Green rust compounds were identified in green corrosion crusts that form on iron and steel surfaces, in alternating aerobic and anaerobic conditions, by water containing anions such as chloride, sulfate, carbonate, or bicarbonate.  They are believed to be intermediates in the oxidative corrosion of iron to form iron(III) oxyhydroxides (ordinary brown rust).  The green rust may be formed either directly from metallic iron or from iron(II) hydroxide () .