Question

Steel is subject to severe cathodic polarisation by ~ -500 mV vs the open circuit potential for 24 hours, in an aerated saline solution.

Measured the pH of the solution after this experiment. Will the pH be the same as the initial pH, will it increase or decrease? Elaborate the answer in 1 sentence.

Answer 1

We analyze this situation by understanding about cathodic polarisation. In this method, a more electropositive metal like zinc is made an anode and steel acts as a cathode. The zinc, being more electropositive loses electrons and these electrons are received by steel through the wires connecting the two. The zinc (anode) is sacrificed to protect the cathode i.e. steel.

Hence, if we analyze the initial and final condition, we observe that initially, we had a steel bar in an aerated saline solution but now we have the corroded zinc in the form of zinc oxide in the solution. Since the metal oxides are basic in nature we will have increased pH than before.