Papers by Keyword: Alumina Scale

Abstract: The oxidation of hot-dip aluminized AISI 1020 steel coated with NaCl in static air at 700°C for a duration of time 49 h was studied by employing thermogravimetry, Scanning Electron Microscopy (SEM), Electron Dispersive Spectroscopy (EDS) and X-ray Diffraction (XRD) analysis. It was found that NaCl deposits markedly accelerated the oxidation of the AISI 1020 steel. The aluminide coating on the bare steel gives the best oxidation protection by forming continuous alumina scale (Al₂O₃). The degradation of aluminide layer and alumina scale on the steel are associated by chloridation/oxidation cyclic reactions. In addition, the released chlorine will be as catalytic actions and leads to the formation of loose Al₂O₃ during corrosion.

Abstract: This small review deals mainly with three issues regarding the nature and protectiveness of alumina scales grown during high-temperature oxidation: (1) sequences of phase transportation of alumina scales formed on Fe-Cr-Al and NiAl alloys, and a few aluminides, (2) combined additions of reactive element (RE) and (3) convolution of α-Al₂O₃ scales. Though the general phase transformation sequence of alumina scales is γ to θ to α phases at intermediate temperatures, variations have been reported. Directional growth of transient aluminas such as γ-Al₂O₃ and θ-Al₂O₃ is discussed with a particular emphasis on its driving force. Parabolic rate constants for the growth of α-Al₂O₃ scales are smaller when the period of transient alumina is longer because of larger α-Al₂O₃ grains. The effect of RE in slowing the parabolic oxidation saturates at a certain concentration, however combined addition further decreases the oxidation rate. The α-Al₂O₃ scales on Fe-Cr-Al alloys without RE are highly convoluted, however those on NiAl and other aluminides are not so convoluted. The α-Al₂O₃ layer beneath the outer NiO layer or NiAl₂O₄ layer is flat in the oxidation of Ni₃Al. Directions for future work are proposed.

Abstract: The oxidation protection of TiAl-alloys at temperatures above 750°C can be improved by the fluorine effect. The results of thermodynamical calculations predict a corridor for a positive fluorine effect. Ion implantation of F was performed because of giving the best results. After a high F-loss during heating a thin protective alumina scale acts as a diffusion barrier. The F-depth profiles show a distinct maximum at the metal/oxide interface. The diffusion coefficient of F in TiAl for 900°C was determined. The stability of the F-effect after long oxidation time at 900°C and 1000°C can be explained by the existence of a constant F-amount at the metal/oxide interface ensuring a slow growth of the alumina scale. The fluorine effect was also applied to Ni-base superalloys to improve their high temperature oxidation resistance by forming a dense continuous protective alumina scale.

Abstract: The early stages of the scale growth process were studied of two FeCrAl alloys: one synthetic (Fe23Cr5Al) and one commercial (Kanthal APM alloy). In addition, Yttrium was implanted to the Fe23Cr5Al alloy. Oxidation exposures were carried out at 1000°C using two-stage-oxidation exposures in atmospheres containing significantly different amounts of ¹⁸O₂-tracer. The scales were analyzed in terms of SIMS, PLS and SEM. The distribution of oxygen isotopes which corresponded to the location of new oxide formation, the scale phase composition, scale morphology and microstructure were determined which enabled description of the scale evolution on all studied alloys. Similar evolution stages were observed, but minor differences were related to the rate of disappearing of the transient aluminium oxides.

Abstract: A new method is proposed to achieve a dense protective alumina scale for Ni-base superalloys with an Al-content lower than 10 wt.% at temperatures above 1000°C. The method is based on the halogen effect. Thermodynamical calculations show the existence of a region for a positive fluorine effect at temperatures between 900-1200°C for the alloys IN738 and IN939. By using fluorine ion implantation in combination with Monte Carlo simulation of the fluorine profiles these results were transformed into a region of F-concentrations at the metal surface. A dense protective alumina scale was formed for IN738 after oxidation at 1050°C. Due to the very low Al-content no alumina scale was found for IN939.

