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Definition:

Corrosion is the degeneration of

materials by reaction with environment.› Rusting of automobiles

› Buildings and bridges

› Fogging of silverware

› Patina formation on copper

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Metals corrode because we use them inenvironments where they are chemicallyunstable.

Any spontaneous reaction in theuniverse is associated with a lowering inthe free energy of the system.

› a negative free energy change All metalsexcept the noble metals have free energiesgreater than their compounds. So they tendto become their compounds through theprocess of corrosion.

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All metallic corrosion are electrochemicalreactions

› metal is converted to its compound with atransfer of electrons.

› The overall reaction may be split intooxidation (anodic) and,

reduction (cathodic) partial reactions.

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Dust particles and man-made pollution› CO, NO, methane, etc.

Temperature

› high T & high humidity accelerates corrosion. Rainfall

› excess washes corrosive materials and debris butscarce may leave water droplets.

Proximity to sea Air pollution

› NaCl, SO2, sulfurous acid, etc.

Humidity› cause condensation.

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Material selection

Improvements in material

Design of structures

Alteration of environment

Cathodic & Anodic protection

Coatings

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Lower temperature› but there are cases where increasing T

decreases attack. E.g hot, fresh or salt water is

raised to boiling T and result in decreasing O2solubility with T.

Reduce velocity›

exception ; metals & alloys that passivate (e.gstainless steel) generally have better resistanceto flowing mediums than stagnant. Avoid veryhigh velocity because of erosion-corrosioneffects.

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Remove oxygen/oxidizers› boiler feedwater was deaerated by passing it thru a

large mass of scrap steel. Modern practice  –  vacuum

treatment, inert gas sparging, or thru the use ofoxygen scavengers. However, not recommended foractive-passive metals or alloys. These materialsrequire oxidizers to form protective oxide films.

Change concentration› higher concentration of acid has higher amount

of active species (H ions). However, for materialsthat exhibit passivity, effect is normally negligible.

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Add Inhibitors

› Adsorption type, (Organic amines, azoles)

› H evolution poisons, (As & Sb)› Scavengers, (Sodium sulfite & hydrazine)

› Oxidizers, (Chromates, nitrates, ferric salts)

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Corrosion is a natural degenerative processaffecting metals, non metals and evenbiological systems like the human body.

Corrosion of engineering materials lead to

significant losses. An understanding of the basic principles of

corrosion and their application in the designand maintenance result in reducing lossesconsiderably.

Proper selection of methods is essential foroptimizing cost.

One or a number of methods may be usedsimultaneously for effective protection.