Tarnishing and Scaling Processes - Environmental Degradation of Metals - page 267

and eventually becomes linear. The onset of breakaway oxidation appears to be
markedly dependent on impurities in the metal, and for high-purity zirconium
the breakaway oxidation is significantly delayed. The loss in protective properties
of ZrO 2 scale is of particular concern as regards the use of zircalloys in nuclear
reactors.
The ZrO 2 films formed on zirconium have been found to consist of three modi-
fications depending on experimental conditions. Most x-ray diffraction studies
on oxide films formed below 1273 K have revealed only the presence of mono-
clinic ZrO 2 , whereas electron diffraction studies on thin films have also indicated
the formation of a cubic (or tetragonal) modification. The monoclinic form is
thermodynamically stable below about 1473 K, whereas the tetragonal modifica-
tion becomes stable at higher temperatures. It is generally accepted that ZrO 2
may be oxygen-deficient, but the extent of nonstoichiometry of ZrO 2 in equilib-
rium with oxygen-saturated metal is not exactly known. From marker studies on
oxidation of zirconium it is concluded that inward migration of oxygen ions
through oxygen vacancies of the ZrO 2 film/scale is the predominant factor in
scale growth process. However, exact nature of the defect structure of ZrO 2 ,
the transport properties and possibility of any complex defect formation are still
controversial issues.
For temperatures above 873 K, a number of researchers [57] have experimen-
tally determined the amount of oxygen going into solution as a function of time
during oxidation of zirconium. In all of these investigations it has been shown
that oxygen dissolution process follows a parabolic rate, which leads to the con-
clusion that the dissolution is governed by volume diffusion of oxygen in zirco-
nium. The oxygen solubility has been reported to be about 60% of the reacting
oxygen at 1123 K and this dissolution becomes increasingly important with a
rise in temperature.
The temperature dependence of the total parabolic rate constant and that for
oxygen volume diffusion in -Zr as reported by different investigators are sum-

marized in Fig. 5.34. It is observed that the activation energy of the dissolution
process is larger (212.5 kJ/mol) than that of the total oxidation process (151 kJ/
mol). Consequently, the dissolution process becomes increasingly important at
higher temperatures of oxidation, whereas at temperatures below 773-873 K the
dissolution process is of minor importance. It is appropriate to point out that the
importance of oxygen dissolution will in reality be smaller than that indicated
in Fig. 5.34 because of the simultaneous oxide formation, which has been ne-
glected in the theoretical estimation of the contribution due to oxygen dissolution.
For temperatures below 873 K Hussey and Smeltzer [61] have determined the
fraction of the reacting oxygen that goes into solution by means of mass gain
measurements and from estimates of oxide film thickness. Such results clearly
reveal that large experimental errors are involved in estimating the thickness
of thin films. If it is assumed that the estimated values are more or less correct,

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