4. Electrolytic chromium plating of black iron

Chromium-plated iron differs from tinplate in that the tin layer is replaced by a thin layer of metallic chromium (approx. 0.01 µm or 70 mg chromium on a 1 m ² face) topped by a layer of partially hydrated chromium (III) oxide (approx. 15 mg chromium on a 1 m ² face), both formed by electrolysis.

Deposition takes place in a conventional dilute chromium plating bath [50 to 200 g/L chromium trioxide (CrO ₃ chromic anhydride)] catalyzed by sulfuric acid (mass ratio H ₂ SO ₄ /CrO ₃ = 1/100) and/or complex fluorides. The higher the density and the lower the bath temperature, the higher the current...