Corrosion may be defined in a number of ways:

Deterioration of metals and alloys by chemical or electrochemical reactions with its environment

Eating away of construction materials

Deterioration of construction materials by means other than straight mechanical

Undesirable interaction of material with its environment

Physico-chemical interaction between a metal and its environment which results in changes in the properties of the metal and which may often lead to impairment of the function of the metal, the environment, or the technical system of which these form a part. (according to ISO)

Since corrosion involves chemical change, the student must be familiar with principles of chemistry in order to understand corrosion reactions. Because corrosion processes are mostly electrochemical, an understanding of electrochemistry is also important. Furthermore, since structure and composition of a metal often determine corrosion behavior, the student should be familiar with the fundamentals of physical metallurgy as well.

The corrosion scientist studies corrosion mechanisms to improve (a) the understanding of the causes of corrosion and (b) the ways to prevent or at least minimize damage caused by corrosion.

The corrosion engineer , on the other hand,applies scientific knowledge to control corrosion. For example, the corrosion engineer uses cathodic protection on a large scale to prevent corrosion of buried pipelines, tests and develops new and better paints, prescribes proper dosage of corrosion inhibitors, or recommends the correct coating g.

Change in any part of the corrosion system caused by corrosion.

Corrosion Damage

Corrosion effect which is considered detrimental to the function of the metal, the environment or the technical system of which these form a part.

Corrosion has been classified in many different ways. One method divides corrosion into low-temperature and high temperature corrosion. Another method separates corrosion into direct combination (or oxidation) and electrochemical corrosion. NACE International (National Association of Corrosion Engineers) has identified as many as eighty forms of corrosion, which can be grouped into eight forms as proposed by Fontana.

The preferred classification is

Wet corrosion or aqueous corrosion, and

Dry corrosion i.e. oxidation in the absence of water e.g. reaction between metal and any oxidizing gas such as carbon dioxide, oxygen, oxides of sulphur etc. at elevated temperatures

Wet corrosion occurs when a liquid is present. This usually involves aqueous solutions or electrolytes and accounts for the greatest amount of corrosion by far. A common example is corrosion of steel by water and oxygen (air).

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

Most corrosion processes are electrochemical in nature. Corrosion principles are discussed for understanding the science of corrosion. Corrosion engineering is the application of science and art to control corrosion damage economically and efficiently. In addition to the knowledge of corrosion science, the corrosion engineer must have sufficient knowledge of chemical, metallurgical, physical and mechanical properties of materials.

The National Bureau of Standards (NBS) has estimated that cost of corrosion in the United States of America in 1975 was $70 billion plus or minus 30% with about 10-45% of the total ($70 billion) as avoidable. The NBS, result follows the extrapolation from earlier percentage values since 1947 and works out to about 4% of US GNP in 1975. The %age seems to be some kind of constant for all economies. The BatteleColumbus Laboratories (BCL) have determined the total cost of corrosion to the United States $70 billion or about 4% of the GNP in 1978 and $10 billion of this cost could be avoided by the use of presently available corrosion control technology (See Fig 1.4).

Direct economic losses constitute the costs of replacements of corroded structures and machinery or their components e.g. condenser tubes, mufflers, pipelines, metal roofing, repainting of structures against rusting, cathodic protection and its upkeep for underground pipe systems. Direct losses also include the extra cost of using (i) corrosion resistant materials in place of carbon steel or other cheaper materials with adequate mechanical properties but not sufficient corrosion resistance, (ii) adding inhibitors to enclosed systems, (iii) protective systems for metal structures etc

The indirect economic losses constitute the financial losses described under various heads as follow:a. Loss of Product

Considerable losses of oil, gas or water may occur through a corroded pipe system until repairs are made. Similarly leaks in industry for different solvents and other liquids result in loss of significant value.

b. Loss of Production

For the repair or replacement of a corroded piece of equipment with a relatively small value, the whole plant may be shutdown for a day or more. Under these circumstances shutdown time must be kept to minimum. Thus the higher cost of corrosion resistant metals/alloys is justified in return for longer productive cycle and maintenance-free periods.

The deposition of corrosion products can decrease the efficiency of operating a plant. Examples include the loss of pumping capacity due to partial clogging of the interior of water pipes due to accumulation of corrosion product, reduction in heat transfer through corrosion deposits in heat exchangers, loss of critical dimensions in internal combustion engines through corrosion.

Fine chemicals, dye-stuffs, food processing and drug industries cannot tolerate the pick up of even small traces of metal ions in their product due to corrosion. Thus to avoid this contamination, these plants have to incorporate lined pipeline, reaction vessels, storage tanks and in some cases the whole plants are constructed of suitable grade of stainless steel, thereby raising the capital cost.

