Question

Sodium chloride (salt) is NOT corrosive. None the less, it commonly gets the blame for corroding metals. Please clearly explain how salt makes the corrosion of steel considerably easier. What is the corrosive agent in the rusting of steel?

Answer 1

Corrosion is a natural process, which converts a refined metal to a more chemically-stable form, such as its oxide, hydroxide, or sulfide.

The combination of moisture, oxygen and salt, especially sodium chloride, damages metal worse than rust does. This combination corrodes, or eats away at, the metal, weakening it and causing it to fall apart. Saltwater corrodes metal five times faster than fresh water does.

Now let us understand stand why does this happens.

When we keep steel in a fresh water. We know Fresh water ionise into H⁺ and OH^- ions. But the ionisation is very less. Therefore the amount of H⁺ and OH^- ions is very less in fresh water.

The process of corrosion takes place as following

Step1 electrons are released from Fe

Step2 electrons are transferred to O₂ and water and form hydroxyl ions

Step3 these hydroxy ions react with ferrous ion to give ferrous hydroxide which is oxidised to ferric oxide (red rust)

Now when the fresh water is considered the transfer of electron is a very slow process as the electrons from Fe are first transferred to H⁺ and then further to O₂. As the amout of H⁺ ions on the fresh water is very less therfore this process takes place very slowly.

HOWEVER WE CAN INCREASE RATE OF CORROSION BY ADDING SALT

We increase rate of corrosion by introducing a salt. As NaCl is a strong electrolytes and thus dissociate in aqueous solution furnishing Na⁺ and Cl^- ions. Now the process of transfer of electrons become faster and easier as there are lot of free ions in solution.

So the salt is not consumed in the reaction it just increases the rate of reaction by helping in transferring of electrons.

IN corrosion of steel the corrosive agent is NaCl. As it increases the rate of corrosion of steel by furnishing free ions which increases rate of corrosion by carrying charge to allow reaction to proceed easier.