Types+of+corrosion

8/10/2019 Types+of+corrosion

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Types of corrosion

Dr. Syed Hassan Javed


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:

Corrosion is the primary means by which metals

deteriorate. Most metals corrode on contact with

water (and moisture in the air), acids, bases,salts, aggressive metal polishes, and other solid

and liquid chemicals. Metals will also corrode

when exposed to gaseous materials like acid

vapors, formaldehyde gas, ammonia gas, andsulfur containing gases.


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Corrosion Mechanism :

Corrosion Mechanism is the destructive attack,or deterioration, of a metal by chemical or

electrochemical reaction with its environment.Corrosive attack of metals is an electrochemicalprocess. In a galvanic cell, two dissimilar metals(e.g., iron and copper) are placed in electricalcontact in the presence of oxygen and moisture.Separate chemical reactions take place at thesurfaces of the two metals, creating a flow ofelectrons through the connecting wire.


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Uniform Corrosion


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Uniform corrosion

This type of corrosion includes the

commonly recognized rusting of iron and

other metals.


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Pitting


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Pitting corrosion

This is a localized type of attack, with the rate of corrosion beinggreater at some areas than at others. If appreciable attack isconfined to a relatively small, fixed area of metal, acting as anode,the resultant pits are described as deep. If the area of attack isrelatively larger and not so deep, the pits are called shallow. Depthof pitting is sometimes expressed by the pitting factor , the ratio of

deepest metal penetration to average metal penetration asdetermined by the weight loss of the specimen. A pitting factor ofunity represents uniform attack.

Iron buried in the soil corrodes with formation of shallow pits,whereas stainless steels immersed in seawater characteristically

corrode with formation of deep pits.


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Erosion-Corrosion


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Erosion corrosion

when subjected to high - velocity liquids,

undergo a pitting type of corrosion called

impingement attack , or erosion corrosion .

Copper and brass condenser tubes, for

example, are subject to this type of

attack.


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Fretting Corrosion


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Fretting corrosion , which results from

slight relative motion (as in vibration) of

two substances in contact, one or both

being metals, usually leads to a series ofpits at the metal interface. Metal - oxide

debris usually fills the pits so that only

after the corrosion products are removeddo the pits become visible


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Cavitation Erosion


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Cavitation erosion is the loss of material

caused by exposure to cavitation, which is

the formation and collapse of vapor

bubbles at a dynamic metal liquidinterface for example, in rotors of

pumps or on trailing faces of propellers.

This type of corrosion causes a sequenceof pits


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Selective Corrosion


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Dealloying is the selective removal of an element froman alloy by corrosion. One form of dealloying,dezincification, is a type of attack occurring with zincalloys (e.g., yellow brass) in which zinc corrodes

preferentially, leaving a porous residue of copper andcorrosion products. The alloy so corroded often retainsits original shape, but its tensile strength and ductility areseriously reduced. Dezincified brass pipe may retainsufficient strength to resist internal water pressures until

an attempt is made to uncouple the pipe, or a waterhammer occurs, causing the pipe to split open.


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Parting is similar to dezincification in that one or more reactivecomponents of the alloy corrode preferentially, leaving a porousresidue that may retain the original shape of the alloy. Parting isusually restricted to such noble metal alloys as gold copper or goldsilver and is used in gold refining. For example, an alloy of Au Ag containing more than 65% gold resists concentrated nitric acid as

well as does gold itself. However, on addition of silver to form analloy of approximately 25% Au 75% Ag, reaction with concentratedHNO3 forms silver nitrate and a porous residue or powder of puregold.

Copper - base alloys that contain aluminum are subject to a form of

corrosion resembling dezincification, with aluminum corrodingpreferentially.


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Intergranular Corrosion


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This is a localized type of attack at the grain boundariesof a metal, resulting in loss of strength and ductility.Grain boundary material of limited area, acting asanode, is in contact with large areas of grain acting ascathode. The attack is often rapid, penetrating deeply

into the metal and sometimes causing catastrophicfailures. Improperly heat - treated 18 - 8 stainless steelsor alloys (4% Cu Al) are among the alloys subject tointergranular corrosion. At elevated temperatures,intergranular corrosion can occur because, under some

conditions, phases of low melting point form andpenetrate along grain boundaries; for example, whennickel - base alloys are exposed to sulfur - bearinggaseous environments catastrophic failures can takeplace. This type of attack is usually called sulfidation


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Corrosion Fatigue


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If a metal cracks when subjected to repeated oralternate tensile stresses in a corrosiveenvironment, it is said to fail by corrosion fatigue. In the absence of a corrosive environment, the

metal stressed similarly, but at values below acritical stress, called the fatigue limit orendurance limit , will not fail by fatigue even aftera very large, or infinite, number of cycles. A trueendurance limit does not commonly exist in a

corrosive environment: The metal fails after aprescribed number of stress cycles no matterhow low the stress.


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Galvanic Corrosion


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Galvanic corrosion (also called dissimilarmetal corrosion) refers to corrosiondamage induced when two dissimilar

materials are coupled in a corrosiveelectrolyte. In a bimetallic couple, the lessnoble material becomes the anode andtends to corrode at an accelerated rate,

compared with the uncoupled conditionand the more noble material will act as thecathode in the corrosion cell.


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Crevice Corrosion


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Crevice corrosion, corrosion take place increvices bcz solutions retained at thisplace and takes longer time to dry out.

When this occurs, the severity of attack ismore severe at crevices. Crevices areformed bcz of the metal contact withanother piece of the same or other metal

or with a nonmetallic material. Corrosion increvice is due to deficiency of O2, Aciditychanges,


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Selective corrosion

Selective corrosion can be found in alloys

and resulting from the fact that the

components of the alloy corrode at

different rates.

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