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RustFrom Wikipedia, the free encyclopedia

For other uses, see Rust (disambiguation).

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article  by adding citations to reliable sources. Unsourced material may be challenged removed. (June 2012)

Colors and porous surface texture of rust

Steels and other iron–carbon alloy phases

• Ferrite

• Austenite

• Cementite

• Graphite

• Martensite

Microstructures

• Spheroidite

• Pearlite

• Bainite

• edeburite

• Tempered martensite

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• !idmanstatten structures

Classes

• Crucible steel

• Carbon steel

• Spring steel

• Alloy steel

• Maraging steel

• Stainless steel

• !eathering steel

• Tool steel

Other iron-based materials

• Cast iron

• Gray iron

• !hite iron

• "uctile iron

• Malleable iron

• !rought iron

• v

• t

• e

Rust is an iron oxide, usually red oxide formed by the redox reaction of iron and oxygen in the

presence of water or air moisture. Several forms of rust are distinguishable both visually andby spectroscopy, and form under different circumstances.[! "ust consists of hydrated iron#$$$%oxides Fe&'()n*&' and iron#$$$% oxide+hydroxide #Fe'#'*%, Fe#'*%(%.

iven sufficient time, oxygen, and water, any iron mass will eventually convert entirely to rust anddisintegrate. Surface rust is flaky and friable, and it provides no protection to the underlying iron,unlike the formation of patina on copper surfaces. "usting is the common term for corrosion of ironand its alloys, such as steel. -any other metals undergo euivalent corrosion, but the resultingoxides are not commonly called rust.

'ther forms of rust exist, like the result of reactions between iron and chloride in an environmentdeprived of oxygen / rebar used in underwater concrete pillars is an example / whichgenerates green rust.

Contents

• Chemical reactions

o .'xidation of iron

o .&0ssociated reactions

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• &1revention

o &."ust+resistant alloys

o &.&alvani2ation

o &.(Cathodic protection

o &.3Coatings and painting

o &.45luing

o &.6$nhibitors

o &.7*umidity control

• (8reatment

• 39conomic effect

• 4Cultural symbolism

• 6allery

• 7"eferences

• :9xternal links

Chemical reactions[edit!

*eavy rust on the links of a chain near the olden ate 5ridge in San Francisco; it was continuously exposed

to moisture and salt spray, causing surface breakdown, cracking, and flaking of the metal.

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'utdoors Rust Wedge display at the9xploratorium shows the enormous expansive force of rusting iron

"ust is another name for iron oxide, which occurs when iron or an alloy that contains iron, like steel,is exposed to oxygen and moisture for a long period of time. 'ver time, the oxygen combines withthe metal at an atomic level, forming a new compound called an oxide and weakening the bonds ofthe metal itself. 0lthough some people refer to rust generally as <oxidation<, that term is much moregeneral; although rust forms when iron undergoes oxidation, not all oxidation forms rust. 'nly iron or 

alloys that contain iron can rust, but other metals can corrode in similar ways.

8he main catalyst for the rusting process is water. $ron or steel structures might appear to be solid,but water molecules can penetrate the microscopic pits and cracks in any exposed metal. 8hehydrogen atoms present in water molecules can combine with other elements to form acids, whichwill eventually cause more metal to be exposed. $f chloride ions are present, as is the case withsaltwater, the corrosion is likely to occur more uickly. -eanwhile, the oxygen atoms combine withmetallic atoms to form the destructive oxide compound. 0s the atoms combine, they weaken themetal, making the structure brittle and crumbly.

Oxidation of iron[edit!

When impure #cast% iron is in contact with water, oxygen, other strong oxidants, or acids, it rusts.$f salt is present, for example in seawater or salt spray, the iron tends to rust more uickly, as a result

of electrochemical reactions. $ron metal is relatively unaffected by pure water or by dry oxygen. 0swith other metals, like aluminium, a tightly adhering oxide coating, a passivation layer , protects thebulk iron from further oxidation. 8he conversion of the passivating ferrous oxide layer to rust resultsfrom the combined action of two agents, usually oxygen and water.

'ther degrading solutions are sulfur dioxide in water and carbon dioxide in water. =nder thesecorrosive conditions, iron hydroxidespecies are formed. =nlike ferrous oxides, the hydroxides do notadhere to the bulk metal. 0s they form and flake off from the surface, fresh iron is exposed, and the

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corrosion process continues until either all of the iron is consumed or all of the oxygen, water, carbondioxide, or sulfur dioxide in the system are removed or consumed. [&!

When iron rusts, the oxides take up more volume than the original metal; this expansion cangenerate enormous forces, damaging structures made with iron. See 9conomic effect for moredetails.

