Difference between revisions of "Corrosion" - New World Encyclopedia

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[[Image:2006-01-14 Rust and dirt.jpg|thumb|right|300px|Rust, the most familiar example of corrosion.]]
 
[[Image:2006-01-14 Rust and dirt.jpg|thumb|right|300px|Rust, the most familiar example of corrosion.]]
  
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'''Corrosion''' is the deterioration of a material's essential properties as a result of reactions with its [[environment]]. The rusting of iron is a well-known example of corrosion. Other [[metal]]s may be similarly damaged, typically producing their [[oxide]]*s, [[hydroxide]]*s, and [[salt]]s. Corrosion also refers to the dissolution of [[ceramic]] materials or the discoloration and weakening of [[polymer]]s affected by the [[Sun]]'s [[ultraviolet]] light. A substance that causes corrosion is called a '''corrosive substance''', or simply, a '''corrosive''' (as a noun). A corrosive can damage living tissue as well as inanimate materials.
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'''Corrosion''' is the deterioration of a material's essential properties as a result of reactions with its [[environment]]. It can be concentrated locally to form a pit or crack, or it can extend across a wide area to produce general deterioration. Many materials corrode merely upon exposure to air, moisture, and sunlight, but the process can be accelerated by exposure to certain chemicals. A substance that causes corrosion is called a '''corrosive substance,''' or simply, a '''corrosive.''' A corrosive can damage living tissue as well as inanimate materials.
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* It is the loss of an electron of [[metal]]s reacting with [[water]] or [[oxygen]]. Weakening of [[iron]] due to oxidation of the iron [[atom]]s 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.
 
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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 [[Corrosion#Pitting corrosion|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 coating|chromate-conversion]] will increase a material's corrosion resistance.
 
  
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The [[rust]]ing of iron is a well-known example of corrosion. Other [[metal]]s may be similarly damaged, typically producing their [[oxide]]s, [[hydroxide]]s, and [[salt]]s. Corrosion also refers to the dissolution of [[ceramic]] materials or the discoloration and weakening of [[polymer]]s, such as by exposure to [[ultraviolet]] light from the [[Sun]].
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{{Toc}}
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Corrosion of a material can be controlled by applying a coating, such as by [[electroplating]] or [[paint]]ing, or by specialized techniques such as [[#Anodization|anodization]] or [[#Cathodic protection|cathodic protection]]. The annual cost of corrosion has been estimated to be on the order of hundreds of billions of dollars in the United States alone.
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[[Image:Rusted floorboards.jpg|thumb|300px|Rust damage in automobiles can create hidden dangers.]]
 
==Corrosive substances==
 
==Corrosive substances==
 
[[Image:Dangclass8.png|thumb|Warning symbol for a corrosive.]]
 
[[Image:Dangclass8.png|thumb|Warning symbol for a corrosive.]]
  
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Corrosive chemicals can be solids, liquids, or gases, and they can belong to any of several classes of materials, such as the following:
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Corrosive chemicals can be solids, liquids, or gases, and they can belong to any of several classes of materials. The following are some examples.
  
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* [[Acid]]s
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* [[Acid]]s, such as [[sulfuric acid]], nitric acid, and hydrochloric acid
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* [[Base (chemistry)|Base]]s ("caustics" or "alkalis")
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* [[Base (chemistry)|Base]]s ("caustics" or "alkalis"), such as sodium hydroxide and potassium hydroxide
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* Dehydrating agents, such as [[phosphorus]] [[pentoxide]]* or [[calcium oxide]]*
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* Dehydrating agents, such as [[phosphorus]] [[pentoxide]] or [[calcium oxide]]
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* [[Halogen]]s and certain halogen salts, such as [[bromine]], [[iodine]], [[zinc chloride]]*, and [[sodium hypochlorite]]*
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* [[Halogen]]s—such as [[bromine]] or [[iodine]]—and certain halogen salts, such as [[zinc chloride]] or [[sodium hypochlorite]]
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* Organic halides and organic acid halides, such as [[acetyl chloride]]* and [[benzyl chloroformate]]*
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* Organic halides and organic acid halides, such as [[acetyl chloride]] and [[benzyl chloroformate]]
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* Acid [[anhydride]]*s
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* Acid [[anhydride]]s, such as acetic anhydride
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* Some organic materials, such as [[phenol]]* ("carbolic acid").
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* Some organic materials, such as [[phenol]] ("carbolic acid").
  
 
==Corrosion of nonmetals==
 
==Corrosion of nonmetals==
  
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Most [[ceramic]] materials are almost entirely immune to corrosion.  
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Most [[ceramic]] materials are nearly immune to corrosion. Their atoms are bound together by strong ionic or covalent bonds that resist disruption. When corrosion does occur, it is almost always a simple dissolution of the material or a chemical reaction. By contrast, the corrosion of metals is an [[#Electrochemical corrosion of metals|electrochemical]] process, as discussed below.
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A common example of corrosion protection in ceramics is the lime (calcium oxide) added to soda-lime [[glass]] to reduce its solubility in water. Though it is not nearly as soluble as pure [[sodium silicate]], normal glass forms submicroscopic flaws when exposed to moisture. Given the [[brittle]]ness of glass, such flaws dramatically reduce the strength of a glass object during its first few hours at room temperature.
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In the case of [[polymer|polymeric]] materials, their degradation can be caused by any of a wide array of complex and often poorly understood physicochemical processes. These are strikingly different from the other processes discussed below, so the term "corrosion" is applied to them in just a loose sense of the word.
  
