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Corrosion of mild steel in sodium sulfate solutions (1000 mg SO42- L-1) in the presence of various phosphate inhibitors was studied gravimetrically. Studies were performed under static conditions at room temperature. The known sodium polyphosphate inhibitor has been found to have low efficacy under these conditions. The inhibitor efficiency of its does not exceed 30.5 % at a concentration of 100 mg P2O5 L-1. It has been shown that a more effective inhibitor is calcium-manganese polyphosphate (Са, Mn)(PO3)2 obtained on the basis of manganese ore enrichment wastes of the Zhayrem deposit. The inhibitor efficiency of its already at a concentration of 20 mgP2O5 L-1 is 47.3 %. At a concentration of 100 mgP2O5 L-1, the inhibitor efficiency (Са, Mn)(PO3)2 is 96.2 %. Electrochemical tests confirmed the results of the gravimetric study. Infrared spectroscopic analysis showed that phosphate protective coatings were formed on the steel surface in solutions of sodium sulfate with additives of calcium-manganese polyphosphate. [ABSTRACT FROM AUTHOR]