The compositions of nonmetallic phases in samples of medium-carbon steel, which differed in production technology and level of contamination with corrosive nonmetallic inclusions, have been studied in detail by scanning electron microscopy. It is shown that the influence of nonmetallic inclusions on the tendency of steel to local corrosion can be related to the degree of non-stoichiometricity of their composition. The mechanism of formation of non-stoichiometricity of inclusions at steel production under the modern technological scheme is proposed.