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The strong ionic or covalent bonds that hold their atoms together leave very little free chemical energy in the structure. When corrosion does occur, it is almost always a simple dissolution of the material or chemical reaction, rather than an electrochemical process.
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Given their high molecular weight, polymers are generally quite difficult to dissolve. In those instances where dissolution is a problem, it is relatively simple to design against. A more common and related problem is "swelling," in which small molecules infiltrate the structure, reducing strength and stiffness and causing a volume change. Conversely, many polymers (notably flexible [[polyvinyl chloride|vinyl]]) are intentionally swollen with [[plasticizer]]s, and when these get leached out, the structure becomes brittle or undergoes other undesirable changes.
  
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A common example of corrosion protection in ceramics is the lime (calcium oxide) added to soda-lime [[glass]] to reduce its solubility in water. Though it is not nearly as soluble as pure [[sodium silicate]]*, normal glass forms submicroscopic flaws when exposed to moisture. Given the [[brittle]]*ness of glass, such flaws dramatically reduce the strength of a glass object during its first few hours at room temperature.
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The most common form of degradation of polymers is a decrease in chain length. In the case of [[DNA]], agents that break their chains include [[ionizing radiation]] (most commonly [[ultraviolet]] light), [[Radical (chemistry)|free radical]]s, and [[redox|oxidizer]]s (such as [[oxygen]], [[ozone]], and [[chlorine]]). [[Additive]]s can slow these processes effectively, and they can be as simple as a UV-absorbing [[pigment]] (such as [[titanium dioxide]] or [[carbon black]]). [[Plastic shopping bag]]s often do not include these additives, so they can break down more easily as [[litter]].
  
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The degradation of [[polymer|polymeric]] materials is due to a wide array of complex and often poorly understood physicochemical 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 high 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.
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==Electrochemical corrosion of metals==
  
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Dissolution is a problem in some polymer applications, but 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 [[polyvinyl chloride|vinyl]]) are intentionally swelled with [[plasticizer]]s, which can be leached out of the structure, causing brittleness or other undesirable changes.
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Consider a metal in contact with an electrolyte—that is, a medium that can conduct electricity by allowing the flow of ions. Corrosion of the metal involves electrochemical changes. In other words, the metal is chemically changed by a process that involves the flow of an electric current, by the movement of electrons and ions.
  
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The most common form of degradation, however, is a decrease in polymer chain length. Mechanisms that break polymer chains are familiar to biologists because of their effect on [[DNA]]: [[ionizing radiation]] (most commonly [[ultraviolet]] light), [[Radical (chemistry)|free radical]]s, and [[redox|oxidizer]]s such as [[oxygen]], [[ozone]], and [[chlorine]]. [[Additive]]s can slow these process very effectively, and can be as simple as a UV-absorbing [[pigment]] (such as [[titanium dioxide]] or [[carbon black]]). [[Plastic shopping bag]]s often do not include these additives, so that they break down more easily as [[litter]].
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A good example is the rusting of iron or steel. When iron comes in contact with water (especially water with a high concentration of ions), an electrochemical corrosion process begins. This process can be summarized as follows.
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# Iron (Fe) atoms on the surface release electrons to form iron(II) ions (Fe<sup>2+</sup>)&mdash;a process called oxidation
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# The electrons released travel to the edges of the water droplet, where there is plenty of dissolved oxygen. They combine with the oxygen and water to form hydroxide ions (OH<sup>−</sup>).
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# The hydroxide ions react with iron(II) ions to form iron(II) hydroxide (Fe(OH)<sub>2</sub>)
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# The iron(II) hydroxide in turn reacts with more dissolved oxygen to form hydrated iron(III) oxide (Fe<sub>2</sub>O<sub>3</sub>.''x''H<sub>2</sub>O)
  
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The remainder of this article is about electrochemical corrosion.
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===Galvanic corrosion===
  
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==Electrochemical theory==
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If two metals are placed in an electrolyte (such as aerated [[seawater]]) and electrically connected, one metal will be more "active" while the other will be more "noble" (less active). The more active metal will experience what is called "galvanic corrosion."
  
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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 "[[electron gas|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.
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The activity of the metals is based on how strongly their ions are bound to their surfaces. When the two metals are in electrical contact, they 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 tends to take electrons from the active one, and the electrolyte facilitates the flow of electricity by hosting a flow of ions.
  
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===Galvanic series===
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Based on the relative activity of metals within a given environment, they can be arranged in a hierarchy known as a ''[[galvanic series]].'' This series can be a useful guideline for choosing materials for electrochemical processes.
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{{main|Galvanic series}}
 
  
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In a given environment (one standard medium is aerated, room-temperature [[seawater]]), one metal will be either more ''[[noble metal|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
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==Protection from corrosion==
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[[Image:GoldNuggetUSGOV.jpg|right|thumb|250px|Gold nuggets do not corrode, even on a geological time scale.]]
  
