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

التآكل هو التأكسد التلقائي للمعدن بوجود عامل مؤكسد،عادة ما يكون الأكسجين. وهي الظاهرة ذاتها التي تُسبب اللون الأخضر المائل للزرقة لتمثال الحرية أو لمرساة صدئة. بعض التآكل تمنح الحماية.على سبيل المثال،يتأكسد النحاس بوجود الأكسجين،والماء،وثاني أكسيد الكربون،ليشكّل كربونات النحاس القاعدية،وهي طبقة خضراء مائلة للزرقة مستقرة من حيث بنيتها و توفر حماية. أما أنواع التآكل الأخرى فهي مُضرة. على سبيل المثال،يتأكسد الحديد بوجود الأكسجين،والماء،وأيونات الهيدروجين،فيتحول إلى أكاسيد معدنية غير مستقرة في بنيتها،تُسمى الصدأ،والتي تتقشر باستمرار،معرّضة المعدن الذي تحتها إلى مزيد من التأكسد.الصدأ تفاعل كهروكيميائي تلقائي. العيوب على السطح،تؤدي دور المناطق المصعدية حيث الحديد له طاقة قطب كامنة تساوي سالب 0.44،يتأكسد بسرعة إلى حديد ثنائي التكافؤ. ترحل الإلكترونات باتجاه المنطقة المهبطية،حيث الأكسجين،بطاقة قطب كامنة مقدارها 1.23 فولت،يُختزل إلى ماء في بيئة حمضية.تأتي أيونات الهيدروجين من التفاعل،عند الضغط الجوي القياسي،بين ثاني أكسيد الكربون والماء،لتكوين حمض الكربونيك. بعد ذلك ترحل أيونات الحديد ثنائي التكافؤ باتجاه منطقة المهبط،عبر رطوبة السطح،وتتأكسد من جديد إلى هيدرات أكسيد الحديد الثلاثي،المعروف عمومًا بالصدأ. تكافؤ الهيدرات تتفاوت باختلاف كمية الماء التي يتعرض لها الحديد ثنائي التكافؤ.تزداد سرعة الصدأ بزيادة الرطوبة،والأحماض،والكهروليت التي تعزز تدفق الشحنات،وتركيز أيونات الهيدروجين. من طرق الحماية من التآكل،طلاء سطع المعدن لمنع التلامس مع الماء والأكسجين،أو استخدام السبائك،وهي خليط من عدة معادن. الفولاذ المقاوم للصدأ،هو سبيكة من الحديد بها كميات صغيرة من الكروم الذي يتآكل ليشكل طبقة من أكسيد خامل على السطح تحمي الحديد.كما تُستخدم الجلفنة كبديل أيضًا. هنا،برغي حديدي مغلف بطبقة من معدن يتأكسد بسهولة أكثر كالزنك،الذي يشكل طبقة من أكسيد الزنك تحمي الحديد. وأخيرًا،الحماية المهبطية التي تتحكم في التآكل بتحويل المعدن المحمي ليصبح هو المهبط.أنابيب الحديد التي توضع تحت الأرض غالبًا ما توصل بمعدن يتأكسد بسهولة أكثر كالزنك،حيث يؤدي الزنك دور مصعد قرباني فيتأكسد،ويصبح الحديد هو المهبط المحمي من التآكل.

