What is Corrosion?

What is Corrosion?

• Corrosion is the gradual destruction of metals by chemical, electrochemical, or biochemical interaction with the environment.

Causes of Corrosion

• Metals occur in nature as their oxides, sulfides, carbonates, etc. and are thermodynamically more stable in these forms.
• When extracted from ores, metals are in a higher energy state, which is thermodynamically unstable.
• Metals try to return to their stable state by chemical or electrochemical interaction with the environment.

Types of Metal Occurrence

Native State

• Metals occur in a native, free, uncombined state or pure metal and are highly stable and non-reactive with the environment.
• Examples include Au, Pt, Ag, Pd, Cu, etc.

Combined State

• Except for noble metals, all other metals are reactive and form stable compounds as their oxides, sulfides, chlorides, and carbonates.
• Examples include FeO, ZnO, PbS, CaCO₃, etc.

Effects of Corrosion

• Corrosion results in the wastage of metal in the form of its compounds.
• Valuable metallic properties like conductivity, malleability, ductility, etc. are lost due to corrosion.
• Life span and efficiency of metallic parts of machinery and fabrications are reduced.
• Efficiency of the machine decreases.
• Plant shutdowns occur due to failure.
• Product contamination can occur.
• Toxic products of corrosion can cause health hazards.
• Overdesign is necessary to allow for corrosion.

Classification or Theories of Corrosion

Chemical Corrosion

• Direct chemical action of atmospheric gases like oxygen, halogens, H₂S, etc. on metals in a dry environment.
• Oxidation corrosion: direct attack of oxygen on metal surface in the absence of moisture.
• Alkali and alkaline earth metals are corroded at low temperatures.
• Most metals except Au, Pt, and Ag are oxidized at high temperatures.
• Corrosion due to other gases: SO₂, CO₂, Cl₂, H₂S, F₂, etc.

Electrochemical Corrosion

• Wet corrosion occurs due to the electrochemical action of moisture and oxygen on metals.
• Occurs when two dissimilar metals or alloys are in contact with each other in the presence of an aqueous solution or moisture.
• Occurs when the metal is exposed to an electrolyte with varying amounts of oxygen.