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 maximum of 4.5 mils per year.
Protective hydride films are black, vary widely in the degree of protection they offer, and appear to be highly sensitive to surface preparation. Chemically polished surfaces produce porous hydride films which offer little protection whereas electrolytically polished specimens produce dense films which are very resistant to hydrochloric acid.
The various mechanisms of corrosion are described and discussed in the light of the work of other investigators. Photomicrographs of the surface of various characteristic types of film formation are shown and their appearance is explained in terms of the postulated corrosion mechanisms. 6.3.20
