Effect of Thiourea and some of its Derivatives on the Corrosion Behaviour of Cobalt in Concentrated Hydrochloric Acid

Abstract

The corrosion rate of cobalt in 8·4 M hydrochloric acid with and without the presence of thiourea, phenyl thiourea, o-tolyl thiourea or sym di-isopropyl thiourea has been studied by gravimetric and electrochemical methods over the temperature range, 20–80°c.

In all cases these compounds brought about enhancement of corrosion so that, at the higher temperatures, corrosion rates some twenty times those experienced in acid alone were attained. These matched the degree of corrosion stimulation of cobalt developed in the presence of hydrogen sulphide and it is suggested that the thiourea compounds act as depolarisers for the hydrogen evolution reaction, with H₂S being a product of the chemical depolarisation stage. The H₂S is considered to be adsorbed in the molecular state, forming activated sites where anodic dissolution is thereby enhanced.