of corrosion in complex components because
of the wide use of dissimilar metals in the design and manufacture of equipment
and structures.
HHyyddrrooggeenn EEmmbbrriittttlleemmeenntt OOvveerrvviieeww
Hydrogen embrittlement can result from absorption of hydrogen that promotes
brittle fracture of the metal. Hydrogen embrittlement is more prevalent in alloys
with a high yield strength.
EExxffoolliiaattiioonn aanndd IInntteerrggrraannuullaarr OOvveerrvviieeww
This is an example of exfoliation, which is intergranular corrosion occurring
along an elongated grain structure, resulting in a flaking off of the grain
layers.
201A Ch1 Introduction to Corrosion
Section 5: Identifying the Types
The aluminum alloy shown is particularly susceptible to exfoliation when joined
using steel fasteners. If the fasteners were aluminum, intergranular corrosion
would have progressed more slowly. As time progresses, you can see the aluminum
around the periphery of the fastener begin to lift and eventually appears to come
apart.
Intergranular corrosion. Metals and alloys have crystalline structures. This
means that the atoms have periodic alignments or stacking of many atoms in atomic
planes to make a crystal. Most commonly used alloys are composed of aggregates of
small crystals called grains, where one periodic stack of atoms forms a crystal
interface with other stacks having slightly different orientations. The
resulting aggregate is called a polycrystalline metal. The regions where these
grains meet are called grain boundaries and can have a different composition and
structure than the individual grains themselves. When this difference in
chemistry and structure leads to a more corrosion-prone grain boundary, corrosion
occurs preferentially at the grain boundaries. This is called intergranular
corrosion.
SSttrreessss CCoorrrroossiioonn CCrraacckkiinngg OOvveerrvviieeww
Stress corrosion cracking is the corrosion-induced propagation of cracks when the
material is under a sustained tensile stress while being exposed to a corrosive
environment.
The tensile stress may be due to an applied load, residual stress from forming
and fabrication, or a combination of the two. It is particularly insidious
because the combined stresses and corrosion can cause unexpected structural
failure.
SSttrraayy CCuurrrreenntt CCoorrrroossiioonn OOvveerrvviieeww
Stray current corrosion occurs when a metal structure inadvertently gets in the
path of current flowing in the environment between 2 other structures.
201A Ch1 Introduction to Corrosion
Section 5: Identifying the Types
This shows an underground tank, such as might be used to store gasoline, with an
impressed current cathodic protection system to reduce the rate of corrosion.
Cathodic Protection will be described later, but it involves the passage of
current from an anode to the part being protected through the ground as in this
case or maybe through seawater.
If there is a nearby metallic structure, such as a buried pipe, the path of least
resistance for the current might be through the pipe. Where that current leaves
the pipe to go to the tank, the pipe is polarized to a higher potential and can
corrode at an accelerated rate. Stray Current Corrosion can be mitigated by
connecting the pipe to the tank electrically, and thus protecting both the pipe
and the tank. This type of corrosion can affect most metals buried in soil or
immersed in water. Sources of electricity throughout equipment and facilities can
provide stray alternating or direct current which may cause corrosion of metal
materials in the path of the stray current.
201A Ch1 Introduction to Corrosion
Section 6: Understanding the Corrosion Cell
TThhee CCoorrrroossiioonn CCeellll
All electrochemical cells consist of four components: First, there must be an
anode where electrons are generated through an oxidation reaction such as metal
corrosion. Second, there must be a cathode where the electrons liberated from the
anode are consumed by a cathodic reaction. Third, there must be a metallic
conductive path that electrically connects the anode and cathode. The anode and
cathode might be two different metals that are electrically connected or they
might be different sites on the same piece of metal. Finally, the metal must be
exposed to an electrolyte consisting of water or a solution that contains and
transports ions and contains a cathodic reactant.
These four components must be present for corrosion to occur or for the operation
of any electrochemical cell, like a battery.
