rancid if ication

8/12/2019 Rancid if Ication

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RancidificationRancidification, the product of which can be described as rancidity, is thechemicaldecompositionoffats,oilsand otherlipids(this degradation also occurs in mechanicalcuttingfluids). Specifically, it is the hydrolysis and/or oxidation of fatty acids into short-chainaldehydesandketoneswhich are generally objectionable in taste and odor. When theseprocesses occur in food, undesirable odors and flavors can result. In some cases, however, theflavors can be desirable (as inaged cheeses).[1]In processed meats, these flavors arecollectively known aswarmed-over flavor.Rancidification can also detract from the nutritionalvalue of the food. Somevitaminsare highly sensitive to degradation.[2]

Contents

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1 Rancidification pathwayso 1.1 Hydrolytic rancidityo 1.2 Oxidative rancidityo 1.3 Microbial rancidity

2 Reducing rancidification 3 Measurement of oxidative stability 4 See also 5 References 6 Further reading 7 External links

Rancidification pathways[edit]Three pathways for rancidification are recognized:[3]

Hydrolytic rancidity[edit]

Hydrolyticrancidity refers to the odor that develops whentriglyceridesare hydrolyzed andfree fatty acids are released. This reaction of lipid with water sometimes requires acatalyst,but results in the formation freefatty acidsandsaltsof free fatty acids. In particular,short-chain fatty acids,such as common butter fats, are odorous. Rancidity in foods may be veryslight, indicated by a loss of freshness to very severe, indicated by objectionable odors and/orflavors. Slight degrees of rancidity are much more common in foods than severe rancidity, yet

slight rancidity is a much more a practical concern. A slight degree of rancidity may not beobjectionable to consumers, but products which do not seem fresh will not attract repeatpurchases. If customers do not return to a product, the long-term effects of a slight degreeof rancidity can be very serious. Even though meat is held under refrigeration or in a frozenstate, the poly-unsaturated fat will continue to oxidize and slowly become rancid. The fatoxidation process, potentially resulting in rancidity, begins immediately after the animal isslaughtered and the muscle, intra-muscular, inter-muscular and surface fat becomes exposed
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lityhttp://en.wikipedia.org/wiki/Rancidity#Reducing_rancidificationhttp://en.wikipedia.org/wiki/Rancidity#Microbial_rancidityhttp://en.wikipedia.org/wiki/Rancidity#Oxidative_rancidityhttp://en.wikipedia.org/wiki/Rancidity#Hydrolytic_rancidityhttp://en.wikipedia.org/wiki/Rancidity#Rancidification_pathwayshttp://en.wikipedia.org/wiki/Rancidityhttp://en.wikipedia.org/wiki/Rancidity#cite_note-2http://en.wikipedia.org/wiki/Vitaminhttp://en.wikipedia.org/wiki/Warmed-over_flavorhttp://en.wikipedia.org/wiki/Rancidity#cite_note-1http://en.wikipedia.org/wiki/Aged_cheesehttp://en.wikipedia.org/wiki/Ketoneshttp://en.wikipedia.org/wiki/Aldehydeshttp://en.wikipedia.org/wiki/Cutting_fluidhttp://en.wikipedia.org/wiki/Cutting_fluidhttp://en.wikipedia.org/wiki/Lipidhttp://en.wikipedia.org/wiki/Oilhttp://en.wikipedia.org/wiki/Fathttp://en.wikipedia.org/wiki/Chemical_decompositionhttp://en.wikipedia.org/wiki/Chemical_decomposition


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to oxygen of the air. This chemical process continues during frozen storage, though moreslowly at lower temperature. Air tight packaging will slow rancidity development.

Oxidative rancidity[edit]

Oxidativerancidity is associated with the degradation by oxygen in the air. Via a free radical

process, thedouble bondsof anunsaturated fatty acidcan undergo cleavage, releasingvolatilealdehydesandketones.This process can be suppressed by the exclusion of oxygen orby the addition ofantioxidants.Oxidation primarily occurs with unsaturated fats.

Microbial rancidity[edit]

Microbialrancidity refers to a process in which microorganisms, such as bacteria, use theirenzymes such aslipasesto break down fat. This pathway can be prevented by sterilization.

Reducing rancidification[edit]

Thefree radicalpathway for the first phase of the oxidative rancidification of fats.

