Corrosion is a term given to a group of several processes that leads to the degradation of a material (usually metals). The deterioration is a result of the material’s interaction with the environment. In technical terms, corrosion is a natural phenomenon in which a material electrochemically reacts with the surrounding environment to form a more chemically stable compound such as oxide, hydroxide, and sulfide. Unfortunately for metals, almost all environments can cause corrosion to some extent since the corroded state is the more stable state. Most metals are prone to oxidation as they tend to lose electrons to oxygen (and other substances) in the air or water. As oxygen is reduced (gains electrons), it forms an oxide with the metal. Generally, corrosion occurs when most or all the atoms of the same metal surface are oxidized, thereby damaging the entire surface. Corrosion is the reverse process of metallurgy. In other words, the energy used to transform ore into a metal is reversed as the metal is exposed to oxygen and water. Corrosion is a major cost component of maintaining infrastructure. There is a special interdisciplinary field of study, involving organic chemistry, microbiology, electrochemistry, and metallurgy dedicated to control and prevent corrosion called Corrosion Engineering. Protection against it involves the use of surface coatings, alloying materials, and using cheap sacrificial materials that corrode rather than the structure being protected. This phenomenon can commonly be observed in our everyday life. Let’s take a look at a few of its examples:
1. Rusting of Iron
Iron is the 26th element with the symbol Fe in the modern periodic table of elements and has a significant role in sustaining life on planet earth. We come across several applications of iron in our daily life, such as being a crucial building block in infrastructure that provides strength, nourishes plants, and helps carry oxygen in our blood. However, the iron masses we observe around us are not in stable thermal equilibrium and tend to go under oxidation reaction to form a more stable compound. Given sufficient time, any iron mass present in the vicinity of water and oxygen is susceptible to corrosion and form a reddish-brown iron oxide, commonly known as rust. It is formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. The general chemical composition of rust is hydrated iron (III) oxide ({Fe}_{2}{O}_{3}.n{H}_{2}{O}); however, under humid conditions, it may include iron (III) oxide-hydroxide (FeO(OH)). Several types of rust are visually different based on the circumstances under which these are formed. For instance, the most familiar form of rust is the reddish coating that forms flakes on iron and steel ({Fe}_{2}{O}_{3}), but it can also come in other colors including yellow, brown, orange, and even green, depending on the chemical composition of the environment in which they are present. Although rust is considered as the result of an oxidation reaction, it is important to note that not all iron oxides are rust. There are many effects of corrosion in our daily life that often goes unnoticed. At home, for instance, doors, pipes, and several other iron infrastructure are damaged by corrosion over time, which affects aesthetically, economically, and can potentially lead to accidents that have serious consequences. To address the issue, paint coatings are applied to act as a barrier between the air and the iron surface.
2. The Changing Colour of the Statue of Liberty
Corrosion is often a slow change and can be observed over a great period of time. The iconic bluish-green Statue of Liberty on the Liberty island of the New York Harbor, USA is a great example of such an observation. The 305-feet (93 meters) statue was built over nine years in sections of copper skin on top of an iron skeleton. When France gifted the Statue of liberty to the US as a way of commemorating the US’s fight for independence in 1886, it was actually brown in color reflecting the shiny copper surface of the skin. Over the next 30 years, though, it slowly turned to the green color you witness today. The change in appearance is a direct consequence of corrosion that took place over this period. The whole phenomenon can be understood by the series of reactions mentioned below:
- Initially, the copper went under oxidation reaction by donating its electrons to the oxygen present in the air. This led to the formation of a reddish-pink oxide mineral known as cuprite or copper (I) oxide ( {Cu}_{2}{O}).
2Cu + {O}_{2} → {Cu}_{2}{O}
- Then the copper(I) oxide continues to react with oxygen to form copper oxide (CuO), which is black in color
2{Cu}_{2}{O}+ {O}_{2} → 4CuO (black)
- Also, due to the burning of coal for energy during that time, the air contained a lot of sulfur. As a result, atmospheric sulfur trioxide, carbon dioxide, and water all reacted with the copper oxide as follow:
2CuO + {CO}_{2} + {H}_{2}{O} → {Cu}_{2}{CO}_{3}{(OH)}_{2} (green)
3CuO + 2{CO}_{2} + {H}_{2}{O} → {Cu}_{3}{({CO}_{3})}_{2}{(OH)}_{2} (blue)
4CuO + {SO}_{3} +3{H}_{2}{O} → {Cu}_{4}{SO}_{4}{(OH)}_{6} (green)
These three compounds are responsible for the characteristic blue-green patina seen on the Statue of Liberty. Fortunately, the formation of patina creates a protective layer on the copper surface, preventing further corrosion of the underlying copper.
