Corrosion is the disintegration of an engineered material into
its constituent atoms due to chemical reactions with its surroundings.
In the most common use of the word, this means electrochemical oxidation
of metals in reaction with an oxidant such as oxygen. Formation of an
oxide of iron due to oxidation of the iron atoms in solid solution is a
well-known example of electrochemical corrosion, commonly known as
rusting. This type of damage typically produces oxide(s) and/or salt(s)
of the original metal. Corrosion can also occur in materials other than
metals, such as ceramics or polymers, although in this context, the term
degradation is more common..
In other words, corrosion is the wearing away of metals due to a chemical reaction.
Many structural alloys corrode merely from exposure to moisture in the air, but the process can be strongly affected by exposure to certain substances (see below). Corrosion can be concentrated locally to form a pit or crack, or it can extend across a wide area more or less uniformly corroding the surface. Because corrosion is a diffusion controlled process, it occurs on exposed surfaces. As a result, methods to reduce the activity of the exposed surface, such as passivation and chromate-conversion, can increase a material's corrosion resistance. However, some corrosion mechanisms are less visible and less predictable.
In this photo, a 5-mm thick aluminum alloy plate is physically (and
hence, electrically) connected to a 10-mm thick mild steel structural
support. Galvanic corrosion occurred on the aluminium plate along the
joint with the mild steel. Perforation of aluminum plate occurred within
2 years due to the large acceleration factor in galvanic corrosion.
Factors such as relative size of anode, types of metal, and operating conditions (temperature, humidity, salinity, etc.) affect galvanic corrosion. The surface area ratio of the anode and cathode directly affects the corrosion rates of the materials. Galvanic corrosion is often utilized in sacrificial anodes.
Corrosion removal should not be confused with Electropolishing which removes some layers of the underlying metal to make a smooth surface. For example phosphoric acid (again) may be used to electropolish copper but it does this by removing copper, not the products of copper corrosion.
The materials most resistant to corrosion are those for which
corrosion is thermodynamically unfavorable. Any corrosion products of
gold or platinum tend to decompose spontaneously into pure metal, which
is why these elements can be found in metallic form on Earth, and is a
large part of their intrinsic value. More common "base" metals can only
be protected by more temporary means.
Some metals have naturally slow reaction kinetics, even though their corrosion is thermodynamically favorable. These include such metals as zinc, magnesium, and cadmium. While corrosion of these metals is continuous and ongoing, it happens at an acceptably slow rate. An extreme example is graphite, which releases large amounts of energy upon oxidation, but has such slow kinetics that it is effectively immune to electrochemical corrosion under normal conditions.
Passivation is primarily determined by metallurgical and environmental factors. The effect of pH is recorded using Pourbaix diagrams, but many other factors are influential. Some conditions that inhibit passivation include: high pH for aluminium and zinc, low pH or the presence of chloride ions for stainless steel, high temperature for titanium (in which case the oxide dissolves into the metal, rather than the electrolyte) and fluoride ions for silicon. On the other hand, sometimes unusual conditions can bring on passivation in materials that are normally unprotected, as the alkaline environment of concrete does for steel rebar. Exposure to a liquid metal such as mercury or hot solder can often circumvent passivation mechanisms.
Certain conditions, such as low concentrations of oxygen or high
concentrations of species such as chloride which complete as anions, can
interfere with a given alloy's ability to re-form a passivating film.
In the worst case, almost all of the surface will remain protected, but
tiny local fluctuations will degrade the oxide film in a few critical
points. Corrosion at these points will be greatly amplified, and can
cause corrosion pits of several types, depending upon conditions.
While the corrosion pits only nucleate under fairly extreme
circumstances, they can continue to grow even when conditions return to
normal, since the interior of a pit is naturally deprived of oxygen and
locally the pH decreases to very low values and the corrosion rate
increases due to an auto-catalytic process. In extreme cases, the sharp
tips of extremely long and narrow corrosion pits can cause stress
concentration to the point that otherwise tough alloys can shatter; a
thin film pierced by an invisibly small hole can hide a thumb sized pit
from view. These problems are especially dangerous because they are
difficult to detect before a part or structure fails. Pitting remains
among the most common and damaging forms of corrosion in passivated
alloys , but it can be prevented by control of the alloy's environment.
