Abstract Short term (144 hour) corrosion tests of zirconium subjected to differing surface treatments prior to immersion in aqueous hydrochloric acid solutions suggested that factors involved in film formation play a dominant role in the corrosion behavior of zirconium. Long duration tests (2000 hours) revealed two general types of protective films: one, a protective primordial zirconium oxide film, and the other, a face-centered cubic zirconium monohydride film. The oxide film is subject to failure by a “breakaway” mechanism involving the formation of an interfacial hydride layer at the oxide-metal interface which eventually causes the protective film to suddenly loosen and flake away. The time for this to occur may vary from less than 144 hours to greater than 2000 hours depending on the integrity of the oxide film. The integrity of the film is in turn dependent on the history of its formation. After “breakaway” the corrosion rate loss in boiling 20 percent hydrochloric acid may range from 3 to a maximu...