Effect of Cu and Ni undercoatings on the electrochemical corrosion behaviour of Cr-C-coated steel samples in 0.1 M H₂SO₄ solution with 1 g/L NaCl

The electrochemical corrosion behaviour of trivalent Cr-C-coated steel samples with Ni and Cu undercoatings was studied in the 0.1 M H₂SO₄ + 1 g/L NaCl solution. The corrosion resistance of Cr-C-coated samples depends strongly on the undercoating material and the polarised anodic potential. With a soft undercoating, the cracks and crack width in the Cr-C coating can be obviously reduced. Different corrosion potentials were measured from Cr-C-coated steel samples with Ni and Cu-undercoatings. Better electrochemical corrosion resistance of a Cr-C/Cu-coated steel sample was detected at 0 V, but it easily corroded at 0.5 V. On the contrary, the Cr-C/Ni-coated steel sample had better electrochemical corrosion resistance at 0.5 V, and poor at 0 V. The through-coating cracks in the Cr-C coating could provide active corrosion paths for dissolution of the Ni or Cu undercoating during potentiostatic tests polarised at 0 and 0.5 V. The reduction behaviour of Cu²⁺ ions dissolved from the Cu undercoating of the heat-hardened Cr-C-coated sample was studied and recognised by means of the immersion test in a solution composed of 200 g/L CuSO⁴·5H²O and 70 g/L H²SO⁴.