Associated reactions[edit!8he rusting of iron is an electrochemical process that begins with the transfer of electrons from ironto oxygen.[(! 8he iron is the reducing agent #gives up electrons% while the oxygen is the oxidisingagent #gains electrons%. 8he rate of corrosion is affected by water and accelerated by electrolytes, asillustrated by the effects of road salt on the corrosion of automobiles. 8he key reaction is thereduction of oxygen>

'& ? 3 e@ ? & *&' A 3 '*@

5ecause it forms hydroxide ions, this process is strongly affected by the presence of acid.$ndeed, the corrosion of most metals by oxygen is accelerated at low p*. 1roviding the electronsfor the above reaction is the oxidation of iron that may be described as follows>

Fe A Fe&? ? & e@

8he following redox reaction also occurs in the presence of water and is crucial to theformation of rust>

3 Fe&? ? '& A 3 Fe(? ? & '&@

$n addition, the following multistep acid+base reactions affect the course of rustformation>

Fe&? ? & *&' ⇌ Fe#'*%& ? & *?

Fe(? ? ( *&' ⇌ Fe#'*%( ? ( *?

as do the following dehydration euilibria>

Fe#'*%& ⇌ Fe' ? *&'

Fe#'*%( ⇌ Fe'#'*% ? *&'& Fe'#'*% ⇌ Fe&'( ? *&'

From the above euations, it is also seen that the corrosionproducts are dictated by the availability of water and oxygen. Withlimited dissolved oxygen, iron#$$%+containing materials are favoured,including Fe' and black lodestone or magnetite #Fe('3%. *ighoxygen concentrations favour ferric materials with the nominalformulae Fe#'*%(+x'xB&. 8he nature of rust changes with time,reflecting the slow rates of the reactions of solids.

Furthermore, these complex processes are affected by thepresence of other ions, such as Ca&?, both of which serve as anelectrolyte, and thus accelerate rust formation, or combine withthe hydroxides and oxides of iron to precipitate a variety of Ca+Fe+'+'* species.

'nset of rusting can also be detected in laboratory with the useof ferroxyl indicator solution. 8he solution detects both Fe&? ions andhydroxyl ions. Formation of Fe&? ions and hydroxyl ions areindicated by blue and pink patches respectively.

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Prevention[edit!

Cor+8en is a special iron alloy that rusts, but still retains its structural

integrity

5ecause of the widespread use and importance of iron and steelproducts, the prevention or slowing of rust is the basis of maoreconomic activities in a number of speciali2ed technologies. 0 briefoverview of methods is presented here; for detailed coverage, seethe cross+referenced articles.

"ust is permeable to air and water, therefore the interior metalliciron beneath a rust layer continues to corrode. "ust prevention thusreuires coatings that preclude rust formation.

Rust-resistant alloys[edit!See also: Stainless steel  and  Weathering steel 

Stainless steel forms a passivation layer of chromium#$$$% oxide.Similar passivation behavior occurswith magnesium, titanium, 2inc, 2inc oxides,aluminium, polyaniline,and other electroactive conductive polymers.[citation needed !

Special <weathering steel< alloys such as Cor+8en rust at a muchslower rate than normal, because the rust adheres to the surface of the metal in a protective layer. Designs using this material mustinclude measures that avoid worst+case exposures, since thematerial still continues to rust slowly even under near+idealconditions.[citation needed !

Galvanization[edit!Main article: Galvanization

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$nterior rust in old galvani2ed iron water pipes can result in brown and

black water.

alvani2ation consists of an application on the obect to beprotected of a layer of metallic 2inc by either  hot+dipgalvani2ing or electroplating. Einc is traditionally used because it ischeap, adheres well to steel, and provides cathodic protection tothe steel surface in case of damage of the 2inc layer. $n more

corrosive environments #such as salt water%, cadmium plating ispreferred. alvani2ation often fails at seams, holes, and ointswhere there are gaps in the coating. $n these cases, the coating stillprovides some partial cathodic protection to iron, by acting asa galvanic anode and corroding itself instead of the underlyingprotected metal. 8he protective 2inc layer is consumed by thisaction, and thus galvani2ation provides protection only for a limitedperiod of time.

-ore modern coatings add aluminium to the coating as zinc-alume; aluminium will migrate to cover scratches and thus provideprotection for a longer period. 8hese approaches rely on thealuminium and 2inc oxides re+protecting a once+scratched surface,

rather than oxidi2ing as a sacrificial anode as in traditionalgalvani2ed coatings. $n some cases, such as very aggressiveenvironments or long design life, both 2inc and a coating areapplied to provide enhanced corrosion protection.

8ypical galvani2ation of steel products which are to subect tonormal day to day weathering in an outside environment consists of a hot dipped :4 m 2inc coating. =nder normal weather conditions,this will deteriorate at a rate of m per year, giving approximately:4 years of protection.[citation needed !

