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

Korozja to spontaniczne utlenianie metalu w obecności czynnika utleniającego, zwykle tlenu. To samo zjawisko kryje się za niebiesko-zielonym kolorem Statuy Wolności lub zardzewiałą kotwicą.
Pewna korozja ma działanie ochronne. Na przykład miedź utlenia się w obecności tlenu, dwutlenku węgla i wody, tworząc zasadowy węglan miedzi, niebiesko-zieloną i strukturalnie stabilną warstwę ochronną.
Inna korozja jest szkodliwa. Na przykład żelazo utlenia się w obecności tlenu, wody i jonów wodorowych do strukturalnie niestabilnych tlenków metali zwanych rdzą, które w sposób ciągły się łuszczą, narażając metal znajdujący się pod spodem na dalsze utlenianie.
Rdzewienie jest spontaniczną reakcją elektrochemiczną. Defekty na powierzchni służą jako obszary anodowe, w których żelazo, o potencjale elektrody -0,44 wolta, łatwo utlenia się do żelaza(II).
Elektrony migrują w kierunku obszaru katodowego, w którym tlen o potencjale elektrody +1,23 V jest redukowany do wody w środowisku kwaśnym. Jony wodorowe pochodzą z reakcji atmosferycznej między dwutlenkiem węgla a wodą, tworząc kwas węglowy.
Następnie jony żelaza(II) migrują do obszaru katodowego poprzez wilgoć powierzchniową i są dalej utleniane do hydratu tlenku żelaza(III), powszechnie znanego jako "rdza". Stechiometria hydratu zmienia się w zależności od ilości wody, na którą wystawione jest żelazo(II).
Rdzewienie jest przyspieszane przez dodatkową wilgoć, kwasy i elektrolity, które poprawiają przepływ ładunków i stężenie jonów H+.
Metody zapobiegania korozji obejmują malowanie powierzchni metalu w celu zapobieżenia kontaktowi z wodą i tlenem lub stosowanie stopów, kombinacji wielu metali. Stal nierdzewna to stop żelaza zawierający niewielką ilość chromu, który koroduje, tworząc niereaktywną warstwę tlenku na powierzchni chroniącej żelazo.
Alternatywnie stosuje się również cynkowanie. Tutaj żelazna śruba jest pokryta warstwą łatwiej utleniającego się metalu, takiego jak, który tworzy ochronną warstwę tlenku chroniącą żelazo.
Wreszcie, ochrona katodowa kontroluje korozję poprzez przekształcenie chronionego metalu w katodę. Podziemne rury żelazne są często połączone z łatwiej utlenionym metalem, takim jak, gdzie działa jak anoda protektorowa utleniana, a żelazo jest katodą chronioną przed korozją.

korozja

Degradacja metali w wyniku naturalnych procesów elektrochemicznych nazywana jest korozją. Rdza na żelazie, matowienie srebra i niebiesko-zielona patyna powstająca na miedzi to przykłady korozji. Korozja polega na utlenianiu metali. Czasami ma charakter ochronny, na przykład w przypadku utleniania miedzi lub aluminium, podczas którego na powierzchni tworzy się warstwa ochronna z tlenku metalu lub jego pochodnych, chroniąc leżący pod spodem metal przed dalszym utlenianiem. W innych przypadkach korozja powoduje uszkodzenie metalu, na przykład rdzewienie żelaza.

Niepożądana reakcja redoks: rdzewienie żelaza

Rdzewienie powstaje w wyniku narażenia żelaza na działanie tlenu i wody. Powstawanie rdzy polega na tworzeniu się ogniwa galwanicznego na powierzchni żelaza, w wyniku czego wytwarza się żelazo(II). Odpowiednie reakcje redoks zachodzące w obszarach anodowych (utlenianie żelaza) i katodowych (redukcja tlenu) utworzonych na powierzchni żelaza obejmują:

<img alt="Eq1" src="/files/ftp_upload/11440/11440_Equation_1.png" style="text-align: center; height: 150px;" />

Żelazo(II) dalej reaguje z wilgotnym powietrzem, tworząc hydrat tlenku żelaza(III), powszechnie znany jako rdza.

<img alt="Eq2" src="/files/ftp_upload/11440/11440_Equation_2.png" style="text-align: center; height: 50px;" />

Stechiometria hydratu zmienia się w zależności od ilości wody, na działanie której żelazo (II) jest wystawione. Wskazuje na to użycie x we wzorze związku. Wilgoć, obecność kwasów i elektrolitów zwiększa szybkość powstawania rdzy. W przeciwieństwie do patyny na miedzi rdza nie tworzy warstwy ochronnej, w związku z czym korozja żelaza trwa nadal, gdy rdza odpada i wystawia świeże żelazo na działanie atmosfery.

Zapobieganie korozji

Aby zapobiec korozji, można zastosować różne metody. Jednym ze sposobów jest pomalowanie metalowej powierzchni, aby uniknąć kontaktu z wodą i tlenem. Dodawanie stopów metali, na przykład mieszanie żelaza z niewielkimi ilościami chromu w stali nierdzewnej, to kolejna skuteczna metoda zapobiegania korozji. Chrom gromadzi się przy powierzchni i ulega utlenieniu, skutecznie chroniąc żelazo przed korozją.

Żelazo i inne metale można również chronić przed korozją poprzez cynkowanie – proces, w którym zabezpieczany metal jest pokrywany warstwą łatwiej utlenionego metalu, zwykle cynku. Gdy warstwa cynku jest nienaruszona, zapobiega kontaktowi powietrza z żelazem znajdującym się pod spodem, a tym samym zapobiega korozji. Jeżeli warstwa cynku zostanie naruszona w wyniku korozji lub ścierania mechanicznego, żelazo można nadal chronić przed korozją poprzez proces ochrony katodowej, który opisano w następnym akapicie.

Ochrona katodowa wykorzystuje zasadę przekształcania chronionego metalu w katodę w reakcji elektrochemicznej. Osiąga się to poprzez połączenie chronionego metalu z bardziej aktywnym lub łatwiej utlenionym metalem, takim jak cynk lub magnez, zwanym anodą protektorową. Anoda ulega korozji i zużywa się, chroniąc metal pełniący funkcję katody. Ochrona katodowa jest najczęściej stosowana w urządzeniach gospodarstwa domowego, takich jak podgrzewacze wody i podziemne zbiorniki magazynujące wodę. Co ważne, ochronę katodową można stosować w przypadku metali innych niż samo żelazo.

Korozja

Degradacja metali w wyniku naturalnych procesów elektrochemicznych jest znana jako korozja. Powstawanie rdzy na żelazie, matowienie srebra i niebiesko-zielone ...

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Elektrochemia

KEY TERMS AND CONCEPTS

Kluczowe terminy i pojęcia

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...

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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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