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Corrosion

2007 Schools Wikipedia Selection. Related subjects: Engineering

                                                  Mechanical failure modes
                                                                  Buckling
                                                                 Corrosion
                                                                     Creep
                                                                   Fatigue
                                                                  Fracture
                                                                   Melting
                                                             Thermal shock
                                                                      Wear

   Corrosion is deterioration of essential properties in a material due to
   reactions with its environment. It is the loss of an electron of metals
   reacting with water or oxygen. Weakening of iron due to oxidation of
   the iron atoms is a well-known example of electrochemistry (a branch of
   chemistry that studies the reactions that take place when an ionic and
   electronic conductor interfere) corrosion. This is commonly known as
   rust. This type of damage usually affects metallic materials, and
   typically produces oxide(s) and/or salt(s) of the original metal.
   Corrosion also includes the dissolution of ceramic materials and can
   refer to discoloration and weakening of polymers by the sun's
   ultraviolet light.

   Most 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 to produce general
   deterioration. While some efforts to reduce corrosion merely redirect
   the damage into less visible, less predictable forms, controlled
   corrosion treatments such as passivation and chromate-conversion will
   increase a material's corrosion resistance.
   Rust, the most familiar example of corrosion.
   Enlarge
   Rust, the most familiar example of corrosion.

Corrosive substances

   Corrosive warning symbol
   Enlarge
   Corrosive warning symbol

   Corrosive chemicals include the following classes:
     * Acids
     * Bases ("caustics" or "alkalis")
     * Dehydrating agents such as phosphorous pentoxide and calcium oxide
     * Halogens and halogen salts such as bromine, iodine, zinc chloride,
       and sodium hypochlorite
     * Organic halides and organic acid halides such as acetyl chloride
       and benzyl chloroformate
     * Acid anhydrides
     * Some organic materials such as phenol ("carbolic acid)

Corrosion in nonmetals

   Most ceramic materials are almost entirely immune to corrosion. The
   strong ionic and/or covalent 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.

   The degradation of polymeric materials 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. 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.

   The remainder of this article is about electrochemical corrosion.

Electrochemical theory

   One way to understand the structure of metals on the basis of particles
   is to imagine an array of positively-charged ions sitting in a
   negatively-charged " gas" of free electrons. Coulombic attraction holds
   these oppositely-charged particles together, but there are other sorts
   of negative charge which are also attracted to the metal ions, such as
   the negative ions ( anions) in an electrolyte. For a given ion at the
   surface of a metal, there is a certain amount of energy to be gained or
   lost by dissolving into the electrolyte or becoming a part of the
   metal, which reflects an atom-scale tug-of-war between the electron gas
   and dissolved anions. The quantity of energy then strongly depends on a
   host of variables, including the types of ions in a solution and their
   concentrations, and the number of electrons present at the metal's
   surface. In turn, corrosion processes cause electrochemical changes,
   meaning that they strongly affect all of these variables. The overall
   interaction between electrons and ions tends to produce a state of
   local thermodynamic equilibrium that can often be described using basic
   chemistry and a knowledge of initial conditions.

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 electron gas,
   so that the tug-of-war at each surface is translated into a competition
   for free electrons between the two materials. The noble metal will tend
   to take electrons from the active one, while the electrolyte hosts a
   flow of ions in the same direction. 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

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

   Gold nuggets do not corrode, even on a geological time scale.
   Enlarge
   Gold nuggets do not corrode, even on a geological time scale.

   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

   Given the right conditions, a thin film of corrosion products can form
   on a metal's surface spontaneously, acting as a barrier to further
   oxidation. When this layer stops growing at less than a micrometre
   thick under the conditions that a material will be used in, the
   phenomenon is known as passivation (rust, for example, usually grows to
   be much thicker, and so is not considered passivation, because this
   mixed oxidized layer is not protective). While this effect is in some
   sense a property of the material, it serves as an indirect kinetic
   barrier: the reaction is often quite rapid unless and until an
   impermiable layer forms. Passivation in air and water at moderate pH is
   seen in such materials as aluminium, stainless steel, titanium, and
   silicon.

   These conditions required for passivation are specific to the material.
   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, 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.

Surface treatments

   Galvanized surface
   Enlarge
   Galvanized surface

Applied coatings

   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 a more active metal such as zinc or cadmium.

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, and a wide range of
   specially-designed chemicals that resemble surfactants (i.e. long-chain
   organic molecules with ionic end groups).
   This figure-8 descender is annodized with a yellow finish. Climbing
   equipment is available in a wide range of anodized colors.
   Enlarge
   This figure-8 descender is annodized with a yellow finish. Climbing
   equipment is available in a wide range of anodized colors.

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.

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.

   It is a method used to protect metal structures from corrosion.
   Cathodic protection systems are most commonly used to protect steel,
   water, and fuel pipelines and tanks; steel pier piles, ships, and
   offshore oil platforms.

   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.

   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 (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.

Corrosion in passivated materials

   Passivation is extremely useful in alleviating corrosion damage, but
   care must be taken not to trust it too thoroughly. Even a high-quality
   alloy will corrode if its ability to form a passivating film is
   compromised. Because the resulting modes of corrosion are more exotic
   and their immediate results are less visible than rust and other bulk
   corrosion, they often escape notice and cause problems among those who
   are not familiar with them.

Pitting corrosion

   Certain conditions, such as low availability of oxygen or high
   concentrations of species such as chloride which compete 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. In extreme
   cases, the sharp tips of extremely long and narrow pits can cause
   stress concentration to the point that otherwise tough alloys can
   shatter, or 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, which often includes ensuring that the material is exposed
   to oxygen uniformly (i.e., eliminating crevices).

Fretting

   Many useful passivating oxides are also effective abrasives,
   particularly TiO[2] and Al[2]O[3]. Fretting corrosion occurs when
   particles of corrosion product continuously abrade away the passivating
   film as two metal surfaces are rubbed together. While this process does
   often damage the frets of musical instruments, they were named
   separately.

Weld decay and knifeline attack

   Stainless steel can pose special corrosion challenges, since its
   passivating behaviour relies on the presence of a minor alloying
   component (Chromium, typically only 18%). 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 near
   welds) in highly corrosive environments. 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 applies, this
   is limited to a small zone, often only a few micrometres across, which
   causes it to proceed more rapidly. This zone is very near the weld,
   making it even less noticeable^1.

Microbial corrosion

   Microbial corrosion, or bacterial corrosion, is a corrosion caused or
   promoted by microorganisms, usually chemoautotrophs. It can apply to
   both metals and non-metallic materials, in both the presence and lack
   of oxygen. Sulfate-reducing bacteria are common in lack of oxygen; they
   produce hydrogen sulfide, causing sulfide stress cracking. In presence
   of oxygen, some bacteria directly oxidize iron to iron oxides and
   hydroxides, other bacteria oxidize sulfur and produce sulfuric acid.
   Concentration cells can form in the deposits of corrosion products,
   causing and enhancing galvanic corrosion.

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, sulphur 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.

Economic impact

   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 3.1% of the US gross domestic product). FHWA
   Report Number:FHWA-RD-01-156. The NACE International website has a
   summary slideshow of the report findings. Jones^1 writes that
   electrochemical corrosion causes between $8 billion and $128 billion in
   economic damage per year in the United States alone, degrading
   structures, machines, and containers.
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