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Mechanism of corrosion

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الكلية كلية الهندسة/المسيب     القسم هندسة السيارات     المرحلة 3
أستاذ المادة جعفر هادي عبود المياح       31/10/2018 12:32:54
Polarization:
2.1 Activation polarization:
2.2 Concentration Polarization:
3 Types of Corrosion:
3.1 Galvanic Corrosion
3.2 Concentration cell Corrosion
3.3 Pitting corrosion
3.4 Crevice Corrosion
3.5 Fretting Corrosion
3.6 Intergranular Corrosion
3.7 Stress Corrosion Cracking (SCC)
3.8 Erosion corrosion
Corrosion Mechanism (Electrochemical Aspects)
Before we are going see the various types of corrosion, it is essential to now the mechanics of corrosion.
The electrochemical nature of corrosion can be illustrated by the attack of zinc by hydrochloric acid. When
zinc is placed in dilute hydrochloric acid a vigorous reaction occurs; hydrogen gas is evolved and the zinc is
dissolves forming a solution of zinc chloride. The reaction is as follows
Zn + 2HCl ——-> Zncl + H
From the above equation, it is found that the chloride ion is not involved in the reaction. Hence the above
equation can be rewritten as follows
2 2
+ +2
10/8/2018 Corrosion ? Basics, Mechanism, Types ? Mechanical Engineering Site
http://www.mechanicalengineeringsite.com/corrosion?basics?mechanism?types/ 3/11
Zn + 2H ——-> Zn + H
From this equation when the zinc ion reacts with the hydrogen ions in the acid solution to form zinc ions
and hydrogen gas. During the reaction, zinc is oxidized to zinc ions and hydrogen ion is reduced to
hydrogen. The reaction can be divided into two reactions.
Oxidation
Reduction
In the above reaction oxidation (Anodic reaction)
Zn ——> Zn + 2e
The reduction reaction (Cathodic reaction)
2H + 2e ——> H
An oxidation or anodic reaction is indicated by an increase in valence or a production of an electron. A
decrease in valence charge or the consumption of electrons signifies a reduction or cathodic reaction.
During metallic corrosion, the rate of oxidation will equal to the rate of reduction.
The above concept is illustrated in the figure below. Here the zinc atom has been transformed into Zinc ion
and two electrons. These electrons which remain in the metal are immediately consumed during the
reduction of hydrogen ions. These two reactions will occur simultaneously at the same point on the surface
of the metal.

المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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