Corrosion vs Rusting
Corrosion and rusting are two chemical processes, which result in disintegration of materials.
When a material reacts with the external environment, over a time, its structure will be deteriorated, and breaks down into small pieces. Ultimately, it can disintegrate into the atomic level. This is known as corrosion. Most commonly this happens to metals. When exposed to the external environment, metals will undergo oxidation reactions with oxygen in the atmosphere. Other than metals, materials like polymers, ceramics can also undergo disintegration. However, in this case, it is known as degradation. The external factors that cause metals to corrode are water, acids, bases, salts, oils, and other solid and liquid chemicals. Other than these, metals corrode when exposed to gaseous materials like acid vapors, formaldehyde gas, ammonia gas, and sulfur containing gases. The basis of the corrosion process is an electrochemical reaction. In the metal where corrosion is taking place, a cathodic and anodic reaction takes place. When metal atoms are exposed to water, they give up electrons to oxygen molecules and form positive metal ions. This is the anodic reaction. The produced electrons are consumed by the cathodic reaction. The two places where cathodic reaction and anodic reaction take place can be close to each other or far apart depending on the circumstances. Some materials are resistance to corrosion, while some are prone to corrosion. However, corrosion can be prevented by certain methods. Coating is one of the methods to protect materials from corrosion. This includes painting, plating, applying enamel on the surface, etc.
Rusting is a chemical process, which is common with the metals containing iron. In other words, the corrosion process taking place when there is iron, it is known as rusting. For rusting to take place, there should be certain conditions. In the presence of oxygen and moisture or water, iron undergoes this reaction and form a series of iron oxide. This reddish-brown color compound is known as rust. So, rust contains hydrated iron (III) oxide Fe2O3·nH2O and iron (III) oxide-hydroxide (FeO(OH), Fe(OH)3). If rusting starts at one place, it will eventually spread, and the whole metal will be disintegrated. Not only iron, but the metals containing iron (alloys) also undergo rusting.
Rusting begins with the transfer of electrons from iron to oxygen. Iron atoms transfer two electrons and form iron (II) ions as follows.
Fe → Fe2+ + 2 e−
Oxygen forms hydroxide ions by accepting electrons in the presence of water.
O2 + 4 e– + 2 H2O → 4 OH–
Above reactions are accelerated in the presence of acids. Further, when there are electrolytes like salts, the reaction is further enhanced. Rust contains iron (III) ions, so the formed Fe2+ undergoes redox reaction, to give Fe3+ as follows.
4 Fe2+ + O2 → 4 Fe3+ + 2 O2−
Fe3+ and Fe2+ undergo following acid base reactions with water.
Fe2+ + 2 H2O ⇌ Fe(OH)2 + 2 H+
Fe3+ + 3 H2O ⇌ Fe(OH)3 + 3 H+
Ultimately, a series of hydrated iron oxides is formed as rust.
Fe(OH)2 ⇌ FeO + H2O
Fe(OH)3 ⇌ FeO(OH) + H2O
2 FeO(OH) ⇌ Fe2O3 + H2O
What is the difference between Corrosion and Rusting?
• Rusting is a type of corrosion.
• When iron or materials containing iron undergo corrosion, it is known as rusting.
• Rusting produce a series of iron oxide, whereas corrosion can result in salts or oxides of the metal.