introduction of corrosion facts about corrosion causes of corrosion types of corrosion

25
Introduction of Corrosion Facts about Corrosion Causes of Corrosion Types of corrosion E ffects of corrosion Factors affecting corrosion Prevention or control of corrosion CHAPTER 4 CORROSION AND ITS PREVENTION

Upload: paco

Post on 24-Feb-2016

329 views

Category:

Documents


14 download

DESCRIPTION

CHAPTER 4 CORROSION AND ITS PREVENTION. Introduction of Corrosion Facts about Corrosion Causes of Corrosion Types of corrosion E ffects of corrosion Factors affecting corrosion Prevention or control of corrosion. Introduction of Corrosion. - PowerPoint PPT Presentation

TRANSCRIPT

Page 1: Introduction of Corrosion Facts about Corrosion Causes of Corrosion Types of corrosion

Introduction of CorrosionFacts about CorrosionCauses of CorrosionTypes of corrosionE ffects of corrosionFactors affecting corrosionPrevention or control of corrosion

CHAPTER 4CORROSION AND ITS PREVENTION

Page 2: Introduction of Corrosion Facts about Corrosion Causes of Corrosion Types of corrosion

Introduction of CorrosionThe branch of Engineering that deals with the study of corrosion and its prevention safely is known as corrosion Engineering.Corrosion:The loss of material(metals or alloys) or its useful properties by chemical interaction with its environment is known as corrosion.Examples;oRusting of iron.oBlackening of silver articles in atmosphere.oFogging of nickel.

Page 3: Introduction of Corrosion Facts about Corrosion Causes of Corrosion Types of corrosion
Page 4: Introduction of Corrosion Facts about Corrosion Causes of Corrosion Types of corrosion

Facts about corrosion1. Corrosion is a natural process.2. Corrosion is an electrochemical process.3. Corrosion leads to a tremendous loss.4. Corrosion is the reverse of metal extraction.

Page 5: Introduction of Corrosion Facts about Corrosion Causes of Corrosion Types of corrosion

Most metals exist in nature in combined forms that is in the form of ore. So extraction of metal from its ores is an unnatural process in which component of a stable system is being separated.Ore extraction Metal corrosion corrosion productStable gain es mteta stable loss of es stableDuring the process of extraction a number of steps like concentration, Roasting ,and Smelting are involved and finally the ore is reduced into metal. therefore metals in their finally refined state are highly unstable and have a tendency to revert back in their original state that is to stable state by the process of corrosion(oxidation).

Causes of corrosion

Page 6: Introduction of Corrosion Facts about Corrosion Causes of Corrosion Types of corrosion

Example; Rusting of iron is the most popular example of corrosion.

Usually the Surface of metal is covered with a thin layer of oxide. When this layer is broken, bare surface is exposed to the environment and this area act as an anode while the remaining area act as cathode. since the medium is exposed to the atmosphere, it contain dissolved oxygen. Thus at cathode oxygen reduction take place while at anode iron corrodes. 4 Fe 4Fe+2 +8 e Oxid. At anodeO2 + 4H2O + 8 e 8OH- Red. At cathode___________________________4Fe + 6H2O + 3O2 4Fe(OH)3 Fe2O3. nH2O

Page 7: Introduction of Corrosion Facts about Corrosion Causes of Corrosion Types of corrosion

Types of corrosion1. Uniform corrosionThis is also called general corrosion. The surface effect produced by most direct chemical attacks (e.g., as by an acid) is a uniform etching of the metal.

Page 8: Introduction of Corrosion Facts about Corrosion Causes of Corrosion Types of corrosion

2. Galvanic Corrosion• Galvanic corrosion is an electrochemical action of

two dissimilar metals in the presence of an electrolyte and an electron conductive path. It occurs when dissimilar metals are in contact.

Page 9: Introduction of Corrosion Facts about Corrosion Causes of Corrosion Types of corrosion

3. Crevice or contact corrosionCrevice or contact corrosion is the corrosion producedat the region of contact of metals with metals ormetals with nonmetals. It may occur at washers,under barnacles, under applied protective films.

Page 10: Introduction of Corrosion Facts about Corrosion Causes of Corrosion Types of corrosion

4. Corrosion in ConcreteConcrete is a widely-used structural material that isfrequently reinforced with carbon steel reinforcingrods, post-tensioning cable or pre-stressing wires. Thesteel is necessary to maintain the strength of thestructure, but it is subject to corrosion.

Page 11: Introduction of Corrosion Facts about Corrosion Causes of Corrosion Types of corrosion

5. Erosion corrosionErosion corrosion is the result of a combination of anaggressive chemical environment and high fluid-surfacevelocities.

6. DealloyingDealloying is a rare form of corrosion found in copper alloys, gray cast iron, and some other alloys. Dealloyingoccurs when the alloy loses the activecomponent of the metal and retains the more corrosion resistant component in a porous "sponge" on the metal surface.

