A mechanism for the undercutting of scratched lacquers on black plate (uncoated low‐carbon steel sheet) is proposed. Experimental evidence leading to its postulation and some tests carried out to confirm it are summarized. It is believed that the undercutting at breaks in coatings on black plate is initiated by the electrochemical reduction of the air‐formed iron oxide film between metal and lacquer effectively to enlarge the break in the film. The subsequent corrosion of exposed steel results in the final undercutting. The stability to reduction of the chromium oxide in the chrome coating of TFS and the absence of iron oxide therein, are the reasons for the resistance of TFS to undercutting.