Name: rusting and metal protection
Uploaded: 2020-09-24T15:06:52.000Z
Duration: 2 min 49 s
Channel: JoVE

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Corrosion

Corrosie is de spontane oxidatie van metaal in aanwezigheid van een oxidatiemiddel, meestal zuurstof. Hetzelfde fenomeen achter de blauwgroene kleur van het Vrijheidsbeeld of een roestig anker. Enige corrosie is beschermend.Koper oxideert bijvoorbeeld in aanwezigheid van zuurstof, koolstofdioxide en water om basisch kopercarbonaat te vormen, een blauwgroene en structureel stabiele beschermende laag. Andere corrosie is schadelijk. IJzer oxideert bijvoorbeeld in aanwezigheid van zuurstof, water en waterstofionen tot structureel onstabiele metaaloxiden, roest genaamd, die continu afbladderen, waardoor het onderliggende metaal aan verdere oxidatie wordt blootgesteld.Roesten is een spontane elektrochemische reactie. Defecten aan het oppervlak dienen als de anodische gebieden waar ijzer, met een elektrodepotentiaal van 0, 44 volt, gemakkelijk oxideert tot ijzer(II)De elektronen migreren naar het kathodische gebied, waar zuurstof, met een elektrodepotentiaal van 1, 23 volt, wordt gereduceerd tot water in een zure omgeving. De waterstofionen zijn afkomstig van de atmosferische reactie tussen kooldioxide en water, waarbij koolzuur wordt gevormd.Vervolgens migreren de ijzer(II)ionen naar het kathodische gebied via het oppervlaktevocht en worden ze verder geoxideerd tot ijzer(III)oxidehydraat, algemeen bekend als roest'De stoichiometrie van het hydraat varieert met de hoeveelheid water waaraan ijzer(II)wordt blootgesteld. Het roesten wordt versneld door extra vocht, zuren en elektrolyten, die de ladingsstroom en de concentratie van H+ionen verbeteren. Methoden om corrosie te voorkomen zijn onder meer het schilderen van het metalen oppervlak om contact met water en zuurstof te voorkomen of het gebruik van legeringen, een combinatie van meerdere metalen.Roestvrij staal is een legering van ijzer die een kleine hoeveelheid chroom bevat, die corrodeert en een niet-reactieve oxidelaag vormt op het oppervlak die het ijzer beschermt. Als alternatief wordt ook galvanisatie gebruikt. Hier is een ijzeren schroef bedekt met een laag gemakkelijker geoxideerd metaal, zoals zink, dat een beschermende laag zinkoxide vormt die het ijzer beschermt.Ten slotte houdt kathodische bescherming corrosie tegen door het metaal dat wordt beschermd om te zetten in de kathode. Ondergrondse ijzeren buizen zijn vaak verbonden met een gemakkelijker geoxideerd metaal, zoals zink, waar het zink als een opofferingsanode werkt die wordt geoxideerd, en het ijzer als kathode wordt beschermd tegen corrosie.