Electrons, liberated during oxidation, move freely along the metallic path but do
not move freely in the electrolyte. Conversely, ions move freely in the
electrolyte but not along the metallic path. It should be noted that an ion is an
atom or molecule that has lost or gained one or more electrons, making it
positively or negatively charged. A negatively-charged ion, which has more
electrons than protons, is known as an anion. A positively-charged ion, which
has fewer electrons than protons, is known as a cation.
In corrosion, the corrosive environment is the electrolyte. A typical corrosive
environment is seawater, which contains chloride and many other types of ions.
Other corrosive environments might be a liquid held in place by a solid substance
such as soil or concrete.
Metal corrosion is the destructive dissolution of metal through an oxidation
reaction at the anode of an electrochemical cell. Metal atoms oxidize to produce
positively charged metal cations such as Zn+2 or Fe+2 that are released into the
environment. The electrons, released by the oxidation reaction, travel through
the metal to the cathode. As with all electrochemical cells, corrosion requires a
cathode where the electrons liberated by the corrosion reaction at the anode are
consumed by some reduction or cathodic reaction. The rate of metal corrosion or
oxidation at the anode is always equal to the rate of reduction at the cathode.
This reduction reaction consumes cathodic reactants usually supplied from the
electrolyte.
Typical cathodic reactants in corrosion are chloride ions, dissolved oxygen but
can involve other species. In acids, the primary cathodic reaction is the
reduction of H+ ions to form hydrogen gas. In neutral environments, such as
seawater, the concentration of H+ ions is low and the cathodic reaction is often
the reduction of dissolved oxygen gas molecules to form hydroxyl anions. Cations
201A Ch1 Introduction to Corrosion
Section 6: Understanding the Corrosion Cell
migrate and diffuse through the electrolyte to the cathode while anions migrate
and diffuse toward the anode.
Electrochemical cells that form on corroding metal surfaces or in metal cracks or
crevices require these four components and behave in the same manner as
batteries. The difference is that the electrical current produced by a battery is
captured to light a bulb, start an engine in a car, or power a device such as a
laptop computer.
Corrosion cells behave like a shorted battery, where the anodes and cathodes are
directly in contact with each other. The local anodes and cathodes might move
across the surface of the piece of metal resulting in uniform attack. In this
arrangement, the electrons cannot be captured as the short circuit prevents this.
In a battery the separation of anode and cathode and the travel of electrons from
the anode to cathode through this circuit, enable the electrons and electrical
energy to be used such as in the example of the light bulb. So, in our attempts
to thwart corrosion, we try to design structures and components to eliminate at
least one of the four electrochemical cell components, and thus prevent the
oxidation and reduction reactions characteristic of electrochemical cells.
201A Ch2 External Factors
Section 1: Metallurgy and Material Microstructure
PPaassssiivvee MMeettaall CCoonnssiiddeerraattiioonnss
There are two important considerations when using passive metals to prevent
corrosion: (a) the corrosion resistance of the alloy, and (b) the corrosivity of
the environment. For success, (a) must be great enough to withstand (b).
CCoorrrroossiioonn TTyyppeess:: MMeettaalllluurrggyy
Metallurgy can affect forms of corrosion. Three forms of corrosion damage are
related to metallurgy and metallurgical behavior.
Pitting corrosion. In this sample, manganese sulfide inclusions from the steel
production process corrode and create pits on the metal surface.
Intergranular corrosion, in this sample of a stainless steel alloy, chromium was
depleted at the grain boundaries resulting in preferential corrosion.
Dealloying. Grey cast iron is composed of ferrite and graphite platelets forming
a lamellar structure. Since ferrite is prone to corrosion and graphite is
corrosion resistant, the ferrite phase corrodes away, leaving a weak structure of
graphite flakes.
IInntteerrggrraannuullaarr CCoorrrroossiioonn:: CChhrroommiiuumm CCaarrbbiiddee PPrreecciippiittaattiioonn
Intergranular corrosion is directly related to the metallurgical behavior of
grain boundaries. Precipitation of chromium carbides (adjacent grain boundaries)
and subsequent migration of these chromium carbides into the grain boundary
regions, may lead to intergranular corrosion when exposed to a corrosive
environment.