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==Resistance to corrosion==
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Some metals are intrinsically more resistant to corrosion than others, based on the fundamental nature of the electrochemical processes involved or the manner in which the reaction products are formed. If a material is susceptible to corrosion, it may be protected from damage by various techniques.
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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===
 
===Intrinsic chemistry===
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[[Image:GoldNuggetUSGOV.jpg|left|thumb|150px|[[Gold]] nuggets do not corrode, even on a geological time scale.]]
 
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The materials most resistant to corrosion are those for which corrosion is [[thermodynamics|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.
 
  
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Some metals have naturally slow [[reaction]] [[chemical kinetics|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.
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The materials most resistant to corrosion are those for which corrosion is [[thermodynamics|thermodynamically]] unfavorable. For example, corrosion products of [[gold]] or [[platinum]] tend to decompose spontaneously into the corresponding pure metal. These elements, therefore, can be found in metallic form in nature, and their resistance to corrosion is a large part of their intrinsic value. By contrast, the more common "base" metals can be protected in ways that are more temporary.
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For some metals, their corrosion may be thermodynamically favorable, but the rate of reaction is acceptably slow. Examples of such metals are [[zinc]], [[magnesium]], and [[cadmium]]. In the case of [[graphite]], it releases large amounts of energy upon [[oxidation]], but the reaction rate is so slow that it is effectively immune to electrochemical corrosion under normal conditions.
  
 
===Passivation===
 
===Passivation===
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{{main|Passivation}}
 
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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]].
 
  
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These conditions required for passivation are specific to the material. The effect of pH is recorded using [[Pourbaix diagram]]s, but many other factors are influential. Some conditions that inhibit passivation include: high [[pH]] for aluminum, 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 (element)|mercury]] or hot [[solder]] can often circumvent passivation mechanisms.
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For some metals (and metalloids) under appropriate conditions, a thin film of a hard, corrosion product forms spontaneously on the metal's surface, acting as a barrier to further oxidation. This protective film is usually an oxide or nitride of the metal. If this layer stops growing after reaching a thickness of less than one micrometer (under the conditions in which the material will be used), the phenomenon is known as '''[[passivation]].'''
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This effect serves as an indirect kinetic barrier: The reaction is often quite rapid until an impermeable layer is formed. Passivation can be observed with materials such as [[aluminum]], [[magnesium]], [[copper]], stainless [[steel]], [[titanium]], and [[silicon]], when they are exposed to air and water at moderate [[pH]]. By contrast, the rusting of [[iron]], which involves the formation of mixed oxides, is not considered passivation because the layer is not protective and usually grows to be much thicker.
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Conditions required for passivation are specific to each material. Some conditions that inhibit passivation include high [[pH]] for aluminum, low pH or the presence of [[chloride]] ions for stainless steel, high temperature for titanium (in which case the oxide dissolves into the metal), 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 (element)|mercury]] or hot [[solder]] can often circumvent passivation mechanisms.
  
 
===Surface treatments===
 
===Surface treatments===
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[[Image:Galvanized surface.jpg|thumb|180px|right|[[Galvanization|Galvanized]] surface]]
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[[Image:Galvanized surface.jpg|thumb|250px|right|A galvanized surface.]]
  
 
====Applied coatings====
 
====Applied coatings====
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{{main|Galvanization}}
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The most common anti-corrosion treatments are [[electroplating]], [[paint]]ing, and the application of [[vitreous enamel|enamel]]. They function by providing a barrier of corrosion-resistant material between the damaging environment and the (often cheaper, tougher, 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 (such as [[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.
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[[Plating]], [[paint]]ing, and the application of [[vitreous enamel|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====
 
====Reactive coatings====
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If the environment is controlled (especially in recirculating systems), [[corrosion inhibitor]]s 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 [[Eifel Aqueduct#The aqueduct as a stone quarry|mineral deposits]]), [[chromate]]s, [[phosphate]]s, and a wide range of specially-designed chemicals that resemble [[surfactant]]s (i.e. long-chain organic molecules with ionic end groups).
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If the environment is controlled, [[corrosion inhibitor]]s can often be added to it. These inhibitors form an electrically insulating or chemically impermeable coating on exposed metal surfaces, thereby suppressing electrochemical reactions. Such methods make the system less sensitive to scratches or defects in the coating. Chemicals that inhibit corrosion include some of the salts in [[hard water]], [[chromate]]s, [[phosphate]]s, and a wide range of chemicals that are designed to resemble [[surfactant]]s (long-chain organic molecules with ionic end groups).
  
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[[Image:Belaying8.jpg|thumb|left|This figure-8 descender is annodized with a yellow finish. [[Climbing equipment]] is available in a wide range of anodized colors.]]
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[[Image:Belaying8.jpg|thumb|200px|right|This figure-8 descender, used in rock climbing, is anodized with a yellow finish. Climbing equipment is available in a wide range of anodized colors.]]
  