A stainless steel is said to be sensitized if chromium carbides are formed in the microstructure. A typical microstructure of a normalized type 304 stainless steel shows no signs of sensitization while a heavily sensitized steel shows the presence of grain boundary precipitates. The dark lines in the sensitized microstructure are networks of chromium carbides formed along the grain boundaries.
Special alloys, either with low carbon content or with added carbon "getters" such as titanium and niobium (in types 321 and 347, respectively), can prevent this effect, but the latter require special heat treatment after welding to prevent the similar phenomenon of knifeline attack. As its name implies, corrosion is limited to a very narrow zone adjacent to the weld, often only a few micrometres across, making it even less noticeable.
Crevice corrosion is a localized form of corrosion occurring in
confined spaces (crevices) to which the access of the working fluid from
the environment is limited and a differential aeration cell is set up,
leading to the active corrosion inside the crevices. Examples of
crevices are gaps and contact areas between parts, under gaskets or
seals, inside cracks and seams, spaces filled with deposits and under
sludge piles.
This photo shows that corrosion occurred in the crevice between the tube and tube sheet (both made of type 316 stainless steel) of a heat exchanger in a sea water desalination plant.
Crevice corrosion is influenced by the crevice type (metal-metal, metal-nonmetal), crevice geometry (size, surface finish), and metallurgical and environmental factors. The susceptibility to crevice corrosion can be evaluated with ASTM standard procedures. A critical crevice corrosion temperature (CCT) is commonly used to rank a material's resistance to crevice corrosion.
Accelerated Low Water Corrosion (ALWC) is a particularly aggressive form of MIC that affects steel piles in seawater near the low water tide mark. It is characterised by an orange sludge, which smells of hydrogen sulphide when treated with acid. Corrosion rates can be very high and design corrosion allowances can soon be exceeded leading to premature failure of the steel pile. Piles that have been coating and have cathodic protection installed at the time of construction are not susceptible to ALWC. For unprotected piles, sacrificial anodes can be installed local to the affected areas to inhibit the corrosion or a complete retrofitted sacrificial anode system can be installed. Affected areas can also be treated electrochemically by using an electrode to first produce chlorine to kill the bacteria, and then to produced a calcareous deposit, which will help shield the metal from further attack.
The products of high temperature corrosion can potentially be turned to the advantage of the engineer. The formation of oxides on stainless steels, for example, can provide a protective layer preventing further atmospheric attack, allowing for a material to be used for sustained periods at both room and high temperature in hostile conditions. Such high temperature corrosion products in the form of compacted oxide layer glazes have also been shown to prevent or reduce wear during high temperature sliding contact of metallic (or metallic and ceramic) surfaces.
For effective CP, the potential of the steel surface is polarized
(pushed) more negative until the metal surface has a uniform potential.
With a uniform potential, the driving force for the corrosion reaction
is halted. For galvanic CP systems, the anode material corrodes under
the influence of the steel, and eventually it must be replaced. The
polarization is caused by the current flow from the anode to the
cathode, driven by the difference in electrochemical potential between
the anode and the cathode.
The US Federal Highway Administration released a study, entitled Corrosion Costs and Preventive Strategies in the United States,
in 2002 on the direct costs associated with metallic corrosion in
nearly every U.S. industry sector. The study showed that for 1998 the
total annual estimated direct cost of corrosion in the U.S. was
approximately $276 billion (approximately 3.2% of the US gross domestic
product).
Rust is one of the most common causes of bridge accidents. As rust has a much higher volume than the originating mass of iron, its build-up can also cause failure by forcing apart adjacent parts. It was the cause of the collapse of the Mianus river bridge in 1983, when the bearings rusted internally and pushed one corner of the road slab off its support. Three drivers on the roadway at the time died as the slab fell into the river below. The following NTSB investigation showed that a drain in the road had been blocked for road re-surfacing, and had not been unblocked so that runoff water penetrated the support hangers. It was also difficult for maintenance engineers to see the bearings from the inspection walkway. Rust was also an important factor in the Silver Bridge disaster of 1967 in West Virginia, when a steel suspension bridge collapsed in less than a minute, killing 46 drivers and passengers on the bridge at the time.