201A Ch2 External Factors
Section 1: Metallurgy and Material Microstructure
Heat treatment forms precipitates that significantly strengthen aluminum. These
precipitates decrease the corrosion resistance of the alloys. As corrosion
progresses, exfoliation begins, and thin sheets of alloy peel away from the
material structures.
FFaabbrriiccaattiioonn PPrroocceessss:: WWeellddiinngg
The fabrication process and resulting structure can also affect an alloys
vulnerability to corrosion and physical characteristics. Welds can be highly
prone to corrosion because of differences in structural composition and
morphology. Welding can also result in high residual stresses in the weld and the
adjacent heat affected zone of the metal.
201A Ch2 External Factors
Section 2: Electrical Potential
EElleeccttrriicc PPootteennttiiaall DDiiffffeerreennccee
Previously, we discussed the electrochemical corrosion cell and the fact that the
difference in electrical potential between two areas of the metal surface is the
driving force behind the corrosion cell performance. This was evident in crevice
corrosion where loss of a passive film forms anodic and cathodic areas. Now we
address the corrosion cell where potential difference is due to dissimilar metals
in contact with each other, different environments, and different locations.
GGaallvvaanniicc SSeerriieess:: SSeeaawwaatteerr
Galvanic corrosion results from the potential difference created by dissimilar
metals in electrical contact.
This graph shows the potential difference between metals in seawater. The
horizontal scale shows electrical potential compared to a reference electrode in
seawater from most negative at the left to more positive at the right.
When two metals in the series come in contact, they form a galvanic couple. The
more positive member of the couple will be the cathode. The more negative will be
the anode. The graph also shows the position of active and passive alloys in the
solid boxes.
201A Ch2 External Factors
Section 2: Electrical Potential
GGaallvvaanniicc CCoorrrroossiioonn:: CCaatthhooddee--AAnnooddee RRaattiioo
The relative areas of the electrodes in a galvanic corrosion cell have a
significant impact on the rate of corrosion. The design of structures and systems
with dissimilar metals should consider the galvanic couple characteristics and
avoid high ratios of cathode to anode areas.
GGaallvvaanniicc CCoorrrroossiioonn:: SShhiippss
The classic case of galvanic corrosion is potential difference due to dissimilar
materials. A common example is associated with ship hulls.
Ship hulls are fabricated from steel. Hulls can be in electrical contact with
propellers, which are fabricated from bronze, which is an alloy of copper. Copper
alloys are more noble than steel, which promotes steel corrosion. Ship hull
corrosion can be off set by using sacrificial anodes. Sacrificial anodes are made
of zinc, a more active metal than copper. The zinc anodes corrode instead of the
steel. An example would be to use enough zinc beneficial galvanic action to
offset bronze detrimental galvanic action.
GGaallvvaanniicc CCoorrrroossiioonn:: AAiirrccrraafftt
Another example of dissimilar metal galvanic corrosion is corrosion of aluminum
surfaces and structures on aircraft where aluminum and steel used in aircraft
fabrication become galvanically coupled. Aluminum corrodes because the steel is
more noble. Electromagnetic interference (EMI) materials can contain silver or
other conductive particles. These conductive particles may be more noble and
therefore trigger galvanic action with the aluminum.
201A Ch2 External Factors
Section 3: Environmental and Solution Chemistry
CCrreevviiccee AAeerraattiioonn CCeellllss
Aeration cells, commonly formed during crevice corrosion, can create a potential
difference caused by dissimilar oxygen concentration environments.
Low oxygen concentration produces a negative potential.
High oxygen concentration produces a higher potential.
Since the potential of the anodic area in the crevice corrosion concentration
cell is much more negative than the potential in the cathodic area, the potential
difference accelerates corrosion cell activity.
CCoorrrroossiioonn IInniittiiaattiioonn aanndd GGrroowwtthh FFaaccttoorrss
Environmental differences and the associated chemistry affect the initiation and
growth of corrosion.
DDiiffffeerreennttiiaall AAeerraattiioonn CCeellll
Aeration cells are oxygen concentration cells where the amount of oxygen varies
from one location to another.
