Electrolytic chromium plating of black iron
Packaging steels

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 m² face) topped by a layer of partially hydrated chromium (III) oxide (approx. 15 mg chromium on a 1 m² face), both formed by electrolysis.

Deposition takes place in a conventional dilute chromium plating bath [50 to 200 g/L chromium trioxide (CrO₃ chromic anhydride)] catalyzed by sulfuric acid (mass ratio H₂SO₄ /CrO₃ = 1/100) and/or complex fluorides. The higher the density and the lower the bath temperature, the higher the current efficiency, but it hardly exceeds 30%: as an example, we quote a value of 18% for 80 A /dm² at 60°C.

We therefore need to modify the sequence of operations on the tinning line to produce chromium-plated iron (we'll confine ourselves...