Embrittlement of high carbon steel in sulphuric acid containing thio-compounds

The influence of some mercaptans and sulphide on the cracking susceptibility of cold rolled high carbon steel (0.86% C) in IN H2SO4 at 40°C was investigated. The results indicate that with increase in inhibitor concentration, the inhibition efficiency increased in all cases except 2-mercaptobenzothiazole, 2-mercaptoaethanol and thioglycollic acid which showed a decrease in inhibition efficiency beyond a concentration of 3.2 x 10-5 M. Increase in branching in case of aliphatic mercaptans had an effect in increasing the inhibitor efficiency compared to unbranched compounds. The inhibition efficiency of these compounds was explained on the basis of structure dependent on electron donating properties and the influence of substitution and decomposition on the properties at compounds. Increase in concentration of 2-mercaptobenzothiazole, 2-mercaptoaethanol and thioglycollic acid enhanced the cracking susceptibility of the steel which was attributed to H2S formation by their cathodic reduction. Iso-butylmercaptan, S-butylmercaptan and T-octylmercaptan were found to be excellent inhibitors when cracking due to H2-embrittlement and % inhibition efficiency were taken into consideration. All the compounds studied showed predominantly cathodic inhibition. (All the authors belong to National Metallurgical Laboratory)