**3.2.2 Nucleation sites and lateral growth of passive film**

In most alloys, nucleation and growth are the mechanism of oxide scale formation during oxidation [44,45,101-103]. It is widely reported that nucleation of oxide is favoured at the high energy sites, i.e., surface defects in the form of dislocations, grain boundaries, triple points, impurities etc. Since nanocrystalline materials are composed of the large fraction of surface defects therefore they offer a large fraction of closely spaced nucleation sites. During the lateral growth of oxide, these nucleation sites become very important as the presence of closely spaced nucleation sites reduces the lateral distance necessary for the lateral growth of a uniform oxide layer to cover the entire surface.

Lobb and Evans [104] have reported an improvement in the oxidation resistance of fine grained conventional microcrystalline materials as a result of increased grain boundary area fraction as the grain boundaries acts as the preferential nucleation sites for Cr2O3. They reported that the oxide film nucleated at the grain boundaries needs to grow laterally for the formation of a uniform oxide layer and reported that the finer the grain size, the better the uniformity of oxide should be. Later it was shown that above a critical grain size, formation of a uniform Cr layer was not possible which could be understood based on the combined role of diffusion and nucleation site densities in the alloys [44,45,98-100]. However, these studies were performed on the material where minimum grain size was a few microns. Nanocrystalline materials offer a huge amount of nucleation sites. Therefore, formation of a homogenous compact layer able to cover whole surface is facilitated, which is expected to result in a significant improvement in the oxidation resistance.

#### **3.2.3 Structure of the oxide scale**

Mechanical properties, microstructure, adhesion and growth of oxide scale has been reviewed recently [105,106] and it has been found that oxidation resistance of a metal largely

Certain alloys can develop a continuous layer of the protective oxide of more reactive alloying elements which forms basis for the development of oxidation resistanct alloys. Such alloys are Fe, Ni, Co based, with Al, Cr or Si as the reactive alloying additives. For example, Iron-chromium alloys (such as stainless steels) are the most commonly employed oxidation resistant materials. It has been established that when time-dependent inward flux of oxygen is less than the time-dependent outward flux of solute (Cr), a continuous layer of Cr-oxide is formed at or very near the surface. Formation of such oxide layer and therefore oxidation resistance of Fe-Cr alloys depend upon the supply of solute from the alloy to alloy/oxide scale. It was established in the literature that a fine grain (~17 µm or less) stainless steel easily developed a uniform layer of Cr2O3. For an alloy with grain sizes greater than ~40 *µ*m, this protective layer of Cr2O3 was difficult to form due to insufficient grain boundary diffusion and inadequate chromium supply [98-100]. In nanocrystalline materials where grain size is very fine and diffusion coefficients are high, such Cr oxide formation should be facilitated by a large extent. Since enhanced diffusion of alloying elements in the nanocrystalline structure facilitates the formation of protective oxides, therefore alumina, chromia and/or silica forming alloys should have improved oxidation resistance in their

In most alloys, nucleation and growth are the mechanism of oxide scale formation during oxidation [44,45,101-103]. It is widely reported that nucleation of oxide is favoured at the high energy sites, i.e., surface defects in the form of dislocations, grain boundaries, triple points, impurities etc. Since nanocrystalline materials are composed of the large fraction of surface defects therefore they offer a large fraction of closely spaced nucleation sites. During the lateral growth of oxide, these nucleation sites become very important as the presence of closely spaced nucleation sites reduces the lateral distance necessary for the lateral growth

Lobb and Evans [104] have reported an improvement in the oxidation resistance of fine grained conventional microcrystalline materials as a result of increased grain boundary area fraction as the grain boundaries acts as the preferential nucleation sites for Cr2O3. They reported that the oxide film nucleated at the grain boundaries needs to grow laterally for the formation of a uniform oxide layer and reported that the finer the grain size, the better the uniformity of oxide should be. Later it was shown that above a critical grain size, formation of a uniform Cr layer was not possible which could be understood based on the combined role of diffusion and nucleation site densities in the alloys [44,45,98-100]. However, these studies were performed on the material where minimum grain size was a few microns. Nanocrystalline materials offer a huge amount of nucleation sites. Therefore, formation of a homogenous compact layer able to cover whole surface is facilitated, which is expected to

Mechanical properties, microstructure, adhesion and growth of oxide scale has been reviewed recently [105,106] and it has been found that oxidation resistance of a metal largely

**3.2.1 Enhanced diffusivity and oxidation resistance** 

**3.2.2 Nucleation sites and lateral growth of passive film**

of a uniform oxide layer to cover the entire surface.

result in a significant improvement in the oxidation resistance.

**3.2.3 Structure of the oxide scale** 

nanocrystalline form.

depends upon their physical properties (e.g., crystal size, morphology and crystallographic orientation of oxides formed, lattice mismatch with the base metal, adhesion of oxide layer) of oxide [105,106]. For example, fine grained oxide scales often show a fast creep rate at high temperatures, releasing the stresses accumulated in the scales and, therefore, decrease in scale spallation tendency. This may have important implication in reducing the spallation of oxides at high temperature. Since nanocrystalline materials provide several orders of magnitude more nucleation sites than the microcrystalline materials therefore grain size of the oxide developed on nanocrystalline materials is expected to be finer. This finer grain size of oxide suppresses oxide scale spallation in nanocrystalline form. This effect has been demonstrated in high temperature oxidation tests of several nanocrystalline alloys [27- 30,47].

It was also proposed that fine nano-sized oxide structure reduces conductivity which suppresses the transport of the oxidizing species, enhancing oxidation resistance. This proposal was successfully used to explain improved oxidation resistance of Zr and Zr based alloys [107]. However, it is not clear if this model also applies for other alloys.

It is important to note that the above factors can operate both indifferently and in combination [12]. The nature of the influence of nanostructure on the diffusion-assisted corrosion (viz., high temperature oxidation) depends on the role of the predominantly diffusing species in a given alloy. For example, oxidation resistances of an ironaluminide and an Fe-B-Si alloy in the nanocrystalline state are reported to be superior to that in their microcrystalline state [87,108]. This behaviour is attributed to Al and Si, the well-known protective oxide film formers, being the predominantly diffusing species respectively in the two alloys, and the nanostructure facilitating their diffusion and expedited formation of protective films (of Al/Si oxide). Therefore, it is important to consider all the factors effecting the oxidation resistant, and the net effect of all the parameters would determine the change in the oxidation rate caused by nanocrystalline structure.
