Effect of Thiourea and Some of its Derivatives on the Corrosion Behaviour of Nickel in 50% v/v (5·6M) Hydrochloric Acid

The corrosion rate of commercial grade, coid-rolled nickel in 50%v/v (5·6 M) hydrochloric acid, with and without the presence of thiourea or one of several of its derivatives, has been studied by gravimetric and electrochemical methods over the temperature range, 23–79°c. All the compounds exhibited some ability to inhibit corrosion but, with the exception of sym di-o-tolyl thiourea, they also enhanced corrosion at lower concentrations. The concentration range producing corrosion stimulation increased with increasing temperature and only o-tolyl thiourea and sym di-isopropyl thiourea were capable of maintaining an efficiency of ∼90% inhibition over the full temperature range.

For nickel dissolving in uninhibited acid, the Arrhenius relationship was linear and yielded a value of 68·2 ± 3·3 kJ/mol for the activation energy. For acid containing low and medium concentrations of inhibitor. a high proportion of the A rrhenius plots exhibited a change in gradient, reflecting a change in mechanism. Only at high concentration of a high-efficiency inhibitor was the linear relationship restored over the complete temperature range. There was no evidence of conformity with the Langmuir or Freundlich adsorption isothermsfor the inhibition process.