Chapter 21 – Corrosion and its prevention

These are the factors that cause corrosion:

• Moisture is one of the most common reasons causing metals to corrode.
• Corrosive gases like ammonia, sulphur oxides, hydrogen oxides, and chlorine cause corrosion too.
• Often, the presence of bacteria and dirt in the atmosphere also accelerate the process of metal corrosion.
• The presence of acids and impurities plays a role in corrosion too.
• When the temperature is high, the chances of corrosion increases.

1. Uniform corrosion – This is one of the most common forms of corrosion where the chemical process occurs uniformly throughout the entire surface that is exposed to the surrounding atmosphere. For example, when zinc is submerged in sulphuric acid, the reaction will occur uniformly across the entire surface.
2. Two-metal corrosion – It is also called galvanic corrosion. This happens when two electrochemically dissimilar metals are in electric contact in a corrosive solution. There is electron flow between the metals; as a result, a galvanic reaction sets-in, in which one metal will corrode at the joint. The metal which is less-resistant is anodic, and the more-resistant metal becomes cathodic.
3. Crevice corrosion – This type of degradation occurs in the crevices of reactive metals when exposed to corrosives.
4. Stress corrosion – When a metal crack as it is left exposed to a corrosive environment, is called stress corrosion. This type mostly happens at a higher temperature.
5. Intergranular corrosion – when there are impurities in the granular structure, Intergranular corrosion sets in. It results in the formation of depleted layers at the grain boundaries.

• Painting and oiling – Applying a good-quality layer of paint or oil or grease on the surface of the metal prevents its surface from being exposed to factors that cause corrosion.
• Galvanizing – In this method, a thin layer of zinc covers the surface of the iron, which prevents it from rusting.
• Anodising – In this process, where an oxide coating made from aluminum is produced on the surface of the metal through an electrolytic process. It is a thick layer that acts as a protective covering and resists wear and tear and corrosion of the surface.
• Alloying – In this process, the metal is converted into alloys – for example, pure iron rusts; therefore, by mixing with 0.05% carbon, an alloy is formed. Stainless steel is an iron alloy formed by mixing iron with nickel and chromium, and stainless steel does not rust.
• Chrome plating – In this process, a thin layer of chromium is electroplated onto the surface of the metal which opposes corrosion.