Factors influencing corrosion rate

Primary factors related to metal:

 1. Nature of the metal:

The tendency of a metal to undergo corrosion is dependent on the nature of the metal. Metals with lower reduction potential undergo corrosion easily whereas metals with higher reduction potential do not undergo corrosion easily. The reactive metals like Na, K, Mg, Zn are more susceptible for corrosion. The noble metals like Ag, Au, Pt, Pd are less susceptible for corrosion.

2. Surface state of the metal:

Corrosion is surface phenomenon, larger the surface area or finer the grain size of the metal, more will be the corrosion. Smooth surface resist corrosion than the rough surface. Due to ups and downs on the rough surface, there will be formation of large number of air concentration cells with anodic and cathodic regions. Hence the metal suffers corrosion.

3. Nature of the corrosion product: 

It largely decides the rate of corrosion. If the corrosion product is insoluble, stable, uniform and nonporous, it acts as a protective film preventing the further corrosion. If the corrosion product formed is soluble, unstable, porous and non uniform, the corrosion continues.

4. Hydrogen over voltage:

If the hydrogen over voltage of metal is low, it is more susceptible for corrosion. When the cathodic reaction is of hydrogen evolution type with lower hydrogen over voltage, hydrogen gas is evolved easily and thus cathodic reaction is faster and corrosion of metal becomes fast. In metals with higher hydrogen over voltage, cathodic reaction is slow and corrosion of metal becomes slow.

Secondary factors related to environment:

1. pH of the medium:

In general, lower the pH higher is the rate of corrosion. If the pH is greater than 10, corrosion of iron is very less due to the formation of protective coating of hydrous oxides of iron. If pH is between 10 and 3, then presence of oxygen is essential for corrosion of iron. If the pH is 3 or lower than 3 severe corrosion occurs in the absence of air due to the continuous evolution of H₂ at cathode. However metals like Al, Zn etc undergo fast corrosion in highly alkaline medium.

2. Temperature:

As temperature increases, rate of corrosion also increases. This is because increase in temperature increases the conductance of the aqueous medium .As a result rate of diffusion also increases.

3. Presence of oxidizing agents:

The presence of oxidizing agents increases the corrosion rate of the metal. Even noble metals undergo corrosion in the presence of oxidizing agents.

4. Humidity:

Most of the metals corrode faster in a humid atmosphere than in dry air. There is a particular value of humidity called critical humidity above which corrosion rate steeply increases. Humidity (moisture) provides conducting medium which helps in

(i)           Formation of electrochemical cell on the surface.

(ii)          Dissolution of gases like O₂, CO₂, SO₂ etc that help in corrosion.

5. Presence of impurities in the atmosphere:

Presence of impurities like SO₂, HCl in the environment increases the rate of corrosion due to acidic conditions created by their dissolution. For example, when SO₂ is present as impurity in the atmosphere, it combines with moisture or rain water forming sulphuric acid. In the presence of an acid metals like iron are more susceptible for corrosion.

6. Conductance of the medium:

Presence of conducting species in the atmosphere increases the rate of corrosion. This is because, higher the conductivity of the medium, faster the ions can migrate between anodic and cathodic regions of the corrosion cell, in turn faster will be the exchange of electrons at the electrode surfaces. Therefore, corrosion problem is more in the sea water than in fresh water.

7. Area effect:

Smaller the anodic area, larger the cathodic area, faster will be the rate of corrosion and conversely, larger the anodic area, smaller the cathodic area, slower will be the rate of corrosion. This is because electrons liberated at anode (smaller the anodic area) are consumed quickly by the large cathodic area and hence, the rate of corrosion will be more.

8. Polarization at anodic and cathodic area:

Polarization of cathode or anode decreases the rate of corrosion. If anodic polarization takes place due to some reaction, then tendency of metal to undergo oxidation decreases hence dissolution of metals as metal ion decreases. This is usually due to increase in concentration of ions of the dissolved metals in the vicinity of electrode or also due to the anodic passivity.

Cathode polarization decreases the cathodic reaction hence hindering the combination of cathode reactant and electron. For the corrosion to continue both anodic and cathodic reaction should take place simultaneously if any one reaction is slower then the rate of corrosion is slower. Use of depolarizes reduces the polarization effect hence the rate of corrosion reaction increases.