Dissolution of steel and absorption of hydrogen by steel during acid pickling in presence of inhibitor

Pickling in acid to remove the scale or rust is an essential step in the fabrication of steel articles. To reduce the attack on the metal during pickling, inhibitors or restrainers are added to the acid. The present investigation has been taken up to study the effect of inhibitors on metal dissolution and hydrogen pick-up by the metal during pickling. The paper embodies the observations made during the studies on the effects of mild steel when picked in sulphuric acid. The effect of addition of these inhibitors on hydrogen pick-up by the metal during pickling has been determined using modified vacuum heating technique for the determination of hydrogen. The inhibitors studied are urea, thiourea and di-o-tolyl thiorea. Of the three inhibitors studied, urea was found to have poor inhibitive properties. Of the other two e.g. thiorea and di-o-totyl thiorea was found to possess excellent inhibitive properties, the latter being more efficient than the former. The thiourea group of inhibitors showed reduction in total hydrogen pick-up by the metal with increase in inhibitor concentration. The ratio of hydrogen pick-up/weight loss was found to increase with increase in concentration of the inhibitor. (Mr. A.K. Lahiri, Senior Scientific Officer, Mr. N.G. Banerjee, Senior Scientific Officer and Dr. T. Banerjee, Deputy Director, National Metallurgical Laboratory