Synthesis and corrosion behavior of nanocrystalline Fe-Cr alloys
thesisposted on 15.01.2017 by Gupta, Rajeev Kumar
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Nanocrystalline (nc) materials, a single or multi-phase polycrystalline solids with a grain size of a few nanometers, typically less than 100 nm have attracted considerable attention of research community. However, investigation of corrosion/oxidation resistance of these materials has attracted limited research attention. Motivation of the current study is to understand the effect of nanocrystalline structure on the corrosion/oxidation resistance and to validate the hypothesis that passivity in the Fe-Cr based alloys can be achieved at lower Cr content if the grain size is reduced to the nano level. However, to investigate the oxidation/corrosion resistance bulk samples of nc materials are required and preparation of bulk samples is not a trivial task. Current study presents a technique to prepare consolidated bulk samples of Fe-Cr based nc materials after a systematic study of grain growth and mechanical (hardness) behavior of nc Fe-10Cr alloys. Oxidation resistance of nc Fe-Cr alloys is compared with that of microcrystalline (mc) Fe-Cr alloys at various temperatures for a time period of up to 52 hours. Nanocrystalline Fe-Cr alloys showed considerably greater oxidation resistance than the mc alloys of the same compositions. This study presents novel results to prove that it is possible to develop a protective layer of chromium oxide at only 10 wt% chromium during oxidation of a nc Fe-Cr alloy. Thus the thesis provides evidence to validate the hypothesis that nc structure can confer considerable oxidation/corrosion resistance at much lower chromium contents (as compared to 18-20 wt% chromium in the common mc Fe-Cr alloys). Oxidation resistance of discs of nc and mc alloys was compared by subjecting them to oxidation in air, and post-oxidation characterization of the oxide scales by secondary ion mass spectrometry (SIMS). The thesis also presents a theoretical treatment to arrive at the minimum chromium content required for establishing a protective layer of chromium oxide in an Fe-Cr alloy of a given nanometric grain size. The electrochemical corrosion of nc and mc Fe-Cr alloys prepared by high energy ball milling (followed by pre-annealing, compaction and sintering) was investigated at room temperature in various solutions using potentiodynamic polarization tests. Nanocrystalline Fe-Cr alloys have shown improved corrosion resistance over their mc counterparts as characterized by passivation potential, critical current density and passive current density. Interestingly passivation is achieved at comparatively lower potential in nc form of alloys. Compositional characterization of passive films carried out by X-ray photoelectron spectroscopy (XPS) have shown higher Cr content in the passive film of nc form of the alloys than that of their mc counterparts. Based on the XPS, polarization results and theoretical understanding, possible mechanisms for the observed enhancement in the corrosion resistance of the nc Fe-Cr alloys are suggested.