 
====Anodization====
 
====Anodization====
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{{main|Anodising}}
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Items made with aluminum or its alloys are often given a surface treatment known as [[anodising|anodization]] in a chemical bath toward the end of the manufacturing process. Through this process, the existing layer of aluminum oxide on the object's surface is made thicker.
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Aluminium alloys often undergo a surface treatment known as [[anodising|anodization]] in a chemical bath near the end of their manufacture. Electrochemical conditions in the bath are carefully adjusted so that uniform pores several [[nanometer]]s 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 close (sealed), forming a harder-than-usual (and therefore more protective) surface layer. If this coating is scratched, normal passivation processes take over to protect the damaged area.
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The aluminum object is made to function as the anode (positively charged electrode) within an electrochemical cell, with an acid solution (usually sulfuric acid in water) serving as the electrolyte. When an electric current (direct current) is passed, the water breaks down (to hydrogen and oxygen), and oxygen at the anode combines with the aluminum to form aluminum oxide. The acid in solution reacts with the oxide, causing it to become a porous film.
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Electrochemical conditions in the bath are carefully adjusted so that uniform pores several [[nanometer]]s 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 close, forming a harder-than-usual (and therefore more protective) surface layer. If this coating is scratched, normal passivation processes take over to protect the damaged area.
  
 
===Cathodic protection===
 
===Cathodic protection===
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{{main|Cathodic protection}}
 
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Cathodic protection (CP) is a technique to control the corrosion of a metal surface by making that surface the [[cathode]] of an [[electrochemical cell]].
 
  
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It is a method used to protect metal structures from corrosion. Cathodic protection systems are most commonly used to protect [[steel]], water, and fuel [[pipeline transport|pipelines]] and tanks; steel pier [[piles]], ships, and [[offshore]] [[oil platform]]s.
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Cathodic protection (CP) is a technique to control the corrosion of a metal surface by making that surface the [[electrolysis|cathode]] (negative electrode) of an [[electrochemical cell]]. This technique is most commonly used to protect [[steel]] [[pipeline transport|pipelines]] and tanks (for water and fuels), steel pier [[piles]], ships, and [[offshore]] [[oil platform]]s.
  
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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.
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In this technique, the steel surface is given a uniform, negative electrical potential relative to another material that acts as the anode. With the flow of current, the corrosion of steel is halted, while the anode material corrodes and must be replaced eventually.
  
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For larger structures, galvanic anodes cannot economically deliver enough current to provide complete protection. [[Cathodic protection#Impressed Current CP|Impressed Current Cathodic Protection]] (ICCP) systems use anodes connected to a [[direct current|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.
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For larger structures, galvanic anodes cannot economically deliver enough current to provide complete protection. [[Cathodic protection#Impressed Current CP|Impressed Current Cathodic Protection]] (ICCP) systems use anodes connected to a [[direct-current]] power source (a [[cathodic protection rectifier]]). Anodes for ICCP systems are tubular and solid rods of various specialized materials, such as high-[[silicon]] cast [[iron]], [[graphite]], mixed [[metal]] [[oxide]], or [[platinum]]-coated titanium.
  
 
==Corrosion in passivated materials==
 
==Corrosion in passivated materials==
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[[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.
 
  
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===Pitting corrosion===
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[[Passivation]] is extremely useful in alleviating corrosion damage, but one must be careful not to trust it too thoroughly. Even a high-quality alloy will corrode if its ability to form a passivating film is compromised. The modes of corrosion may be more exotic and their immediate results less visible than [[rust]] and other bulk forms of corrosion. Consequently, they may escape detection and cause problems.
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{{main|Pitting corrosion}}
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Certain conditions, such as low availability of oxygen or high concentrations of species such as [[chloride]] which compete as [[anion]]s, 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 [[nucleation|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 [[structural failure|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).
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===Pitting===
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Pitting is among the most common and damaging forms of corrosion in passivated alloys. In the worst case, almost the entire surface remains protected, but tiny local fluctuations degrade the oxide film at a few critical points. Corrosion at these points can be greatly amplified, causing ''corrosion pits'' of several types, depending on conditions in the environment. Some conditions&mdash;such as low availability of oxygen or high concentrations of [[ion|anion]]s (for example, [[chloride]] ions)&mdash;can interfere with a given alloy's ability to re-form a passivating film.
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Although corrosion pits may begin to be formed under unusual circumstances, they can continue to grow even when conditions return to normal, because the interior of each pit is naturally deprived of oxygen. In extreme cases, the sharp tips of long, narrow pits can cause [[stress concentration]] to the point that otherwise tough alloys may shatter. Alternatively, a thin film pierced by an invisibly small hole may hide a thumb-sized pit from view. These problems are especially dangerous because they are difficult to detect before failure of the structure or its part.
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Pitting can be prevented by controlling the alloy's environment. This approach often includes ensuring that the material is exposed to oxygen uniformly (with the elimination of crevices).
  
 
===Fretting===
 
===Fretting===
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Many useful passivating oxides are also effective abrasives, particularly [[titanium dioxide|TiO<sub>2</sub>]] and [[alumina|Al<sub>2</sub>O<sub>3</sub>]]. '''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 [[fret]]s of musical instruments, they were named separately.
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Many useful passivating oxides are also effective abrasives, particularly titanium dioxide (TiO<sub>2</sub>) and alumina (Al<sub>2</sub>O<sub>3</sub>). '''Fretting corrosion''' occurs when particles of corrosion product continuously abrade away the passivating film, as two metal surfaces are rubbed together. Although this process often damages the [[fret]]s of musical instruments, they were named separately.
  
