9 Methods for the Prevention of Corrosion

 When a metal is exposed to certain environment, it may get deteriorated due to its reaction with the environment and its surface may become rough. This phenomenon is called corrosion.

A large number of metals lose their shine due to deposition of the compounds such as oxides, sulfides, sulfates, carbonates, etc., on their surface. 

Some metals lose their strength and become weak an brittle. 

The tarnishing of silver (loss of shine), rusting of iron (deposition of a brown colored layer on its surface), development of green coating on copper and bronze, tarnishing of aluminum are some familiar examples of corrosion.

Corrosion is a wide term and not only includes the deterioration of a metal by atmospheric changes but it also includes deterioration of metal by any environment surrounding the metal is covered by this term. Corrosion may be caused by gases, liquids and solutions present around the metal.

The slow but spontaneous deterioration of metals due to their interaction with certain environment is called corrosion.

Corrosion of metals is not desirable because tarnishes the metal and reduces it's strength. 

Prevention of corrosion can be done by the following eight methods.

By coating the metal surface with paints.

By coating the metal surface with oil or grease.

By coating the metal surface with non-corroding metals.

By coating the metals with certain chemicals.

By galvanizing iron

By tinning of iron and copper

By using cathodic protection 

By using anti-rusting solutions

Barrier Protection

This is one of the simplest methods to prevent corrosion. The method is largely used to protect iron from rusting. In this method, a suitable barrier is placed between the metal surface and atmospheric air. Thus, metal surface remains protected by the action of air, water and carbon dioxide and does not corrode. The barrier method can be achieved by any of the following methods.

By coating the metal surface with paints.

A thin coating of paints, enamels, lacquers, etc., protects the metal surface form being corroded. Iron sheets are usually protected from rusting by this method. For example, the iron sheets used in bicycles, cars, buses, etc., are always painted with suitable paint to protect them from rusting.

By coating the metal surface with oil or grease.

Rusting of iron can be prevented by applying a thin film of oil or grease on its surface. Iron tools and machinery parts are usually protected by this method.

By coating the metal with non-corroding metals

A thin coating of non-corroding metals such as nickel, chromium etc., on a corroding metal saves it from being corroded. For example, iron can be protected from being rusted by coating it with nickel or chromium through electroplating technique.

By coating the metal with certain chemicals

Rusting of iron can also be prevented by coating its surface with FePO4 or other chemicals which gives a tough adherent insoluble film. This film prevents the metal to interact with air and moisture and the metal is saved from being rusted.

Sacrificial Protection

This method involves the protection of a metal at the expense of some other more electropositive metal coated on the surface.

When the metal to be protected is covered with a layer of more active metal, the more active metal loses electrons in the preference to the metal to be protected and thus converts itself into the  ionic state.

Consequently, the more active metal goes on consuming with the passage of time and saves the host metal from being corroded so long as it is present on the host metal.

Galvanization of Iron

Sacrificial protection technique is widely used to protect iron from being rusted. Zinc being more reactive (more electropositive) than iron is generally used for covering iron surfaces. The process of deposition of a thin layer of zinc on iron surface is called galvanization. Galvanization is done by either of the two techniques given below.

By dipping

Iron sheets are dipped in molten zinc and then passed through hot rollers when the extra zinc sticking to them is removed and a thin uniform later of zinc is obtained.

By sherardizing

In this technique, zinc dust is heated strongly and vapors of zinc thus obtained are allowed to condense on the sheets of iron when a thin uniform layer of zinc is obtained.

The thin layer of zinc present on the surface of iron prevents it to come in contact with atmospheric oxygen and moisture. 

Iron does not rust even when zinc coating develops scratches or cracks. This is because the standard reduction potential of zinc is less than that of iron.

This shows that zinc has a greater tendency to undergo oxidation in comparison to iron. Hence, in case of a crack, the zinc layer acts as anode and undergoes oxidation in preference to iron. The iron acts as cathode. The electrons released in the oxidation of zinc migrate to iron cathode and reduce O2 to water present at the exposed part of iron which is working as cathode. Due to oxidation, zinc layer may be converted to basic zinc carbonate, ZnCO3.Zn (OH)2 by the action of oxygen, CO2 and moisture. This layer protects the exposed part further.

Protection of iron by tin

Iron can also be protected by depositing a thin layer of tin on it. This process is called tinning. Tinning can protect iron as long as the coating is intact. If the coating develops scratches or cracks, the iron is not protected anymore. One point to note here is that the protective action of tin is different from that of zinc which protects iron even when the coating develops cracks. This is because the standard reduction potential of iron is less than that of tin.

This shows that iron possesses a greater tendency to get oxidized as compared to tin. Therefore, if tin coating develops scratches or cracks, iron present in the exposed part will undergo oxidation in preference to tin and will get rusted. In such a case the rusting is much more rapid as compared to that in case of an unprotected iron piece.

Protection of copper by tin

Tin can protect copper as efficiently as zinc protects iron. This is due to the fact the standard reduction potential of tin is less than that of copper.

Thus, tin has a much greater tendency to get oxidized as compared to copper. Therefore, if tin layer develops crack, tin itself undergoes oxidation in preference to copper an copper remains protected.

From the above discussion, it may be concluded that a more electropositive metal (having lower value of standard reduction potential) can sacrificially protect a lesser electropositive metal when used in the form of a thin layer on the metal to be protected. This is why highly electropositive metals like Al, Zn, Mg etc., in powdered form are mixed with paints used to protect inferior metals like iron.

Cathodic protection

This method involves the use of a more reactive metal as sacrificial anode along with the lesser reactive metal be to protected. The method is generally used for the protection of underground pipes and tanks.

In this method a plate or block of a reactive metal (Zn or Mg) is burned beside the iron pipe or tank connected to it by a wire.

Since the more reactive metal has a greater tendency to get oxidized it undergoes oxidation in preference to iron. Thus, the more reactive metal acts as anode.

The electrons thus released migrate to the iron object which starts acting as cathode. At iron cathode, electrons reduce O2 into OH- ions.

The anode gradually disappears due to oxidation of the reactive metal. Thus iron object gets protected from being rusted and the reactive anode is sacrificed to protect it. The iron object remains protected as long as the active metal is present. The sacrificial reactive metal anode is replaced from time to time.

Protection by Anti-Rust Solutions

Corrosion of iron can also be prevented by using come anti-rust solutions. These are usually alkaline phosphate and chromate solutions.

The alkaline medium of the solution prevents the availability of H+ ions which are essential for rusting. Moreover, the phosphates tend to deposit and insoluble protective film of iron phosphate on the metal. 

This film protects the metal from being rusted. Anti-rust solutions are used to prevent the rusting of automobile engine parts and car radiators.

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