Internal Oxidation of a Ag-l.3 at.% Se Alloy. Part I: Composition and Appearance of Oxidation Products

The internal oxidation of a two-phase Ag-1.3 at.% Se alloy in pure oxygen was studied at 750, 800 and 830°C. The alloy is composed of a dilute solid solution of selenium in silver) and Ag₂Se intermetallic particles. The internal oxidation of this alloy proceeds through gradual in-situ oxidation of the Ag₂Se particles as well as through diffusive internal oxidation of selenium from solid solution. Gradual in-situ internal oxidation of Ag₂Se particles reflects itself in the appearance of two internal-oxidation fronts, inner and outer, marking the beginning and end of the in-situ oxidation of Ag₂Se particles. The oxide phase formed during in-situ oxidation is the molten double oxide, Ag₂SeO₃. A theoretical treatment of the phase relationships in a hypothetical Ag–Ag₂Se–Ag₂SeO₃ ternary phase diagram is presented to assist the explanation of the mechanism of in-situ oxidation. The kinetics of oxidation are presented in Part II.     

Hot Corrosion Behavior of Superalloy IN718 at 550 and 650 °C

This investigation was undertaken to evaluate oxidation and hot corrosion behavior of the Fe-Ni-based superalloy IN718, at 550 and 650 °C, to explore its performance as turbine engine components under marine environment. Uncoated and different salt-coated samples (100 wt.% NaCl, 75 wt.% Na₂SO₄ + 25 wt.% NaCl, and 90 wt.% Na₂SO₄ + 5 wt.% NaCl + 5 wt.% V₂O₅) were exposed in air at 550 and 650 °C under cyclic heating and cooling for 100 h. Weight gain was studied for both uncoated and salt-coated samples. X-ray diffraction, scanning electron microscopy, and electron dispersive spectroscopy were used to characterize the oxidation and corrosion products. A possible mechanism of corrosion, based on the corrosion compounds, is discussed. The variation in weight gain with time showed a parabolic growth of oxides. Coating with NaCl was found to be detrimental both at 550 °C as well as 650 °C. On the other hand, the salt mixture of NaCl and Na₂SO₄ had no effect at 550 °C; however, it was detrimental at higher temperature of 650 °C. Coatings of salt mixture of Na₂SO₄, NaCl, and V₂O₅ caused very slow oxidation at both the temperatures. Increase in thickness of salt coating was observed to enhance the rate of hot corrosion. Among the three types of salt coatings, the coating of NaCl was found to be most damaging both at 550 and 650 °C.     

Effect of Sulphur on the Oxidation Behaviour of Possible Construction Materials for Heat Exchangers in Oxyfuel Plants in the Temperature Range 550–700 °C

During oxyfuel combustion metallic heat exchangers are subjected to service environments which substantially differ from those prevailing during the conventional air firing process. In the present study the behaviour of three selected construction materials (P92, super S304HCu and alloy 617) during exposure in simulated oxyfuel gas with and without addition of SO₂ at temperatures between 550 and 700 °C has been investigated. The alloy microstructure and the corrosion products formed during exposures up to 1000 h were studied by SEM/EDX and correlated with gravimetric data collected during the discontinuous exposures. It was found that the behaviour of the martensitic steel was hardly affected by the presence of SO₂; however, in the case of the austenitic steel S304HCu the SO₂ suppressed internal oxidation occurring at 650 °C in the SO₂-free gas, thus promoting formation of a protective chromium-rich oxide. In the case of the nickel base alloy 617 the SO₂ addition increased the corrosion rates at 550 and 650 °C due to replacement of the external chromia scale by a multiphase scale with sulphur-containing surface nodules. At 700 °C the alloy formed a chromia base surface scale and SO₂ addition suppressed the formation of volatile Cr species. The results are explained using classical oxidation theory related to conditions for external scale formation in combination with thermodynamic considerations of phase stability as well as relative rates of adsorption of various gas species.     