Similarly, corrosion of concrete-covered steel and iron can cause the concrete to spall, creating severe structural problems. It is one of the most common failure modes of reinforced concrete bridges. Measuring instruments based on the half-cell potential are able to detect the potential corrosion spots before total failure of the concrete structure is reached.
Polymer degradation is due to a wide array of complex and often
poorly-understood physiochemical processes. These are strikingly
different from the other processes discussed here, and so the term
"corrosion" is only applied to them in a loose sense of the word.
Because of their large molecular weight, very little entropy can be
gained by mixing a given mass of polymer with another substance, making
them generally quite difficult to dissolve. While dissolution is a
problem in some polymer applications, it is relatively simple to design
against. A more common and related problem is swelling, where
small molecules infiltrate the structure, reducing strength and
stiffness and causing a volume change. Conversely, many polymers
(notably flexible vinyl) are intentionally swelled with plasticizers,
which can be leached out of the structure, causing brittleness or other
undesirable changes. The most common form of degradation, however, is a
decrease in polymer chain length. Mechanisms which break polymer chains
are familiar to biologists because of their effect on DNA: ionizing
radiation (most commonly ultraviolet light), free radicals, and
oxidizers such as oxygen, ozone, and chlorine. Ozone cracking is a
well-known problem affecting natural rubber for example. Additives can
slow these process very effectively, and can be as simple as a
UV-absorbing pigment (i.e., titanium dioxide or carbon black). Plastic
shopping bags often do not include these additives so that they break
down more easily as litter.
In other words, corrosion is the wearing away of metals due to a chemical reaction.
Many structural alloys corrode merely from exposure to moisture in the air, but the process can be strongly affected by exposure to certain substances (see below). Corrosion can be concentrated locally to form a pit or crack, or it can extend across a wide area more or less uniformly corroding the surface. Because corrosion is a diffusion controlled process, it occurs on exposed surfaces. As a result, methods to reduce the activity of the exposed surface, such as passivation and chromate-conversion, can increase a material's corrosion resistance. However, some corrosion mechanisms are less visible and less predictable.
Galvanic corrosion
Galvanic corrosion
Galvanic corrosion occurs when two different metals and/or alloys
have physical or electrical contact with each other and are immersed in a
common electrolyte. In a galvanic couple, the more active metal (the
anode) corrodes at an accelerated rate and the more noble metal (the
cathode) corrodes at a retarded (؟) rate. When immersed separately, each
metal corrodes at its own rate. What type of metal(s) to use is readily
determined by following the galvanic series. For example, zinc is often
used as a sacrificial anode for steel structures. Galvanic corrosion is
of major interest to the marine industry and also anywhere water (via
impurities such as salt) contacts pipes or metal structures.Factors such as relative size of anode, types of metal, and operating conditions (temperature, humidity, salinity, etc.) affect galvanic corrosion. The surface area ratio of the anode and cathode directly affects the corrosion rates of the materials. Galvanic corrosion is often utilized in sacrificial anodes.
Galvanic series
Galvanic series
In a given environment (one standard medium is aerated, room-temperature seawater), one metal will be either more noble or more active
than the next, based on how strongly its ions are bound to the surface.
Two metals in electrical contact share the same electrons, so that the
"tug-of-war" at each surface is analogous to competition for free
electrons between the two materials. Using the electrolyte as a host for
the flow of ions in the same direction; the noble metal will take
electrons from the active one. The resulting mass flow or electrical
current can be measured to establish a hierarchy of materials in the
medium of interest. This hierarchy is called a galvanic series, and can be a very useful in predicting and understanding corrosion.Corrosion removal
Often it is possible to chemically remove the products of corrosion to give a clean surface, but one that may exhibit artifacts of corrosion such as pitting. For example phosphoric acid in the form of naval jelly is often applied to ferrous tools or surfaces to remove rust.Corrosion removal should not be confused with Electropolishing which removes some layers of the underlying metal to make a smooth surface. For example phosphoric acid (again) may be used to electropolish copper but it does this by removing copper, not the products of copper corrosion.
