ADEBISI JELEEL ADEKUNLE 109046004

DEPARTMENT OF METALLURGICAL AND MATERIALS ENGINEERING

MME815: ELECTROCHEMISTRY AND CORROSION

CORROSION AND RUSTING

Whenever people are discussing about corrosion, they usually reference their old cars.

Everyone who owns a car over five years old has firsthand experience with rusting, along with

the bitter knowledge of what it costs in reliability and resale value. Corrosion is the deterioration

of a material due to interaction with its environment. It is the process in which metallic atoms

leave the metal or form compounds in the presence of water and gases. Metal atoms are removed

from a structural element until it fails, or oxides build up inside a pipe until it is plugged. All

metals and alloys are subject to corrosion. Even the noble metals, such as gold, are subject to

corrosive attack in some environments. The corrosion of metals is a natural process. Most metals

are not thermodynamically stable in their metallic form; they want to corrode and revert to the

more stable forms that are normally found in ores, such as oxides. Metal is usually mentioned in

the definition of corrosion, but any material can be damaged by its environment: plastics swell in

solvents, concrete dissolves in sewage, wood rots, and so on. These situations are all very serious

problems that occur by various mechanisms, but they are not included in this definition. The

damage/deterioration is specified purposely to exclude processes such as chemical milling,

anodizing of aluminium, and bluing of steel, which modify the metal intentionally. All sorts of

chemical and electrochemical processes are used industrially to react with metals, but they are

designed to improve the metal, not damage it. Thus these processes are not considered to be

corrosion. The environment that corrodes a metal can be anything; air, water, and soil are

common but everything from tomato juice to blood contacts metals, and most environments are

corrosive.

Corrosion is of primary concern in areas like nuclear reactor plants, oil and gas,

petrochemicals, food and beverages, construction, automobile, even at homes. It occurs

anywhere metal is used without good corrosion prevention measures. Corrosion occurs

continuously throughout all these areas, and every metal is subject to it. Even though this

corrosion cannot be eliminated, it can be controlled. In recent years, automobile manufacturers

have faced the problem and have begun to control corrosion by improving design, by sacrificial

and inhibiting coatings, and by greater use of plastics. Chemical plants, with their tremendous

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variety of aqueous, organic, and gaseous corrodants, come up with nearly every type of corrosion

imaginable. It becomes quite a challenge to control corrosion of the equipment without

interfering with chemical processes. Petroleum refineries have the best reputation for corrosion

control, partly because the value of their product gives them the money to do it correctly and

partly because the danger of fire is always present if anything goes wrong. The cost of corrosion-

resistant materials and expensive chemical inhibitors is considered to be necessary insurance.

Ships, especially the huge supertankers, illustrate another type of corrosion problem. Seawater is

very corrosive to steel and many other metals. Some metals that corrode only slightly, such as

stainless steels, are likely to crack in seawater by the combination of corrosion and high stresses.

Corrosion can cause the loss of a ship and its crew as well as damage to a fragile environment.

Corrosion control commonly involves several coats of paint plus cathodic protection, as well as

designing to minimize stress concentration.

Rusting is a type of corrosion but it is the corrosion of ferrous metals (irons and steels)

only, producing that familiar brownish-red corrosion product, rust. Rust is scientifically called

oxidation, which occurs when oxygen comes in long-term contact with certain metals. Over time,

the oxygen combines with the metal at an atomic level, forming a new compound called an oxide

and weakening the bonds of the metal itself. If the base metal is iron or steel, the resulting rust is

properly called iron oxide. Rusted aluminum would be called aluminum oxide, copper forms

copper oxide and so on. The main catalyst for the rusting process is dihydrogen oxide, but we

know it better as water. Iron or steel structures may appear solid, but water molecules can easily

penetrate the microscopic pits and cracks in any exposed metal. The hydrogen atoms present in

water can combine with other elements to form acids, which will eventually cause more metal to

be exposed. If sodium is present, as is the case with saltwater, corrosion will likely occur more

quickly. Meanwhile, the oxygen atoms combine with metallic atoms to form the destructive

oxide compound. As the atoms combine they weaken the metal, making the structure brittle and

crumbly. Placing a steel wool pad in water and exposing it to air will cause almost-immediate

rusting. The air around the pad will actually feel several degrees warmer. Eventually the

individual iron bonds will be destroyed from the heat and the entire pad will disintegrate. Rust

formation cannot be stopped easily, but metals can be treated to resist the most damaging effects.

