Corrosion and Its Prevention

The loss of material(metals or alloys) or its useful properties by chemical interaction with its environment is known as corrosion. Corrosion is an electrochemical process. Corrosion leads to a tremendous loss. Corrosion is the reverse of metal extraction.

Examples:

 Rusting of iron. 
Blackening of silver articles in atmosphere.
 Fogging of nickel.

Causes 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 product Stable gain es mteta stable loss of es stable During 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).

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.

Types of corrosion

 Uniform corrosion:

This 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.

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.

 Crevice or contact corrosion:

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

Corrosion in Concrete Concrete is a widely-used structural material that is frequently reinforced with carbon steel reinforcing rods, post-tensioning cable or pre-stressing wires. The steel is necessary to maintain the strength of the structure, but it is subject to corrosion.

 Erosion corrosion :
Erosion corrosion is the result of a combination of an aggressive chemical environment and high fluid-surface velocities.

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

Factors affecting corrosion

1. Presence of impurities in metals Speed of corrosion increases with the presence of impurities in the metals because these impurities help in setting up the voltaic cells.

 2. Presence of electrolyte Electrolytes present in water also increases the rate of corrosion e.g. corrosion of iron in sea water takes place in large extent than in distilled water because sea water contains salts i.e.electrolytes.

Position of metals in electrochemical series

Highly reactive metals undergo corrosion faster than least reactive metals. Reactivity of metals can be found from the electrochemical series. e.g.Au Cu Pb Fe Zn Al Mg Na K Reactivity increases 4. Presence of carbon dioxide in water Presence of carbon dioxide in natural water also increases the rusting of iron because it acts as an electrolyte and increases the flow of electron from one place to another.

 Presence of protective coating

When the iron surface is coated with the metal, which is more reactive than the iron, then the rate of corrosion is retarded e.g. when iron is coated with zinc, iron is protected from rusting.

PREVENTION OF CORROSION

Corrosion 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

 Methods of preventing corrosion and rusting

Tarring

 Painting

Enameling

 Galvanizing

 Sheradising

 Tin plating

 electroplating

 Tarring: 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.

 Painting: 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.

Enameling: 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.

Galvanizing: 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.

Sheradising: 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.

Tin plating: 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.

Electroplating: 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.

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