Conditions For Corrosion

What Is Corrosion?

Corrosion is a process by which deterioration of metal occurs, which is the result of the reaction of metal with air and water surrounding it. In the case of iron, the corrosion process is commonly known as rusting. Corrosion slowly coats the surfaces of metallic objects with oxides or other salts of the metals. The name for the corrosion process varies from metal to metal, like rusting for iron, tarnishing for silver etc.  The formation of a green coating on the surface of copper and bronze are some more examples of corrosion. Corrosion causes very large amounts of damage to buildings, bridges, ships and all such objects that are made of metals and their alloys, especially iron. In this article, we are going to briefly discuss conditions for corrosion and measures taken to prevent conditions for corrosion.

Conditions for corrosion

Atmospheric gases attack the surface of metals which result in formation of undesirable compounds such as oxides, sulphides etc. This causes decomposition of the metal and is called corrosion.

All the metals present in the environment, except the metals which are least active in nature such as gold, platinum and palladium, are attacked by air and water present in the environment and undergo corrosion. For example, silver, lead and stainless steel lose their lustre due to corrosion. It has been observed that rusting of iron only takes place when iron is in contact with moist air. Iron does not rust in vacuum or in presence of dry air. A thin layer of moisture with CO2 dissolved in it, constitutes an electrolytic solution which covers the metal surface at different places. Here are some conditions that usually cause corrosion.

1. Position of metals in e.m.f. series or reactivity of metals: Reactivity of any element mainly depends upon its position in the electrochemical series. More reactive metals are readily corroded. In other words we can say that more the reactivity of metal, the more will be the possibility of the metals getting corroded.
2. Presence of impurities in metals: It increases the speed of corrosion because the impurities help in setting up Voltaic cells.
3. Presence of electrolytes: When electrolytes are present in water, it increases the rate of corrosion. For example, corrosion of iron in seawater takes place to a larger extent than in distilled water.
4. Presence of CO2 in water: When CO2 is present in natural water, It increases the efficiency of water in corroding iron. Water which contains CO2 acts as an electrolyte which results in increased electrons that flow from one place to another.
5. Presence of protective coating: When the iron surface is coated with layers of metals more active than iron itself, then the rate of corrosion is retarded. For example, when we coat the metal with different elements like zinc, it can prevent corrosion altogether.
6. Stains in metals: Corrosion takes place rapidly at bends, scratches, nicks and cuts in the metals.

Consequences of Corrosion 

1. Economic consequences

Several studies have been conducted in various nations to calculate the national cost of corrosion. The most comprehensive of these studies, conducted in the United States in 1976, discovered that the overall yearly cost of metallic corrosion to the US economy was $70 billion, or 4.2 percent of GDP. 

2. Effects on safety

A far more serious issue is structural corrosion, which can result in serious injuries or even death. Corrosion compromises safety by contributing to the breakdown of bridges, aircraft, vehicles, gas pipelines, and so on – the entire complex of metal structures and equipment that comprise the contemporary world.

3. Effects on health

Metal prosthetic devices in the body, such as pins, plates, hip joints, pacemakers, and other implants, have become more common in recent years. New alloys and improved implantation procedures have been developed, yet corrosion remains an issue.

Measures to prevent Corrosion 

1. Barrier protection: The simplest way to prevent corrosion is by not allowing metal surface  to come in contact with moisture, oxygen and carbon dioxide. This can be achieved by:

• Applying a thin film of oil or grease on the metal surface.
• Electroplating with metals like tin, nickel, zinc, chromium, aluminium, etc.

2. Sacrificial protection: In this method iron is covered with a layer of metal which is more active than iron and thus prevents iron from losing electrons, for example with tin, zinc, magnesium etc.
3. Electrical protection: This method is used when iron articles are in contact with water such as underground water pipes, storage tanks, canals etc. The articles of iron are connected with more active metals like magnesium, zinc or aluminium which act as anodes. The iron pipe acts as a cathode in this case.
4. Using anti-rust solutions: Solutions of alkaline phosphatase and alkaline chromate are used in car-radiators to prevent rusting of iron in different parts of the engine.

Conclusion 

Corrosion occurs when metals come in contact with atmospheric gases or with water or air. There are three important conditions for corrosion to occur:

• an electrolyte
• an exposed metal surface
• an electron acceptor

We can prevent corrosion by removing one of these conditions. When we coat a metal surface with paint and enamel, they provide a barrier between the metal and moisture present around it. Corrosion is an irreversible process of metal, thus corrosion can only be prevented.