M.P. Hierro

The influence of implanted silicon on the cyclic oxidation behaviour of two different stainless steels

Abstract High-temperature alloys are frequently used in power plants, gasification systems, the petrochemical industry, combustion processes and in aerospace applications. Depending on the application, materials are subjected to contaminated atmospheres and/or thermal cycling. Thermal cyclic experiments were carried out in order to study the influence of implanted silicon on the adherence of the scale. The effect of silicon was tested on two different stainless steels in air at 1173xa0K. The oxidized specimens were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDS). The experimental results confirm that silicon implantation at a nominal dose of 10 16 xa0Sixa0ionsxa0cm −2 does not play a significant role on the cyclic oxidation behaviour of the austenitic AISI 304 steel in air at 1173xa0K. However, it appears to enhance the oxidation resistance of the ferritic AISI 430 at oxidation cycles longer than 250.

High-Temperature Oxidation Studies of Low-Nickel Austenitic Stainless Steel. Part I: Isothermal Oxidation

The oxidation behavior of low-nickel austenitic stainless steel (LNiSS) in air at 873 and 973 K was investigated for 500 hr. The oxide scales formed during the process were examined by a wide range of experimental techniques including SEM/EDS, XRD, and EPMA, in order to determine their influence on kinetics behavior. Kinetics laws were close to parabolic at both temperatures, but the morphology of scales showed important differences with temperature. At 873 K the oxide scale was thinner, with irregular growth, intrusions, and without spallation. It was concluded that slower kinetics and advantageous scale morphology suggest that LNiSS is a suitable material for isothermal oxidation in air at 873 K. At higher temperatures, uniformly thick scales plus iron-rich nodules were observed with different composition regions. The most destructive feature was the formation of Fe-rich nodules, which were vulnerable to spalling during cooling.

Effect of yttrium and erbium ion implantation on the oxidation behaviour of the AISI 304 austenitic steel

Abstract The beneficial effect of the addition of yttrium and erbium by ion implantation on the oxidation behaviour of AISI 304 stainless steel at 1173 K has been investigated. Isothermal oxidation tests have been conducted for up to 500 h. The effect of ‘rare earth elements’ (REE) have been studied previously in order to enhance the oxidation behaviour of different alloys. The results show that yttrium and erbium have similar effects, reducing the rate of continuing scale growth and inhibiting scale failure processes. It is concluded in this study, that both reactive elements inhibit the growth of the poorly protective and adherent oxides rich in iron and chromium, which help the spalled behaviour, together with a smaller oxide grain size.

Effects of yttrium and erbium ion implantation on the AISI 304 stainless steel passive layer

In this work, the response of AISI 304 stainless steel after having been implanted with 1017 ions/cm2 doses of Y and Er at an acceleration potential of 150 keV is examined. Computational estimates of the implanted profile have been performed using the PROFILE code. The nature of the outermost surface of the steel, i.e. the passive layer and onwards is studied by means of Auger Electron Spectroscopy as well as X-Ray Photoelectron Spectroscopy techniques, supported by thermodynamics calculations. It is shown that the incorporation of both reactive elements in the steel surface brings about different changes in the nature of their respective passive layers. Both implanted ions induce oxidation of the base material to a certain extent, probably due to the implantation process itself giving rise to Cr2O3 and FeO and/or Fe·OH species, together with different RE oxides as well as Y(OH)3.

Soft X-ray absorption spectroscopy study of the effects of Si, Ce, and Mo ion implantation on the passive layer of AISI 304 stainless steel

Abstract The chemical modifications introduced in the passive layer of AISI 304 stainless steel after Si, Ce, and Mo ion implantation were investigated and compared with non-implanted steel by soft X-ray absorption spectroscopy. The influence of ion implantation on the passive properties was evaluated by measuring soft X-ray absorption spectra at the Cr, Fe, Ni, Mn and Si 2p in addition to oxygen 1s thresholds. All ion implanted samples show a relative Cr-enrichment at the surface as compared with non-implanted samples. Fe 2p as well as O 1s spectral changes reveal chemical differences in the passive layer as a function of the element ion-implanted.