Page 12: Introduction of Corrosion Facts about Corrosion Causes of Corrosion Types of corrosion
Page 13: Introduction of Corrosion Facts about Corrosion Causes of Corrosion Types of corrosion

• Reduced Strength• Downtime of equipment• Lost surface properties• Reduced value of goods

Effects of corrosion

Page 14: Introduction of Corrosion Facts about Corrosion Causes of Corrosion Types of corrosion

Factors affecting corrosion1.Presence of impurities in metalsSpeed of corrosion increases with the presence ofimpurities in the metals because these impurities helpin setting up the voltaic cells.

2. Presence of electrolyteElectrolytes present in water also increases the rate ofcorrosion e.g. corrosion of iron in sea water takesplace in large extent than in distilled water becausesea water contains salts i.e.electrolytes.

Page 15: Introduction of Corrosion Facts about Corrosion Causes of Corrosion Types of corrosion

3. Position of metals in electrochemical series Highly reactive metals undergo corrosion faster thanleast reactive metals. Reactivity of metals can befound from the electrochemical series.e.g.Au Cu Pb Fe Zn Al Mg Na KReactivity increases

4. Presence of carbon dioxide in waterPresence of carbon dioxide in natural water alsoincreases the rusting of iron because it acts as anelectrolyte and increases the flow of electron fromone place to another.

Page 16: Introduction of Corrosion Facts about Corrosion Causes of Corrosion Types of corrosion

5. Presence of protective coatingWhen the iron surface is coated with the metal,which is more reactive than the iron, then the rate ofcorrosion is retarded e.g. when iron is coated withzinc, iron is protected from rusting.

Page 17: Introduction of Corrosion Facts about Corrosion Causes of Corrosion Types of corrosion

PREVENTION OF CORROSIONCorrosion is slow but continuously eating away the metal and consequence of rusting. Rusting can be avoided by not letting moist air come in contact with the metal surface .Following methods could gain fully be adopted to minimize rusting and corrosion

Page 18: Introduction of Corrosion Facts about Corrosion Causes of Corrosion Types of corrosion

Methods of preventing corrosion and rusting1. Tarring2. Painting3. Enameling4. Galvanizing5. Sheradising6. Tin plating7. electroplating

Page 19: Introduction of Corrosion Facts about Corrosion Causes of Corrosion Types of corrosion

metal is dipped in hot coal tar so that a film of it is sticks to the surface which protect the surface from rusting and corrosion.Pipes or ends of poles to be embedded in the earth are usually given this protective treatment.

1. Tarring:

Page 20: Introduction of Corrosion Facts about Corrosion Causes of Corrosion Types of corrosion

Paints usually the lead paints are applied on the surface to be protected. Exposed metal surfaces as in case of roof and bridge structure are given this treatment which has to be repeated after regular interval of time.

2. Painting:

Page 21: Introduction of Corrosion Facts about Corrosion Causes of Corrosion Types of corrosion

Enamels consist of high grade bases like zinc oxide or lead oxide ground in oil or varnish. they dry slowly leaving a hard tough and elastic film which is smooth and durable.Enamle painted surfaces are washable and are not affected by acids,alkali,gases or steam. Even tough they are more costly than ordinary paints yet because of their durability. They are equally good for use both on external and internal work.

3.Enameling:

Page 22: Introduction of Corrosion Facts about Corrosion Causes of Corrosion Types of corrosion

Depositing a fine film of zinc on the iron/steel surfaces is termed as galvanizing.The surfaces to be galvanizing is first cleared of all foreign matter by giving it an acid wash to be followed by a wash of clean water. the surface is then dried and dipped in molten zinc.The fine film of zinc that get deposited protect the surface from contact with atmosphere and consequence oxidation.Removal of the zinc film caused by scratches.

4.Galvanizing:

Page 23: Introduction of Corrosion Facts about Corrosion Causes of Corrosion Types of corrosion

Surface to be treated is cleaned of all foreign deposits by washing it with acid solution and then with clean water.It is then dried and covered with zinc dust and enclosed in steel boxes to be heated in a furnace under controlled temperatures.Molten zinc spreads over the whole surface and on cooling forms a thin protective layer.Sherading gives better protection than galvanizing.

5.Sheradising:

Page 24: Introduction of Corrosion Facts about Corrosion Causes of Corrosion Types of corrosion

After cleaning the surface with acid wash followed by wash with clean water and drying, it is dipped in a bath of molten tin.A protective covering of tin layer if left on the surface.

6.Tin plating:

Page 25: Introduction of Corrosion Facts about Corrosion Causes of Corrosion Types of corrosion

By the process of electrolysis a thin film of nickel,cromium,copper or zinc is deposited on the surface to be protected.The surface to be protected is made the cathode and the metal to be deposited is made the anode.

6. Electroplating: