Sébastien Perrier

In-situ characterization of the oxide scale formed on yttrium-coated 304 stainless steel at 1000 °C

Abstract In-situ X-ray diffraction was combined with scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analyses to characterize the oxides formed on yttrium-coated 304 stainless steel during oxidation at 1000 °C in air. Results are compared with those obtained on uncoated specimens. Care has also been taken on the structural transformations during the cooling process. At 1000 °C, yttrium leads to the formation of YCrO 3 and YCrO 4 oxides. These oxides are mainly located at the external interface. Moreover, silicon segregation at the oxide–metal interface is also observed. The yttrium coating seems to promote a favorable effect on the continuity of the silicon-rich subscale limiting the oxide scale growth and the formation of iron oxides. Thermogravimetric studies reveal that yttrium addition leads to a lower weight gain which is related to a limitation of the initial transient oxidation stage and to a reduction of the parabolic rate constant.

Yttrium sol–gel coating effects on the cyclic oxidation behaviour of 304 stainless steel

Abstract The present results reveal the interest of sol–gel coating technique to improve 304 steel high temperature oxidation resistance. An yttrium sol–gel coating appears to enhance the oxidation resistance during isothermal oxidation test, to decrease widely the oxide weight gain and to reduce the initial transient oxidation stage generally observed in the case of blank steels. Moreover, the experimental results confirm that yttrium sol–gel coating also plays a significant role on the cyclic oxidation behaviour of the 304 steel. In fact, the yttrium addition promotes remarkably the prolongation of the period during which the oxide scale still remains adherent to the substrate.

Yttrium implantation effect on 304L stainless steel high temperature oxidation at 1000°C

Scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDXS) and in situ X-ray diffraction techniques were carried out to observe the oxide scale evolutions of yttrium implanted and unimplanted commercial 304L stainless steels during and after their high temperature oxidation at 1000°C for 100 h. Our results clearly demonstrate that yttrium implantation promotes a faster oxide scale growth and the formation of a more uniform chromia layer due to a higher chromium selective oxidation compared to unimplanted 304L stainless steel. Moreover, the presence of yttrium also leads to the formation of an enriched silicon layer at the metal-oxide interface limiting the growth of iron-based oxides which were not detected (even during cooling) in the case of yttrium implanted samples. These results allow to understand the low weight gain of yttrium implanted 304L stainless steel observed by thermogravimetry and underline the beneficial effect of yttrium implantation on the 304L oxidation resistance at high temperature.

Selectivity of bacterial ionophore monensin for monovalent metal cations. Solvent effects in methanol and biphasic water-organic systems

Interactions of ionophore monensin with heavy metal monovalent cations, Ag+, Tl+, Hg22+, were studied in methanol and in various biphasic waterorganic solvent systems to supplement previous data on alkali-metal cations. The species formed were identified and the corresponding formation constants determined. Enthalpies of formation were also obtained in methanol for Ag+ and Tl+ from calorimetric measurements. The results for monovalent cations in general are discussed in terms of cation size and solvation, and structure of the ionophore anion.

X-Ray Diffraction to Study the Oxidation Mechanism of Chromium at Elevated Temperatures

It is demonstrated that the oxidation behaviour of chromium was significantly different according to the temperatures, i.e. 800, 900, and 1000 °C. Under isothermal condition, the formation of a chromia scale on pure PM chromium follows parabolic kinetics, indicative of a diffusion-controlled growth mechanism. At each temperature the mass gain curves showed some discontinuities that can be explained by the formation of cracks, that gives a direct access to nonoxidized metallic surfaces. Nevertheless, under the experimental conditions, chromium was able to form a rather protective oxide scale, preventing the underlying substrate against severe corrosion by a healing process. Cross-section examinations revealed that under the oxide layer a nitrogen-rich sub-surface layer was formed in the substrate. X ray diffraction results prove that due to inward diffusion of nitrogen, a solid solution of nitrogen in chromium was formed, which finally reacts with each other forming chromium nitrides CrN and Cr2N. The presence of this nitrogen rich layer changes the metallic matrix hardness and then a decrease of the oxide scale adherence was observed.