The area of higher oxygen concentration has a higher electrical potential.
The area with low oxygen concentration has a lower electrical potential.
Current flows from the area of lower potential the anode through the
solution to the area of higher potential the cathode, forming an
electrochemical corrosion cell.
Micro scale corrosion cells can form in naturally formed crevices and under
deposits of precipitates created during fabrication. Oxygen is consumed beneath
those crevices and deposits, and cannot be replenished because of the
microstructure but there is a high amount of oxygen remaining in the bulk
material.
201A Ch2 External Factors
Section 3: Environmental and Solution Chemistry
CCrreevviiccee CCoorrrroossiioonn:: CCrriittiiccaall CCrreevviiccee CChheemmiissttrryy
Critical crevice chemistry is the primary
mechanism that causes crevice corrosion. This
classic experiment demonstrates the results of
critical crevice chemistry. The solution
chemistry is shown at the bottom.
The solution in the crevice becomes more corrosive.
Critical chemistry develops where the passive film
breaks down. The crevice becomes more corrosive because
of oxygen depletion. Chloride ions build up and the
solution becomes more acidic.
As the process continues, the anodic area becomes
increasingly more negative than the freely exposed
metal, and this further accelerates the critical
chemistry activity.
AAggggrreessiivvee CChheemmiiccaall SSppeecciieess
Aggressive chemical species, such as chlorides, are often present naturally in
the bulk environments or are added during production processes. Chlorides are
particularly detrimental because they can break down passive films and increase
solution conductivity so current can flow more rapidly. Oxygen can also be
detrimental because of the aeration effects. Inhibitors are important to thwart
the effects of such aggressive chemicals.
MMiiccrroobbiiaall--IInnfflluueenncceedd CCoorrrroossiioonn
Microbial action in biofilms, or within deposits on metal surfaces, can change
the chemical environment and significantly increase corrosivity. For example,
sulfate ions are moderately non-corrosive chemical species. Sulfate reducing
microbes will convert sulfate ions to sulfide ions, which are very aggressive.
201A Ch2 External Factors
Section 3: Environmental and Solution Chemistry
Pipelines, storage tanks and other locations that have sulfate in the water, and
where conditions are conducive to microbial activity, can suffer from severe
Microbial Influenced Corrosion (MIC).
HHeeaatt EEffffeeccttss
Hot metal surfaces and surfaces with high heat transfer rates require special
consideration.
Water dripping onto a hot metal surface can cause very corrosive conditions.
A moist surface or a moist material, such as insulation in contact with a hot
metal surface, can cause increased corrosivity.
Corrosion under insulation is a classic example, where moisture wetting the
insulation comes in contact with the hot metal surface and the resulting
concentrated solution triggers corrosion. When high heat levels are transferred
across a metal surface, corrosion rates are significantly higher than if the
metal is immersed in the solution that is at the same temperature as the metal.
201A Ch3 Stress and Hydrogen Damage
Section 1: Mechanical Stress and Wear
CCoorrrroossiioonn TTyyppeess:: MMeecchhaanniiccaall SSttrreessss aanndd WWeeaarr
Lets consider mechanical stress and wear related problems where a combination of
the mechanical stress and corrosive environments result in corrosion damage.
Additionally, we will address hydrogen related problems where hydrogen absorbed
into the metal causes damage.
SSttrreessss CCoorrrroossiioonn CCrraacckkiinngg
In those forms of corrosion caused by mechanical stress and wear, mechanical
stresses work in conjunction with the environment and can cause cracking of
otherwise durable materials. Mechanical stress and wear can cause Stress
Corrosion cracking and corrosion fatigue. Abrasive wear from impact type forces
can cause erosion corrosion and fretting.
Stress corrosion cracking is also called environmental cracking, anodic stress
corrosion cracking, chloride stress cracking, sulfide corrosion cracking,
hydrogen stress cracking, and so on. Regardless of the terminology, three
conditions must prevail for stress corrosion cracking to occur.
1. Tensile stresses are always present on the outer fibers of the materials.
These stresses act to pull apart the outer surfaces.