Kinetic Features of Iron Oxidation in Liquid Lead Saturated with Oxygen Below and Above the Chaudron Point (570 °C)

The oxidation kinetics of Armco–Fe in stagnant lead melts saturated with oxygen at 550 and 650 °C was investigated and the peculiarities of structure, phase and elemental composition of scales were determined. At 550 °C oxidation follows a parabolic law up to 1,500 h and then (1,500–2,000 h) oxidation accelerated. At 650 °C during 100 h the scale grew rapidly according to a quadratic time dependence. Then (100–500 h) the oxidation rate decreased sharply. From 500–1,000 h oxidation accelerated. The scales formed at 550 and 650 °C consisted of Fe₃O₄ and FeO/Fe₃O₄, respectively. The scales had a duplex structure. The outer-oxide layer grew from the initial solid metal–liquid metal interface towards the melt while the inner layer grew towards the iron substrate. The defectiveness of scale altered with time. The possible reaction mechanisms in the Fe–Pb[O] system are discussed.     

Thermodynamic Properties of Superionic Phase Ag<Subscript>4</Subscript>HgSe<Subscript>2</Subscript>I<Subscript>2</Subscript> Determined by the EMF Method

Triangulation of the Ag-Hg-Se-I system in the vicinity of quaternary phase Ag₄HgSe₂I₂ was performed by differential thermal analysis, X-ray diffraction and electromotive force (EMF) methods. The spatial position of the phase region Ag₄HgSe₂I₂-Se-HgI₂ regarding the figurative point of silver was used to write the chemical reaction of formation of Ag₄HgSe₂I₂. The EMF measurements were carried out by applying an electrochemical cell: (–) C|Ag|Ag₂GeS₃ glass|Ag₄HgSe₂I₂, HgI₂, Se|C (+), where C is graphite and Ag₂GeS₃ glass is the fast purely Ag⁺ ions conducting electrolyte. The linear dependence of the EMF of the electrochemical cell on temperature was used to determine the standard thermodynamic values of Ag₄HgSe₂I₂ for the first time.     

Selective Oxidation of Chromium by O2 Impurities in CO2 During Initial Stages of Oxidation

This study shows that the corrosion behaviour of 12 wt% Cr steel in CO₂ at 550 °C is determined in the first stage of oxidation by reaction with O₂ impurities. Depending on the amount of theses impurities and the thermal ramp rate, selective oxidation of chromium could lead to the formation of a protective chromium-rich oxide. An oxidation model describing qualitatively the nature of the oxide layer formed in the initial period of oxidation is presented. From these observations, surface engineering processes for protecting 9–12 wt% chromium steels from fast corrosion rate have emerged.     

Hot Corrosion Behavior of Some Superalloys in a Simulated Incinerator Environment at 900 °C

Incinerators are being used to burn solid waste of all types. This burning of waste creates a very aggressive environment at extremely high temperature. This environment attacks the various components of the incinerators. Some studies have been reported regarding behavior of steels in simulated incinerator environment at 550 °C. In present work superalloys Superco 605, Superni 600, and Superni 718 have been subjected to cyclic oxidation in 40 wt.% K₂SO₄ + 40 wt.% Na₂SO₄ + 10 wt.% KCl + 10 wt.% NaCl environment at 900 °C under cyclic condition. Weight change measurements have been done and weight change has been plotted against the numbers of cycles. The oxide scales formed on the surface of the corroded superalloys have been characterize by FESEM, EDS, XRD, cross-sectional analysis, and x-ray mapping. The nickel-based superalloys Superni 600 and Superni 718 indicated better resistance to corrosion in the above environment whereas Superco 605 lead to massive weight gain.     

The high temperature corrosion of nickel in SO2 at 500–800°C

The reaction of nickel with SO₂ has been studied at 500–800°C and different pressures of SO₂ (0.4–100 kPa). The reaction products are NiO and nickel sulphides. The reaction rate goes through a maximum at about 600°C at and above 13 kPa SO₂, while the maximum is absent at lower SO₂ pressures. It is concluded that the reaction takes place through different reaction paths: (i) the direct reaction of Ni with SO₂ and (ii) a reaction path via NiSO₄ as an intermediate reaction product. The latter path is the more rapid one and gives rise to the very rapid reaction rates and maximum in the reaction rate at 600°C.     

