Henri Buscail

Oxidation of alumina formers at 1173 K: effect of yttrium ion implantation and yttrium alloying addition

Abstract The oxidation behaviour of three alumina forming FeCrAl alloys has been investigated during isothermal exposures in air at 1173 K. Two of them were Kanthal A1, differing by the presence or not of implanted yttrium. The third one, Kanthal AF contains alloying additions of yttrium. Kinetic results indicate that only yttrium implantation significantly reduces the growth rate of the oxide scale during the early oxidation stage. For longer oxidation times, the reactive element markedly influences the oxidation rate and the composition of the oxide scale, whatever its introduction mode in the alloy. In situ X-ray diffraction shows that yttrium suppresses the formation of transition alumina and promotes the growth of α-Al 2 O 3 , thereby leading to the earlier formation of a protective oxide scale.

The effect of gas composition on the isothermal oxidation behaviour of PM chromium

Abstract The effect of small amounts of water vapour on the isothermal high-temperature oxidation behaviour of powder-metallurgical (PM) chromium in Ar/20% O 2 and N 2 /20% O 2 has been studied. Additional oxidation experiments were performed with isotopically enriched gases, i.e. Ar/20% 18 O 2 and 15 N 2 /20% 18 O 2 under controlled humidity using H 2 16 O in combination with secondary ion mass spectrometry. In argon/oxygen, water vapour slightly increased the oxidation rate and was the main oxygen source for the oxidation process. The presence of water did not influence the type of kinetics and growth mechanism of oxide formation on pure chromium. In nitrogen/oxygen, the addition of water vapour reduced the oxide growth rate by about 50%. Under these conditions, nitrogen affects the reactivity of water molecules participating in the formation of an oxide layer. The mass transport mechanisms responsible for the formation of an oxide layer did not change, however. When nitrogen was present, a nitrogen-rich sub-layer formed near the scale/substrate interface, influencing the stress relaxation mechanism. Here, repetitive scale cracking occurred during isothermal exposure, which finally resulted in a more adherent and protective oxide layer with less internal stress and withstanding high thermal stresses accumulated during cooling from the test to ambient temperature.

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.

Influence of yttrium-alloying addition on the oxidation of alumina formers at 1173 K

The oxidation behavior of three commercial Fe–Cr–Al alloys, Kanthal APM, Kanthal A1, and Kanthal AF (containing alloying additions of yttrium), has been investigated during isothermal exposures in air at 1173 K. After an initial transient stage, a diffusional process appears to predominantly control the oxidation kinetics of both alloys. During the transient stage, relatively important mass gains have been registered and the presence of yttrium does not seem to have a significant effect on the oxidation rate. On the contrary, the reactive element markedly influences the parabolic oxidation rate and the composition of the oxide scale. In situ X-ray diffraction (XRD) shows that yttrium promotes the transformation of transition alumina into α-Al2O3, leading to the formation of a more protective oxide scale.

Comparison of the oxidation resistances of yttrium implanted low manganese and low manganese–carbon steels at high temperature

Abstract Yttrium implanted and unimplanted low manganese and low manganese–carbon steel samples were analyzed at T =700°C and under an oxygen partial pressure P O 2 =0.04 Pa for 24 h to show the yttrium implantation effect on sample high temperature oxidation resistances. Sample oxidation weight gains were studied by thermogravimetry and structural analyses were performed by in-situ high temperature X-ray diffraction (XRD) 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 temperature according to the initial sample treatment (yttrium implanted or unimplanted). The results obtained by in-situ high temperature XRD will be compared to those by thermogravimetry to a better understanding of the weight gain curves observed during the oxidation test at 700°C and P O 2 =0.04 Pa. This study allows us to show the improved corrosion resistance of yttrium implanted low manganese and low manganese–carbon steel at high temperature.

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.

The influence of yttrium implantation on the oxidation of impure iron containing C and Mn at 700°C

A study of impurity-yttrium interactions hasbeen performed during iron oxidation [p(O2)= 0.04 Pa, T = 700°C]. Yttrium-implanted specimensalways exhibit better oxidation behavior compared withblank specimens. On pure iron or the Fe 0.054 wt.%C alloy thebeneficial effect is attributed toFe2YO4 formation. With themanganese-containing alloys (Fe 0.2 wt.%Mn), theprotective effect of yttrium is attributed to YMnO3 formation. The best oxidationbehavior is obtained with implanted Fe0.18 wt.%Mn-0.041wt.%C alloys due to the formation of an YMnO3oxide subscale at the scale-alloy interface. Yttriumimplantation also hinders carbon segregation at theoxide-alloy interface. This effect ensures better scaleadherence. With the most-impure alloy, yttriumimplantation also changes the growth process fromexternal cation diffusion to predominant inward-oxygendiffusion.

Elaboration and characterization of yttrium implanted low manganese steel.

Abstract Low manganese steel samples were yttrium implanted using ion implantation technique. Sample compositions and structures were investigated before and after yttrium implantations to determine the yttrium distribution in low manganese steel. Yttrium implantation depth profiles were characterized using conventional techniques such as X-ray photoelectron spectroscopy (XPS), reflection high energy electron diffraction (RHEED), X-ray diffraction (XRD), glancing angle X-ray diffraction (GAXRD) and a nuclear analytical method: Rutherford backscattering spectroscopy (RBS). The aim of this study is to show that correlation between composition and structural analyses allows to understand the effect of implanting yttrium in low manganese steel.