Antioxidantsare often added to fat-containing foods to delay the onset or slow thedevelopment of rancidity due to oxidation. Natural antioxidants includepolyphenols(forinstance flavonoids),ascorbic acid(vitamin C) andtocopherols(vitamin E). Syntheticantioxidants includebutylated hydroxyanisole(BHA),butylatedhydroxytoluene(BHT),TBHQ,propyl gallateandethoxyquin.The natural antioxidants tend tobe short-lived[citation needed], so synthetic antioxidants are used when a longer shelf-life ispreferred. The effectiveness of water-soluble antioxidants is limited in preventing directoxidation within fats, but is valuable in intercepting freeradicalsthat travel through theaqueous parts of foods. A combination of water-soluble and fat-soluble antioxidants is ideal,

usually in the ratio of fat to water.In addition, rancidification can be decreased, but not completely eliminated, by storing fatsand oils in a cool, dark place with little exposure to oxygen or free radicals, since heat and lightaccelerate the rate of reaction of fats with oxygen. The addition of antimicrobialagents[clarification needed]can also delay or prevent rancidification by inhibiting the growth ofbacteria or other micro-organisms.
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Oxygen scavengingtechnology can be used to remove oxygen from food packaging andtherefore prevent oxidative rancidification.

Measurement of oxidative stability[edit]

Oxidative stability is a measure of an oil or fat's resistance to oxidation. Because the processtakes place through a chain reaction, the oxidation reaction has a period when it is relativelyslow, before it suddenly speeds up. The time for this to happen is called the "induction time",and it is repeatable under identical conditions (temperature, air flow, etc.). There are a numberof ways to measure the progress of the oxidation reaction. One of the most popular methodscurrently in use is the Rancimat method.

The Rancimat method is carried out using an air current at temperatures between 50 and220 C. The volatile oxidation products (largelyformic acid[4]p. 47) are carried by the aircurrent into the measuring vessel, where they are absorbed (dissolve) in the measuring fluid(distilled water). By continuous measurement of the conductivity of this solution, oxidation

curves can be generated. Thecusp pointof the oxidation curve (the point where a rapid rise inthe conductivity starts) gives the induction time of the rancidification reaction,[5]and can betaken as an indication of the oxidative stability of the sample.

The Rancimat method, the oxidative stability instrument (OSI) and the oxidograph were alldeveloped as automatic versions of the more complicated AOM (active oxygen method), whichis based on measuring peroxide values,[5]for determining the induction time of fats and oils.Over time, the Rancimat method has become established, and it has been accepted into anumber of national and international standards, for exampleAOCSCd 12b-92 and ISO 6886.
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CorrosionFrom Wikipedia, the free encyclopedia

For the Front Line Assembly album, seeCorrosion (album).

[hide]VTE

Mechanical failure modes

BucklingCorrosionCorrosion fatigueCreepFatigueFoulingFractureHydrogen embrittlementImpactMechanical overloadStress corrosion crackingThermal shock

WearYielding

Corrosionis the gradual destruction of materials (usuallymetals)by chemical reaction with itsenvironment.

In the most common use of the word, this means electrochemicaloxidationof metals inreaction with an oxidant such asoxygen.Rusting,the formation ofironoxides, is a well-knownexample of electrochemical corrosion. This type of damage typically producesoxide(s)
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orsalt(s) of the original metal. Corrosion can also occur in materials other than metals, suchasceramicsorpolymers,although in this context, the term degradation is more common.Corrosion degrades the useful properties of materials and structures including strength,appearance and permeability to liquids and gases.

Many structuralalloyscorrode merely from exposure to moisture in air, but the process can bestrongly affected by exposure to certain substances. Corrosion can be concentrated locally toform apitor crack, or it can extend across a wide area more or less uniformly corroding thesurface. Because corrosion is a diffusion-controlled process, it occurs on exposed surfaces. Asa result, methods to reduce the activity of the exposed surface, suchaspassivationandchromate conversion,can increase a material's corrosion resistance.However, some corrosion mechanisms are less visible and less predictable.

Rust, the most familiar example of corrosion.

Volcanic gaseshave accelerated the corrosion of this abandoned mining machinery.
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Corrosion on exposed metal.

