Technology of black coloring for stainless steel by electrochemical method

Abstract

The technology of black coloring for stainless steel by electrochemical method was studied. The optimum bath compositions and operating conditions were obtained as follows: 40 – 50 g/L K₂Cr₂O₇, 15 – 20 g/L MnSO₄, 15 – 20 g/L (NH₄)₂SO₄, 20 – 40 g/L H₃BO₃, 20 – 30 g/L additive A, 2 g/L (NH₄)₆Mo₇O₂₄; time 9 – 20 min; temperature 15 – 30 °C; potential 3 V and current density 1 – 2 mA/cm². The effects of the compositions of the bath on the quality of black colored film were discussed. The influences of passivation process on the black coloring velocity and performances of black colored film layer were investigated. The results show that the passivation process can improve the corrosion resistance and the stability to bear color-change; (NH₄)₂SO₄ can cantrol the black coloring velocity and prolong black coloring bath life remarkably; and additive A can improve the evenness and compactness of black colored film layer. The results of scanning electron microscopy and energy dispersive spectra show that the microstructure is of cylindrical lump, the filling process can decrease the crackles, and the main elements of black colored film are Fe, Cr, Mn and Ni.

Keywords

stainless steel / electrochemical method / black coloring / corrosion resistance

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