 
===Weld decay and knifeline attack===
 
===Weld decay and knifeline attack===
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{{main|Intergranular corrosion}}
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[[Stainless steel]] can pose special corrosion challenges, since its passivating behavior 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 [[carbide]]s can form in the [[crystallite|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 [[electrochemistry|galvanic couple]] with the well-protected alloy nearby, which leads to ''weld decay'' (corrosion of the grain boundaries near [[welding|welds]]) in highly corrosive environments. Special alloys, either with low carbon content or with added carbon "[[getter]]s" 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<sup>1</sup>.
+
Stainless [[steel]] can pose special corrosion challenges, because its passivating behavior relies on the presence of a minor alloying component ([[chromium]], typically only 18 percent). During the elevated temperatures of [[welding]] or during improper [[heat treatment]], chromium [[carbide]]s can form in the [[crystallite|grain boundaries]] of stainless steel alloys. This chemical reaction robs the material of chromium at the grain boundaries, making those areas much less resistant to corrosion. In highly corrosive environments, the result is "weld decay," that is, corrosion of the grain boundaries near welds.
 +
 
 +
Special alloys, with low carbon content or with added carbon "[[getter]]s"&mdash;such as [[titanium]] and [[niobium]]&mdash;can prevent this effect. If carbon getters are used, the alloy requires special heat treatment after welding to prevent the phenomenon of "knifeline attack." As its name implies, this type of corrosion is limited to a small zone, often only a few micrometers across, causing it to proceed more rapidly. This zone is very near the weld, making it less noticeable (Jones, 1996).
  
 
==Microbial corrosion==
 
==Microbial corrosion==
−
{{main|Microbial corrosion}}
 
−
[[Microbial corrosion]], or bacterial corrosion, is a corrosion caused or promoted by [[microorganism]]s, usually [[chemoautotroph]]s. 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 cell]]s can form in the deposits of corrosion products, causing and enhancing [[galvanic corrosion]].
 
  
−
==High temperature corrosion==
+
Microbial (or bacterial) corrosion is that caused or promoted by [[microorganism]]s, usually [[chemoautotroph]]s. It can apply to both metals and nonmetals, in both the presence or absence of [[oxygen]]. In conditions lacking oxygen, sulfate-reducing bacteria are common, and they produce [[hydrogen sulfide]], causing [[sulfide stress cracking]]. In the presence of oxygen, some bacteria directly oxidize iron to iron oxides and hydroxides, while other bacteria oxidize sulfur and produce sulfuric acid. [[Concentration cell]]s can form in the deposits of corrosion products, causing and enhancing [[galvanic 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.
+
 
 +
==High-temperature corrosion==
  
−
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 glaze]]s have also been shown to prevent or reduce wear during high temperature sliding contact of metallic (or metallic and ceramic) surfaces.
+
High-temperature corrosion is chemical deterioration of a material (typically a metal) at very high temperatures. This nongalvanic form of corrosion can occur when a metal is subjected to a high-temperature atmosphere containing oxygen, sulfur, or other substances capable of oxidizing (or assisting the oxidation of) the metal. For example, materials used in [[aerospace]], power generation, and automobile engines have to resist sustained periods at high temperatures, when they may be exposed to an atmosphere containing highly corrosive products of combustion.
 +
 
 +
The products of high-temperature corrosion can potentially be turned to the engineer's advantage. The formation of oxides on stainless steels, for example, can provide a protective layer preventing further atmospheric attack, so that the material can be used for sustained periods at both ordinary and elevated temperatures in hostile conditions. Such high-temperature corrosion products, in the form of [[compacted oxide layer glaze]]s, have also been shown to prevent or reduce wear during high-temperature sliding contact of metallic (or metallic and ceramic) surfaces.
  
 
==Economic impact==
 
==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]] [http://www.nace.org website]  has a [http://www.nace.org/nace/content/publicaffairs/cost_corr_pres/cost_corrosion_files/frame.htm summary] slideshow of the report findings.  Jones<sup>1</sup> 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.
+
 
 +
In 2002, the U.S. [[Federal Highway Administration]] released a study entitled, "Corrosion Costs and Preventive Strategies in the United States." It gave the direct costs associated with metallic corrosion in nearly every U.S. industry sector. It showed that for 1998, the total annual estimated direct cost of corrosion in the United States was approximately $276 billion (about 3.1 percent of the U.S. [[gross domestic product]]). Jones (1996) 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.
  