Contents

[hide]

1 Galvanic corrosiono 1.1 Galvanic series

2 Corrosion removal 3 Resistance to corrosion

o 3.1 Intrinsic chemistryo 3.2 Passivation

4 Corrosion in passivated materialso 4.1 Pitting corrosiono 4.2 Weld decay and knifeline attacko 4.3 Crevice corrosion

5 Microbial corrosion 6 High-temperature corrosion 7 Metal dusting 8 Protection from corrosion

o 8.1 Surface treatments 8.1.1 Applied coatings 8.1.2 Reactive coatings 8.1.3 Anodization 8.1.4 Biofilm coatings

o 8.2 Controlled permeability formworko 8.3 Cathodic protection

8.3.1 Sacrificial anode protection 8.3.2 Impressed current cathodic protection

o 8.4 Anodic protectiono 8.5 Rate of corrosion

9 Economic impact 10 Corrosion in nonmetals

o 10.1 Corrosion of polymerso 10.2 Corrosion of glasses

10.2.1 Glass corrosion tests 11 See also 12 References 13 Further reading 14 External links

Galvanic corrosion[edit]
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ki/Corrosion#Anodic_protectionhttp://en.wikipedia.org/wiki/Corrosion#Impressed_current_cathodic_protectionhttp://en.wikipedia.org/wiki/Corrosion#Sacrificial_anode_protectionhttp://en.wikipedia.org/wiki/Corrosion#Cathodic_protectionhttp://en.wikipedia.org/wiki/Corrosion#Controlled_permeability_formworkhttp://en.wikipedia.org/wiki/Corrosion#Biofilm_coatingshttp://en.wikipedia.org/wiki/Corrosion#Anodizationhttp://en.wikipedia.org/wiki/Corrosion#Reactive_coatingshttp://en.wikipedia.org/wiki/Corrosion#Applied_coatingshttp://en.wikipedia.org/wiki/Corrosion#Surface_treatmentshttp://en.wikipedia.org/wiki/Corrosion#Protection_from_corrosionhttp://en.wikipedia.org/wiki/Corrosion#Metal_dustinghttp://en.wikipedia.org/wiki/Corrosion#High-temperature_corrosionhttp://en.wikipedia.org/wiki/Corrosion#Microbial_corrosionhttp://en.wikipedia.org/wiki/Corrosion#Crevice_corrosionhttp://en.wikipedia.org/wiki/Corrosion#Weld_decay_and_knifeline_attackhttp://en.wikipedia.org/wiki/Corrosion#Pitting_corrosionhttp://en.wikipedia.org/wiki/Corrosion#Corrosion_in_passivated_materialshttp://en.wikipedia.org/wiki/Corrosion#Passivationhttp://en.wikipedia.org/wiki/Corrosion#Intrinsic_chemistryhttp://en.wikipedia.org/wiki/Corrosion#Resistance_to_corrosionhttp://en.wikipedia.org/wiki/Corrosion#Corrosion_removalhttp://en.wikipedia.org/wiki/Corrosion#Galvanic_serieshttp://en.wikipedia.org/wiki/Corrosion#Galvanic_corrosionhttp://en.wikipedia.org/wiki/Corrosion


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Main article:Galvanic corrosion

Galvanic corrosion of aluminium. A 5-mm-thick aluminium alloy plate is physically (and hence, electrically)

connected to a 10-mm-thick mild steel structural support. Galvanic corrosion occurred on the aluminium plate

along the joint with the steel. Perforation of aluminium plate occurred within 2 years.[1]

Galvanic corrosion occurs when two different metals have physical or electrical contact witheach other and are immersed in a commonelectrolyte,or when the same metal is exposed toelectrolyte with different concentrations. In agalvanic couple,the more active metal (theanode) corrodes at an accelerated rate and the morenoble metal(the cathode) corrodes at aretarded rate. When immersed separately, each metal corrodes at its own rate. What type ofmetal(s) to use is readily determined by following thegalvanic series.For example, zinc isoften used as a sacrificial anode for steel structures. Galvanic corrosion is of major interest tothe marine industry and also anywhere water (containing salts) contacts pipes or metalstructures.

Factors such as relative size ofanode,types of metal, and operating conditions(temperature,humidity,salinity,etc.) affect galvanic corrosion. The surface area ratio of theanode andcathodedirectly affects the corrosion rates of the materials. Galvanic corrosion isoften prevented by the use ofsacrificial anodes.

