(702c) Study of Sulfidation Behavior On the Ordered Metallic Alloy Surface Using the Reaxff Force Field

Although sulfur as an impurity on metallic alloys
plays an important role in reducing the stability and corrosion resistance of
the protective oxide layer and degrading the mechanical properties of the
material, the underlying mechanism has not been clearly described yet. To investigate this
mechanism, we have expanded our ReaxFF force fields for Fe/Al/Ni alloy[1] and the alloy oxide
to Al/Ni//O/S interactions. This ReaxFF force field for
Al/Ni//O/S interactions is fitted against quantum mechanical (QM) calculations. The QM data includes equation of state for
crystal phases, heat of formation, adsorption on various metal
surface sites and diffusion for the development of the force fields capable of
describing the surface reactions as well as bulk properties. Although QM
methods have served as useful tools for predicting molecular properties in
small chemical systems, their computationally expensive cost has been a major
practical obstacle to the application to large systems. On the other hand, ReaxFF
performs the reactive molecular dynamics simulations at much cheaper
computational cost. ReaxFF implemented in parallel molecular dynamics (MD)
simulation packages is suitable for large scale systems larger than 10⁶
atoms over time periods in the nanosecond regime.

In order to
understand the sulfide formation process, we
investigated the formation of sulfide layer on an Al₂O₃
layer and a metal surface at high temperatures (1000-2000 K) by
performing ReaxFF MD simulations. We
examined the sulfide structure (e.g.,
connectivity and angle distribution) and the charge distribution across the sulfide
structure at the interface as well as the sulfide formation behavior on the surface. Our simulations indicate that
this formation process is dominated by the
outward diffusion of aluminum in the alloy/metal. Understanding the mechanism
of the sulfide formation at high temperature provides a theoretical basis for corrosion protection of the
metallic alloy in sulfidizing environments.

[1] Y. Shin, S. Kwak, A. Vasenkov and A.C.T. van Duin, Development of ReaxFF force field for Fe/Al/Ni alloys
and the atomistic study of segregation, manuscript in preparation.