Brass is an alloy of copper and zinc, and is used in large quantities on ships. Sea water can de-alloy brass, removing all the zinc, if the fraction of zinc is high — but traces of arsenic and boron, in equal quantities, can stabilize the metal. They probably do it by combining as As-B pairs to occupy double vacancies in the zinc, stopping these vacancies from diffusing through the infinite zinc cluster that is present in the alloy.