Galvanic series[edit]Main article:Galvanic series

In a given environment (one standard medium is aerated, room-temperatureseawater), onemetal will be either more nobleor more activethan others, based on how strongly its ions arebound to the surface. Two metals in electrical contact share the same electrons, so that the"tug-of-war" at each surface is analogous to competition for free electrons between the twomaterials. Using the electrolyte as a host for the flow of ions in the same direction, the noblemetal will take electrons from the active one. The resulting mass flow or electrical current canbe measured to establish a hierarchy of materials in the medium of interest. This hierarchy iscalled agalvanic seriesand is useful in predicting and understanding corrosion. This method isexpensive but offers maximum protection against corrosion.
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Corrosion removal[edit]

Often it is possible to chemically remove the products of corrosion. For examplephosphoricacidin the form ofnaval jellyis often applied to ferrous tools or surfaces to remove rust.Corrosion removal should not be confused withelectropolishing,which removes some layers of

the underlying metal to make a smooth surface. For example, phosphoric acid may also be usedto electropolish copper but it does this by removing copper, not the products of coppercorrosion.

Resistance to corrosion[edit]

Some metals are more intrinsically resistant to corrosion than others (for some examples,seegalvanic series). There are various ways of protecting metals from corrosion (oxidation)including painting,hot dip galvanizing,and combinations of these.[2]

Intrinsic chemistry[edit]

Gold nuggetsdo not naturally corrode, even on a geological time scale.

The materials most resistant to corrosion are those for which corrosionisthermodynamicallyunfavorable. Any corrosion products ofgoldorplatinumtend todecompose spontaneously into pure metal, which is why these elements can be found in metallicform on Earth and have long been valued. More common "base" metals can only be protected bymore temporary means.

Some metals have naturally slowreaction kinetics,even though their corrosion isthermodynamically favorable. These include such metals aszinc,magnesium,andcadmium.While corrosion of these metals is continuous and ongoing, it happens at an acceptably slowrate. An extreme example isgraphite,which releases large amounts of energy uponoxidation,but has such slow kinetics that it is effectively immune to electrochemical corrosion under

normal conditions.

Passivation[edit]Main article:Passivation (chemistry)

Passivation refers to the spontaneous formation of an ultrathin film of corrosion productsknown as passive film, on the metal's surface that act as a barrier to further oxidation. Thechemical composition and microstructure of a passive film are different from the underlying
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metal. Typical passive film thickness on aluminium, stainless steels and alloys is within 10nanometers. The passive film is different from oxide layers that are formed upon heating andare in the micrometer thickness range the passive film recovers if removed or damagedwhereas the oxide layer does not. Passivation in natural environments such as air, water andsoil at moderatepHis seen in such materials asaluminium,stainless steel,titanium,andsilicon.

Passivation is primarily determined by metallurgical and environmental factors. The effect ofpH is summarized usingPourbaix diagrams,but many other factors are influential. Someconditions that inhibit passivation include high pH for aluminium and zinc, low pH or thepresence ofchlorideions for stainless steel, high temperature for titanium (in which case theoxide dissolves into the metal, rather than the electrolyte) andfluorideions for silicon. On theother hand, unusual conditions may result in passivation of materials that are normallyunprotected, as the alkaline environment ofconcretedoes forsteelrebar.Exposure to a liquidmetal such asmercuryor hotsoldercan often circumvent passivation mechanisms. Passivationis primarily determined by metallurgical and environmental factors.

Corrosion in passivated materials[edit]Passivation is extremely useful in mitigating corrosion damage, however even a high-qualityalloy will corrode if its ability to form a passivating film is hindered. Proper selection of theright grade of material for the specific environment is important for the long-lastingperformance of this group of materials. If breakdown occurs in the passive film due tochemical or mechanical factors, the resulting major modes of corrosion may includepittingcorrosion,crevice corrosionandstress corrosion cracking.