The less corrosion conscious organizations may suffer heavy commercial losses due to this type of contamination. Contamination by the corrosion products in fuel storage tanks of aircrafts and automobiles may cause serious quality problems.

The principle of overdesign is applied to allow for ravages of corrosion and consequently much thicker sections are used than would normally be required for mechanical strength. In case of water treatment and oil industries, corrosion allowances ranging between 50 to 100% are made in corrosion susceptible areas of plant, which means higher capital costs for extra consumption of materials and are against the concept of conservation of resources.

Therefore, in terms of overall economic balance, the concept of overdesign is less preferable than alternate use of protective measures for the prevention of corrosion unless the latter are exceedingly expensive and economically prohibitive.

To avoid unnecessary delays in scheduled or unscheduled shutdowns in large factories, replacement sections of plants and standby units have to be maintained in readiness to take over when corrosion failures occur. Similarly heavy inventories of replacement items have to be maintained in case of urgency during normal shutdowns. This also leads to a considerable increase in capital investments.

It is quite obvious that indirect losses form a substantial part of the economic loss suffered through corrosion, but it is quite difficult to arrive at a reasonable estimate of total economic burden within one industry. There are instances where loss of health or life through fire or explosion, unpredictable failure of chemical equipment, resulting in release of toxic vapors, rupture of vessels containing corrosive liquids through sudden failure of critical parts, have occurred.

The cost of human life and material losses alone including invisibles and overheads may amount to a staggering figure in large chemical concerns over the productive life of the plant.

Corrosion

Cost

Applied

Current

Technology

More Hostile

Environments

Environmental

Regulations

Research &

Development

Extentions of

Useful Life

Technology

Transfer

Increased

Performance

Requirments

Deferred

Maintenance

Factors which increase or decrease the costs of corrosion

Corrosion cannot be defined without a reference to environment. All environments are corrosive to some degree. Following is the list of typical corrosive environments:

(1) Air and humidity.

(2) Fresh, distilled, salt and marine water.

(3) Natural, urban, marine and industrial

atmospheres.

(4) Steam and gases, like chlorine.

(5) Ammonia.

(6) Hydrogen sulfide.

(7) Sulfur dioxide and oxides of nitrogen.

(8) Fuel gases.

(9) Acids.

(10) Alkalies.

(11) Soils.

Corrosion may severely affect the following

functions of metals, plant and equipment:

(1) Impermeability: Environmental constituents

must not be allowed to enter pipes , process equipment, food containers, tanks , etc. to minimize the possibility of corrosion.

2. Mechanical strength:

Corrosion should not affect the capability to withstand specified loads, and its strength should not be undermined by corrosion.

3. Dimensional integrity:

Maintaining dimensions is critical to engineering designs and they should not be affected by corrosion.

4. Physical properties:For efficient operation , the physical properties of plants, equipment and materials, such as thermal conductivity and electrical properties, should not be allowed to be adversely affected by corrosion.

5. Contamination:Corrosion, if allowed to build up, can contaminate processing equipment,

food products, drugs and pharmaceutical

products and endanger health and environmental safety.

6. Damage to equipment:

Equipment adjacent to one which has suffered corrosion failure , may be damaged.

1. Materials are precious resources of a country. Our material resources of iron, aluminum, copper, chromium, manganese, titanium, etc. are dwindling fast. Some day there will be an acute shortage of these materials. An impending metal crisis does not seem anywhere to be a remote possibility but a

reality. There is bound to be a metal crisis

and we are getting the signals. To preserve

these valuable resources, we need to understand how these resources are destroyed by corrosion and how they must be preserved by applying corrosion protection technology.

2. Engineering knowledge is incomplete without an understanding of corrosion.

Aeroplanes, ships, automobiles and other

transport carriers cannot be designed without

any recourse to the corrosion behavior of

materials used in these structures.

(3) Several engineering disasters, such as crashing of civil and military aircraft, naval and

passenger ships, explosion of oil pipelines

and oil storage tanks, collapse of bridges and

decks and failure of drilling platforms and

tanker trucks have been witnessed in recent

years. Corrosion has been a very importantfactor in these disasters. Applying the knowledge of corrosion protection can minimize such disasters. In USA, two million miles of pipe need to be corrosion-protected for safety.

(4) The designing of artificial implants for the human body requires a complete understanding of the corrosion science and engineering. Surgical implants must be very corrosion-resistant because of corrosive nature of human blood.

(5) Corrosion is a threat to the environment. For

instance, water can become contaminated

by corrosion products and unsuitable for

consumption. Corrosion prevention is integral

to stop contamination of air, water and

soil. The American Water Works Association

needs US$ 325 billion in the next twenty years

to upgrade the water distribution system.