You can get seed from a head of cabbage if you grow it yourself. It is a biennial vegetable so it will grow the head the first year and if you leave it in the ground over the winter it will sprout seed heads the next year. You can buy seeds to plant or you can also find seedlings to transplant at garden centers.
Copper and bronze do not contain any iron, and only iron can rust (because rust is defined as iron oxide, a compound of iron and oxygen); so the answer is that steel will rust the fastest and copper and bronze will never 'rust'. But, yes, copper and bronze do tarnish, they do corrode.
A small amount of iron usually made it into the castings, but too much iron raised the melting point, so it was minimized. There is no metallurgical standard for pot metal. Common metals in pot metal include zinc, lead, copper, tin, magnesium, aluminium, iron, and cadmium. The primary advantage of pot metal is that it is quick and easy to cast.