There are two factors that influence the rate of corrosion. Hence a knowledge of these factors and the mechanism with which they affect the corrosion rate is essential because the rate of corrosion is different in different atmosphere.
- Nature of the metal
- Nature of the corroding environment
Nature of the metal following factors :
- Physical state.
- Purity of metal.
- Over voltage.
- Nature of surface film.
- Relative areas of the anodic and cathodic parts:
- Position in galvanic series:
- Passive character of metal:
- Solubility of corrosion products:
- Volatility of corrosion products:
Nature of the Corroding Environment :
- Temperature: The rate of corrosion is directly proportional to temperature ie., rise in temperature increases the rate of corrosion. This is because the rate of diffusion of ionsincreases with rise in temperature.
- Humidity of air: The rate of corrosion will be more when the relative humidity of the environment is high. The moisture acts as a solvent for oxygen, carbon dioxide, sulphur dioxide etc. in the air to produce the electrolyte which is required for setting up a corrosion cell.
- Presence of impurities in atmosphere: Atmosphere in industrial areas contains corrosive gases like CO2, H2S, SO2 and fumes of HCl, H2SO4 etc. In presence of these gases, the acidity of the liquid adjacent to the metal surfaces increases and its electrical conductivity also increases, thereby the rate of corrosion increases.
- Presence of suspended particles in atmosphere:
In case of atmospheric corrosion:
- if the suspended particles are chemically active in nature (like NaCl, Ammonium sulphate), they absorb moisture and act as strong electrolytes, thereby causing enhanced corrosion;
- if the suspended particles are chemically inactive in nature (eg., charcoal), they absorb both sulphur gases and moisture and slowly enhance corrosion rate.
- Influence of pH: Generally acidic media (ie., pH<7) are more corrosive than alkaline and neutral media. However, amphoteric metals (like Al, Zd, Pb, etc.) dissolve in alkalinesolutions as complex ions. The corrosion rate of iron in oxygen-free water is slow, until the pH is below 5.
The corresponding corrosion rate in presence of oxygen is much higher. Consequently corrosion of metals, readily attacked by acid, can be reduced by increasing the pH of the attacking environment, eg., Zn (which is rapidly corroded, even in weakly acidic solutions such as carbonic acid suffers minimum corrosion at pH=11.
- Nature of ions present.
- Conductance of the corroding medium.
- Formation of oxygen concentration cell.
- Flow velocity of process stream.
- Polarization of electrodes.