Yan Niu

An Approximate Analysis of the External Oxidation of Ternary Alloys Forming Insoluble Oxides. I: High Oxidant Pressures

The construction and the properties of three-dimensional diagrams showing the regions of stability of the various compounds, which can form as a result of the oxidation of ideal ternary A–B–C alloys by a single oxidant at a constant temperature (kinetics diagrams) are examined for oxidant pressures insufficient to oxidize all possible alloys within the system (low oxidant pressures). For the calculation it is assumed that the various oxides do not dissolve into each other and do not form double oxides and that the alloy has an ideal behavior, while internal oxidation of the most-reactive components is disregarded. The range of meaningful oxidant pressures is divided into six intervals, which correspond to the formation of different types of scales. The simplified two-dimensional (2D) kinetics diagrams presented are obtained by projecting the appropriate three-dimensional (3D) lines of equilibrium between the alloy and the various oxides on the base triangle, which gives the composition of the system in terms of the three metal components only. The kinetics diagrams are correlated with the corresponding equilibrium phase diagrams for the same quaternary A–B–C–O systems.

Oxidation of Four NiAl-Ag Alloys at 900°C in 1 Atm O2

Small amounts of silver have been added to the intermetallic compound β-NiAl for the purpose of improving its mechanical properties. Four ternary NiAl-Ag alloys NiAl-0.5Ag, NiAl-1Ag, NiAl-5Ag and NiAl-10Ag (at.%), and an Ag-free β-NiAl have been oxidized at 900oC for 24 h in 1 atm O2 to study the effect of the presence of silver on the oxidation of β-NiAl. The kinetics of all the alloys were generally composed of two main parabolic stages with slightly larger parabolic rate constants for the second stage, except for NiAl-10Ag, which has an instantaneous parabolic rate constant decreasing with time. A continuous external layer of Al2O3 formed on all the alloys. In particular, the scales formed on NiAl-5Ag and NiAl-10Ag contained a thin and discontinuous layer of silver at the alloy/Al2O3 interface. Furthermore, NiAl-10Ag formed also isolated Ag particles or even a discontinuous Ag layer occasionally surmounting the Al2O3 scale. The addition of minor amounts of silver does not affect significantly the oxidation of β-NiAl, because silver is essentially present as a second phase due to its very small solubility in this intermetallic compound.

Effect of Chromium on the Oxidation of a Fe-10 Al Alloy at 1000°C

The effect of the addition of 5 and 10 at.% Cr on the oxidation of a binary Fe-10 at.% Al alloy (Fe-10Al) was studied in 1 atm O2 at 1000°C. Fe-10Al underwent a very slow initial nearly-parabolic stage followed by a breakaway composed of two subsequent parabolic stages with a smaller rate constant for the final period. The two ternary alloys (Fe-5Cr-10Al and Fe-10Cr-10Al) presented two parabolic stages with final rate constants similar to each other and much lower than that for Fe-10Al. The alumina scale developed initially on Fe-10Al was replaced later by a layered scale containing mixtures of Fe and Al oxides plus many Fe-rich oxide nodules. Fe-5Cr-10Al was mostly covered by a thin alumina layer just above the alloy surface with some Fe-rich nodules formed in the beginning of oxidation, which later on were healed by alumina with a large decrease of the oxidation rate. A continuous alumina layer formed on the whole sample surface without any Fe-rich nodule for Fe-10Cr-10Al. Therefore, the addition of chromium is obviously beneficial for the oxidation resistance of Fe-10Al by inhibiting the formation of fast-growing Fe-containing oxides and promoting the development of an exclusive alumina layer. However, the effect of chromium is different from the classical third-element effect.

Electrochemical Impedance Study of the Corrosion of Fe-Cr Alloys in Molten (Li,K)2CO3 Eutectic Mixture

Electrochemical impedance spectroscopy (EIS) together with thermogravimetric techniques were employed to study the corrosion of iron and Fe-Cr alloys containing 5, 10, 20 and 25 wt.% Cr respectively in a molten eutectic (Li,K) 2 CO 3 mixture at 650°C. The Nyquist plots for the corrosion of iron and Fe-Cr alloys in the melt are similar, and are all composed of a small semicircle at high frequency and a large semicircle at low frequency. The radius of the semicircle at low frequency, which can be used to evaluate the corrosion resistance of the alloys, is significantly larger for Fe-20Cr and Fe-25Cr than for Fe, Fe-5Cr and Fe-10Cr. The Cr-rich alloys undergo slower corrosion than iron and the Cr-poor alloys: this is further confirmed by the thermogravimetric results and is due to the formation of Cr-rich scales An equivalent circuit of oxide capacitance in series with the double-layer capacitance was proposed to describe the impedance spectra.