2. A metal susceptible to stress corrosion cracking in the given environment
like sensitized stainless steel.
3. A corrosive environment containing a specific chemical that can trigger
stress corrosion cracking a chloride ion in solution for example.
These three critical conditions must be present simultaneously and eliminating
any one of these conditions will prevent stress corrosion cracking.
SSttrreessss CCoorrrroossiioonn CCrraacckkiinngg:: EExxaammpplleess
These pictures are examples of stress corrosion cracking failures on different
alloys. They show that stress corrosion cracking is highly localized and the
cracks at the metal surface deeply penetrate the metal. Adjacent metal shows no
damage.
201A Ch3 Stress and Hydrogen Damage
Section 1: Mechanical Stress and Wear
SSttrreessss CCoorrrroossiioonn CCrraacckkiinngg:: MMeettaall aanndd CCoorrrroossiivvee EEnnvviirroonnmmeenntt
This table shows some of the combinations of metal alloy systems and environments
that cause stress corrosion cracking.
Carbon steels are susceptible to stress corrosion cracking in concentrated
caustic alkaline environments, concentrated nitrate solutions, and in carbonate,
bicarbonate type solutions.
Austenitic stainless steels suffer stress corrosion cracking in hot concentrated
chloride solutions.
High strength aluminum alloys are vulnerable to stress corrosion cracking in
marine environments and/or corrosive environments of chloride, bromide, or iodide
solutions.
201A Ch3 Stress and Hydrogen Damage
Section 1: Mechanical Stress and Wear
SSttrreessss CCoorrrroossiioonn CCrraacckkiinngg:: DDeeggrreeee ooff DDaammaaggee
The degree of stress corrosion cracking varies with several parameters as shown
here.
As the magnitude of tensile stress on the outer surface of the metal increases,
the more rapidly cracking occurs. If threshold stress is not exceeded, no stress
corrosion will occur. As the concentration of critical chemicals in solution
increases, cracking occurs more readily. Geometries such as a notch, a crack, or
a sharp corner on the material, can concentrate stress, and accelerate crack
initiation and propagation.
SSttrreessss CCoorrrroossiioonn CCrraacckkiinngg MMiittiiggaattiioonn
There are several ways in which to prevent or control stress corrosion cracking.
The best approach is to choose the right materials for a given environment. If
susceptible materials are used, design and production process specifications may
be created that avoid stresses. Austentic stainless steels with precipitated
chromium carbides and the resultant chromium depletion is just such a susceptible
material, so additional mitigation needs to be considered.
Likewise, high-strength aluminum alloys can also be susceptible depending on
their composition and heat treatment. Shot peening is a process to generate
compressive stresses at the surface and can reduce susceptibility to stress
corrosion cracking.
201A Ch3 Stress and Hydrogen Damage
Section 1: Mechanical Stress and Wear
CCoorrrroossiioonn FFaattiigguuee
Corrosion fatigue is the combined action of corrosion and cyclic stresses that
initiates cracks through metal that would otherwise be ductile and durable.
Frequent landings and take-offs contributed to corrosion fatigue and failure of
the Aloha Airlines aircrafts aluminum alloy fuselage. Crevice corrosion
contributed to the initiation of the cracking. During flight, an entire section
of the fuselage broke away from the aircraft.
CCoorrrroossiioonn FFaattiigguuee:: SSttrreessss CCoonncceennttrraattoorrss
Stress concentrators can promote corrosion fatigue as shown here.
Stress concentration occurs at the root of scratches and at sharp corners. Cracks
initiate more readily at these sites resulting in increased fracture rate,
shortened corrosion fatigue life, and reduced strength.
CCoorrrroossiioonn FFaattiigguuee MMiittiiggaattiioonn
In order to control and mitigate corrosion fatigue, select materials with
enhanced corrosion fatigue strength or higher endurance limits; reduce residual
tensile stresses in manufactured parts by selecting effective design and
fabrication processes.