Resistance to corrosion
Some metals are more intrinsically resistant to corrosion than others, either due to the fundamental nature of the electrochemical processes involved or due to the details of how reaction products form. For some examples, see galvanic series. If a more susceptible material is used, many techniques can be applied during an item's manufacture and use to protect its materials from damage.Intrinsic chemistry
Some metals have naturally slow reaction kinetics, even though their corrosion is thermodynamically favorable. These include such metals as zinc, magnesium, and cadmium. While corrosion of these metals is continuous and ongoing, it happens at an acceptably slow rate. An extreme example is graphite, which releases large amounts of energy upon oxidation, but has such slow kinetics that it is effectively immune to electrochemical corrosion under normal conditions.
Passivation
Passivation
Passivation refers to the spontaneous formation of an ultra-thin film
of corrosion products known as passive film, on the metal's surface
that act as a barrier to further oxidation. The chemical composition and
microstrucure of a passive film are different from the underlying
metal. Typical passive film thickness on aluminum, stainless steels and
alloys is within 10 nanometers. The passive film is different from oxide
layer or scale that are frequently formed at high temperatures and are
in the micrometer thickness range. The passive film has the unique
property of self-healing while the oxide layer or oxide scale does not.
For example, when you scratch the surface of a stainless steel, the
damaged passive film will be healed spontaneously by the instantaneous
oxidation of chromium from the underlying metal. Passivation in natural
environments such as air, water and soil at moderate pH is seen in such
materials as aluminium, stainless steel, titanium, and silicon.Passivation is primarily determined by metallurgical and environmental factors. The effect of pH is recorded using Pourbaix diagrams, but many other factors are influential. Some conditions that inhibit passivation include: high pH for aluminium and zinc, low pH or the presence of chloride ions for stainless steel, high temperature for titanium (in which case the oxide dissolves into the metal, rather than the electrolyte) and fluoride ions for silicon. On the other hand, sometimes unusual conditions can bring on passivation in materials that are normally unprotected, as the alkaline environment of concrete does for steel rebar. Exposure to a liquid metal such as mercury or hot solder can often circumvent passivation mechanisms.
Corrosion in passivated materials
Passivation is extremely useful in mitigating corrosion damage, however even a high-quality alloy will corrode if its ability to form a passivating film is hindered. Proper selection of the right grade of material for the specific environment is important for the long-lasting performance of this group of materials. If breakdown occurs in the passive film due to chemical or mechanical factors, the resulting major modes of corrosion may include pitting corrosion, crevice corrosion and stress corrosion cracking.Pitting corrosion
Pitting corrosion
Weld decay and knifeline attack
Intergranular corrosion
Stainless steel can pose special corrosion challenges, since its
passivating behavior relies on the presence of a major alloying
component (Chromium, at least 11.5%). Due to the elevated temperatures
of welding or during improper heat treatment, chromium carbides can form
in the grain boundaries of stainless alloys. This chemical reaction
robs the material of chromium in the zone near the grain boundary,
making those areas much less resistant to corrosion. This creates a
galvanic couple with the well-protected alloy nearby, which leads to weld decay (corrosion of the grain boundaries in the heat affected zones) in highly corrosive environments.A stainless steel is said to be sensitized if chromium carbides are formed in the microstructure. A typical microstructure of a normalized type 304 stainless steel shows no signs of sensitization while a heavily sensitized steel shows the presence of grain boundary precipitates. The dark lines in the sensitized microstructure are networks of chromium carbides formed along the grain boundaries.
Special alloys, either with low carbon content or with added carbon "getters" such as titanium and niobium (in types 321 and 347, respectively), can prevent this effect, but the latter require special heat treatment after welding to prevent the similar phenomenon of knifeline attack. As its name implies, corrosion is limited to a very narrow zone adjacent to the weld, often only a few micrometres across, making it even less noticeable.