Cathodic protection[edit!Main article: Cathodic rotection

Cathodic protection is a techniue used to inhibit corrosion onburied or immersed structures by supplying an electrical chargethat suppresses the electro+chemical reaction. $f correctly applied,corrosion can be stopped completely. $n its simplest form, it isachieved by attaching a sacrificial anode, thereby making the ironor steel the cathode in the cell formed. 8he sacrificial anode mustbe made from something with a more negative electrodepotential than the iron or steel, commonly 2inc, aluminium, ormagnesium. 8he sacrificial anode will eventually corrode away,

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ceasing its protective action unless it is replaced in a timelymanner.

Cathodic protection can also be provided by using a special+purpose electrical device to appropriately induce an electric charge.

Coatings and painting[edit!

Main article: Rustroo!ing 

Flaking paint, exposing a patch of surface rust on sheet metal

"ust formation can be controlled with coatings, suchas paint, lacuer , or varnish that isolate the iron from theenvironment. Garge structures with enclosed box sections, such asships and modern automobiles, often have a wax+based product#technically a <slushing oil<% inected into these sections. Suchtreatments usually also contain rust inhibitors. Covering steel withconcrete can provide some protection to steel because ofthealkaline p* environment at the steel+concrete interface.

*owever rusting of steel in concrete can still be a problem, asexpanding rust can fracture or slowly <explode< concrete fromwithin.[citation needed !

 0s a closely related example, iron bars were used to reinforcestonework of the 1arthenon in 0thens, reece, but causedextensive damage by rusting, swelling, and shatteringthe marble components of the building. [citation needed !

When only temporary protection is needed for storage or transport,a thin layer of oil, grease, or a special mixture suchas Cosmoline can be applied to an iron surface. Such treatmentsare extensively used when <mothballing< a steel ship, automobile,or other euipment for long+term storage.

Special anti+sei2e lubricant mixtures are available, and are appliedto metallic threads and other precision machined surfaces toprotect them from rust. 8hese compounds usually contain greasemixed with copper, 2inc, or aluminum powder, and other proprietaryingredients.[citation needed !

Bluing[edit!Main article: "luing (steel)

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5luing is a techniue that can provide limited resistance to rustingfor small steel items, such as firearms; for it to be successful, awater+displacing oil is rubbed onto the blued steel and other steel.

Inhibitors[edit!Main article: Corrosion inhibitor 

Corrosion inhibitors, such as gas+phase or volatile inhibitors, canbe used to prevent corrosion inside sealed systems. 8hey are noteffective when air circulation disperses them, and brings in freshoxygen and moisture.

umidity control[edit!See also: #ehumidi!ier  and  #esiccant 

"ust can be avoided by controlling the moisture in the atmosphere. 0n example of this is the use of  silica gel packets to controlhumidity in euipment shipped by sea.

Treatment[edit!

"ust removal from small iron or steel obects by electrolysis can bedone in a home workshop using simple materials such as a plasticbucket, tap water , lengths of rebar ,washing soda, baling wire, anda battery charger .[3!

"ust may be treated with commercial products known as rustconverter  which contain tannic acid which combines with rust.

Economic effect[edit!

Main article: Corrosion

"usting rebar  has expanded andspalled concrete off the surface of

thisreinforced concrete support

"ust is associated with degradation of iron+based tools and

structures. 0s rust has a much higher volume than the originatingmass of iron, its build+up can also cause failure by forcing apartadacent parts H a phenomenon sometimes known as <rustpacking<. $t was the cause of the collapse of the -ianus riverbridge in I:(, when the bearings rusted internally and pushed onecorner of the road slab off its support.

"ust was an important factor in the Silver 5ridge disaster of I67in West Jirginia, when a steel suspension bridge collapsed in lessthan a minute, killing 36 drivers and passengers on the bridge at

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the time. 8he Kin2ua 5ridge in 1ennsylvania was blown down bya tornado in &LL(, largely because the central base bolts holdingthe structure to the ground had rusted away, leaving the bridgeanchored by gravity alone.

"einforced concrete is also vulnerable to rust damage. $nternalpressure caused by expanding corrosion of concrete+covered steeland iron can cause the concrete to spall, creating severe structuralproblems. $t is one of the most common failure modes of reinforcedconcrete bridges and buildings.

  !tructural failures caused by rust

•

8he collapsed Silver 5ridge, as seen from the'hio side

 

•

8he Kin2ua 5ridge after it collapsed

Cultural symbolism[edit!

"ust is a commonly used metaphor  for slow decay due to neglect,since it gradually converts robust iron and steel metal into a softcrumbling powder. 0 wide section of the industriali2ed  0merican-idwest and  0merican Mortheast, once dominated by steelfoundries, the automotive industry, and other manufacturers, hasexperienced harsh economic cutbacks that have caused the regionto be dubbed the <"ust 5elt<.

$n music, literature, and art, rust is associated with images of fadedglory, neglect, decay, and ruin.

Gallery [edit!

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•

"usted struts of the 7L+year+old Mandu "iver $ron 5ridge

 

•

Concentric rust patterns breaking through a painted surface

 

•

 0 leaking water pump caused severe corrosion of this engine block