 
==References==
 
==References==
−
* {{cite book
+
* Brimi, Marjorie A. ''Electrofinishing.'' New York: American Elsevier Publishing Company, Inc., 1965.
−
    | title = Principles and Prevention of Corrosion
+
* Budinski, Kenneth G. ''Surface Engineering for Wear Resistance.'' Englewood Cliffs, NJ: Prentice Hall, 1988. ISBN 978-0138779375
−
    | edition = 2nd edition
+
* Jones, Denny. ''Principles and Prevention of Corrosion.'' Upper Saddle River, NJ: Prentice Hall, 1996. ISBN 0133599930
−
    | last = Jones
+
* NACE International. [https://www.nace.org/uploadedFiles/Publications/ccsupp.pdf Corrosion Costs and Preventive Strategies in the United States]. Retrieved June 16, 2017.
−
    | first = Denny
 
−
    | authorlink = Denny A. Jones
 
−
    | publisher = [[Prentice Hall]]
 
−
    | location = [[Upper Saddle River, New Jersey]]
 
−
    | year = [[1996]]
 
−
    | id = ISBN 0-13-359993-0 }}
 
−
* [http://www.llnl.gov/es_and_h/hsm/doc_14.08/doc14-08.html Working Safely with Corrosive Chemicals]
 
−
 
 
−
== See also ==
 
−
* [[Chemical hazard label]]
 
−
* [[Copper band]] corrosion.
 
−
* [[Rust|Rusting]]
 
−
* [[Electronegativity]]
 
−
* [[Cathodic protection]]
 
−
* [[Galvanization]]
 
−
* [[Oxidation]]
 
−
* [[Periodic table]]
 
−
* [[STLE|Society of Tribologists and Lubrication Engineers]]
 
  
 
== External links ==
 
== External links ==
−
 
+
All links retrieved January 7, 2024.
−
*[http://www.nace.org/ NACE International] -Professional society for corrosion engineers ( [[NACE International|NACE]] )
+
*[http://www.corrosion-doctors.org/ Corrosion-doctors.org] - Site dedicated to corrosion of all types.
−
*[http://www.corrosion-doctors.org/ corrosion-doctors.org] -Site dedicated to corrosion of all forms
+
*[http://www.nace.org/ NACE International] - Professional society for corrosion engineers.
−
*[http://www.efcweb.org/Member_Societies.html] -[[European Federation of Corrosion]]
+
*[http://www.efcweb.org/Member_Societies.html European Federation of Corrosion].
−
*[http://www.worldstainless.org/About+stainless/What+is/Corrosion/]-Corrosion Properties of Stainless Steel
+
*[https://www.corrosionpedia.com/ Corrosionpedia]
  
 
[[Category:Physical sciences]]
 
[[Category:Physical sciences]]
 
[[Category:Chemistry]]
 
[[Category:Chemistry]]
 
[[Category:Materials science]]
 
[[Category:Materials science]]
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[[Category:Building engineering]]
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[[Category:Technology]]
−
[[Category:Safety]]
 
  
 
{{credit|86196386}}
 
{{credit|86196386}}

Latest revision as of 03:36, 8 January 2024

Rust, the most familiar example of corrosion.

Corrosion is the deterioration of a material's essential properties as a result of reactions with its environment. It can be concentrated locally to form a pit or crack, or it can extend across a wide area to produce general deterioration. Many materials corrode merely upon exposure to air, moisture, and sunlight, but the process can be accelerated by exposure to certain chemicals. A substance that causes corrosion is called a corrosive substance, or simply, a corrosive. A corrosive can damage living tissue as well as inanimate materials.

The rusting of iron is a well-known example of corrosion. Other metals may be similarly damaged, typically producing their oxides, hydroxides, and salts. Corrosion also refers to the dissolution of ceramic materials or the discoloration and weakening of polymers, such as by exposure to ultraviolet light from the Sun.

Corrosion of a material can be controlled by applying a coating, such as by electroplating or painting, or by specialized techniques such as anodization or cathodic protection. The annual cost of corrosion has been estimated to be on the order of hundreds of billions of dollars in the United States alone.

Rust damage in automobiles can create hidden dangers.

Corrosive substances

Warning symbol for a corrosive.

Corrosive chemicals can be solids, liquids, or gases, and they can belong to any of several classes of materials. The following are some examples.

  • Acids, such as sulfuric acid, nitric acid, and hydrochloric acid
  • Bases ("caustics" or "alkalis"), such as sodium hydroxide and potassium hydroxide
  • Dehydrating agents, such as phosphorus pentoxide or calcium oxide
  • Halogens—such as bromine or iodine—and certain halogen salts, such as zinc chloride or sodium hypochlorite
  • Organic halides and organic acid halides, such as acetyl chloride and benzyl chloroformate
  • Acid anhydrides, such as acetic anhydride
  • Some organic materials, such as phenol ("carbolic acid").

Corrosion of nonmetals

Most ceramic materials are nearly immune to corrosion. Their atoms are bound together by strong ionic or covalent bonds that resist disruption. When corrosion does occur, it is almost always a simple dissolution of the material or a chemical reaction. By contrast, the corrosion of metals is an electrochemical process, as discussed below.

A common example of corrosion protection in ceramics is the lime (calcium oxide) added to soda-lime glass to reduce its solubility in water. Though it is not nearly as soluble as pure sodium silicate, normal glass forms submicroscopic flaws when exposed to moisture. Given the brittleness of glass, such flaws dramatically reduce the strength of a glass object during its first few hours at room temperature.

In the case of polymeric materials, their degradation can be caused by any of a wide array of complex and often poorly understood physicochemical processes. These are strikingly different from the other processes discussed below, so the term "corrosion" is applied to them in just a loose sense of the word.

