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

Korrosion ist die spontane Oxidation von Metall in Gegenwart eines Oxidationsmittels, in der Regel Sauerstoff. Das gleiche Phänomen verbirgt sich hinter der blau-grünen Farbe der Freiheitsstatue oder einem rostigen Anker.
Etwas Korrosion ist schützend. Kupfer oxidiert beispielsweise in Gegenwart von Sauerstoff, Kohlendioxid und Wasser zu basischem Kupfercarbonat, einer blaugrünen und strukturstabilen Schutzschicht.
Andere Korrosion ist schädlich. Zum Beispiel oxidiert Eisen in Gegenwart von Sauerstoff-, Wasser- und Wasserstoffionen zu strukturell instabilen Metalloxiden, die als Rost bezeichnet werden und kontinuierlich abplatzen und das darunter liegende Metall weiterer Oxidation aussetzen.
Rost ist eine spontane elektrochemische Reaktion. Defekte auf der Oberfläche dienen als anodische Bereiche, in denen Eisen mit einem Elektrodenpotential von -0,44 Volt leicht zu Eisen(II) oxidiert.
Die Elektronen wandern in den kathodischen Bereich, wo Sauerstoff mit einem Elektrodenpotential von +1,23 Volt in einem sauren Milieu zu Wasser reduziert wird. Die Wasserstoffionen entstehen durch die atmosphärische Reaktion zwischen Kohlendioxid und Wasser, wobei Kohlensäure entsteht.
Als nächstes wandern die Eisen(II)-Ionen über die Oberflächenfeuchtigkeit in den kathodischen Bereich und werden weiter zu Eisen(III)-oxidhydrat oxidiert, das allgemein als "Rost" bekannt ist. Die Stöchiometrie des Hydrats variiert mit der Menge an Wasser, der Eisen(II) ausgesetzt ist.
Das Rosten wird durch zusätzliche Feuchtigkeit, Säuren und Elektrolyte beschleunigt, die den Ladungsfluss und die Konzentration von H+-Ionen verbessern.
Zu den Methoden zur Vermeidung von Korrosion gehören das Lackieren der Metalloberfläche, um den Kontakt mit Wasser und Sauerstoff zu verhindern, oder die Verwendung von Legierungen, einer Kombination aus mehreren Metallen. Edelstahl ist eine Eisenlegierung, die eine geringe Menge Chrom enthält, das korrodiert und eine nicht reaktive Oxidschicht auf der Oberfläche bildet, die das Eisen schützt.
Alternativ kommt auch eine Verzinkung zum Einsatz. Hier wird eine Eisenschraube mit einer Schicht aus leichter oxidierbarem Metall wie Zink überzogen, die eine Schutzschicht aus Zinkoxid bildet, die das Eisen schützt.
Schließlich kontrolliert der kathodische Schutz die Korrosion, indem er das zu schützende Metall in die Kathode umwandelt. Unterirdische Eisenrohre sind oft mit einem leichter oxidierbaren Metall wie Zink verbunden, wobei das Zink als oxidierende Opferanode fungiert und das Eisen die vor Korrosion geschützte Kathode ist.