201A Ch3 Stress and Hydrogen Damage
Section 2: Abrasive Wear from Impact Type Forces
EErroossiioonn CCoorrrroossiioonn
Erosion corrosion is material deterioration due to the combination of chemical
action and mechanical abrasion or wear. The severity of attack is more than that
of either chemical corrosion or abrasion alone. The degree of erosion can vary
from slight, to moderate, to heavy.
Erosion corrosion is prevalent where high velocity or turbulent fluids of other
materials flow over a metal surface.
EErroossiioonn CCoorrrroossiioonn VVuullnneerraabbiilliittyy
The degree to which a metal surface is vulnerable to erosion corrosion depends on
the following factors:
surface film content and characteristics
passive film durability
corrosion product layers, and
the velocity of liquids or solids over the materials surface. The higher
the velocity; the more aggressive the erosion corrosion.
There is a critical velocity below which there is little or no corrosion.
Similarly, turbulence, which tends to scrape or scour the surface; or impingement
of fluids, gases and particulate matter will increase the erosion. In order to
mitigate erosion corrosion
1. design and build systems to avoid turbulent flow.
2. Place deflector plates in high velocity areas
3. protect welded and other susceptible areas from fluid flow.
Material selection and changing the environment can also mitigate erosion
corrosion.
FFrreettttiinngg CCoorrrroossiioonn
Fretting corrosion is corrosion where two moving
metal surfaces in simple small relative motions make
rubbing contact when the interface is subjected to
vibrations and compressive loads. In this highly
magnified view of metal surfaces in contact, the
surfaces appear smooth but there are mountain
201A Ch3 Stress and Hydrogen Damage
Section 2: Abrasive Wear from Impact Type Forces
tops that contact each other when the surfaces rub together.
When those points of contact fuse, small amounts of metal become dislodged,
causing metal particle buildup. These small metal particles oxidize and
essentially become efficient grinding compounds thus, fretting is self-
propagating since these corrosion products cause more fretting. Disassembly of
the interfacing surfaces reveals the loss and a surface covered with corrosion
products.
Parameters that affect fretting are:
1. the amount of stress on contacting surfaces, which makes movement difficult
2. the amount of oxide debris on the surfaces
3. presence of vibration or surface rubbing during transportation of products
that are in contact with each other
4. degree of relative motion between surfaces, and
5. presence of oxygen and moisture between interfacing surfaces.
FFrreettttiinngg CCoorrrroossiioonn MMiittiiggaattiioonn
In order to mitigate and control fretting:
1. Implement effective design and material selection, fabrication, and
operating procedures.
2. Maintain good alignment of rotating parts.
3. Make surfaces rougher in order to reduce slippage.
4. Apply loads that will lock interfacing parts together and reduce relative
motion.
5. Use low viscosity fluids with corrosion inhibitor and;
6. Apply corrosion preventative compounds.
201A Ch3 Stress and Hydrogen Damage
Section 3: Hydrogen-related Damage
HHyyddrrooggeenn AAbbssoorrppttiioonn
Now, let's consider hydrogen-related damage to metals caused by the metals
absorption of hydrogen.
There are several sources of hydrogen on metal surfaces. Hydrogen, the smallest
atom, can move from the surface into the metal, for example, be absorbed.
Hydrogen atoms can move in the metal along the interstitial sites in the metals
crystalline lattice. The hydrogen atom can collect at points of tensile stress,
causing metal embrittlement and hydrogen stress cracking. Blistering and, in some
alloys, hydride formation are additional forms of hydrogen damage.
HHyyddrrooggeenn AAttoomm
Hydrogen is a by-product of the corrosion reaction. Remember that corrosion
occurs at the anode where metal goes into solution as metal ions or at the
cathode, the reduction of a hydrogen ion [H+] (as in an acid) comes to the metal
surface accepts an electron and becomes a hydrogen atom. The hydrogen atom can
recombine with another hydrogen atom and form a hydrogen gas bubble or can
accumulate on the surface and be absorbed by the metal.