التآكل

يُعرف تحلل المعادن بسبب العمليات الكهروكيميائية الطبيعية بالتآكل. يعتبر تكوين الصدأ على الحديد، وتشويه الفضة، والزنجار الأزرق والأخضر الذي يتطور على النحاس أمثلة على التآكل. يشمل التآكل أكسدة المعادن. أحياناً إنه وقائي، مثل أكسدة النحاس أو الألومنيوم، حيث تتشكل طبقة واقية من أكسيد المعدن أو مشتقاته على السطح، مما يحمي المعدن الأساسي من المزيد من الأكسدة. وفي حالات أخرى، يؤدي التآكل إلى إتلاف المعدن، مثل صدأ الحديد. تفاعل الأكسدة والاختزال غير المرغوب فيه: صدأ الحديد يحدث الصدأ بسبب تعرض الحديد للأكسجين والماء. ينطوي تكوين الصدأ على تكوين خلية جلفانية على سطح الحديد، مما ينتج عنه توليد الحديد (II). تتضمن تفاعلات الأكسدة والاختزال ذات الصلة التي تحدث في مناطق أنودية (أكسدة الحديد) والكاثودية (اختزال الأكسجين) المتكونة على سطح الحديد ما يلي: <img alt="Eq1" src="/files/ftp_upload/11440/11440_Equation_1.png" style="text-align: center; height: 150px;" /> يتفاعل الحديد (II) بشكل أكبر مع الهواء الرطب، مكونًا هيدرات أكسيد الحديد (III) ، المعروف باسم الصدأ. <img alt="Eq2" src="/files/ftp_upload/11440/11440_Equation_2.png" style="text-align: center; height: 50px;" /> يختلف القياس المتكافئ للهيدرات' باختلاف كمية الماء التي يتعرض لها الحديد (II)، كما يتضح من استخدام x في الصيغة المركبة. الرطوبة ووجود الأحماض والإلكتروليتات تزيد من معدل تكون الصدأ. على عكس الزنجار على النحاس، لا يؤدي تكوين الصدأ إلى تكوين طبقة واقية، وبالتالي يستمر تآكل الحديد حيث يتقشر الصدأ وتتعرض طبقات الحديد الحديث لتأثير لجو. منع التآكل يمكن استخدام طرق مختلفة لمنع التآكل. تتمثل إحدى الطرق في الحفاظ على السطح المعدني مطلياً لتجنب ملامسة الماء والأكسجين. يعتبر خلط المعادن، مثل خلط الحديد بكميات صغيرة من الكروم في الفولاذ المقاوم للصدأ، طريقة فعالة أخرى لمنع التآكل. يتجمع الكروم بالقرب من السطح ويتأكسد، وبالتالي يحمي الحديد بشكل فعال من التآكل. يمكن أيضاً حماية الحديد والمعادن الأخرى من التآكل عن طريق الجلفنة، وهي عملية يتم فيها طلاء المعدن المراد حمايته بطبقة من معدن أكثر سهولة في التأكسد، وعادة ما يكون الزنك. عندما تكون طبقة الزنك سليمة، فإنها تمنع الهواء من الاتصال بالحديد الأساسي وبالتالي تمنع التآكل. إذا تم اختراق طبقة الزنك عن طريق التآكل أو التآكل الميكانيكي، فقد يظل الحديد محمياً من التآكل بواسطة عملية الحماية الكاثودية، الموضحة في الفقرة التالية. تستخدم الحماية الكاثودية مبدأ تحويل المعدن المراد حمايته إلى كاثود في تفاعل كهروكيميائي. يتم تحقيق ذلك من خلال توصيل المعدن المحمي بمعدن أكثر نشاطاً أو أكسدة بسهولة مثل الزنك أو المغنيسيوم، والمعروف باسم الأنود المضحّى به. يتآكل الأنود ويستهلك لحماية المعدن الذي يعمل ككاثود. تستخدم الحماية الكاثودية بشكل شائع في الأجهزة المنزلية مثل سخانات المياه وخزانات المياه الجوفية. الأهم من ذلك، يمكن استخدام الحماية الكاثودية مع معادن غير الحديد فقط. هذا النص مقتبس من <a href="https://openstax.org/books/chemistry-2e/pages/17-6-corrosion">OpenStax, Chemistry 2e, Chapter 17.6: Corrosion</a>.

يعرف تدهور المعادن بسبب العمليات الكهروكيميائية الطبيعية بالتآكل. تكوين الصدأ على الحديد ، وتشويه الفضة ، والأزرق والأخضر ...

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واحد

Electrochemistry

الكيمياء الكهربائية

KEY TERMS AND CONCEPTS

المصطلحات والمفاهيم الأساسية

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

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the nernst equation

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concentration cells

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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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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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