EFFECT OF THIOUREA AND SOME OF ITS DERIVATIVES ON THE CORROSION BEHAVIOR OF NICKEL IN 50-PERCENT V-V (5.6M) HYDROCHLORIC-ACID

The corrosion rate of commercial grade, coid-rolled nickel in 50010vlv (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 thefull 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. © 1979 Maney Publishing.