 Cobalt is a weakly reducing metal that in pure metallic form , is protected from oxidation by a passivating oxide film . It is attacked by halogens and sulfur . Heating in oxygen produces Co3O4 which loses oxygen at 900 ° C ( 1 @,@ 650 ° F ) to give the monoxide CoO . The metal reacts with fluorine ( F2 ) at 520 K to give CoF3 ; with chlorine ( Cl2 ) , bromine ( Br2 ) and iodine ( I2 ) , producing equivalent binary halides . It does not react with hydrogen gas ( H2 ) or nitrogen gas ( N2 ) even when heated , but it does react with boron , carbon , phosphorus , arsenic and sulfur . At ordinary temperatures , it reacts slowly with mineral acids , and very slowly with moist , but not with dry , air . 