Iron rust which, generally appears brown, is itself a chemical compound quite different from the

iron itself. The rusting of iron can be prevented. In towns and large cities where iron sheets are

used for roofing houses, rusting can also take place if the iron sheets are not protected from

rusting. Iron rust is a compound of iron. It is formed when iron reacts with the oxygen of the air

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in the presence of moisture (water) to form a brown substance known as iron oxide. The process

is a chemical reaction, which takes place over a period of time. It should be noted that oxygen

and water are necessarily present for iron to rust.

Iron + Oxygen + Water → Iron rust.

Rust is a general term for a series of iron oxides. Colloquially, the term is applied to red

oxides, formed by the reaction of iron and oxygen in the presence of water or air moisture. Yet,

there are also other forms of rust, such as the result of the reaction of iron and chlorine in an

environment deprived of oxygen, such as rebar used in underwater concrete pillars, which

generates green rust. Several forms of rust are distinguishable visually and by spectroscopy, and

form under different circumstances. Rust consists of hydrated iron (III) oxides Fe2O3·nH2O and

iron (III) oxide-hydroxide (FeO(OH), Fe(OH)3).

Conclusively, rusting is the common term for corrosion of iron and its alloys, such as

steel. Many other metals undergo equivalent corrosion, but the resulting oxides are not

commonly called rust. Corrosion is the general term for any degradation on any materials due to

its interaction with the environment.

Some Chemical reactions

The key reaction is the reduction of oxygen: O2 + 4e- + 2H2O → 4OH-

Because it forms hydroxide ions, this process is strongly affected by the presence of acid. Indeed,

the corrosion of most metals by oxygen is accelerated at low pH. Providing the electrons for the

above reaction is the oxidation of iron that may be described as follows:

Fe → Fe2+ + 2 e−

The following redox reaction also occurs in the presence of water and is crucial to the formation

of rust:

4 Fe2+ + O2 → 4 Fe3+ + 2 O2−

Additionally, the following multistep acid-base reactions affect the course of rust formation:

Fe2+ + 2 H2O Fe(OH)⇌ 2 + 2 H+

Fe3+ + 3 H2O Fe(OH)⇌ 3 + 3 H+

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as do the following dehydration equilibria:

Fe(OH)2 FeO + H⇌ 2O

Fe(OH)3 FeO(OH) + H⇌ 2O

2 FeO(OH) Fe⇌ 2O3 + H2O

From the above equations, it is also seen that the corrosion products are dictated by the

availability of water and oxygen. With limited dissolved oxygen, iron (II)-containing materials

are favoured, including FeO and black lodestone (Fe3O4). High oxygen concentrations favour

ferric materials with the nominal formulae Fe(OH)3-xOx/2. The nature of rust changes with time,

reflecting the slow rates of the reactions of solids.

Examples of Rusting

Heavy rust on the links of a

chain near the Golden Gate

bridge in San Francisco; it was

continuously exposed to

moisture and salt-laden spray,

causing surface breakdown,

cracking, and flaking of the

metal.

A rusted (and dirt-encrusted) bolt;

note the surface pitting and gradual

shape-deformation, caused by

severe oxidation

The Kinzua Bridge after it

collapsed. Kinzua Bridge in

Pennsylvania was blown down by a

tornado in 2003 largely because the

central base bolts holding the

structure to the ground had rusted

away, leaving the bridge resting by

gravity alone.

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Rusted pyrite cubes Flaking paint, exposing a patch of

surface rust on sheet-metal

Rusting can completely degrade

iron. Note the galvanization on the

unrusted portions.

The collapsed Silver Bridge, as

seen from the Ohio side

Rust in pipes can result in brown

and black water.

Sources

Bradford, Samuel A. (2001); Corrosion control, second edition, CASTI Publishing Inc.

DOE FUNDAMENTALS HANDBOOK, MATERIAL SCIENCE, Volume 1 of 2

en.wikipedia.org

http://library.unesco-iicba.org/English/SECONDARY_SCIENCE_SERIES/index_pages/

complete_list_of_science_lessons.htmls

http://www.wisegeek.com/topics/rust.htm

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