Highlights

Study the Relationship between Metal Dusting Corrosion and the Growth of Carbon Nano-filaments

Scientific Achievement

Oxide scale, which is essential to protect structural alloys from high temperature degradation such as oxidation, carburization, and metal dusting, is usually considered to consist simply of oxide phases.  However, our recent analyses indicate that the scale is actually a mixture of oxides and metal nanoparticles. 

We observed bright spots in oxide scale on alloy surface using the SEM in EMC.  EDX analysis indicated that these bright spots are rich in iron and nickel but contained less oxygen and chromium.  This finding leads us to further study these bright spots by X-ray nanobeam at APS and magnetic force microscope at CNM. Our results show that metal nanoparticles are indeed present in the scale.  These metal nanoparticles self-assemble into nanonetworks, forming continuous channels for carbon transport through the oxide scales, which leads to the formation of pits on alloys and carbon nanofilament.  To avoid the formation of these metallic particles in the oxide scale, alloys must develop a scale without spinel phase.  Novel alloys have been designed, prepared, and tested in a high-carbon activity environment.  Results show that the incubation time for carbon transport through the oxide scale of the new alloy was extended by >10 times compared to commercial alloys with similar chromium content. 

Significance

Carburization and metal dusting are longstanding problems in energy producing systems.  As energy prices soar to historical highs, it becomes increasingly important to solve these problems and thereby increase the efficiency of energy production.  However, the mechanism of carbon transport through oxide scales was considered to only through the cracks and pores in oxide scale.  The conclusion has led the research community to pay no attention to the effect of metallic particles in oxide scale on alloy degradation for over twenty years.  The problems of defects such as cracks and pores are difficult to find a method to solve.  However, the phase of oxide scale on alloys could be controlled by adjusting the composition of alloys.  We proposed a novel mechanism for alloy degradation due to the carbon transport through metallic networks in oxide scales.  Furthermore, this work uses this insight to present a new alloy that performs far better than commercial alloys.  This innovation could save over one billion dollar annually in the hydrogen industry alone.

Currently, we are using our new mechanism to solve problems in energy conversion systems.  We were invited to present our result in TMS Annual Meeting & Exhibition, Symposium: Materials in Clean Power Systems III: Fuel Cells, Hydrogen, and Clean Coal-Based Technologies, March 9-13, 2008, in New Orleans, Louisiana.  The paper was also published on Nature Materials 7, 641 (2008).

Performers