3. Corrosion of the Eiffel Tower
Eiffel Tower is one of the most remarkable and renowned landmarks in the world. Located at the Champ de Mars in Pairs, France, this 324 meters high wrought-iron tower was built for the Paris World’s Fair of 1889 to commemorate the centennial of the French Revolution. Since then, the tower has been painted every seven years to protect the iron from rusting. It takes around 50 metric tons of three graded tones of paint every 7 years to protect the 200,000 square meters of iron latticework from rust. The application of an anticorrosion treatment lasts almost 18 months. The paints applied to the Eiffel Tower are formulated specifically for this purpose. The purity of the iron used in the construction of the Eiffel Tower is the result of a process carried out in high-temperature furnaces. This process allows the carbon content to be reduced to a very low percentage and to eliminate almost all the sulfur, so the resulting iron is of high purity.
4. Discoloration of the Taj Mahal
The Taj Mahal, one of the new seven wonders of the world, is an iconic monument that attracts many visitors to the Indian city of Agra, Uttar Pradesh, every year. It is a mausoleum made of ivory-white marble built by the Mughal emperor Shah Jahan as a tomb house for one of his beloved wives, Mumtaj Mahal. On the time scale of several years, the white marble domes of the Taj Mahal have turned yellow-brown in color. The issue is addressed as “Discoloration of the Taj Mahal” by the Indian Government. Several studies have shown that the reason for discoloration of the Taj Mahal is increased air pollution level, which causes acid rain. Acid rain, also known as acid deposition, is caused by emissions of sulfur dioxide and nitrogen oxides from power plants, cars, and factories. These chemicals react with water vapors to form sulfuric and nitric acids that fall as acid rain, snow, sleet, or fog. To help control the pollution, the Indian government has set up the “Taj Trapezium Zone (TTZ),” a 10,400-square-kilometre area around the monument where strict emissions standards are in place.
5. Neodymium Magnets
A neodymium magnet is a rare earth permanent magnet made from an alloy of iron, neodymium, and boron to form a {Nd}_{2}{Fe}_{14}{B} tetragonal crystalline structure. Every permanent magnet contains some form of iron as it has the most dramatic ferromagnetic properties of all elements. Because of their high iron content (64-68%), neodymium magnets are also highly susceptible to corrosion in damp environments, especially along with the boundary interface of a sintered magnet. This type of corrosion can cause serious deterioration including crumbling of a magnet into a powder of small magnetic particles or spalling of a surface layer. To prevent corrosion, most neodymium magnets are plated with a three-layer, nickel-copper-nickel plating. This particular plating combination performs better than zinc plating or other solutions in most applications.
6. Marine Corrosion
From supplying cargo and connecting continents to changing the course of the war, ships have played a prominent role in globalization. These massive vessels are primarily exposed to atmospheric corrosion, caused by a combination of high moisture and salt-laden sea spray, both of which directly attack the steel through the smallest deficiencies of the paint layer. Depending on construction metals, vessels are susceptible to different degrees of corrosion. For instance, galvanized steel is much less resistant to corrosion than some grades of stainless steel. Corrosion also depends on exposure to different marine environments. The worse the environmental conditions, the worse will be the impact of corrosion.
7. Efflorescence
Brick is considered one of the most popular building materials in the modern world. Like many other porous materials, brick is also susceptible to corrosion by water. In chemistry, efflorescence (which means “to flower out” in French) is the migration of a salt to the surface of a porous material, where it forms a coating. It is generally a white or off-white color with a powdery appearance. Although it is not much of a risk structurally, it affects the long-term durability of a building. The process involves the dissolving of an internally held salt in water or occasionally in another solvent. After water enters the pores of the material, it gradually dissolves crystal formation, which results in the weakening of connections between the pores and loss of strength of the material. The soluble salts contained in the brickwork can become potentially dangerous as they promote greater deterioration of the technical characteristics and aesthetic qualities of buildings and reduces their longevity.
