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

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Corrosion

A corrosão é a oxidação espontânea do metal na presença de um agente oxidante, geralmente oxigénio. É o mesmo fenómeno que está por detrás da cor azul-verde da Estátua da Liberdade ou uma âncora enferrujada. Alguma corrosão é protetora.Por exemplo, o cobre Oxida-se na presença de oxigénio, carbono dióxido de carbono, e água, para formar carbonato de cobre básico, uma camada protetora azul-esverdeada e estruturalmente estável. Outra corrosão é prejudicial. Por exemplo, o ferro oxida na presença de oxigénio, água, e íons de hidrogénio, em óxidos metálicos estruturalmente instáveis chamados ferrugem, que se desprende continuamente, expondo o metal subjacente para uma maior oxidação.A oxidação é reação uma eletroquímica espontânea. Defeitos na superfície servem como regiões anódicas onde o ferro, com um potencial elétrodo de 0, 44 volts, facilmente oxida em ferro Os elétrons migram em direção à região catódica, onde o oxigénio, com um potencial elétrodo de 1, 23 volts, é reduzido a água num ambiente ácido. Os íons de hidrogénio provêm da reação atmosférica entre o dióxido de carbono e a água, formando ácido carbónico.Em seguida, os íons de ferro migram para a região catódica através da humidade superficial e são depois oxidado em hidrato de óxido de ferro geralmente conhecido como ferrugem'A estequiometria do hidrato varia com a quantidade de água a que o ferro está exposto. A oxidação é acelerada com humidade adicional, ácidos, e eletrólitos, que melhoram o fluxo de cargas, e a concentração de íons H+Os métodos para prevenir a corrosão incluem pintar a superfície metálica para prevenir o contacto com a água e o oxigénio ou a utilização de ligas, uma combinação de múltiplos metais. O aço inoxidável é uma liga de ferro que contém uma pequena quantidade de crómio, que corrói e forma uma camada de óxido não reativo na superfície protegendo o ferro.Em alternativa, também é utilizada a galvanização. Aqui, um parafuso de ferro é revestido com uma camada de um metal mais facilmente oxidado, como o zinco, que forma uma camada protetora de óxido de zinco que protege o ferro. Finalmente, a proteção catódica controla a corrosão convertendo o metal a ser protegido em cátodo.Os tubos de ferro enterrados estão frequentemente ligados a um metal mais facilmente oxidado, como o zinco, onde o zinco atua como um anodo sacrifical a ser oxidado, e o ferro é o cátodo protegido contra a corrosão.

corrosão

A degradação de metais devido a processos eletroquímicos naturais é conhecida como corrosão. A formação de ferrugem no ferro, as manchas em prata, e a pátina azul-verde que se desenvolve sobre cobre são exemplos de corrosão. A corrosão envolve a oxidação de metais. Às vezes é protetora, como a oxidação do cobre ou alumínio, em que uma camada protetora de óxido de metal ou seus derivados se forma à superfície, protegendo o metal subjacente de oxidação adicional. Em outros casos, a corrosão é prejudicial ao metal, como a ferrugem do ferro.
Reação Redox Indesejável: A Ferrugem do Ferro
A ferrugem ocorre devido à exposição do ferro ao oxigénio e à água. A formação de ferrugem envolve a criação de uma célula galvânica na superfície do ferro, o que resulta na geração de ferro(II). As reações redox relevantes que ocorrem nas regiões anódicas (oxidação do ferro) e catódicas (redução do oxigénio) formadas na superfície do ferro incluem:
<img alt="Eq1" src="/files/ftp_upload/11440/11440_Equation_1.png" style="text-align: center; height: 150px;" />
O ferro(II) reage ainda mais com ar húmido, formando um óxido de ferro(III) hidratado, conhecido normalmente como ferrugem.
<img alt="Eq2" src="/files/ftp_upload/11440/11440_Equation_2.png" style="text-align: center; height: 50px;" />
A estequiometria do hidrato varia com a quantidade de água a que o ferro(II) é exposto, conforme indicado pelo uso de x na fórmula do composto. A humidade, a presença de ácidos e eletrólitos aumenta a velocidade de formação de ferrugem. Ao contrário da pátina sobre cobre, a formação de ferrugem não cria uma camada protetora, e assim a corrosão do ferro continua à medida a ferrugem descai e expõe ferro fresco à atmosfera.
Prevenção da Corrosão
Podem ser utilizados vários métodos para evitar a corrosão. Uma maneira é manter a superfície metálica pintada para evitar o contacto com água e oxigénio. A liga de metais, como a mistura de ferro com pequenas quantidades de crómio em aço inoxidável, é outro método eficaz para evitar a corrosão. O crómio acumula-se perto da superfície e sofre oxidação, protegendo eficazmente o ferro da corrosão.
O ferro e outros metais também podem ser protegidos contra a corrosão por galvanização, processo no qual o metal a proteger é revestido com uma camada de um metal mais facilmente oxidado, geralmente zinco. Quando a camada de zinco está intacta, impede que o ar entre em contacto com o ferro subjacente e, assim, evita a corrosão. Se a camada de zinco for rompida por corrosão ou abrasão mecânica, o ferro ainda poderá ser protegido contra a corrosão por um processo de proteção catódica, descrito no parágrafo seguinte.
A proteção catódica utiliza o princípio da conversão do metal a ser protegido em um cátodo em uma reação eletroquímica. Isto é conseguido através da ligação do metal protegido a um metal mais ativo ou facilmente oxidado, como o zinco ou magnésio, conhecido como ânodo sacrificial. O ânodo corrói e é utilizado para proteger o metal que serve como cátodo. A proteção catódica é mais comumente usada em eletrodomésticos, como aquecedores de água e tanques subterrâneos de armazenamento de água. A ter em conta, a proteção catódica pode ser usada para outros metais que não apenas ferro.
Este texto é adaptado de <a href="https://openstax.org/books/chemistry-2e/pages/17-6-corrosion">OpenStax, Chemistry 2e, Chapter 17.6: Corrosion</a>.

Corrosão

A degradação de metais devido a processos eletroquímicos naturais é conhecida como corrosão. Formação de ferrugem no ferro, manchas de prata e o azul-esverdeado ...

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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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