Oxidation of injection molding 316L stainless steel at high temperature

AISI 316L stainless steel has been injection molded for the fabrication of small parts of complex geometry. This work consists in pioneer effort to evaluate the high temperature corrosion rate of such material. To that end, corrosion tests have been carried out for 200 h at different temperatures (700 °C and 900 °C). The nature of the corrosion products was analyzed by optical and scanning electron microscopy, X-ray diffraction, and EDS microprobe analysis. The results indicated that porosity not only increased the kinetics of the corrosion process, but also determined the nature of the resulting oxide layer.

Corrosion Protection of Low-Nickel Austenitic Stainless Steel by Yttrium and Erbium-Ion Implantation Against Isothermal Oxidation

The beneficial effect of ion-implanted yttrium and erbium on the oxidation behavior of low-nickel austenitic stainless steel (LNiSS) at 973 K has been investigated up to 500 hr of oxidation test in air. The resulting oxide scales were examined by a wide range of experimental techniques, including SEM/EDS, XRD, and EPMA. The results indicate that both reactive elements have similar effects. The most significant effects have been to significantly reduce the corrosion rate and to improve the oxide scale adhesion. It is concluded that ion implantation is a powerful tool as surface-modification process introducing reactive elements in the top surface.

Surface modification of ion-implanted AISI 304 stainless steel after oxidation process: X-ray absorption spectroscopy analysis

Abstract The influence of implanted Si, Mo and Ce vs. the as-received austenitic AISI 304 stainless steel has been studied after isothermal oxidation in air at 900 °C for 32 h. The oxide layer formed was characterised by means of conventional X-ray diffraction, scanning electron microscopy/energy-dispersion spectroscopy and X-ray absorption spectroscopy (XAS) techniques. The projected ranges of the implantation were calculated using the trim code. The results obtained by the most sensitive technique, XAS, show slight differences in the chemical composition of the oxide layer of the different ion-implanted samples. However, these chemical differences could determine a threshold between acceptable and non-acceptable oxidation behaviour. The evolution of the chemical composition from the oxide–metal interface to the oxide surface has also been studied. XAS spectra show that Cu diffusion is favoured in the oxide layer for the non-implanted sample, which does not occur for implanted samples. Both Si and Ce ion implantation promotes active diffusion of Cr and Mn from the parent steel to form a protective oxide layer, whereas Mo implantation induces major participation of Fe in the oxide scale. This may have been caused because of volatilisation of molybdenum oxides.

Hot Corrosion Monitoring of Waste Incineration Corrosion Processes Using Electrochemical Techniques

In order to evaluate the damage of molten salt mixtures in waste incineration environments, the alloy 625 was exposed to a molten KCl-ZnCl2 mixture at 650 °C for 200 hours. The corrosion process was monitored by electrochemical impedance spectroscopy (EIS). After exposure the corrosion products were analyzed by X-ray diffraction and SEM. Two different electrochemical impedance models were found to describe the electrochemical processes. In the early stages of corrosion the alloy developed a protective Cr2O3 scale which turned into a porous spinel oxide scale after 100 hours.

Coating Design Using Computational Codes for Steam Oxidation Conditions

Thermodynamical calculations were performed using Thermo-Calc codes to obtained equilibrium diagrams of the heterogeneous system (steel P92, Fe-30Al-6Cr coating (wt. %))|(oxide compounds)|(O2 + H2O gas mixture). Calculations were made for 650 o C, constant water vapour pressure, equal 10 -1 atm, and variable oxygen partial pressure from 1⋅10 -20 to 1⋅10 -1 atm. From obtained results indicate that in the steam gas environments the highest volatile compounds form Mo, W and Cr oxides. Formation of volatile products depends on the oxygen pressure in the O2 + H2O gas mixture. There is change of the principle volatile species in the Fe-Cr system, when at constant water vapour pressure, the oxygen partial pressure decrease. In such conditions there is a decrease of CrO2(OH)2 vapour pressure, which is accompanied by an increase of Fe(OH)2 vapour pressure. At higher oxygen partial pressure vapour pressure of CrO2(OH)2 exceeds that of the most volatile aluminium species, Al(OH)3, by many orders of magnitude.