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Corrosion ppt...

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Assignment A. Explain corrosion. State the mechanism of dry and wet corrosion. Corrosion is a natural process that converts a refined metal into a more chemically stable form such as oxide, hydroxide or sulphide. It is the gradual destruction of materials (usually a metal) by chemical and/or electrochemical reaction with their environment. There are 2 types of corrosion - Dry Corrosion and Wet Corrosion which are further divided into : Oxidation, Molten Salt Corrosion and Hydrogen Attack which come under Dry Corrosion and Aqueous Corrosion, Atmospheric Corrosion and Non Aqueous Corrosion which come under Wet Corrosion. DRY CORROSION Oxidation : Oxidation is defined as a corrosion reaction in which the corroded metal forms an oxide. Usually, this reaction is with a gas containing oxygen, such as air. Metal systems need to form dense, well-adherent oxide films. MECHANISM :

WET CORROSION : Corrosion of metals immersed in an aqueous medium is an example of wet corrosion, which proceeds electrochemically. Atmospheric corrosion also belongs to the class of wet corrosion, since it is caused by moisture deposited on the metal surface. Another division of wet corrosion is the degradation of metals and alloys in non aqueous media by chemical pathways.

MECHANISM :

B.

Write short notes on 1. Galvanic corrosion 2. Concentration corrosion 3. Stress corrosion 4. Pitting corrosion GALVANIC CORROSION

Galvanic Corrosion, also known as bimetallic corrosion is an electrochemical process whereby one metal corrodes in preference to another metal that it is in contact with through an electrolyte. A good example of this would be placing aluminium and steel in seawater and connecting them. We will observe that Aluminium is corroding faster and steel doesn’t corrode. CONCENTRATION CORROSION Concentration cell corrosion occurs when two or more areas of a metal surface are in contact with different concentrations of the same solution. There are three general types of concentration cell corrosion: 1. metal ion concentration cells 2. oxygen concentration cells, and 3. active-passive cells.

Examples - Galvanic Cell is an example of concentration corrosion STRESS CORROSION CRACKING Stress corrosion cracking (SCC) is caused by the simultaneous effects of tensile stress and a specific corrosive environment. Stresses may be due to applied loads, residual stresses from the manufacturing process, or a combination of both. Example - Stress Corrosion Cracking on insulated steel condensate lines. Water wets the insulation and the chloride from the insulation leaches onto the hot metal thus causing it to crack. PITTING CORROSION In this type of corrosion, random pits are formed in the metals or alloys which happened due to the corrosive reaction. This occurs generally in presence of chloride ions combined with depolarizers such as oxygen or oxidizing salts. This type of corrosion is noticed in passive metals such as stainless steel.

C. Factors affecting and prevention of corrosion  Exposure of the metals to air containing gases like CO2, SO2, SO3 etc.  Exposure of metals to moisture especially salt water (which increases the rate of corrosion).  Presence of impurities like salt (eg. NaCl).  Temperature: An increase in temperature increases corrosion.  Nature of the first layer of oxide formed: some oxides like Al2O3 forms an insoluble protecting layer which can prevent further corrosion. Others like rust easily crumble and expose the rest of the metal. SWAMITA SACHAN 2029038 A25 CSCE...