Yes, it takes a while for lead to oxidize, but oxidized pellets are not what's dangerous to the chickens. As birds, chickens will eat small pebbles for their craws (gizzards) where they are used to break up seeds, etc, in place of teeth for chewing. If they eat a pellet or two, it will be in there with the grit and, eventually, some of the lead will be ground off. I believe that's what affect puddle ducks in marshlands.