Abstract: The paper describes an examination of the effect of the addition of zirconium as a third element on the heat-resisting properties and explains the high temperature oxidation mechanism of Fe3Al intermetallic compounds. The Fe3Al and Fe3Al-0,05Zr specimens have been isothermally oxidized in the temperature range of 1173-1473 K in synthetic air for 100 hrs. The formed oxide layer, about 1,5-2 μm thick, was Al2O3. An examination of the cross-sectioned scales by SEM-EDS showed that the alumina layer consisted of a small inner columnar layer and an outer equiaxed grain layer. Additionally, very fine (50-150 nm) oxide grains rich in Zr, further identified as ZrO2, were found across the alumina scales. To understand the role of Zr on the growth mechanism of α–Al2O3 oxide scale on Fe3Al materials, two-stage oxidation experiments were performed (16O2/18O2), followed by SIMS and TEM-SAD observations. Particular attention was paid to the use of TEM in order to precisely characterize the products on samples prepared using the FIB (Focused Ion Beam) method. A combination of analytical techniques revealed that ZrO2 particles, most of which were formed along alumina grain boundaries, enhanced oxygen diffusion along grain boundaries due to oxygen-deficient composition of zirconium oxide (ZrO2-y).

Abstract: Alumina scale adhesion on high temperature alloys is known to be affected primarily by sulfur segregation and reactive element additions. However adherent scales can become partially compromised by excessive strain energy and cyclic cracking. With time, exposure of such scales to moisture can lead to spontaneous interfacial decohesion, occurring while the samples are maintained at ambient conditions. Examples of this Moisture-Induced Delayed Spallation (MIDS) are presented for NiCrAl and single crystal superalloys, becoming more severe with sulfur level and cyclic exposure conditions. Similarly, delayed failure or Desk Top Spallation (DTS) results are reviewed for TBC’s, culminating in the water drop failure test. Both phenomena are discussed in terms of moisture effects on bulk alumina and bulk aluminides. A mechanism is proposed based on hydrogen embrittlement and is supported by a cathodic hydrogen charging experiment. Hydroxylization of aluminum from the alloy interface appears to be the relevant basic reaction.

Abstract: TiAl-based alloys have attractive properties as light weight heat-resisting material. In the present work, the influence of Cu, Zn, Ag and Se on the oxidation behavior of TiAl was investigated by ion implantation at acceleration voltage of 50 kV and ion doses of 1019 to 2x1021 ions/m2. The oxidation behavior was assessed by a cyclic oxidation test at 1200 K in a flow of purified oxygen under atmospheric pressure. The oxidation products were analyzed by conventional methods including X-ray diffractometry, SEM and EPMA. The implantation of Zn and Cu improves the oxidation resistance significantly by forming virtually Al2O3 scales, while Ag and Se enhance the oxidation. The improvement by Zn is attributable to the formation of complex oxide of Zn in the initial stage of oxidation. The oxygen partial pressure under the layer seems to be very low, resulting in the formation of alumina scale due to a selective oxidation of Al. The influence of Cu is not certain. The influence of Ag and Se is explained in terms of Al depletion in the implanted layer.

Abstract: Coupon specimens of 30Nb-40Al-(30-x)Cr-xMo (mol %), with x being 0, 7.5, 15, 22.5 or 30, and of 30Nb-40Al-(22.5-y)Cr-7.5Mo-ySi, with y being 5 or 10, were oxidised in air in a temperature range 1473 to 1773 K for 72 ks. The influence of additions of 0.05 and 0.1% Hf to 30Nb-40Al-22.5Cr-7.5Mo was also examined. Conventional metallographic examinations were performed for the specimens before and after the oxidation. The major phase of the specimens is AlNbCr, and their minor phases are Cr2Al and NbAl3, except for high Mo-content specimens which form AlNbMo, Nb2Al, Mo3Al and MoAl3. The additions of Si lead to additional formation of Nb5Si3. Virtually Al2O3 scales (a dense and continuous Al2O3 layer with a thin layer consisting of AlNbO4, CrNbO4 and/or SiO2 on it) are formed at temperatures up to 1673 K, except for the high Mo-content specimens which form poorly protective scales. The additions of Hf increase the mass gain to some degree. 5Si forms protective scales at temperatures up to 1723 K, and 30Cr and 10Si up to 1773 K.

Abstract: Ru coating prior to aluminizing is one of the effective methods to reduce the harmful intermediate layer that forms under the coating (SRZ) on a 4th generation Ni-base SC superalloy. This study examined the short-term isothermal oxidation behavior of this Ru-modified coating at 1373 K in air. Surface observation by SEM showed that the scale becomes flat and uniform in comparison to simple aluminide coating. XRD and cross-sectional analysis results also showed that phase transformation from β-NiAl to γ’-Ni3Al seldom occurs in the Ru-modified coating layer leading to the prevention of local oxidation.