Estudo das propriedades químicas, elétricas e da microestrutura de filmes de óxidos coloridos crescidos sobre aço inoxidável por processos químico e eletroquímicos
Abstract
In the present work, it was studied the chemical composition,
morphology, structure and semiconductor properties of oxide films coloured by
the chemical - CHE and electrochemical processes (triangular current scan -
TCS, alternating potential pulse - APP and superposition of ac and dc signals -
SPS) before and after a hardening treatment. Oxide films with two distinct
values of thickness (~130 nm e ~200 nm) were obtained using the CHE, TCS
and APP colouration processes. The SPS process produced oxide films with
only one value of thickness (~150 nm). The hardening treatment led to an
increase in the thickness values of the oxide films produced by all the
investigated processes (CHE, TCS, APP and SPS). Polarization curves showed
that the hardening treatment also promotes enrichment in chromium oxide in the
oxide films. This was supported by analyses of chemical composition obtained
by Auger spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS).
Moreover, these analyses also showed that iron and chromium oxides mainly
constitute all oxide films, before and after the hardening treatment. By other
hand, images obtained by electron transmission microscopy (TEM) revealed that
more compact oxide films are produced after the hardening treatment. Patterns
of electron diffraction revealed that the oxide films are constituted of
nanocrystalline FeCr2O4 spinel oxide, before and after the hardening treatment.
The Mott-Schottky plots presented a quite complex behavior with three straight
regions for all the oxide films studied. From these plots two distinct flat band
potentials were obtained, being one in the passive region (-0,50 V vs. ECS) and
other in the transpassive region (0.50 V vs. SCE) of the stainless steel. These flat
band potentials are characteristics of iron and chromium oxides, respectively.
Besides, two semiconductivity transitions were observed: transition from p-type
to n-type (p-n heterojunction) at -0.50 V vs. ECS and transition from n-type to ptype
at 0.0 V vs. SCE. Values of band gap energy in the range 2.94 eV - 3.49 eV
were calculated assuming direct transitions only for the hardened oxide films
produced by the distinct processes.