Effect of Lanthanum Sol-Gel Coating on the Oxidation Behaviour of the AISI 304 Steel at 1000°C

The aim of the present work is to investigate the effect of Lanthanum surface addition on the oxidation behaviour of the AISI 304 stainless steel, in air, at 1000°C. The in situ X-ray diffraction (XRD) analyses on the blank steel reveal that after the first 10h oxidation, a change in the structural composition of the oxide scale occurs. During the first ten hours oxidation an initial growth of chromia and Mn1,5Cr1,5O4 is observed. After 10 h oxidation, chromia is not detected anymore and iron-containing oxides such as hematite (Fe2O3) and iron chromite (FeCr2O4) are observed in the outer part of the scale. With blank AISI 304 specimens, the iron-containing oxides are generally not very protective and show severe spallation during cooling to room temperature due to thermal stresses. They do not allow a good adherence of the corrosion layer under thermal cycling. On the Lanthanum coated AISI 304 Stainless Steel the oxidation rate is 10 times lower. In situ XRD analyses show the absence of iron containing oxides. It reveals the formation of a fine convoluted Cr2O3 layer associated with the formation of the mixed oxides Mn1,5Cr1,5O4 and LaCrO3. LaCrO3 is found to be located at the oxide/steel interface. Our results show that, even though the scale formed under isothermal conditions is not composed of iron containing oxides, Lanthanum sol-gel coating does not prevent spallation during thermal cycling at 1000°C.

The Nitridation, A Way To Improve High Temperature Oxidation Behaviour Of AISI 304

The present study focuses on the high temperature oxidation of a AISI 304 chromiaforming nitrided alloy. Isothermal oxidations were performed in air, at 800°C. The effect of nitridation on the steel surface depends on the temperature of the treatment. It leads whether to a γN solid solution formation or to CrN formation. In situ X-ray diffraction has been used to follow the oxides formation. Results show the concomitant growth of CrN and Fe2O3 at the beginning of the test. Then, Cr2O3 quickly appears which leads to the formation of a protective oxide scale (a parabolic rate law is observed). Our conclusions suggest that nitridation increases the high temperature oxidation resistance of 304 steels at 800°C.

In situ high-temperature X-ray diffraction characterization of yttrium-implanted extra low-carbon steel

Abstract Yttrium-implanted and unimplanted extra low-carbon steel samples were analyzed at T = 700°C and under an oxygen partial pressure PO2 = 0.04Pa for 24 h to show the yttrium implantation effect on extra low-carbon steel high-temperature corrosion resistance. Sample oxidation weight gains were studied by thermogravimetry, and structural analyses were performed using in situ high-temperature X-ray diffraction with the same experimental conditions. The aim of this paper is to show the initial nucleation stage of the main compounds induced by oxidation at high temperatures according to the initial sample treatment (yttrium-implanted or unimplanted). The results obtained by in situ high-temperature X-ray diffraction will be compared to those by thermogravimetry to show the existing correlation between weight gain curves and structural studies. Our results allow one to understand the improved corrosion resistance of yttrium-implanted extra low-carbon steel at high temperatures.

Manganese effect on isothermal high temperature oxidation behaviour of AISI 304 stainless steel

Chromia-forming steels are excellent candidates to resist to high temperature oxidizing atmospheres because they form protective oxide scales. The oxide scale growth mechanisms are studied by exposing AISI 304 stainless steel to high temperature conditions in air, and the analyses were carried out by means of thermogravimetry and in situ X-rays diffraction. The in situ XRD analyses carried out during high temperature AISI 304 steel oxidation in air reveals the accelerated growth of iron-containing oxides such as hematite Fe2O3 and iron-chromite FeCr2O4, when the initial germination of the oxide layer contains the presence of a manganese-containing spinel compound (1000°C). When the initial growth shows the only chromia formation (800°C), hematite formation appears differed in time. Protection against corrosion is thus increased when the initial germination of manganese-containing spinel oxide is inhibited in the oxide scale.

Yttrium Implantation and Additional Element Effects on the Oxidation Behaviour of Reference Steels at 973 K

Yttrium implantation effects on reference steels (extra low carbon and low manganese steel) were studied by Rutherford Backscattering Spectrometry (RBS), Reflection High Energy Electron Diffraction (RHEED), X-ray Diffraction (XRD) and Glancing Angle X-ray Diffraction (GAXRD). Thermogravimetry and in situ X-Ray Diffraction at 700°C and PO2=0.04 Pa for 24h were used to determine the yttrium implantation and addition element effects on sample oxidation resistance at high temperatures. This study clearly shows that yttrium implantation and subsequent high temperature oxidation induced the formation of several yttrium mixed oxides which closely depend on the reference steel addition elements. Moreover, these yttrium mixed oxides seem to be responsible for the improved reference steel oxidation resistance at high temperatures.