Annealing oxides on duplex stainless steels

Abstract: In this report, the relation between the oxide formation and the pickling
behaviour of duplex stainless steels is investigated.

A duplex stainless steel 2205 (EN 1.4462), and two single phases, ferritic
and austenitic stainless steels with the same composition as the phases
inside the 2205, have been annealed. The surface and composition of the oxide
have been analyzed using scanning electron microscopy (SEM), energy
dispersive X-ray (EDS), focused ion beam (FIB) and glow discharge optical
emission spectroscopy (GDOES).

The thickness of the oxide scales increased when the annealing time and
temperature were increased. The ferritic stainless steel has the thickest
oxide, whereas the austenitic has the thinnest one. Nodules in the middle of
the grains and thick oxides on the grain boundaries are observed on the
austenitic stainless steel but also in a lower content on the austenitic
phase of duplex stainless steel. The oxide scale on the 2205 is not
homogenous, showing different types of oxide depending on the phase.

The pickling behaviour of the same samples is also reviewed: oxide surface
study and relative weight loss is discussed. The austenitic stainless steel
has the highest pickling rate, followed by the 2205 and the ferritic
stainless steel respectively. The oxide on the austenitic stainless steel is
spalled from the surface mostly on the zones where nodules are placed,
whereas the oxide on the ferritic stainless steel is dissolved during the
process. The 2205 exhibit a mixture of both pickling behaviours: some grains
are pickled fast, whereas in others the oxide still remains on the surface.

The existence of thicker oxide on the top of grain boundaries, iron-enriched
nodules with a porous structure, thick chromium depleted region and non
protective silicon oxide could be attributed to the faster pickling rate on
the austenitic stainless steel and the austenitic phase regions on the 2205.