201A Ch3 Stress and Hydrogen Damage
Section 3: Hydrogen-related Damage
An electroplating process can generate hydrogen, and large amounts of hydrogen
atoms can be absorbed by the metal. If very high pressure gaseous hydrogen is
present, hydrogen molecules can disassociate and hydrogen atoms can be absorbed
by the metal. If hydrogen sulfide is present in the solution when corrosion is
occurring, absorption of hydrogen by the metal occurs more readily and greater
amounts are absorbed.
HHyyddrrooggeenn EEmmbbrriittttlleemmeenntt
Let's distinguish the three forms of hydrogen damage.
Hydrogen embrittlement is a loss of ductility from the absorption of hydrogen.
Reactions that take place during the corrosion process or during electroplating
cause absorption of large quantities of hydrogen, which are absorbed and retained
in the metals crystalline structure. If a load is applied to the structure, or
there are residual tensile stresses, the metal can crack and break without
warning. The higher the materials strength, the more susceptible it is to
hydrogen embrittlement.
HHyyddrrooggeenn EEmmbbrriittttlleemmeenntt MMiittiiggaattiioonn
Hydrogen embrittlement mitigation methods include:
1. Baking: Heat the parts in an oven at moderate temperature for a length of
time that allows the hydrogen to leave the metal before it cracks.
2. Material selection: Select lower strength metals for the particular
application, if possible. And,
3. Processing: change the processing or exposure conditions to avoid hydrogen
absorption.
HHyyddrrooggeenn BBlliisstteerriinngg
Hydrogen blistering occurs as a result of internal delaminations, or blisters,
forming from the recombination of hydrogen within the metal. As hydrogen is
absorbed by the metal, it forms a gas in a weak area within the metal.
Recombination of the hydrogen atoms into gas causes the buildup of very high
pressures, which can deform the metal.
Corrosion is the primary source of hydrogen in many cases and hydrogen sulfide
can exacerbate the problem.
201A Ch3 Stress and Hydrogen Damage
Section 3: Hydrogen-related Damage
HHyyddrriiddee FFoorrmmaattiioonn
Hydride formation is another form of hydrogen damage that occurs in materials
such as titanium and zirconium. High levels of hydrogen absorption in these
alloys can cause the formation of metal hydrides. Metal hydrides form along
crystallographic planes. When the hydride forms, its volume is greater than the
metal, which creates high stresses in the material. Hydrides tend to be brittle
and crack readily under stress.
As shown in the micrographs, hydrides formed in a very orderly pattern along
crystallographic planes and the resulting crack or fracture reduces material
ductility.
201A Ch4 Stray Current and Miscellaneous
Section 1: Stray Current Corrosion
SSttrraayy CCuurrrreenntt CCoorrrroossiioonn OOvveerrvviieeww
Now, we will discuss a few special cases of corrosion.
The first of which is Stray Current Corrosion caused by currents that can flow
from Direct Current (DC) sources such as electric railroads or between pipelines
through the soil, seawater or other conductive environments.
Cathodic protection currents on pipelines in close proximity can result in stray
currents.
There may also be induced AC currents, resulting from the co-location of buried
metal structures and high power transmission lines.
Current can enter and flow through the buried or immersed metal structures
because they have lower electrical resistance than the environment. Severe
corrosion damage occurs when the current flows back into the environment. The
damage tends to be highly localized and penetration rates are rapid.
SSttrraayy CCuurrrreenntt CCoorrrroossiioonn MMiittiiggaattiioonn
In order to mitigate or control stray current corrosion:
1. Ground the stray current so it doesnt flow from the metal surface directly
into the electrolyte but flows through the grounding device.
2. Electrically bond together co-located systems to avoid areas where current
can flow to and from electrolytes.
3. Use sacrificial anodes along with impressed current in cathodic protection
systems. The sacrificial anodes provide a path to ground.
AACC IInndduucceedd CCuurrrreennttss
AC induced currents are caused by high voltage AC power lines that induce current
flow in the buried support structures and co-located buried steel structures and
pipelines that can propagate induced current flow.
The corrosion damage occurs where there are induced currents flowing from the
metal and to the soil or other electrolyte. Corrosion damage by induced currents
is exacerbated by High AC voltage levels.
