The general process of the decay of material due to the different environmental factors is called corrosion.
The rate of corrosion depends on the reactivity of metals.
The higher the reactivity of metals, the more rapidly it will corrode.
The most important case of corrosion is rusting.
The slow but continuous process of eaten away of the surface of the metals by the gases present in the atmosphere is called rusting.
Rusting is an electrochemical process where different parts of a metal act as anode and cathode and moist air acts as electrolyte.
At anode the metal is oxidized and releases electrons. These electrons travel to the other part of the metal, where they are picked up by the H+ ions of the metal and they are reduced along with the moisture to form OH- ions. Hence different parts of the metal act as anode and cathode and surface of the metal acts like a galvanic cell. The oxidized metal and hydroxide ions combine to form metal hydroxide. These metal hydroxides are further oxidized by the air forming the metal oxide along with the water of crystallization, which is called the rust.
The reactions taking place at the surface of the metal during the rusting process are as
At anode (oxidation)
Fe Fe + 2e
At cathode (reduction)
+ _ _
H2O + H + 2e 2OH
Fe + 2OH Fe (OH) 2 (s)
Fe (OH) 2 (s) + O2 + XH2O Fe2O3.XH2O
Moist air Rust
All the metals like iron undergo rusting when these metals are exposed to moist air, because all the metals when exposed to air are oxidized easily. However, the oxidation of these metals is slow depending upon the oxidation potential of the metals.
The metals like Na, K are so extremely reactive that these metals can't be exposed in the air. As these metals rapidly tarnish in the air, so the rusting process with these extremely reactive metals is out of question.
The moderately reactive metals like Zn, Fe, Cu, Al etc undergo oxidation when expose to air, and form the oxide layer of rust at their surface. The rusting of these metals other than iron is only a surface phenomenon, that is the oxide layer of the rust of these metals is a nonporous layer, due to which the process of rusting only occurs at the surface, and interior bulk of the metal is protected by the layer of rust. But in case of iron the oxide layer of rust form at the surface is a porous layer, due to which the contact of the moist air with the interior bulk of the mass remains continuous.
Due to this reason the rusting of iron is slow and continuous process. This process remains continuous till whole of the metal is eaten away. `
The process of rusting of iron is a slow process but it is accelerated many folds in the coastal areas, like Dubai. In the rusting process water (moist) is used as an electrolyte. The moisture which is present in the coastal areas contains high contents of salt in it. Due to which it is an excellent electrolyte which makes the process of rusting quick and fast.
Similarly the metals like Silver and Gold also undergoes rusting when exposed to moist air. As these metals have very low oxidation potential, so these are not easily oxidized when exposed to air and their rusting process is very very slow. Due to this reason these metals are most abundantly used metals in the formation of ornaments and decorative material.
Methods for the reduction of rusting:
The process of rusting especially in case of iron is although slow but continuous, due to which we cannot completely eradicate the process of rusting. However it can be reduced or avoided to some extent by taking the following measures….
It is the technique in which the less reactive metal is coated over the surface of more reactive metal by using the process of electrolysis. For this purpose the metals like gold, silver, tin, zinc etc are used for the coating purpose as the rusting of these metals is only a surface phenomenon. The coating of tin over the surface of other metals is called tinning and the coating of Zn metal on the surface of other metals is called galvanization. The coating of these metals not only serves for the purpose of prevention of rusting but also serves for the purpose of decoration as well.
Alloy is the mixture of metals. By mixing the metals and making the alloys also helps in the reduction of corrosion. Therefore when steel is made of iron the rusting of iron is avoided in steel.
3 Sacrificial rusting:
It is the process in which the more reactive metals connected with the other less reactive metals undergoes rusting. The more reactive metal loses electrons readily and becomes anode while the other metal becomes cathode. The more reactive metal is oxidized and undergoes rusting while the less reactive metal remains intact as cathode. For example Mg, attached to the supplies pipes underground. Mg being more reactive than iron undergoes oxidation and act as anode while iron remains intact as cathode. Such type of rusting is called sacrificial rusting, where one metal is sacrificed to safe the other metal.
The paint is also one of the solution to avoid rusting of metals, especially when they are exposed to water and air continuously, like the metals used in ship surface and the metals used in making bridges. Paint avoids the contact of moist air with the surface of the metal, due to which the rusting is avoided to some extent.
5 High temperature corrosion:
This type of corrosion occurs due to deep deterioration of material under high temperature, and it occurs when the metal is subjected to high temperature atmosphere, which contains O. Due to high temperature the metal is oxidized and hence goes into rusting. High temperature corrosion is frequently found in the materials used in power generation stations and aerospace. For example stainless steel under ordinary temperature does not undergo corrosion, but at high temperature when the contact is made with the combustion product, it undergoes corrosion, due to which oxides are formed on its surface and prevents the further attack. Due to this reason stainless steel can be used under high temperature conditions such as weirs of the machine which have sliding contact and the running machinery where the heat is produced by the machinery.
6 Corrosion in nonmetals:
Some nonmetallic materials such as ceramic also undergo little corrosion.
This corrosion is non galvanic; that, it does not involve electrochemical process, It occurs due to dissolution and deterioration of materials. Such corrosion can easily be prevented if the solubility of material in water is reduced. For example, when lime is added to soda glass, it becomes soda lime glass which is less soluble as compare to soda glass. That is now it is not easily soluble as pure sodium silicate (Soda Glass Na2SiO3).