Pitting corrosion[edit]Main article:Pitting corrosion

The scheme of pitting corrosion

Certain conditions, such as low concentrations of oxygen or high concentrations of species suchas chloride which complete asanions,can interfere with a given alloy's ability to re-form apassivating film. In the worst case, almost all of the surface will remain protected, but tinylocal fluctuations will degrade the oxide film in a few critical points. Corrosion at these pointswill be greatly amplified, and can cause corrosion pitsof several types, depending uponconditions. While the corrosion pits onlynucleateunder fairly extreme circumstances, they can
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continue to grow even when conditions return to normal, since the interior of a pit is naturallydeprived of oxygen and locally the pH decreases to very low values and the corrosion rateincreases due to an autocatalytic process. In extreme cases, the sharp tips of extremely longand narrow corrosion pits can causestress concentrationto the point that otherwise toughalloys can shatter; a thin film pierced by an invisibly small hole can hide a thumb sized pit from

view. These problems are especially dangerous because they are difficult to detect before apart or structurefails.Pitting remains among the most common and damaging forms ofcorrosion in passivated alloys[citation needed], but it can be prevented by control of the alloy'senvironment.

Pitting results when a small hole, or cavity, forms in the metal, usually as a result of de-passivation of a small area. This area becomes anodic, while part of the remaining metalbecomes cathodic, producing a localized galvanic reaction. The deterioration of this small areapenetrates the metal and can lead to failure. This form of corrosion is often difficult todetect due to the fact that it is usually relatively small and may be covered and hidden bycorrosion-produced compounds.

Weld decay and knifeline attack[edit]

Normal microstructure

Sensitized microstructure

Main article:Intergranular corrosion

Stainless steel can pose special corrosion challenges, since its passivating behavior relies onthe presence of a major alloying component (chromium,at least 11.5%). Because of the elevated
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temperatures ofweldingand heat treatment, chromiumcarbidescan form in thegrainboundariesof stainless alloys. This chemical reaction robs the material of chromium in thezone near the grain boundary, making those areas much less resistant to corrosion. Thiscreates agalvanic couplewith the well-protected alloy nearby, which leads to welddecay(corrosion of the grain boundaries in the heat affected zones) in highly corrosive

environments.

A stainless steel is said to be sensitized ifchromium carbidesare formed in themicrostructure. A typical microstructure of a normalized type-304 stainless steel shows nosigns of sensitization while a heavily sensitized steel shows the presence of grain boundaryprecipitates. The dark lines in the sensitized microstructure are networks of chromiumcarbides formed along the grain boundaries.[3]

Special alloys, either with low carbon content or with added carbon "getters" such as titaniumandniobium(in types 321 and 347, respectively), can prevent this effect, but the latterrequire special heat treatment after welding to prevent the similar phenomenon of knifelineattack. As its name implies, corrosion is limited to a very narrow zone adjacent to the weld,often only a few micrometers across, making it even less noticeable.

Crevice corrosion[edit]Main article:Crevice corrosion

Corrosion in the crevice between the tube and tube sheet (both made of type-316 stainless steel) of a heat

exchanger in a seawater desalination plant.[4]

Crevice corrosionis a localized form of corrosion occurring in confined spaces (crevices), towhich the access of the working fluid from the environment is limited. Formation of adifferential aeration cell leads to corrosion inside the crevices. Examples of crevices are gapsand contact areas between parts, under gaskets or seals, inside cracks and seams, spaces filled

with deposits and under sludge piles.

Crevice corrosion is influenced by the crevice type (metal-metal, metal-nonmetal), crevicegeometry (size, surface finish), and metallurgical and environmental factors. The susceptibilityto crevice corrosion can be evaluated with ASTM standard procedures. A critical crevicecorrosion temperature is commonly used to rank a material's resistance to crevice corrosion.

Microbial corrosion[edit]
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Main article:Microbial corrosion

Microbial corrosion,or commonly known as microbiologically influenced corrosion (MIC), is acorrosion caused or promoted bymicroorganisms,usuallychemoautotrophs.It can apply toboth metallic and non-metallic materials, in the presence or absence of oxygen.Sulfate-reducing bacteriaare active in the absence of oxygen (anaerobic); they producehydrogensulfide,causingsulfide stress cracking.In the presence of oxygen (aerobic), some bacteriamay directly oxidize iron to iron oxides and hydroxides, other bacteria oxidize sulfur andproduce sulfuric acid causingbiogenic sulfide corrosion.Concentration cellscan form in thedeposits of corrosion products, leading to localized corrosion.