201A Ch4 Stray Current and Miscellaneous
Section 1: Stray Current Corrosion
AACC IInndduucceedd CCuurrrreenntt CCoorrrroossiioonn:: CCaauussee aanndd EEffffeecctt
These photos and illustrations show some causes and effects of AC induced current
corrosion. Sharing the right-ofway between power lines and pipelines - note
trench operations for construction of a buried pipeline between power lines.
Induced current corrosion a defect in the coating along with induced AC
currents caused severe corrosion, which penetrated the pipeline and caused a leak
18 months after installation. Disbonded coating was also observed. If an induced
current flows in a structure with a high quality coating one with very few
defects, the current density at the isolated defect will be much higher than in a
structure with many defects.
Induced AC current superimposed on the DC current in a typical cathodic
protection system can exacerbate corrosion initiation and damage.
201A Ch4 Stray Current and Miscellaneous
Section 2: Miscellaneous Forms
MMiisscceellllaanneeoouuss TTyyppeess ooff CCoorrrroossiioonn
There are a number of instances of corrosion that cannot be conveniently
classified as one of the forms of corrosion previously identified.
Sequential corrosion processes. Corrosion of a multi-component, multi-layer
coating systems. Special Case: filiform corrosion, which is a specific type of
corrosion degradation of coated metal products.
Coupled corrosion processes, where one form triggers another specific form.
SSeeqquueennttiiaall CCoorrrroossiioonn PPrroocceessss
These photos depict an example of the sequential corrosion process. A manganese
sulfide inclusion in the steel was at the outer surface of a metal sample. When
the metal sample was placed in a corrosive brine solution, the sulfide corroded
and a pit was formed. A pit in the metal surface is shown at high magnification
in the upper figure. This and other pits became initiation sites for stress
corrosion cracks growing into the metal.
The lower figure shows multiple corrosion sites (black oxide-covered surface on
the fractured face). Rapid fracture occurred (light area) by mechanical overload
of the uncracked area.
201A Ch4 Stray Current and Miscellaneous
Section 2: Miscellaneous Forms
MMuullttiillaayyeerr CCooaattiinngg SSyysstteemm
There are multiple coating layers, each to protect each other. For example, in
an automobile theres cleaning, a pretreatment which is mostly in organic and the
pretreatment is meant to protect the metals substrate against corrosion. A primer
goes on top of that after the pretreatment is dried and cured and the primer is
meant to protect the pretreatment. A top coat goes on top of that and the top
coat is meant to protect the primer. So in other words, its a three separate
coating steps all working as a system.
MMuullttiillaayyeerr CCooaatteedd MMeettaall CCoorrrroossiioonn
If some external damage occurs which cuts through those coating layers, corrosion
of the steel will be determined by the depth and width of the penetration, the
life of the organic coating and primer, and the zinc layer cathodic protection.
201A Ch4 Stray Current and Miscellaneous
Section 2: Miscellaneous Forms
The actual corrosion of the steel itself follows after the galvanic corrosion of
zinc stops thus multi-layer coated metal corrosion is also a sequential
corrosion process.
FFiilliiffoorrmm CCoorrrroossiioonn
Filiform corrosion of both coated steel and coated aluminum is shown in the
schematic. Defects in coatings allow the electrolyte to contact the metal
surface.
Anodes and cathodes on the metal surface under the coating complete the formation
of corrosion cells. These filiform corrosion cells migrate along the surface of
the metal in a pattern similar to mole tunnels in a lawn. The corrosion cells
leave corrosion products that discolor the metal surface beneath the coating. The
result does not cause significant structural damage but causes user concern due
to the unpleasant appearance.
CCaavviittaattiioonn CCoorrrroossiioonn
Cavitation corrosion is a somewhat unique form of corrosion that results from the
formation and collapse of bubbles on a metal surface. These bubbles form and
collapse in areas of rapid pressure drops such as on the blades of ship
propellers. Low pressure generates the bubbles and a following wave of high
pressure collapses them. Under magnification, the mechanical damage to the
surface appears like metal peening or grit blasting processes.