korrosion

Der Zerfall von Metallen aufgrund natürlicher elektrochemischer Prozesse wird als Korrosion bezeichnet. Rostbildung auf Eisen, Anlaufen von Silber und die blaugrüne Patina, die auf Kupfer entsteht, sind Beispiele für Korrosion. Unter Korrosion versteht man die Oxidation von Metallen. Manchmal ist es schützend, wie etwa bei der Oxidation von Kupfer oder Aluminium, wobei sich auf der Oberfläche eine Schutzschicht aus Metalloxid oder seinen Derivaten bildet, die das darunter liegende Metall vor weiterer Oxidation schützt. In anderen Fällen schädigt Korrosion das Metall, beispielsweise durch Rosten von Eisen. Unerwünschte Redoxreaktion: Das Rosten von Eisen Rost entsteht, wenn Eisen Sauerstoff und Wasser ausgesetzt wird. Bei der Rostbildung entsteht an der Eisenoberfläche eine galvanische Zelle, die zur Bildung von Eisen(II) führt. Zu den relevanten Redoxreaktionen, die an den auf der Eisenoberfläche gebildeten anodischen (Oxidation von Eisen) und kathodischen (Reduktion von Sauerstoff) Bereichen stattfinden, gehören: <img alt="Eq1" src="/files/ftp_upload/11440/11440_Equation_1.png" style="text-align: center; height: 150px;"> Das Eisen(II) reagiert weiter mit feuchter Luft und bildet ein Eisen(III)-Oxidhydrat, das allgemein als Rost bekannt ist. <img alt="Eq2" src="/files/ftp_upload/11440/11440_Equation_2.png" style="text-align: center; height: 50px;"> Die Stöchiometrie des Hydrats variiert mit der Wassermenge, der Eisen(II) ausgesetzt ist, was durch die Verwendung von x in der Verbindungsformel angezeigt wird. Feuchtigkeit, das Vorhandensein von Säuren und Elektrolyten beschleunigen die Rostbildung. Anders als bei der Patina auf Kupfer entsteht durch die Rostbildung keine Schutzschicht, und so setzt sich die Korrosion des Eisens fort, da der Rost abblättert und frisches Eisen der Atmosphäre preisgibt. Korrosionsschutz Um Korrosion vorzubeugen, können verschiedene Methoden eingesetzt werden. Eine Möglichkeit besteht darin, die Metalloberfläche lackiert zu lassen, um den Kontakt mit Wasser und Sauerstoff zu vermeiden. Das Legieren von Metallen, etwa das Mischen von Eisen mit kleinen Mengen Chrom in Edelstahl, ist eine weitere wirksame Methode zur Vorbeugung von Korrosion. Das Chrom sammelt sich in der Nähe der Oberfläche und wird oxidiert, wodurch das Eisen wirksam vor Korrosion geschützt wird. Eisen und andere Metalle können auch durch Galvanisierung vor Korrosion geschützt werden. Dabei handelt es sich um einen Prozess, bei dem das zu schützende Metall mit einer Schicht aus einem leichter oxidierbaren Metall, normalerweise Zink, überzogen wird. Wenn die Zinkschicht intakt ist, verhindert sie, dass Luft mit dem darunter liegenden Eisen in Kontakt kommt, und beugt so Korrosion vor. Wenn die Zinkschicht entweder durch Korrosion oder mechanischen Abrieb durchbrochen wird, kann das Eisen dennoch durch einen kathodischen Schutzprozess vor Korrosion geschützt werden, der im nächsten Absatz beschrieben wird. Der kathodische Schutz nutzt das Prinzip der Umwandlung des zu schützenden Metalls in eine Kathode in einer elektrochemischen Reaktion. Dies wird erreicht, indem das geschützte Metall mit einem aktiveren oder leichter oxidierbaren Metall wie Zink oder Magnesium verbunden wird, das als Opferanode bezeichnet wird. Die Anode korrodiert und verbraucht sich, um das Metall zu schützen, das als Kathode dient. Der kathodische Schutz wird am häufigsten in Haushaltsgeräten wie Warmwasserbereitern und unterirdischen Wasserspeichern eingesetzt. Wichtig ist, dass der kathodische Schutz auch für andere Metalle als nur Eisen verwendet werden kann. Dieser Text wurde angepasst von <a href="https://openstax.org/books/chemistry-2e/pages/17-6-corrosion">OpenStax, Chemistry 2e, Chapter 17.6: Corrosion</a>.

Korrosion

Die Zersetzung von Metallen durch natürliche elektrochemische Prozesse wird als Korrosion bezeichnet. Rostbildung auf Eisen, Anlaufen von Silber und dem blaugrünen ...

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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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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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