Ehhh, you haven't read the books at all, right? It's not that Frodo couldn't be corrupted. It was just that he volunteered himself to carry it to the volcano and everyone else agreed. He may have had a bit more resistance than others (like Boromir, for example, who tried to kill him to take it to Gondor), but that's all. In the end he fell to the corruption and it took Gollum to take it from him.