Crevice corrosion
Crevice corrosion
This photo shows that corrosion occurred in the crevice between the tube and tube sheet (both made of type 316 stainless steel) of a heat exchanger in a sea water desalination plant.
Crevice corrosion is influenced by the crevice type (metal-metal, metal-nonmetal), crevice geometry (size, surface finish), and metallurgical and environmental factors. The susceptibility to crevice corrosion can be evaluated with ASTM standard procedures. A critical crevice corrosion temperature (CCT) is commonly used to rank a material's resistance to crevice corrosion.
Microbial corrosion
Microbial corrosion
Microbial corrosion, or commonly known as microbiologically
influenced corrosion (MIC), is a corrosion caused or promoted by
microorganisms, usually chemoautotrophs. It can apply to both metallic
and non-metallic materials, in the presence or absence of oxygen.
Sulfate-reducing bacteria are active in the absence of oxygen
(anaerobic); they produce hydrogen sulfide, causing sulfide stress
cracking. In the presence of oxygen (aerobic), some bacteria may
directly oxidize iron to iron oxides and hydroxides, other bacteria
oxidize sulfur and produce sulfuric acid causing biogenic sulfide
corrosion. Concentration cells can form in the deposits of corrosion
products, leading to localized corrosion.Accelerated Low Water Corrosion (ALWC) is a particularly aggressive form of MIC that affects steel piles in seawater near the low water tide mark. It is characterised by an orange sludge, which smells of hydrogen sulphide when treated with acid. Corrosion rates can be very high and design corrosion allowances can soon be exceeded leading to premature failure of the steel pile. Piles that have been coating and have cathodic protection installed at the time of construction are not susceptible to ALWC. For unprotected piles, sacrificial anodes can be installed local to the affected areas to inhibit the corrosion or a complete retrofitted sacrificial anode system can be installed. Affected areas can also be treated electrochemically by using an electrode to first produce chlorine to kill the bacteria, and then to produced a calcareous deposit, which will help shield the metal from further attack.
High temperature corrosion
High temperature corrosion is chemical deterioration of a material (typically a metal) under very high temperature conditions. This non-galvanic form of corrosion can occur when a metal is subject to a high temperature atmosphere containing oxygen, sulfur or other compounds capable of oxidising (or assisting the oxidation of) the material concerned. For example, materials used in aerospace, power generation and even in car engines have to resist sustained periods at high temperature in which they may be exposed to an atmosphere containing potentially highly corrosive products of combustion.The products of high temperature corrosion can potentially be turned to the advantage of the engineer. The formation of oxides on stainless steels, for example, can provide a protective layer preventing further atmospheric attack, allowing for a material to be used for sustained periods at both room and high temperature in hostile conditions. Such high temperature corrosion products in the form of compacted oxide layer glazes have also been shown to prevent or reduce wear during high temperature sliding contact of metallic (or metallic and ceramic) surfaces.
Methods of protection from corrosion
Surface treatments
Applied coatings
Galvanization
Plating, painting, and the application of enamel are the most common
anti-corrosion treatments. They work by providing a barrier of
corrosion-resistant material between the damaging environment and the
(often cheaper, tougher, and/or easier-to-process) structural material.