Accelerated low-water corrosion (ALWC) is a particularly aggressive form of MIC that affectssteel piles in seawater near the low water tide mark. It is characterized by an orange sludge,which smells of hydrogen sulfide when treated with acid. Corrosion rates can be very high anddesign corrosion allowances can soon be exceeded leading to premature failure of the steelpile.[5]Piles that have been coating and have cathodic protection installed at the time ofconstruction are not susceptible to ALWC. For unprotected piles, sacrificial anodes can beinstalled local to the affected areas to inhibit the corrosion or a complete retrofittedsacrificial anode system can be installed. Affected areas can also be treated electrochemicallyby using an electrode to first produce chlorine to kill the bacteria, and then to produced acalcareous deposit, which will help shield the metal from further attack.

High-temperature corrosion[edit]

High-temperature corrosion is chemical deterioration of a material (typically a metal) as aresult of heating. This non-galvanic form of corrosion can occur when a metal is subjected to ahot atmosphere containing oxygen, sulfur or other compounds capable of oxidizing (or assisting

the oxidation of) the material concerned. For example, materials used in aerospace, powergeneration and even in car engines have to resist sustained periods at high temperature inwhich they may be exposed to an atmosphere containing potentially highly corrosive productsof combustion.

The products of high-temperature corrosion can potentially be turned to the advantage of theengineer. The formation of oxides on stainless steels, for example, can provide a protectivelayer preventing further atmospheric attack, allowing for a material to be used for sustainedperiods at both room and high temperatures in hostile conditions. Such high-temperaturecorrosion products, in the form ofcompacted oxide layer glazes,prevent or reduce wearduring high-temperature sliding contact of metallic (or metallic and ceramic) surfaces.

Metal dusting[edit]Main article:Metal dusting

Metal dustingis a catastrophic form of corrosion that occurs when susceptible materials areexposed to environments with high carbon activities, such as synthesis gas and other high-COenvironments. The corrosion manifests itself as a break-up of bulk metal to metal powder. Thesuspected mechanism is firstly the deposition of a graphite layer on the surface of the metal,
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usually from carbon monoxide (CO) in the vapour phase. This graphite layer is then thought toform metastable M3C species (where M is the metal), which migrate away from the metalsurface. However, in some regimes no M3C species is observed indicating a direct transfer ofmetal atoms into the graphite layer.

Protection from corrosion[edit]

The US Armyshrink wrapsequipment such as helicopters to protect them from corrosion and thus save millions

of dollars.

Surface treatments[edit]

Applied coatings[edit]Main article:Galvanization

Galvanizedsurface

Plating,painting,and the application ofenamelare the most commonanti-corrosiontreatments.They work by providing a barrier of corrosion-resistant material between the damagingenvironment and the structural material. Aside from cosmetic and manufacturing issues, theremay be tradeoffs in mechanical flexibility versus resistance to abrasion and high temperature.Platings usually fail only in small sections, but if the plating is more noble than the substrate(for example, chromium on steel), agalvanic couplewill cause any exposed area to corrode muchmore rapidly than an unplated surface would. For this reason, it is often wise to plate withactive metal such as zinc or cadmium.

Painting either by roller or brush is more desirable for tight spaces; spray would be better forlarger coating areas such as steel decks and waterfront applications. Flexible polyurethanecoatings, like Durabak-M26 for example, can provide an anti-corrosive seal with a highly
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durable slip resistant membrane. Painted coatings are relatively easy to apply and have fastdrying times although temperature and humidity may cause dry times to vary.

Reactive coatings[edit]

If the environment is controlled (especially in recirculating systems),corrosion inhibitorscan

often be added to it. These chemicals form an electrically insulating or chemically impermeablecoating on exposed metal surfaces, to suppress electrochemical reactions. Such methods makethe system less sensitive to scratches or defects in the coating, since extra inhibitors can bemade available wherever metal becomes exposed. Chemicals that inhibit corrosion include someof the salts inhard water(Roman water systems are famous for theirmineraldeposits),chromates,phosphates,polyaniline,otherconducting polymersand a wide range ofspecially-designed chemicals that resemblesurfactants(i.e. long-chain organic molecules withionic end groups).

Anodization[edit]Main article:Anodizing

Thisclimbing descenderis anodized with a yellow finish.