corrosie

De afbraak van metalen als gevolg van natuurlijke elektrochemische processen staat bekend als corrosie. Roestvorming op ijzer, verkleuring van zilver en de blauwgroene patina die op koper ontstaat zijn voorbeelden van corrosie. Corrosie omvat de oxidatie van metalen. Soms is het beschermend, zoals bij de oxidatie van koper of aluminium, waarbij zich een beschermende laag van metaaloxide of zijn derivaten op het oppervlak vormt, die het onderliggende metaal beschermt tegen verdere oxidatie. In andere gevallen is corrosie schadelijk voor het metaal, zoals het roesten van ijzer. Ongewenste redoxreactie: het roesten van ijzer Roestvorming ontstaat door blootstelling van ijzer aan zuurstof en water. Roestvorming omvat de vorming van een galvanische cel aan het ijzeroppervlak, wat resulteert in de vorming van ijzer (II). De relevante redoxreacties die optreden in de anodische (oxidatie van ijzer) en kathodische (reductie van zuurstof) gebieden die op het ijzeroppervlak worden gevormd, zijn onder meer: <img alt="Eq1" src="/files/ftp_upload/11440/11440_Equation_1.png" style="text-align: center; height: 150px;"> Het ijzer(II) reageert verder met vochtige lucht, waarbij een ijzer(III)oxidehydraat ontstaat, beter bekend als roest. <img alt="Eq2" src="/files/ftp_upload/11440/11440_Equation_2.png" style="text-align: center; height: 50px;"> De stoichiometrie van het hydraat varieert met de hoeveelheid water waaraan ijzer (II) wordt blootgesteld, zoals aangegeven door het gebruik van x in de samengestelde formule. Vocht, de aanwezigheid van zuren en elektrolyten verhogen de snelheid van roestvorming. In tegenstelling tot de patina op koper, creëert de vorming van roest geen beschermende laag, en dus gaat de corrosie van het ijzer door terwijl de roest afbladdert en vers ijzer aan de atmosfeer blootstelt. Preventie van corrosie Er kunnen verschillende methoden worden gebruikt om corrosie te voorkomen. Eén manier is om het metalen oppervlak geverfd te houden om contact met water en zuurstof te voorkomen. Het legeren van metalen, zoals het mengen van ijzer met kleine hoeveelheden chroom in roestvrij staal, is een andere effectieve methode om corrosie te voorkomen. Het chroom verzamelt zich nabij het oppervlak en ondergaat oxidatie, waardoor het ijzer effectief tegen corrosie wordt beschermd. IJzer en andere metalen kunnen ook tegen corrosie worden beschermd door galvanisatie, een proces waarbij het te beschermen metaal wordt bedekt met een laag van een gemakkelijker geoxideerd metaal, meestal zink. Wanneer de zinklaag intact is, voorkomt deze dat lucht in contact komt met het onderliggende ijzer en voorkomt zo corrosie. Als de zinklaag wordt doorbroken door corrosie of mechanische slijtage, kan het ijzer nog steeds tegen corrosie worden beschermd door een kathodisch beschermingsproces, dat in de volgende paragraaf wordt beschreven. Kathodische bescherming maakt gebruik van het principe van het omzetten van het te beschermen metaal in een kathode in een elektrochemische reactie. Dit wordt bereikt door het beschermde metaal te verbinden met een actiever of gemakkelijk te oxideren metaal zoals zink of magnesium, bekend als de opofferingsanode. De anode corrodeert en raakt opgebruikt en beschermt het metaal dat als kathode dient. Kathodische bescherming wordt het meest gebruikt in huishoudelijke apparaten zoals boilers en ondergrondse wateropslagtanks. Belangrijk is dat kathodische bescherming kan worden gebruikt voor andere metalen dan alleen ijzer. Dit tekst is aangepast van <a href="https://openstax.org/books/chemistry-2e/pages/17-6-corrosion">OpenStax, Chemistry 2e, Chapter 17.6: Corrosie</a>.

Corrosie

The degradation of metals due to natural electrochemical processes is known as corrosion. Rust formation on iron, tarnishing of silver, and the blue-green ...

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rusting and metal protection

Corrosion is the natural breakdown of metals through electrochemical reactions. It can appear as rust on iron, tarnish on silver, or a blue-green patina on copper. In each case, the metal is oxidized. Some corrosion products can protect the surface, while others keep damaging the metal.

Rusting is the harmful type of corrosion seen in iron. It happens when iron is exposed to oxygen and water. A galvanic cell forms on the iron surface, and this drives oxidation at the anodic region and reduction of oxygen at the cathodic region. The iron(II) that forms then reacts with humid air to make iron(III) oxide hydrate, which is rust.

The amount of water in rust can vary, so its formula includes x to show changing stoichiometry. Moisture, acids, and electrolytes all speed up rust formation. Unlike the protective patina on copper, rust does not seal the metal. It flakes away and exposes fresh iron to air, so corrosion continues.

Metal corrosion can be slowed in several ways. One method is painting the surface so water and oxygen cannot reach it. Another is alloying, such as adding chromium to iron to make stainless steel. The chromium moves near the surface and oxidizes there, which helps protect the iron underneath.

Metals can also be protected by galvanization, where a layer of a more easily oxidized metal, usually zinc, coats the surface. If the zinc layer stays intact, it blocks air from the iron. If the coating is damaged, cathodic protection can still help. In that method, the protected metal is connected to a more active sacrificial anode, such as zinc or magnesium, which corrodes instead. This approach is used in water heaters and underground water storage tanks, and it can protect metals other than iron.

Rusting and Metal Protection

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corrosion

Corrosion is the natural breakdown of metals through electrochemical reactions. It can appear as rust on iron, tarnish on silver, or a blue-green patina on copper. In each case, the metal is oxidized. Some corrosion products can protect the surface, while others keep damaging the metal. Rusting is the harmful type of corrosion seen in iron. It happens when iron is exposed to oxygen and water. A galvanic cell forms on the iron surface, and this drives oxidation at the anodic region and...

electrolysis

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Electrolysis uses electrical energy to drive a nonspontaneous redox reaction. In a galvanic cell, a spontaneous redox reaction does electrical work on the surroundings by sending electrons through an external circuit. In electrolysis, an external circuit does work on the redox system by applying a voltage large enough to force the reaction in the opposite direction. This process is used in several real-world settings. Recharging a battery is one example, because an outside power source drives...

key terms and concepts

Learn how corrosion affects metals, why iron rusts, and how painting, stainless steel, galvanization, and cathodic protection help prevent damage.

Rusting and Metal Protection | JoVE K12

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Rusting and Metal Protection

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