scope:

This Technical Report describes the methods to evaluate the properties of dark-stain phenomenon observed in chromate conversion-coated coiled and sheet products.

Chromate conversion coatings, with trivalent or hexavalent chromium ions, are applied to hot-dip metallic zinc and electroplated-zinc steel products to enhance corrosion resistance of the components.

When zinc-coated packed coiled and sheet products with conversion coatings are unpacked after long-term storage at elevated temperature and high relative humidity (RH), the surface of the sheet products appears tarnished. This discolouration appears as dark or black stain and influences the appearance and corrosion resistance, when parts are produced using these affected sheets.