Corrosion and material damage

English builds every perfect with one auxiliary; German makes the verb choose, and this list shows the rule at work. An intransitive verb that names a change of state, where something ends up different from how it started, takes sein: der Schweller ist durchgerostet, der Stahl ist in der Kälte versprödet. A verb that names an activity or a condition lasting for a while takes haben, even when nothing and nobody moves: das Eisen hat im Ofen geglüht. The same pairing runs through the whole language — die Rose hat geblüht, die Rose ist verblüht — and a prefix is often what turns the one into the other, because durch and ver add an end point. In a damage report the difference matters: ist durchgerostet records a result that an inspector has found, not a process that somebody watched.