1. Definition of Corrosion
Corrosion is defined as the destruction or deterioration of
a material because of reaction wit environment.
Corrosion can be fast or slow.
Most metals are found in nature as ores which are typically metallic oxides or sulphides. Metals are refined from ores by reduction, a forcing of the metal ions to give up their charges and become metallic atoms in a regular lattice array. For this to take place, energy in the form of heat or electricity must be
supplied. It is the energy stored in the metal during the refining process that provides the driving force for corrosion.

2. The Cost of Corrosion Reliability and Cost:
Estimates of the cost of corrosion have been made by several nations (UK, USA, and Australia and others) and all have reached similar conclusions. The direct cost replacement and costs of corrosion protection come to roughly 4%of a developed nation’s gross national product (GNP) value, which for USA means about $270 billion annually lost to corrosion.
Indirect costs, such as shutdowns, loss or contamination of products, loss of efficiency, and the costs of over-design are much more difficult to estimate but they are believed to be at least as great as the direct costs.

4. Most iron ores contain oxides of iron, and rusting of steel by water and oxygen result in a hydrated iron oxide. Rusting is a term reserved for steel and iron corrosion, although many other metals form their oxides when corrosion occurs.

5. Environments
Practically all environments are corrosive to some degree. Some examples are air and moisture, fresh, distilled, salt and mine water, and steam and other gases such as chlorine, ammonia and hydrogen sulfide, mineral acids, and organic acids. In general, the inorganic materials are more corrosive than the organics.
Higher temperatures and pressures usually involve more sever corrosion conditions.
Eratures and pressures usually involve m

6. Corrosion Damage
Anodizing of aluminum is beneficial corrosion process used to obtain better and more uniform appearance in addition to a protective corrosion product on the surface.
Appearance: Automobiles are painted because rusted surface are not pleasing to the eye.
Maintenance and Operating Cost: Substantial savings can be obtained in many types of plants through the use of corrosion-resistance material of construction.

7. Application of cathodic protection can cut leak rate in existing underground pipelines to practically nil with attendant large savings in repair costs.
Close cooperation between the corrosion engineer and process and design personnel before a plant is built can eliminate or substantially reduce the cost maintenance costs in many cases.
Plant shutdown: Frequently plants are shutdown or portions of process stopped because of unexpected corrosion failure.
Corrosion monitoring of a plant process is helpful in preventing unexpected corrosion failure and plant shutdown. This can be done periodically examine corrosion specimens that are continually exposed to the process or by using corrosion probe that continuously records the corrosion rate.
Periodic inspection of equipment during the schedule down times can help prevent unexpected shutdown.
Effect on Safety and Reliability.

8. Classification of Corrosion
Corrosion has been classified in many different ways. One methods divides corrosion into low-temperature corrosion and high-temperature corrosion. The preferred classification is wet and dry corrosion.
Wet corrosion occurs when a liquid is present. This usually involves aqueous solutions or electrolytes. A common example is corrosion of steel by water.

9. Example: An engineer was asked to select a better material for a critical stainless steel heat exchanger that had had to be replaced four times. Looking at one of the exchangers on the scrape heap, he saw the tubes were plugged with rust. But under the rust was bright uncorroded stainless steel. The rust has come from carbon steel equipment upstream.
He replaced this steel with a more resistance alloy and the plant operated with scrapped cleaned heat exchangers for many years.

10. Dry corrosion occurs in the absence of a liquid phase or above the dew-point of the environment. Vapours and gases are usually the corrodents.
Dry corrosion is most often associated with high temperatures. An example is attack on steel by furnace gases.

36. Polarization

40. Nature of Metals Inhomogeneities in Metal Surface