Given their high molecular weight, polymers are generally quite difficult to dissolve. In those instances where dissolution is a problem, it is relatively simple to design against. A more common and related problem is "swelling," in which small molecules infiltrate the structure, reducing strength and stiffness and causing a volume change. Conversely, many polymers (notably flexible vinyl) are intentionally swollen with plasticizers, and when these get leached out, the structure becomes brittle or undergoes other undesirable changes.

The most common form of degradation of polymers is a decrease in chain length. In the case of DNA, agents that break their chains include ionizing radiation (most commonly ultraviolet light), free radicals, and oxidizers (such as oxygen, ozone, and chlorine). Additives can slow these processes effectively, and they can be as simple as a UV-absorbing pigment (such as titanium dioxide or carbon black). Plastic shopping bags often do not include these additives, so they can break down more easily as litter.

Electrochemical corrosion of metals

Consider a metal in contact with an electrolyte—that is, a medium that can conduct electricity by allowing the flow of ions. Corrosion of the metal involves electrochemical changes. In other words, the metal is chemically changed by a process that involves the flow of an electric current, by the movement of electrons and ions.

A good example is the rusting of iron or steel. When iron comes in contact with water (especially water with a high concentration of ions), an electrochemical corrosion process begins. This process can be summarized as follows.

  1. Iron (Fe) atoms on the surface release electrons to form iron(II) ions (Fe2+)—a process called oxidation
  2. The electrons released travel to the edges of the water droplet, where there is plenty of dissolved oxygen. They combine with the oxygen and water to form hydroxide ions (OH−).
  3. The hydroxide ions react with iron(II) ions to form iron(II) hydroxide (Fe(OH)2)
  4. The iron(II) hydroxide in turn reacts with more dissolved oxygen to form hydrated iron(III) oxide (Fe2O3.xH2O)

Galvanic corrosion

If two metals are placed in an electrolyte (such as aerated seawater) and electrically connected, one metal will be more "active" while the other will be more "noble" (less active). The more active metal will experience what is called "galvanic corrosion."

The activity of the metals is based on how strongly their ions are bound to their surfaces. When the two metals are in electrical contact, they 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 tends to take electrons from the active one, and the electrolyte facilitates the flow of electricity by hosting a flow of ions.

Based on the relative activity of metals within a given environment, they can be arranged in a hierarchy known as a galvanic series. This series can be a useful guideline for choosing materials for electrochemical processes.

Protection from corrosion

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

Some metals are intrinsically more resistant to corrosion than others, based on the fundamental nature of the electrochemical processes involved or the manner in which the reaction products are formed. If a material is susceptible to corrosion, it may be protected from damage by various techniques.

Intrinsic chemistry

The materials most resistant to corrosion are those for which corrosion is thermodynamically unfavorable. For example, corrosion products of gold or platinum tend to decompose spontaneously into the corresponding pure metal. These elements, therefore, can be found in metallic form in nature, and their resistance to corrosion is a large part of their intrinsic value. By contrast, the more common "base" metals can be protected in ways that are more temporary.

For some metals, their corrosion may be thermodynamically favorable, but the rate of reaction is acceptably slow. Examples of such metals are zinc, magnesium, and cadmium. In the case of graphite, it releases large amounts of energy upon oxidation, but the reaction rate is so slow that it is effectively immune to electrochemical corrosion under normal conditions.

Passivation

For some metals (and metalloids) under appropriate conditions, a thin film of a hard, corrosion product forms spontaneously on the metal's surface, acting as a barrier to further oxidation. This protective film is usually an oxide or nitride of the metal. If this layer stops growing after reaching a thickness of less than one micrometer (under the conditions in which the material will be used), the phenomenon is known as passivation.

This effect serves as an indirect kinetic barrier: The reaction is often quite rapid until an impermeable layer is formed. Passivation can be observed with materials such as aluminum, magnesium, copper, stainless steel, titanium, and silicon, when they are exposed to air and water at moderate pH. By contrast, the rusting of iron, which involves the formation of mixed oxides, is not considered passivation because the layer is not protective and usually grows to be much thicker.

Conditions required for passivation are specific to each material. Some conditions that inhibit passivation include high pH for aluminum, low pH or the presence of chloride ions for stainless steel, high temperature for titanium (in which case the oxide dissolves into the metal), 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

A galvanized surface.

Applied coatings

The most common anti-corrosion treatments are electroplating, painting, and the application of enamel. They function by providing a barrier of corrosion-resistant material between the damaging environment and the (often cheaper, tougher, 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 (such as 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, corrosion inhibitors can often be added to it. These inhibitors form an electrically insulating or chemically impermeable coating on exposed metal surfaces, thereby suppressing electrochemical reactions. Such methods make the system less sensitive to scratches or defects in the coating. Chemicals that inhibit corrosion include some of the salts in hard water, chromates, phosphates, and a wide range of chemicals that are designed to resemble surfactants (long-chain organic molecules with ionic end groups).

This figure-8 descender, used in rock climbing, is anodized with a yellow finish. Climbing equipment is available in a wide range of anodized colors.

