Corrosion inhibitors

Method of inhibiting corrosion of ferrous metals in oil and gas well drilling and production systems comprising adding to said systems an effective corrosion inhibiting amount of a composition comprising a solution, dispersion or emulsion of a compound selected from the group consisting of compounds represented by the formula ##STR5## wherein X represents oxygen or sulfur, R.sub.1 and R.sub.2 are the same or different and R.sub.3 and R.sub.5 are the same and represent hydrogen, alkyl, cycloalkyl, aryl, aralkyl, alkaryl and heterocyclyl, R.sub.4 represents an organic radical, and the acid salts and quaternary ammonium salts thereof.