MULTILAYER COATINGS AS CORROSION PROTECTION OF ZIRCALOY

In single layer coatings, pores and other coating defects are likely t o grow all through the coating. In multilayer coatings, however, the r epetitive renucleation reduces the possibility that pores and defects grow that large. It is conceivable that this would, in turn, obstruct diffusion through the coating, thus reducing, e.g., oxidation and hydr iding of the underlying substrate. The protective function of five dif ferent coatings consisting of ordered stackings of Ti and TIN and one TiN single layer coating (reference), deposited on Zircaloy-4 (Zr-4), were evaluated by means of an autoclave test in pure water at 360 degr ees C and 220 bars for 200 days. The oxidation of coated samples was m uch lower than for uncoated Zr-4. Two different oxidation phenomena we re observed. One corresponding to formation of surface oxides and, sec ondly, a far more serious oxidation in the coating-substrate interface . It is shown that coatings containing many sublayers are more capable in avoiding both of these oxidation phenomena. The uptake of hydrogen was shown to be much lower for coated Zr-4 than for uncoated Zr-4.