Corrosion is the gradual destruction of materials
(usually metals)
by chemical reaction with its environment.
In the most common use of the word, this means
electrochemical oxidation of metals in reaction with an oxidant such as oxygen. Rusting, the
formation of iron
oxides, is a well-known example of electrochemical corrosion. This type of
damage typically produces oxide(s) 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. Corrosion
degrades the useful properties of materials and structures including strength,
appearance and permeability to liquids and gases.
Many structural alloys corrode merely from exposure to moisture in air, but the
process can be strongly affected by exposure to certain substances. 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 of aluminium. A 5-mm-thick aluminium
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 steel. Perforation of aluminium plate occurred
within 2 years.
Galvanic corrosion occurs when two different metals have
physical or electrical contact with each other and are immersed in a common electrolyte,
or when the same metal is exposed to electrolyte with different concentrations.
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 (containing salts) 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
prevented by the use of sacrificial anodes.
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 others, 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 is useful in predicting and
understanding corrosion. This method is expensive but offers maximum protection
against corrosion.
Corrosion removal
Often it is possible to chemically remove the products of
corrosion. 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 may also 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 (for some examples, see galvanic
series). There are various ways of protecting metals from corrosion
(oxidation) including painting, hot dip galvanizing, and combinations of these.
Intrinsic chemistry
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 have long been valued. 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
Passivation refers to the spontaneous formation of an
ultrathin 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 microstructure of a passive film are different from the underlying metal.
Typical passive film thickness on aluminium, stainless steels and alloys is
within 10 nanometers. The passive film is different from oxide layers that are
formed upon heating and are in the micrometer thickness range – the passive
film recovers if removed or damaged whereas the oxide layer does not.
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 summarized 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, unusual conditions may result in passivation of
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
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 autocatalytic 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.
Pitting results when a small hole, or cavity, forms in the
metal, usually as a result of de-passivation of a small area. This area becomes
anodic, while part of the remaining metal becomes cathodic, producing a
localized galvanic reaction. The deterioration of this small area penetrates
the metal and can lead to failure. This form of corrosion is often difficult to
detect due to the fact that it is usually relatively small and may be covered
and hidden by corrosion-produced compounds.
Weld decay and knifeline attack
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%). Because of the elevated temperatures of welding and 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 micrometers across, making it even less noticeable.
Crevice corrosion
Corrosion in the crevice between the tube and tube sheet
(both made of type 316 stainless steel) of a heat
exchanger in a seawater desalination plant.
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. Formation of a differential
aeration cell leads to 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.
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 is commonly used to rank a material's resistance to
crevice 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 characterized by an orange sludge, which smells of hydrogen
sulfide 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) as a result of heating. This non-galvanic form of
corrosion can occur when a metal is subjected to a hot atmosphere containing
oxygen, sulfur or other compounds capable of oxidizing (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 temperatures in hostile conditions. Such
high-temperature corrosion products, in the form of compacted oxide layer glazes, prevent
or reduce wear during high-temperature sliding contact of metallic (or metallic
and ceramic) surfaces.
Metal dusting
Metal dusting is a catastrophic form of corrosion
that occurs when susceptible materials are exposed to environments with high
carbon activities, such as synthesis gas and other high-CO environments. The
corrosion manifests itself as a break-up of bulk metal to metal powder. The
suspected mechanism is firstly the deposition of a graphite layer on the
surface of the metal, usually from carbon monoxide (CO) in the vapour phase.
This graphite layer is then thought to form metastable M3C species
(where M is the metal), which migrate away from the metal surface. However, in
some regimes no M3C species is observed indicating a direct transfer
of metal atoms into the graphite layer.
Protection from corrosion
The US Army shrink
wraps equipment such as helicopters to protect them from corrosion and thus
save millions of dollars.
Surface treatments
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 structural material. Aside from cosmetic and manufacturing issues, there
may be tradeoffs in mechanical flexibility versus resistance to abrasion and
high temperature. Platings usually fail only in small sections, but 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 chemicals form an electrically insulating or chemically impermeable
coating on exposed metal surfaces, to suppress electrochemical reactions. Such
methods 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
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 (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 (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
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.
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,
graphite, mixed metal oxide or platinum coated titanium or niobium coated rod
and wires.
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.
Rate of corrosion
A simple test for measuring corrosion is the weight loss
method.The method involves exposing a clean weighed piece of the metal or alloy
to the corrosive environment for a specified time followed by cleaning to
remove corrosion products and weighing the piece to determine the loss of
weight. The rate of corrosion (R) is calculated as
R = KW/(ρAt)
where k is a constant, W is the weight loss of
the metal in time t, A is the surface area of the metal exposed,
and ρ is the density of the metal (in g/cm³).
