Papers by Author: N. Karimi

Abstract: 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.

Abstract: Manganese addition and subsequent yttrium implantation effects on extra low carbon 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 manganese alloying addition and subsequent yttrium implantation effects on reference steel oxidation resistance at high temperatures. This study clearly shows the combined effect of manganese alloying addition and subsequent yttrium implantation which promotes the formation of several yttrium mixed oxides seem to be responsible for the improved reference steel oxidation resistance at high temperatures.

Abstract: 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.