The influence of implanted yttrium on the cyclic oxidation behaviour of 304 stainless steel
Introduction
Since the introduction of catalytic converters in automotive exhaust lines, the high temperature resistance of the exhaust system materials has been more intensively studied, particularly since the temperature in the upstream region can reach 1273 K. The thermal cycling resistance of these materials must be considered since they are subjected to severe temperature fluctuations [1].
High temperature resistant alloys have to combine with two main requirements: a low scale growing rate and adequate scale adherence on the alloy, especially if thermal cycles are considered. According to the literature the best protective oxides are Cr2O3 and Al2O3 since diffusion processes through them are relatively slow [2], [3], [4], [5].
304 Stainless steel can generate a Cr2O3 layer that protects the alloy against corrosion up to 1100 K. Outward cationic diffusion is the main process responsible of the protective layer formation. As a consequence of this diffusion mechanism, a parabolic rate law is followed [2]. At higher temperatures, and especially when thermal cycles are imposed, the stresses generated in the oxide scale induce spallation and chromium depletion in the alloy surface leading to severe degradation [1].
The beneficial effects of active element additions on the oxidation resistance of heat resistant alloys are well known. Small amounts (usually below 1%) of reactive elements (Sc, Ti, Y, Zr, Ce, La, …) clearly improve the oxidation behaviour of chromia- and alumina-forming alloys [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. Several explanations have been given about this effect, usually called reactive element effect (REE): a modification of the diffusion mechanisms, a reduction of vacancies condensation at the internal interface, a formation of reactive element oxide inclusions trapping alloy impurities, a perovskite-type phase formation [13], [14], [15], [16], [17], [18], [19], [20].
Reactive element ion implantation shows several advantages compared to other surface modification techniques [21], [22], [23]. Doses ranging from 1016 to 1018 ions cm−2 are generally implanted on alloys to improve their corrosion properties at higher temperatures (1150–1350 K). The aim of this paper is to provide an overview of the oxidation properties of 304 stainless steel under isothermal and cyclic conditions in air at 1273 K. With the help of the in situ X-ray diffraction, the present work will also focus on the possible correlation between the presence of implanted yttrium and the oxide structure formed at high temperature. The oxidation resistance of yttrium-implanted specimens will be compared with those of blank specimens. This paper constitutes the next stage of our study concerning the oxidation resistance at high temperature of various model materials [24], [25], [26].
Section snippets
Experimental details
AISI 304 specimen composition is given in Table 1. The samples were cut from cold-rolled plates to give rectangular shape 15 mm × 7.5 mm × 2 mm coupons. Before implantation, all samples were polished on silicon carbide paper up to the 800 grade, cleaned in water, ethanol and dried. The uniform implantation of yttrium, at a nominal dose of 1017 at. cm−2 was undertaken using a 180 keV acceleration potential. The samples were implanted on both large faces, which corresponds to approximately 70% of the total
Thermogravimetry
Fig. 1 exhibits the mass gain curves obtained with implanted and blank 304 specimens after 125 h isothermal oxidation. The growth of the oxide layer formed on both blank and yttrium-implanted specimens follows a parabolic regime after a faster linear transient stage. The parabolic rate constants were only calculated from the parabolic part of the curve. The kp values obtained were kp(Y-implanted) = 3.2 ± 0.2 × 10−7 mg2 cm−4 s−1 and kp(blank) = 2.9 ± 0.2 × 10−6 mg2 cm−4 s−1, for yttrium-implanted and blank
Discussion
On most of industrial plants working under thermal cycling at high temperature, it has been shown that many process parameters, i.e. maximum and minimum temperature, cooling and heating rate and hold-time can play an important role during cyclic oxidation [29]. The relation between mass change and metal loss can be very complicated. Mass gain can be due to oxygen uptake, evaporation and/or spallation caused mass loss. Thus, the resistance to thermal cycling is an important consideration when
Conclusion
In this study, it has been shown that the ion implantation process damages the steel surface generating defects in the austenite structure as well as a modification of the composition of the alloy surface. Compared to blank specimens, yttrium implantation improves the oxidation resistance of AISI 304 specimens under isothermal conditions at 1273 K. Yttrium introduction shortens the transient oxidation stage. A lower parabolic rate constant is also registered and in situ XRD shows a different
Acknowledgement
The authors wish to thank Dr. M.F. Stroosnijder (Joint Researh Center ISPRA, Italy)) for providing implanted 304 specimens.
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