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# Effects of Corrosion

We have seen old metal pipes with brown colour coating on them. We usually call it rust and the phenomenon is known as corrosion. How does metal object cover with this coat and what does the reason behind this? Alloys like steel, copper alloys etc. Have corrosion resistance due to presence of certain metals in them.

We know that metals are highly reactive elements due to less number of valence electrons therefore they tend to lose electrons during chemical reactions. Corrosion is also a chemical reaction in which metal reactions with atmospheric moisture and oxygen gas to form metallic oxides. That is the reason, metals corrode quickly.  Metallurgy is the process of conversion of metallic ores to metal as metals are usually found in combined state in nature. These pure metals are used for various industries.

The deterioration of metallic surface due to corrosion reduces the efficiency of metallic equipments which causes financial lose to industries. Therefore it’s important to note the causes, forms and effects of corrosion.

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## Effects of Corrosion on Metals

The chemistry corrosion that helps to understand the effect of corrosion over metal surface. In general, metals have tendency to loss electrons and convert into metal cations through metal oxidation reaction.

$M \rightarrow M^{+} + e^{-}$

Some metals loss one electron like sodium and some more than one like Fe, Cu etc. The loss of electrons from metal is a common anodic reaction.

$Fe \rightarrow Fe^{2+} + 2e^{-}$

The free electrons produce from metal atoms at move to cathodic site for further reaction. At cathode, electrons react with atmospheric oxygen and moisture (water) to for hydroxyl ions.

$O_{2} + 4e^{-} + 4H_{2}O \rightarrow 4OH^{-}$

These hydroxyl ions quickly react with metal ions to form metallic hydroxides. Overall metal deteriorate to metal oxides. Some of the noble and non-reactive metals like gold, platinum do not react with atmospheric oxygen and do not corrode.

## Causes of Corrosion

The common causes of corrosion:
• Characteristics of water: Chemical and physical properties of water like alkalinity, presence of ions or impurities, temperature, hardness, flow or stability and pH, dissolved solids influence the rate of corrosion of metal. Low pH, soft or with primarily non-carbonate hardness, low alkalinity increases the rate of corrosion. Excess of oxygen, carbon dioxide and dissolved solids in water also increases the rate of corrosion.
• Bacteria: Presence of bacteria in surrounding also accelerates the rate of corrosion. This is because accumulation of bacterial colonies increases the oxygen cell concentration accelerates pitting and tuberculation.  Bacterial colonies also produce carbon dioxide and block the deposition of calcium carbonate scale.
• Other Factors: Characteristics of the metal, electrical currents are some other factors that influence the rate of corrosion.

## Forms of Corrosion

Different forms of corrosions are listed in given table.

 Example Types of  Corrosion Uniform CorrosionGenerally occurs due to direct chemical attacks Galvanic CorrosionAn electrochemical action that occurs between two dissimilar metals which are in contact with other. Concentration Cell CorrosionIt occurs when two metal surfaces are in contact with different concentrations of the same solution. Pitting CorrosionA kind of localized corrosion that occurs with pits at underneath surface formed due to corrosion product accumulation. Crevice CorrosionIt occurs when metals are in contact with nonmetals. Filiform CorrosionIt occurs on painted surfaces due to penetration of coated surface by moisture in the form of filaments. Intergranular CorrosionIt occurs on grain boundaries of a metal or alloy. Stress Corrosion CrackingIt occurs due to applied loads, residual stresses or a combination of both. Corrosion FatigueA special case of stress corrosion that occurs due to combined effects of cyclic stress and corrosion. Fretting CorrosionIt is a rapid corrosion, occurs at the interface between highly loaded metal surfaces due to vibratory motions. Erosion CorrosionIt occurs due to combination of a chemical environment with high fluid-surface velocities.

## Corrosion Resistance

Since corrosion deteriorate the physical and chemical properties of metals, therefore we always try to make corrosion resistance metal surfaces. There are various ways to make metals corrosion resistance.  Few of corrosion control methods are listed below.
• Environmental Modifications
• Metal Selection
• Protective Coatings and plating
• Corrosion Allowances
• Cathodic Protection
Corrosion resistance methods can be classified as,
• Active corrosion protection: Development of corrosion-resistant alloys or addition of inhibitors
• Passive corrosion protection: Coating by protective layer, film
• Permanent corrosion protection: Tin plating, Galvanization, Coating, Copper plating
• Temporary corrosion protection: Protective coating