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

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Corrosion

Коррозия это самопроизвольное окисление металла в присутствии окислителя, обычно кислорода. То же самое явление ответственно за сине-зеленый цвет Статуи Свободы или ржавого якоря. Одни виды коррозии защищают.Например, медь окисляется в присутствии кислорода, углекислого газа и воды с образованием основного карбоната меди, сине-зеленого и структурно устойчивого защитного слоя. Другие виды коррозии разрушают. Например, железо окисляется в присутствии ионов кислорода, воды и водорода до структурно нестабильных оксидов металла, называемых ржавчиной, которые непрерывно отслаиваются, подвергая нижележащий металл дальнейшему окислению.Ржавение это самопроизвольная электрохимическая реакция. Дефекты на поверхности служат анодными областями, где железо с электродным потенциалом 0, 44 вольта легко окисляется до железа Электроны мигрируют к катодной области, где кислород с электродным потенциалом 1, 23 вольта восстанавливается в кислой среде до воды. Ионы водорода образуются в результате атмосферной реакции между диоксидом углерода и водой с образованием угольной кислоты.Затем ионы железа мигрируют в катодную область через поверхностную влагу и далее окисляются до гидрата оксида железа широко известного как ржавчина"Стехиометрия гидрата зависит от количества воды, в которой находится железо Ржавение ускоряется за счет дополнительной влаги, кислот и электролитов, которые увеличивают поток зарядов и концентрацию ионов H+Методы предотвращения коррозии включают окраску металлической поверхности для предотвращения контакта с водой и кислородом или использование сплавов комбинации нескольких металлов. Нержавеющая сталь это сплав железа, содержащий небольшое количество хрома, который подвергается коррозии, образуя слой инертного оксида на поверхности, защищающий железо. В качестве альтернативы также используется оцинковка.Здесь железный винт покрыт слоем более легко окисляемого металла, такого как цинк, который образует слой оксида цинка, защищающий железо. Наконец, катодная защита контролирует коррозию, превращая защищаемый металл в катод. Подземные железные трубы часто соединяются с более легко окисляемым металлом, таким как цинк, где цинк действует как защитный анод при окислении, а железо является катодом, защищенным от коррозии.

Коррозия

Деградация металлов, вызванная естественными электрохимическими процессами, называется коррозией. Примером коррозии являются образование ржавчины на железе, потускание серебра и сине-зеленая патина, которая развивается на меди. Коррозия включает окисление металлов. Иногда она является защитной, например, окисление меди или алюминия, при котором на поверхности образуется защитный слой оксида металла или его производных, защищающий основной металл от дальнейшего окисления. В других случаях коррозия может повредить металл, например, ржавчину железа. 
 Нежелательная окислительно-восстановительные реакции: Ржавчина железа 
 Ржавчина возникает из-за воздействия железа на кислород и воду. Образование ржавчины предполагает создание гальванической ячейки на поверхности железа, что приводит к образованию железа(II). К соответствующим окислительно-восстановительным реакциям, возникаемым на анодической (окисление железа) и катодной (уменьшение кислорода) областях, образованных на поверхности железа, относятся: 
 <img alt="Eq1" src="/files/ftp_upload/11440/11440_Equation_1.png" style="text-align: center; height: 150px;" />
 Утюг(II) также реагирует с влажным воздухом, образуя гидрат оксида железа(III), известный как ржавчина. 
 <img alt="Eq2" src="/files/ftp_upload/11440/11440_Equation_2.png" style="text-align: center; height: 50px;" />
 Стехиометрия гидрата варьируется в зависимости от количества воды, к которой подвергается железо(II), как указано в формуле вещества с помощью x. Влага, наличие кислот и электролитов увеличивает скорость образования ржавчины. В отличие от патины на меди, образование ржавчины не создает защитного слоя, и поэтому коррозия утюга продолжается по мере отслоения ржавчины и подвергает свежий утюг атмосфере. 
 Предотвращение коррозии 
 Для предотвращения коррозии можно использовать различные методы. Одним из способов является сохранение окрашенной металлической поверхности во избежание контакта с водой и кислородом. Легирование металлов, таких как смешивание железа с небольшим количеством хрома в нержавеющей стали, является еще одним эффективным способом предотвращения коррозии. Хром собирается около поверхности и подвергается окислению, таким образом, эффективно защищая утюг от коррозии. 
 Железо и другие металлы также могут быть защищены от коррозии путем гальванизации, процесс, в котором защищаемый металл покрыт слоем более легко окисленного металла, обычно цинка. Если цинковый слой не поврежден, он предотвращает контакт воздуха с основным железом и предотвращает коррозию. Если цинковый слой поврежден коррозией или механическим абразивным покрытием, то железо может быть защищено от коррозии путем катодной защиты, о которой говорится в следующем пункте. 
 Катодная защита использует принцип преобразования металла для защиты в катод в электрохимической реакции. Это достигается за счет соединения защищаемого металла с более активным или легко окисляемым металлом, таким как цинк или магний, известный как жертвенный анод. Анод разъедает и используется для защиты металла, который служит катодом. Катодная защита чаще всего используется в бытовых приборах, таких как водонагреватели и подземные резервуары для хранения воды. Важно отметить, что катодную защиту можно использовать не только для железа, но и для других металлов. 
 Этот текст адаптирован к<a href="https://openstax.org/books/chemistry-2e/pages/17-6-corrosion"> OpenStax, Химия 2е изд., глава 17.6: Коррозия. </a>

Разложение металлов в результате естественных электрохимических процессов известно как коррозия. Образование ржавчины на железе, потускнение серебра и сине-зеленый...

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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

balancing redox equations

Redox Reactions and Half-Reaction Balancing

Redox reactions involve electron transfer and changes in oxidation number. In electrochemistry, these reactions connect chemical changes with electrical energy. They also power technologies such as industrial metal processing and rechargeable batteries used in electric vehicles. Oxidation number is a way to track how an atom's electronic environment changes compared with the pure element, where the oxidation number is zero. For a simple ionic compound, the oxidation number matches the ion...

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Galvanic Cells and Battery Reactions

Galvanic cells use spontaneous redox reactions to produce electric current. A common example is a disposable AA battery, which powers devices by turning chemical energy into electrical energy. Another example is a copper wire placed in silver nitrate solution, where the solution slowly turns bright blue and a grey solid forms on the wire. In the copper and silver reaction, copper is oxidized and silver ions are reduced. Copper forms cuprous ions, which give the solution its blue color. At the...

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cell potential and free energy

Redox Cell Potential, ΔG°, and K

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Cell Potential Under Changing Conditions

Cell potential changes when a redox reaction does not run at standard conditions. Standard cell potentials connect with standard free energy change, ΔG°, and the equilibrium constant K, but those relationships only hold at 1 M concentration, 298 K, and 1 atmosphere. A zinc(II) and tin(II) redox cell with E°cell = +0.291 V and ΔG° = −56.2 kJ is spontaneous under those standard conditions. As a cell discharges, the reactant concentration changes and the cell potential steadily drops. Many redox...

concentration cells

Cell Potential from Concentration Differences

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

Primary, Rechargeable Batteries and Fuel Cells

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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