Oxidation Features of Low-Carbon Steel in Nitrate Solutions

A nitrogen-containing additive MD-3 employed previously in steel phosphating was used to improve the technological properties of magnetite coatings (MCs) formed on low-carbon steel from a dilute solution of ammonium nitrate proposed as an alternative to environmentally polluted and energy-expensive alkaline bluing method. We showed that an increase in duration of the oxidation from 1 to 6 h linearly thickens the MСs in the presence of MD-3 without an increase in the size of the poorly adhered area of the coating. The flicker-noise spectroscopy study of the MC structure showed that MD-3 makes the surface smooth and more uniform. An increase in oxidation time also increases the homogeneity of MCs, although not as much as in the previous case. The corrosion tests according to the Akimov method and in a salt spray chamber correlate well with each other. The best results were obtained for MCs prepared in a solution of 20 g/L NH4NO3 + 0.08 g/L MD-3 (85°C, 4 h) after passivation treatment in a solution of IFKhAN-39U.