Prevention of Rusting of Iron - Methods and Examples
Oxidation is a natural spontaneous process. It occurs because of the presence of oxygen in our atmosphere. The susceptible materials undergo reaction with the molecular oxygen present in the air and convert to their oxidized forms. Iron is one of those susceptible metals which are very prone to oxidation in the presence of moisture. The oxidation of iron, in the presence of atmospheric moisture, to form its hydrated oxidized form (chemically called hydrated iron oxide, and commonly known as rust) is called rusting (or rusting of iron or corrosion of iron). The chemical reaction involved in the process of rusting can be depicted as follows:
Rust appears as a brownish orange discoloration of the iron metal making it look unattractive. Apart from its looks, rusting of iron makes it mechanically weaker as metals possess more strength than their oxidized forms. The rusted iron becomes more brittle than its original metal form, thus creating a lot of problems. The bridges on roads and rivers and a lot of other construction materials which are made up of iron may lose their structural integrity and may pose a great risk of their breakage due to the disintegration and deterioration of iron metal. Since iron is a very important metal which is used for a lot of purposes in our daily life, we need to make use of some additional precautionary measure to prevent its rusting and prolonging its life.
There are a lot of methods which are employed by the industry and other common people to prevent the rusting of iron. Here is a comprehensive list of all these measures:
• The golden rule to prevent rusting is to keep the three components, which cause rusting (viz., iron, oxygen and water), apart from each other. If you are able to keep even one of these components away from the rest of the two, the purpose is solved.
• The iron industry which suffered huge losses in the ancient times due to the rusting of iron came up with a technique to combat iron rusting. They designed special rust-resistant alloys of iron which became very popular and have become an indispensable part of our daily lives. Using alloys as an alternative to pure iron has proven to be very effective in the prevention of rusting. The common alloys of iron are discussed below:
• The most popular rust-resistant alloy of iron is stainless steel. It comprises of a blend of various other metals (such as chromium, nickel, manganese and copper) along with iron and carbon as a non-metal. There are mainly three varieties of stainless steel available in the market nowadays – namely Martensitic (comprising of 12% chromium and up to 1.2% carbon), Ferritic (comprising of 10-27% of chromium) and Austenitic (comprising of 16-26% chromium and 35% nickel).
• Another common type of iron alloy used is the weathering steel comprising of upto 21% of other alloying metals such as chromium, nickel and copper. It contains phosphorus as a non-metal instead of carbon.
These alloys help in forming a protective layer over the actual iron metal thereby preventing it from coming in direct contact with either oxygen or moisture. This prevents iron from corroding or rusting.
• During the construction of anything which employs the use of iron, the designing of the structure is very important. Properly designed construction can help in minimizing the penetration of water through the constructed material. Special care and attention should be given on avoiding the creation of crevices or gaps or cavities anywhere in the structure which may act as a reservoir or path for the flow of water during rainy season. There should be a proper drainage system around the construction for correct outflow of water. In any case, water logging should be avoided. The architecture of the construction unit should be made such that the ventilation of air is high, allowing crossing of the fresh air which does not let atmospheric moisture retain in any particular area of the construction. Even if iron keeps in coming in contact with oxygen (from the air) but no moisture or direct water, rusting can be easily prevented.
• Another common method used to prevent iron from rusting is the process of galvanization. It is a process in which the surface of iron is covered with a protective layer of another metal, especially zinc. The process of galvanization can be carried out by two methods. The first method is known as the hot-dip galvanization. In this method, the iron material which is to be galvanized is dipped into a hot bath of molten zinc metal at a very high temperature (around 449-450oC). This method, however, has a disadvantage of resulting in an uneven coating of zinc over the iron metal surface and therefore is less commonly used these days. The second method, named electroplating galvanization (or simply electrogalvanization), is more commonly used these days because it provides a uniform coating of zinc metal over the iron surface. In this method, a layer of zinc metal is electrostatically bonded onto the surface of the iron. By passing a current of electricity through the zinc solution containing zinc metal as an anode, while the iron is present at the cathode, a coating of zinc is applied over the iron surface.
• A new technique, named bluing, is being employed nowadays to prevent iron from rusting. However, this technique is only useful for small iron parts and provides only partial resistance towards rusting to the “blued” iron. In fact, this technique got its name from the final blue-ish appearance it gives to the treated iron. In this technique, iron is immersed in a solution of potassium nitrate and sodium hydroxide in water. It employs the application of an electrochemical oxidation reaction in which the iron is specifically oxidized to ferric oxide (Fe3O4), also known as magnetite, which unlike the natural rusting process does not form the hydrated oxidized form of iron, hydrated ferrous oxide. Ferric oxide has a lesser affinity to towards moisture as compared to ferrous oxide and hence prevents rusting for quite some time. However, this blue-black oxide of iron is not completely rust-free and hence needs additional treatment with water-repellent chemicals or oils to reduce the possibility of occurrence of rust.
• A common household method used for the prevention of iron from rusting is its coating with materials such as organic paints. This method is easy as well as economical for all people who wish to prevent their iron materials without taking much hassles and spending more money in other processes like galvanization which is comparatively more costly. The coating of paint on the surface of iron acts as a barrier for the contact of iron with oxygen or moisture present in the air. Generally, oil-based paints are preferred more for this purpose due to the inherent tendency of oils to repel water against them. Any organic paint coating which is up to 15 to 25 µm in thickness is enough to prevent the coated iron material from rusting.
• Apart from paint coating, powder coating is another good option to prevent iron materials from rusting. Although paint coating seems the easiest and convenient method of preventing iron rusting, it sometimes becomes ineffective in case of cracked paint coatings or damaged painted surfaces which expose the underlying iron material again to the attack of oxygen and moisture. Instead, powder coating techniques are considered superior to paint coating since it forms a thicker layer, providing dense protection and one coat of powder coating is sufficient to prevent iron material from rusting. In powder coating technique, a dry organic powder is sprayed onto the surface (using the electrostatic technique) of the cleaned iron material which is to be coated. The iron material is then heated to a temperature of the melting point of the organic powder material being used. Once sufficient temperature is achieved, the organic powder melts on the surface of the iron material, thereby forming a uniform thin film over its surface. Generally, the organic powders commonly used in this technique include polyester, vinyl, urethane, acrylic, nylon and epoxy based organic materials. These materials are electrostatically charged before being sprayed onto the surface of the iron material. Since metals like iron are good conductors of electricity, they attract the electrically charged powder particles onto its surface thereby getting covered under their coating. The coating formed by this technique is generally 25-125 µm thick.
• One of the best methods to prevent iron from rusting is taking proper care of the iron materials and doing their regular maintenance. At regular intervals, check for any signs of rust, and if present, remove it using a blade and coat its surface for further prevention.