Economic impact
In 2002, the US Federal Highway Administration
released a study titled Corrosion Costs and Preventive Strategies in the
United States on the direct costs associated with metallic corrosion in the
U.S. industry. In 1998, the total annual direct cost of corrosion in the U.S.
was ca. $276 billion (ca. 3.2% of the US gross domestic product). Broken down into
five specific industries, the economic losses are $22.6 billion in
infrastructure; $17.6 billion in production and manufacturing; $29.7 billion in
transportation; $20.1 billion in government; and $47.9 billion in utilities.
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; as a result, runoff water penetrated
the support hangers. Rust was also an important factor in the Silver
Bridge disaster of 1967 in West
Virginia, when a steel suspension
bridge collapsed within 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 can detect the potential corrosion spots before total failure of
the concrete structure is reached.
Until 20–30 years ago, galvanized steel pipe was used
extensively in the potable water systems for single and multi-family residents
as well as commercial and public construction. Today, these systems have long
ago consumed the protective zinc and are corroding internally resulting in poor
water quality and pipe failures. The economic impact on homeowners, condo
dwellers, and the public infrastructure is estimated at 22 billion dollars as
the insurance industry braces for a wave of claims due to pipe failures.
Corrosion in nonmetals
Most ceramic materials are almost entirely immune to corrosion.
The strong chemical bonds that hold them together leave very
little free chemical energy in the structure; they can be thought of as already
corroded. When corrosion does occur, it is almost always a simple dissolution
of the material or chemical reaction, rather than an electrochemical process. A
common example of corrosion protection in ceramics is the lime
added to soda-lime glass
to reduce its solubility in water; though it is not nearly as soluble as pure sodium
silicate, normal glass does form sub-microscopic flaws when exposed to
moisture. Due to its brittleness, such flaws cause a dramatic reduction in the
strength of a glass object during its first few hours at room temperature.
Corrosion of polymers
Polymer degradation involves several 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.
Corrosion of glasses
Glass disease is the corrosion of silicate glasses in
aqueous
solutions. It is governed by two mechanisms: diffusion-controlled leaching (ion exchange) and hydrolytic
dissolution of the glass network. Both mechanisms strongly depend on the pH of
contacting solution: the rate of ion exchange decreases with pH as 10−0.5pH
whereas the rate of hydrolytic dissolution increases with pH as 100.5pH.
Mathematically, corrosion rates of glasses are characterized
by normalized corrosion rates of elements NRi (g/cm2·d)
which are determined as the ratio of total amount of released species into the
water Mi (g) to the water-contacting surface area S (cm2),
time of contact t (days) and weight fraction content of the element in the
glass fi:
The overall corrosion rate is a sum of contributions from
both mechanisms (leaching + dissolution) NRi=Nrxi+NRh.
Diffusion-controlled leaching (ion exchange) is characteristic of the initial
phase of corrosion and involves replacement of alkali ions in the glass by a
hydronium (H3O+) ion from the solution. It causes an
ion-selective depletion of near surface layers of glasses and gives an inverse
square root dependence of corrosion rate with exposure time. The diffusion-controlled
normalized leaching rate of cations from glasses (g/cm2·d) is given
by:,
where t is time, Di is the i-th cation effective
diffusion coefficient (cm2/d), which depends on pH of contacting
water as Di = Di0·10–pH, and ρ is the density
of the glass (g/cm3).
Glass network dissolution is characteristic of the later
phases of corrosion and causes a congruent release of ions into the water
solution at a time-independent rate in dilute solutions (g/cm2·d):
where rh is the stationary hydrolysis
(dissolution) rate of the glass (cm/d). In closed systems the consumption of
protons from the aqueous phase increases the pH and causes a fast transition to
hydrolysis. However, a further saturation of solution with silica impedes
hydrolysis and causes the glass to return to an ion-exchange, e.g.
diffusion-controlled regime of corrosion.
In typical natural conditions normalized corrosion rates of
silicate glasses are very low and are of the order of 10−7–10−5
g/(cm2·d). The very high durability of silicate glasses in water
makes them suitable for hazardous and nuclear waste immobilisation.
Glass corrosion tests
There exist numerous standardized procedures for measuring
the corrosion (also called chemical durability) of glasses in neutral,
basic, and acidic environments, under simulated environmental conditions, in
simulated body fluid, at high temperature and pressure, and under other
conditions.
The standard procedure ISO 719 describes a test of the
extraction of water-soluble basic compounds under neutral conditions: 2 g of
glass, particle size 300–500 μm, is kept for 60 min in 50 ml de-ionized water
of grade 2 at 98 °C; 25 ml of the obtained solution is titrated against
0.01 mol/l HCl solution. The volume of HCl required for
neutralization is classified according to the table below.
|
Amount of 0.01M HCl needed to neutralize extracted
basic oxides, ml
|
Extracted Na2O
equivalent, μg |
Hydrolytic
class |
|
< 0.1
|
< 31
|
1
|
|
0.1-0.2
|
31-62
|
2
|
|
0.2-0.85
|
62-264
|
3
|
|
0.85-2.0
|
264-620
|
4
|
|
2.0-3.5
|
620-1085
|
5
|
|
> 3.5
|
> 1085
|
> 5
|
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