Improving Oxidation Resistance of Stainless Steel (AISI 316L) by Pack Cementation
Engineering and Technology Journal,
2007, Volume 25, Issue 7, Pages 918-939
AbstractThe cyclic oxidation resistance of austenitic stainless steel (AISI 316L) can be
improved by enriching the surface composition in Al and Si using pack cementation
process. In this work, stainless steel is coated with two different types of coatings,
the first one is Si-modified aluminide coating and the second is the Ce-doped silicon
modified aluminide coating. Aluminum, silicon with and without cerium were
simultaneously deposited by diffusion into St.St.316L substrate by the packcementation
process, using a pack mixture containing (18%A1, 7%Si, 2%NH4C1
and 73%Al2O3) and 0.5% Ce (wt %) when required.
Microstructure and chemical composition of the coated specimens were
analyzed using electron microscopy (SEM) with energy dispersive spectroscopy
(EDS). X-Ray diffraction (XRD) was used to identify phase formed in the surface
layer of as-coated specimens. The coating time was changed, and it was found that
diffusion coating time of 3h at 970C° produces coating thickness of 160-180μm and
consist mainly of FeAl and (Cr4Si4Al13) phases. Also, the surface morphology for the
coated samples after 3h coating time at 970C° are dense, smooth and homogeneous.
Cyclic oxidation tests were conducted on the uncoated St.St.316L , Si-modified
aluminide coating and on Ce-doped silicon modified aluminide coating at a
temperature range between (700-900)C° in (air and H2O) for 120h at 10 h cycle.
The oxidation kinetics for uncoated St.St.316L in air environment are found
to be linear, while the oxidation kinetics at water vapor environment are found to
be nearly parabolic. The linear rate constant (KL) and the parabolic rate constant
(Kp) values obtained at 800C° in air and water vapor are –2.77*10-7(mg/cm2)/s and
2.18*10-5(mg2/cm4)/s respectively. The phases present on the cyclic oxidation of
uncoated St.St.316L surface under most test conditions as revealed by XRD analysis
are chromium (III) oxide, NiFe2O4, NiCr2O4 and iron oxide. Oxide phases that were
formed on coated systems during air and H2O oxidation exposure condition are
FeAl2O4, Fe(Al,Cr)2O4 and Fe2O3. The oxidation kinetics for both coated systems in
air and water vapor are found to be linear and parabolic respectively.
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