The process of corrosion of carbon steels in natural and industrial water containing activating chlorine and sulfate ions was studied by virtue of gravimetric, electrochemical, and physical methods (UV and IR spectroscopy, X-ray phase analysis, electronic and optic microscopy). The presence of chlorine ions leads to a pitting corrosion; it can induce perforating damages and usually puts the pipeline out of action. The sulfate ions induce a general more or less uniform steel corrosion. In both the cases iron is accumulated in the pipeline and lowers the water quality. The process of inhibition of steel corrosion by various chemical substances containing hydrazine derivatives was studied. The substances of various structure and containing different functional groups were used: carbohydrazide, thiocarbohydrazide, thiosemicarbazide, thiosemicarbazide with Na2S2O3, thiosemicarbazide-hydrazine-1,1-diacetic acid, thiosemicarbazone of pyruvic acid, chlorine-(dihydrazidesemicarbazide) of diacetic acid Ni(II) trihydrate. It was shown how the chemical composition and structure of the substance influence the process of the metal ionization, temporal variation of the steady potential, and formation of protective coatings on the corroding steel surface. The interaction of the ionized bivalent and trivalent iron with the inhibitors and the products of their decomposition was proven. In this case the inhibitor is initially absorbed at the corroding steel surface; then the metal becomes ionized; a film of iron(III) oxides and hydroxides and insoluble complexes are formed with their subsequent incorporation in the cover layers. In this case they fill the pores of the oxide and hydroxide film and increase its protective ability [1]. At the same time, when the organic substance contains sulphur, hydrogen sulphide is formed in the solution during the corrosion process. The hydrogen sulphide interacts with iron ions with formation of iron disulphide, which also becomes a component of the cover layer. On the one hand, iron disulphide increases its protective ability, on the other hand, it enhances the inhibitor absorption at the corroding surface and impedes the process of steel ionization. This process is often terminated by 72 hours of the test duration, when the maximal protection of steel from corrosion is ensured, and the pH value is stabilized. When the exposure continues, a dynamic equilibrium is attained, and the metal ionization occurs at its minimal level sharply decreasing the steel corrosion rate [2]. We succeeded to decrease the corrosion losses of metal in closed loop pipelines with water circulation by 5 to 39 times dependent on the inhibitor chemical composition, its structure, various chemical groups in its formula, and its concentration.