**7. References**

208 Corrosion Resistance

Fig. 18 presents the effect of addition of graphite to sintered steel on polarization resistance for the material in the corrosion environment used in the study. The highest corrosion resistance was found for the sinter of 100% graphite, whereas sinter of 100% of 316L steel, compared to graphite, exhibit nearly 1000 time lower polarization resistance in the environment of H2 and several thousand times lower in the environment of O2. The sinters are characterized by higher corrosion resistance in the solution saturated with hydrogen

Fig. 18. Change in polarization resistance depending on proportion of graphite in a

investigate the problems analyzed in the present study.

strength and corrosion resistance;

Based on the investigations, the following conclusions were drawn:

The main task of bipolar plates in fuel cells is to ensure even distribution of reactants on electrode surface. In order to meet this requirement, solid materials are cut the channels with a variety of shapes. In the case of use of porous material, obtained by means of powder metallurgy, surface unevenness forms the channels which supply and discharge media. From the standpoint of economics, the intended porosity of material is beneficial. With consideration of seeking new solutions and opportunities in the field of material choice in order to facilitate and optimize operation of machines, it seems entirely legitimate to


composite.

**6. Conclusions** 

(including the sinter of 100% 316L), compared to the O2 solution.


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**10** 

*Australia* 

**Oxidation Resistance of Nanocrystalline Alloys** 

Nanocrystalline (nc) materials are single or multi-phase polycrystalline solids with a grain size of a few nanometers, typically less than 100 nm. Owing to the very fine grain size, the volume fraction of atoms located at grain boundaries or interfaces increases significantly in nanocrystalline materials [1]. A simple geometrical estimation, where the grains are assumed as spheres or cubes, yields the following values for the volume fraction of the interfaces: 50% for 5 nm grains, 30% for 10 nm grains and about 3% for 100 nm grains [2-5]. These values of interface volume fraction are several orders of magnitude higher than those of conventional microcrystalline materials. Consequently, nanocrystalline materials exhibit properties that are significantly different from and often improved over, their conventional microcrystalline (mc) counterparts. For example, nanocrystalline materials exhibit increased mechanical strength [6-10], enhanced diffusivity [11], improved corrosion resistance (some nanocrystalline materials) [12-20], optical, electrical and magnetic properties [21-24]. Due to their unique properties, nanocrystalline materials have attracted considerable research interests and the field of nanocrystalline materials has now become one of major identifiable

Metals exposed to high temperature oxygen-containing environments form oxides. If an oxide scale can form and the oxide is dense and adherent, then this scale can function as a barrier isolating the metal from the external corrosive atmosphere. This oxide scale is called protective oxide scale. On the other hand, if a non-protective oxide scale is formed, oxygen can penetrate through the scale and the oxidation will extend further into the metal substrate, causing a rapid metal recession. Of all oxides, chromia and alumina are two kinds of oxides thermodynamically and kinetically feasible to meet the requirement for resisting

Alloys containing aluminium or chromium can be selectively oxidised to form alumina or chromia. To form an external oxide scale, the concentration of aluminium or chromium should reach a critical value. Nanocrystalline alloys promote this selective oxidation process by reducing the critical value. For example, in the conventional microcrystalline Ni-20Cr-Al alloy system, > 6 wt% Al is required to form a protective Al2O3 scale at 1000°C [25]. If the Al content is lower than 6 wt%, complex oxide mixtures consisting of Cr2O3, NiCr2O4 and internal Al2O3 form, resulting in high reaction rates and poor oxidation resistance. With a nano-crystalline alloy structure, this value can be substantially reduced to 2 wt% Al, when

**1. Introduction**

activities in materials science and engineering.

high temperature oxidation.

*2School of Materials Science & Engineering, The University of New South Wales,* 

Rajeev Kumar Gupta1, Nick Birbilis1 and Jianqiang Zhang2

*1Department of Materials Engineering, Monash University,* 