Aluminium alloys often undergo a surface treatment. Electrochemical conditions in the bath arecarefully adjusted so that uniform pores, severalnanometerswide, appear in the metal's oxidefilm. 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-usualsurface layer. If this coating is scratched, normal passivation processes take over to protectthe damaged area.

Anodizing is very resilient to weathering and corrosion, so it is commonly used for buildingfacades and other areas that the surface will come into regular contact with the elements.

Whilst being resilient, it must be cleaned frequently. If left without cleaning,panel edgestainingwill naturally occur.

Biofilm coatings[edit]

A new form of protection has been developed by applying certain species of bacterial films tothe surface of metals in highly corrosive environments. This process increases the corrosionresistance substantially. Alternatively, antimicrobial-producingbiofilmscan be used to inhibitmild steel corrosion fromsulfate-reducing bacteria.[6]
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Controlled permeability formwork[edit]Main article:Controlled permeability formwork

Controlled permeability formwork (CPF) is a method of preventing the corrosionofreinforcementby naturally enhancing the durability of thecoverduring concrete placement.

CPF has been used in environments to combat the effects ofcarbonation,chlorides,frostandabrasion.

Cathodic protection[edit]Main article:Cathodic protection

Cathodic protection (CP) is a technique to control the corrosion of a metal surface by makingthat surface the cathode of anelectrochemical cell.Cathodic protection systems are mostcommonly used to protect steel, water, and fuelpipelinesand tanks; steel pierpiles,ships, andoffshoreoil platforms.

Sacrificial anode protection[edit]

Sacrificial anode on the hull of a ship.

For effective CP, the potential of the steel surface is polarized (pushed) more negative untilthe metal surface has a uniform potential. With a uniform potential, the driving force for thecorrosion reaction is halted. For galvanic CP systems, the anode material corrodes under theinfluence of the steel, and eventually it must be replaced. Thepolarizationis caused by thecurrent flow from the anode to the cathode, driven by the difference in electrochemicalpotential between the anode and the cathode.

Impressed current cathodic protection[edit]

For larger structures, galvanic anodes cannot economically deliver enough current to providecomplete protection.Impressed current cathodic protection(ICCP) systems use anodes

connected to aDCpower source (such as acathodic protection rectifier). Anodes for ICCPsystems are tubular and solid rod shapes of various specialized materials. These include highsiliconcast iron,graphite, mixed metal oxide or platinum coated titanium or niobium coated rodand wires.

Anodic protection[edit]Main article:Anodic protection
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Anodic protection impresses anodic current on the structure to be protected (opposite to thecathodic protection). It is appropriate for metals that exhibit passivity (e.g., stainless steel)and suitably small passive current over a wide range of potentials. It is used in aggressiveenvironments, e.g., solutions of sulfuric acid.

Rate of corrosion[edit]A simple test for measuring corrosion is the weight loss method.[citation needed]The methodinvolves exposing a clean weighed piece of the metal or alloy to the corrosive environment for aspecified time followed by cleaning to remove corrosion products and weighing the piece todetermine the loss of weight. The rate of corrosion (R) is calculated as

R = KW/(At)

where kis a constant, Wis the weight loss of the metal in time t, Ais the surface area ofthe metal exposed, andis the density of the metal (in g/cm).

Economic impact[edit]

The collapsed Silver Bridge, as seen from the Ohio side

In 2002, the USFederal Highway Administrationreleased a study titled Corrosion Costsand Preventive Strategies in the United Stateson the direct costs associated withmetallic corrosion in the U.S. industry. In 1998, the total annual direct cost of corrosion inthe U.S. was ca. $276 billion (ca. 3.2% of the USgross domestic product).[7]

Rust is one of the most common causes of bridge accidents. As rust has a much highervolume than the originating mass of iron, its build-up can also cause failure by forcing apartadjacent parts. It was the cause of the collapse of theMianus river bridgein 1983, whenthe bearings rusted internally and pushed one corner of the road slab off its support.

Three drivers on the roadway at the time died as the slab fell into the river below. ThefollowingNTSBinvestigation showed that a drain in the road had been blocked for road re-surfacing, and had not been unblocked; as a result, runoff water penetrated the supporthangers. Rust was also an important factor in theSilver Bridgedisaster of 1967 inWestVirginia,when a steelsuspension bridgecollapsed within a minute, killing 46 drivers andpassengers on the bridge at the time.
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