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

K12 Standards

Arizona Science Standards

High School

Physical Science

HS.P1

HS.P1U3.4

Corrosion

La corrosione è l'ossidazione spontanea del metallo in presenza di un agente ossidante, solitamente ossigeno. Questo è lo stesso fenomeno del colore blu-verde della Statua della Libertà o di un'ancora arrugginita. Un po'di corrosione è protettiva.

Per esempio, il rame si ossida in presenza di ossigeno, anidride carbonica e acqua, per formare carbonato di rame basico, uno strato protettivo blu-verde e strutturalmente stabile. Un'altra tipologia di corrosione è dannosa. Per esempio, il ferro si ossida in presenza di ossigeno, acqua, e ioni idrogeno, in ossidi metallici strutturalmente instabili detti ruggine, che si sbriciolano continuamente, esponendo il metallo sottostante ad un'ulteriore ossidazione.

La ruggine è una reazione elettrochimica spontanea. I difetti sulla superficie fungono da regioni anodiche in cui il ferro, con un potenziale dell'elettrodo di 0, 44 volt, si ossida prontamente a ferro Gli elettroni migrano verso la regione catodica, dove l'ossigeno, con un potenziale dell'elettrodo di 1, 23 volt, viene ridotto ad acqua in un ambiente acido. Gli ioni idrogeno provengono dalla reazione atmosferica fra anidride carbonica e acqua, formando acido carbonico.

Successivamente, gli ioni di ferro migrano nella regione catodica attraverso l'umidità superficiale e vengono ulteriormente ossidati in ossido di ferro idrato, comunemente noto come ruggine. La stechiometria dell'idrato varia con la quantità di acqua a cui è esposto il ferro La ruggine viene accelerata dall'aggiunta di umidità, acidi ed elettroliti, che aumentano il flusso delle cariche e la concentrazione di ioni H+I metodi per prevenire la corrosione comprendono la verniciatura della superficie metallica per prevenire il contatto con acqua e ossigeno o l'uso di leghe, una combinazione di più metalli. L'acciaio inossidabile è una lega di ferro contenente una piccola quantità di cromo, che si corrode formando uno strato di ossido non reattivo sulla superficie, che protegge il ferro.

In alternativa, viene usata anche la zincatura. Qui, una vite di ferro è rivestita con uno strato di metallo più facilmente ossidabile, come lo zinco, che forma uno strato protettivo di ossido di zinco che protegge il ferro. Infine, la protezione catodica controlla la corrosione convertendo il metallo protetto nel catodo.

I tubi di ferro sotterranei sono spesso uniti a un metallo più facilmente ossidabile, come lo zinco, dove lo zinco funge da anodo sacrificale che viene ossidato, e il ferro è il catodo protetto dalla corrosione.

corrosione

La degradazione dei metalli dovuta a processi elettrochimici naturali è nota come corrosione. Esempi di corrosione sono la formazione di ruggine sul ferro, l'ossidazione dell'argento e la patina blu-verde che si sviluppa sul rame. La corrosione comporta l'ossidazione dei metalli. Talvolta ha funzione protettiva, come nel caso dell'ossidazione del rame o dell'alluminio, in cui sulla superficie si forma uno strato protettivo di ossido metallico o suoi derivati, che protegge il metallo sottostante da ulteriore ossidazione. In altri casi, la corrosione danneggia il metallo, come l'arrugginimento del ferro.

Reazione Redox Indesiderata: La Ruggine del Ferro

La ruggine si verifica a causa dell'esposizione del ferro all'ossigeno e all'acqua. La formazione della ruggine comporta la creazione di una cella galvanica sulla superficie del ferro, che si traduce nella generazione di ferro (II). Le reazioni redox rilevanti che si verificano nelle regioni anodica (ossidazione del ferro) e catodica (riduzione dell'ossigeno) formate sulla superficie del ferro includono:

<img alt="Eq1" src="/files/ftp_upload/11440/11440_Equation_1.png" style="text-align: center; height: 150px;" />

Il ferro (II) reagisce inoltre con l'aria umida, formando un ossido di ferro (III) idrato, comunemente noto come ruggine.

<img alt="Eq2" src="/files/ftp_upload/11440/11440_Equation_2.png" style="text-align: center; height: 50px;" />

La stechiometria dell'idrato varia con la quantità di acqua a cui è esposto il ferro (II), come indicato dall'uso di x nella formula del composto. L'umidità, la presenza di acidi ed elettroliti aumentano la velocità di formazione della ruggine. A differenza della patina sul rame, la formazione di ruggine non crea uno strato protettivo, quindi la corrosione del ferro continua mentre la ruggine si sfalda ed espone il ferro fresco all'atmosfera.

Prevenzione della corrosione

È possibile utilizzare vari metodi per prevenire la corrosione. Un modo è mantenere la superficie metallica verniciata per evitare il contatto con acqua e ossigeno. La lega dei metalli, come la miscelazione del ferro con piccole quantità di cromo nell'acciaio inossidabile, è un altro metodo efficace per prevenire la corrosione. Il cromo si raccoglie vicino alla superficie e subisce ossidazione, proteggendo così efficacemente il ferro dalla corrosione.

Il ferro e altri metalli possono anche essere protetti dalla corrosione mediante zincatura, un processo in cui il metallo da proteggere viene rivestito con uno strato di un metallo più facilmente ossidabile, solitamente zinco. Quando lo strato di zinco è intatto, impedisce all'aria di entrare in contatto con il ferro sottostante e quindi previene la corrosione. Se lo strato di zinco viene rotto da corrosione o abrasione meccanica, il ferro può ancora essere protetto dalla corrosione mediante un processo di protezione catodica, descritto nel paragrafo successivo.

La protezione catodica utilizza il principio di convertire il metallo da proteggere in un catodo in una reazione elettrochimica. Ciò si ottiene collegando il metallo protetto a un metallo più attivo o facilmente ossidabile come zinco o magnesio, noto come anodo sacrificale. L'anodo si corrode e si consuma proteggendo il metallo che funge da catodo. La protezione catodica è più comunemente utilizzata negli elettrodomestici come scaldabagni e serbatoi di stoccaggio dell'acqua sotterranei. È importante sottolineare che la protezione catodica può essere utilizzata anche per metalli diversi dal semplice ferro.

Questo testo è adattato da <a href="https://openstax.org/books/chemistry-2e/pages/17-6-corrosion">OpenStax, Chemistry 2e, Chapter 17.6: Corrosion</a>.

Corrosione

La degradazione dei metalli dovuta a processi elettrochimici naturali è nota come corrosione. Formazione di ruggine sul ferro, appannamento dell'argento e del blu-verde ...

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KEY TERMS AND CONCEPTS

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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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Concentration cells are voltaic cells that use two almost identical half-cells. The half-cells use the same electrode and the same half-reaction. They differ only in the concentration of one redox species, so the cell potential depends on that concentration difference. In a concentration cell, the Nernst equation can be used to find the cell potential. For the zinc example shown, the calculated cell potential is positive. A positive cell potential means the overall reaction is spontaneous. <br...

batteries and fuel cells

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Batteries and fuel cells are galvanic cells that supply electrical power for many applications. They range from tiny button batteries in wristwatches to large systems that can provide backup energy to municipal power grids. Some are primary cells, which are single-use and cannot be recharged. Others are secondary cells, which use reversible reactions and can be charged again by an external power source. A common primary battery is the dry cell. It uses zinc as both the container and the anode,...

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

Electrolysis in Batteries and Metal Production

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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