Corrosion

Corrosion

                         Corrosion

INTRODUCTION:
Corrosion is an undesirable phenomena or process. Due to the corrosion there is limitation of progress in
many the area. The cost of replacement of the  materials and equipment lost through corrosion is
unlimited.
Metals and alloys are used as fabrication or construction materials in engineering. If the metals or alloy structures are not properly maintained, they deteriorate slowly by the action of atmospheric gases, moisture and other chemicals. This phenomenon of destruction of metals and
alloys is known as corrosion.
Corrosion of metal is defined as spontaneous destruction of metals in the course of their chemical, electro-chemical or bio-chemical interaction with the environment. Thus, it is exactly the reverse of the extraction of metals from the ores.
Example: Rusting of iron
A layer of reddish scale and powder of oxide (Fe3O4) is formed on the surface of the iron metal.
A green films of the basic carbonate [CuCO3 + Cu(OH)2] is formed on the surface of copper, when it is exposed to moist-air containing carbon dioxide.

CONSEQUENCES (EFFECTS) OF CORROSION:

The economic and social consequences of corrosion include
i) Due to formations of the corrosion product over the machinery, the efficiency of the machine gets failure leads to plant shut down.
ii) The product contamination or losses of products due to corrosion.
iii) The corroded equipments must be replaced
iv) Preventive maintenance like the  metallic coating or organic coating is required.
v) Corrosion releases toxic products.
vi) Health (eg., from the pollution due to a corrosion product or due to escaping chemical from a corroded equipment).

CAUSES OF CORROSION:

In nature, metals occur in two different forms.

1) Native State (2) Combined State

Native State: The metals exist as such in the earth crust then the metals are present in a native state.Native state means free or uncombined state. These metal the are non-reactive in nature. They are the noble metals which have very good corrosion resistance. Example: Au, Pt, Ag, etc.,

Combined State: -Except noble metals, all other metal are the highly reactive in nature which the undergoes the reaction with their the  environment to form the stable compound called ores and minerals. This is combined state of metals. Example: Fe2O3, ZnO, PbS, CaCO3, etc.,

Metallic Corrosion: The metal are extracted from their metallic compounds (ores). During  extraction, ores are reduced to their the  metallic states by applying energy in form of various process. In pure metallic state, the metals are the unstable as they are considered in excited states (higher energy state). Therefore as soon as the metals are extracted from their ores, the reverse process begin and form the metallic compounds, which are the thermodynamically stable (lower energystate). Hence, when metals are used in various forms, they are exposed to the environment, the exposed metal surface begin to the decay (conversion to more stable compound). This is the basic reason for metallic corrosion.

Corrosion-Oxidation

Metal Metallic Compound + Energy

Metallurgy-Reduction

Although corroded metal is the  thermodynamically more stable than the pure metal but due to corrosion, useful properties of a metal like malleability, ductility, hardness, luster and electrical conductivity are lost.

CLASSIFICATION OR THEORIES OF CORROSION

Based on the environment, corrosion is classified into

(i) Dry or Chemical Corrosion .

(ii) Wet or Electrochemical Corrosion.

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