Aside from cosmetic and manufacturing issues, there are tradeoffs in
mechanical flexibility versus resistance to abrasion and high
temperature. Platings usually fail only in small sections, and if the
plating is more noble than the substrate (for example, chromium on
steel), a galvanic couple will cause any exposed area to corrode much
more rapidly than an unplated surface would. For this reason, it is
often wise to plate with active metal such as zinc or cadmium. Painting
either by roller or brush is more desirable for tight spaces; spray
would be better for larger coating areas such as steel decks and
waterfront applications. Flexible polyurethane coatings, like
Durabak-M26 for example, can provide an anti-corrosive seal with a
highly durable slip resistant membrane. Painted coatings are relatively
easy to apply and have fast drying times although temperature and
humidity may cause dry times to vary.Reactive coatings
If the environment is controlled (especially in recirculating systems), corrosion inhibitors can often be added to it. These form an electrically insulating and/or chemically impermeable coating on exposed metal surfaces, to suppress electrochemical reactions. Such methods obviously make the system less sensitive to scratches or defects in the coating, since extra inhibitors can be made available wherever metal becomes exposed. Chemicals that inhibit corrosion include some of the salts in hard water (Roman water systems are famous for their mineral deposits), chromates, phosphates, polyaniline, other conducting polymers and a wide range of specially-designed chemicals that resemble surfactants (i.e. long-chain organic molecules with ionic end groups).Anodization
Anodising
Aluminium alloys often undergo a surface treatment. Electrochemical
conditions in the bath are carefully adjusted so that uniform pores
several nanometers wide appear in the metal's oxide film. These pores
allow the oxide to grow much thicker than passivating conditions would
allow. At the end of the treatment, the pores are allowed to seal,
forming a harder-than-usual surface layer. If this coating is scratched,
normal passivation processes take over to protect the damaged area.
Anodizing is very resilient to weathering and corrosion, so it is
commonly used for building facades and other areas that the surface will
come into regular contact with the elements. Whilst being resilient, it
must be cleaned frequently. If left without cleaning Panel Edge
Staining will naturally occur.Biofilm coatings
A new form of protection has been developed by applying certain species of bacterial films to the surface of metals in highly corrosive environments. This process increases the corrosion resistance substantially. Alternatively, antimicrobial-producing biofilms can be used to inhibit mild steel corrosion from sulfate-reducing bacteria.Controlled permeability formwork
Controlled permeability formwork
Controlled permeability formwork (CPF) is a method of preventing the
corrosion of reinforcement by naturally enhancing the durability of the
cover during concrete placement. CPF has been used in environments to
combat the effects of carbonation, chlorides, frost and abrasion.Cathodic protection
Cathodic protection
Cathodic protection (CP) is a technique to control the corrosion of a
metal surface by making that surface the cathode of an electrochemical
cell. Cathodic protection systems are most commonly used to protect
steel, water, and fuel pipelines and tanks; steel pier piles, ships, and
offshore oil platforms.Sacrificial anode protection
Impressed current cathodic protection
For larger structures, galvanic anodes cannot economically deliver enough current to provide complete protection. Impressed Current Cathodic Protection (ICCP) systems use anodes connected to a DC power source (such as a cathodic protection rectifier). Anodes for ICCP systems are tubular and solid rod shapes of various specialized materials. These include high silicon cast iron, RUST, mixed metal oxide or platinum coated titanium or niobium coated rod and wires.Anodic protection
Anodic protection
Anodic protection impresses anodic current on the structure to be
protected (opposite to the cathodic protection). It is appropriate for
metals that exhibit passivity (e.g., stainless steel) and suitably small
passive current over a wide range of potentials. It is used in
aggressive environments, e.g., solutions of sulfuric acid.Economic impact
Rust is one of the most common causes of bridge accidents. As rust has a much higher volume than the originating mass of iron, its build-up can also cause failure by forcing apart adjacent parts. It was the cause of the collapse of the Mianus river bridge in 1983, when the bearings rusted internally and pushed one corner of the road slab off its support. Three drivers on the roadway at the time died as the slab fell into the river below. The following NTSB investigation showed that a drain in the road had been blocked for road re-surfacing, and had not been unblocked so that runoff water penetrated the support hangers. It was also difficult for maintenance engineers to see the bearings from the inspection walkway. Rust was also an important factor in the Silver Bridge disaster of 1967 in West Virginia, when a steel suspension bridge collapsed in less than a minute, killing 46 drivers and passengers on the bridge at the time.
Similarly, corrosion of concrete-covered steel and iron can cause the concrete to spall, creating severe structural problems. It is one of the most common failure modes of reinforced concrete bridges. Measuring instruments based on the half-cell potential are able to detect the potential corrosion spots before total failure of the concrete structure is reached.
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