Anodization

Items made with aluminum or its alloys are often given a surface treatment known as anodization in a chemical bath toward the end of the manufacturing process. Through this process, the existing layer of aluminum oxide on the object's surface is made thicker.

The aluminum object is made to function as the anode (positively charged electrode) within an electrochemical cell, with an acid solution (usually sulfuric acid in water) serving as the electrolyte. When an electric current (direct current) is passed, the water breaks down (to hydrogen and oxygen), and oxygen at the anode combines with the aluminum to form aluminum oxide. The acid in solution reacts with the oxide, causing it to become a porous film.

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 close, forming a harder-than-usual (and therefore more protective) 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 (negative electrode) of an electrochemical cell. This technique is most commonly used to protect steel pipelines and tanks (for water and fuels), steel pier piles, ships, and offshore oil platforms.

In this technique, the steel surface is given a uniform, negative electrical potential relative to another material that acts as the anode. With the flow of current, the corrosion of steel is halted, while the anode material corrodes and must be replaced eventually.

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 direct-current power source (a cathodic protection rectifier). Anodes for ICCP systems are tubular and solid rods of various specialized materials, such as high-silicon cast iron, graphite, mixed metal oxide, or platinum-coated titanium.

Corrosion in passivated materials

Passivation is extremely useful in alleviating corrosion damage, but one must be careful not to trust it too thoroughly. Even a high-quality alloy will corrode if its ability to form a passivating film is compromised. The modes of corrosion may be more exotic and their immediate results less visible than rust and other bulk forms of corrosion. Consequently, they may escape detection and cause problems.

Pitting

Pitting is among the most common and damaging forms of corrosion in passivated alloys. In the worst case, almost the entire surface remains protected, but tiny local fluctuations degrade the oxide film at a few critical points. Corrosion at these points can be greatly amplified, causing corrosion pits of several types, depending on conditions in the environment. Some conditions—such as low availability of oxygen or high concentrations of anions (for example, chloride ions)—can interfere with a given alloy's ability to re-form a passivating film.

Although corrosion pits may begin to be formed under unusual circumstances, they can continue to grow even when conditions return to normal, because the interior of each pit is naturally deprived of oxygen. In extreme cases, the sharp tips of long, narrow pits can cause stress concentration to the point that otherwise tough alloys may shatter. Alternatively, a thin film pierced by an invisibly small hole may hide a thumb-sized pit from view. These problems are especially dangerous because they are difficult to detect before failure of the structure or its part.

Pitting can be prevented by controlling the alloy's environment. This approach often includes ensuring that the material is exposed to oxygen uniformly (with the elimination of crevices).

Fretting

Many useful passivating oxides are also effective abrasives, particularly titanium dioxide (TiO2) and alumina (Al2O3). Fretting corrosion occurs when particles of corrosion product continuously abrade away the passivating film, as two metal surfaces are rubbed together. Although this process often damages the frets of musical instruments, they were named separately.

Weld decay and knifeline attack

Stainless steel can pose special corrosion challenges, because its passivating behavior relies on the presence of a minor alloying component (chromium, typically only 18 percent). During the elevated temperatures of welding or during improper heat treatment, chromium carbides can form in the grain boundaries of stainless steel alloys. This chemical reaction robs the material of chromium at the grain boundaries, making those areas much less resistant to corrosion. In highly corrosive environments, the result is "weld decay," that is, corrosion of the grain boundaries near welds.

Special alloys, with low carbon content or with added carbon "getters"—such as titanium and niobium—can prevent this effect. If carbon getters are used, the alloy requires special heat treatment after welding to prevent the phenomenon of "knifeline attack." As its name implies, this type of corrosion is limited to a small zone, often only a few micrometers across, causing it to proceed more rapidly. This zone is very near the weld, making it less noticeable (Jones, 1996).

Microbial corrosion

Microbial (or bacterial) corrosion is that caused or promoted by microorganisms, usually chemoautotrophs. It can apply to both metals and nonmetals, in both the presence or absence of oxygen. In conditions lacking oxygen, sulfate-reducing bacteria are common, and they produce hydrogen sulfide, causing sulfide stress cracking. In the presence of oxygen, some bacteria directly oxidize iron to iron oxides and hydroxides, while 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) at very high temperatures. This nongalvanic form of corrosion can occur when a metal is subjected to a high-temperature atmosphere containing oxygen, sulfur, or other substances capable of oxidizing (or assisting the oxidation of) the metal. For example, materials used in aerospace, power generation, and automobile engines have to resist sustained periods at high temperatures, when they may be exposed to an atmosphere containing highly corrosive products of combustion.

The products of high-temperature corrosion can potentially be turned to the engineer's advantage. The formation of oxides on stainless steels, for example, can provide a protective layer preventing further atmospheric attack, so that the material can be used for sustained periods at both ordinary and elevated temperatures 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

In 2002, the U.S. Federal Highway Administration released a study entitled, "Corrosion Costs and Preventive Strategies in the United States." It gave the direct costs associated with metallic corrosion in nearly every U.S. industry sector. It showed that for 1998, the total annual estimated direct cost of corrosion in the United States was approximately $276 billion (about 3.1 percent of the U.S. gross domestic product). Jones (1996) 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.

References
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External links

All links retrieved January 7, 2024.

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