Question

Discuss the simple thermodynamic theory of the corrosion of metals in aqueous solutions based on the scale of standard electrode potentials. In your explanation, use gold, silver, and iron in dilute HCl solution as illustrative examples. Explain the major limitations of thermodynamic theory using Ti and Al as examples.

Answer 1

The galvanic process by which the metals in nature or by chemical means deteriorate via the process of oxidation is known as corrosion. The process of corrosion is electrochemical in nature.

The process of corrosion in the aqueous medium requires the metal to undergo oxidation by losing electrons and the formation of the hydrogen gas which is formed by accepting these electrons by the protons in the medium.

Example:-

The change in the Standard Gibb’s free energy is the measure of the driving force of the reaction to complete. It is defined as:-

Where,

is the change in the Standard Gibb’s free energy

n is the number of electrons involved

F is Faraday’s constant

is the Standard cell potential.

Also, negative value of Standard Gibb’s free energy or, means that the reaction is spontaneous, and no external force is required to drive the reaction.

However, positive value of Standard Gibb’s free energy or, means that the reaction is non-spontaneous, and some external force is required to drive the reaction.

Also, from the formula, positive value of Standard cell potential or, means that the reaction is spontaneous and vice versa.

The Standard cell potential, is the difference of the standard reduction potentials of the cathode and anode.

Thus,

Also,

The equation becomes:-

Explanation of the thermodynamic theory of corrosion of the iron in the acidic medium as:-

At anode:-

At cathode:-

So,

As , it means reaction is spontaneous which also states that iron when placed in acidic medium gets corrodes.

Explanation of the thermodynamic theory of corrosion of the silver in the acidic medium as:-

At anode:-

At cathode:-

So,

As , it means reaction is non-spontaneous or no reaction. It states that silver when placed in acidic medium does not get corrode.

However, if oxygen is present in the medium,

At anode:-

At cathode:-

So,

As , it means reaction is spontaneous or no reaction and states that silver in the presence of oxygen gets corrode.

Explanation of the thermodynamic theory of corrosion of the gold in the acidic medium as:-

At anode:-

At cathode:-

So,

As , it means reaction is non-spontaneous or no reaction. It states that gold when placed in acidic medium does not get corrode.

Thus, the electrode potentials of the metals can help to determine whether it will corrode or not when placed in the acidic medium.

Major limitations of using the metals, Al and Ti as examples are:-

• Both the metals have low density and on exposure to the atmosphere, both forms a very thin and protective layer of their respective oxide on their surface. This layer protects the water or air to get to the metal and prevent them from corroding. Thus, the actual E^0 values of the half cell reactions do not work for these metals.
• Also, the variation of these metals with the temperature is very high because of this layer and thus, more amount of electrical energy is required for the reaction to take place.