Cyclic Oxidation Behavior of the Ti–6Al–4V Alloy

The cyclic oxidation behavior of the Ti–6Al–4V alloy has been studied under heating and cooling conditions within a temperature range from 550 to 850 °C in air for up to 12 cycles. The mass changes, phase, surface morphologies, cross-sectional morphologies and element distribution of the oxide scales after cyclic oxidation were investigated using electronic microbalance, X-ray diffractometry, scanning electron microscopy and energy dispersive spectroscopy. The results show that the rate of oxidation was close to zero at 550 °C, obeyed parabolic and linear law at 650 and 850 °C, respectively, while at 750 °C, parabolic—linear law dominated. The double oxide scales formed on surface of the Ti–6Al–4V alloy consisted of an inner layer of TiO₂ and an outer layer of Al₂O₃, and the thickness of oxide scales increased with an increasing oxidation temperature. At 750 and 850 °C, the cyclic oxidation resistance deteriorated owing to the formation of voids, cracks and the spallation of the oxide scales.     

A study of the Ag(s)/Ag2SO4(l) high temperature reference electrode

Ag(s)/Ag₂SO₄(l) reference electrodes for use in hot corrosion research were fabricated and their properties were studied at 927°C (1200°K). The McDanel^TM MV 30 mullite tube was used as sheath of the reference electrode and concentrations of the tested electrolytes were varied from 0.1 to 10 m/o Ag₂SO₄ (l) + Na₂SO₄ (l). 10 m/o Ag₂SO₄ was always used as electrolyte in the reference electrode. The working electrodes with less than 2 m/o Ag₂SO₄ showed marked potential drift as the Ag₂SO₄ concentration was lowered, showing poor potential stability. However, the initial values of cell voltages obeyed Nernstian behavior for all range of Ag₂SO₄ concentrations in the working electrodes, which showed that Ag(s)/Ag₂SO₄(l) electrodes behaved reversibly. Galvanostatic polarization was performed on a pair of 10 m/o Ag₂SO₄ reference electrodes to test the reversibility. The polarizability was 3.5 mV/10 μA with no hysteresis loop on the polarization curve, showing that the reversibility was satisfactory. The potential of gold electrode in molten Na₂SO₄ was measured under pure oxygen atmosphere with 10 m/o Ag₂SO₄ electrode as a reference potential. The results showed that the potential of gold electrode could be explained by the 2 Na + SO₃ + 1/2 O₂ → Na₂SO₄ cell reaction.     

The Effect of Colored Titanium Oxides on the Color Change on the Surface of Ti-5Al-5Mo-5V-1Cr-1Fe Alloy

In the present investigation, a color change on the surface of Ti-5Al-5Mo-5V-1Cr-1Fe alloy was studied through thermal oxidation experiments in the temperature range of 100-1000 °C with an interval of 50 °C. The phase composition and morphology of oxide layer were characterized by x-ray diffraction and light optical microscopy, respectively. The result shows that the achieved colors after thermal oxidation followed a chromatic scale which went from silver white to light yellow to golden yellow to blue and then to light green and brownish black. The color change on the alloy mainly resulted from the different colored titanium oxides in the oxide layer. The silver white, yellow, and blue on the alloy with the oxidation temperature below 600 °C were the results of TiO₂ white tint, TiO golden tint, and Ti₂O₃ blue color, respectively. The light green was the mixed color of TiO golden tint and Ti₂O₃ blue color in the oxidation temperature range of 600-700 °C. However, at the oxidation temperatures exceeding 750 °C, the color turned to be brownish black. It might be associated with the thick, porous, and multilayered oxide layer. Consequently, it can be suggested that the illustration of the color change is vitally necessary for assessing the quality of the final workpieces according to the color change on titanium alloys.     

The Effect of TaC Reinforcement on the Oxidation Resistance of CNTs/SiC CMCs

This study focuses on a two-stage spark plasma sintering (SPS) of TaC and/or carbon nanotubes (CNTs)-reinforced SiC ceramic matrix composites (CMCs). The oxidation mechanism of SiC-based CMCs with CNTs reinforcement as well as the TaC additives effect on the thermal oxidation resistance of the SiC-CNTs-TaC systems are investigated. The oxidation behavior up to 1500 °C is characterized in terms of mass changes, oxide layer formation, and thickness. The results showed that more disorder occurred in the CNT network with increased oxidation temperature. TaC additives exhibited an enhanced protective effect in increasing the oxidation temperature of CNTs from 460 to 550 °C, and this protective effect was effective at 1200 °C achieved by the crystalized Ta₂O₅ which grew with a preferred orientation giving rise to the phase separation in the glassy protective layer. Degraded oxidation resistance was found at 1500 °C.     

The salt-induced corrosion behaviour of FeCr alloys at elevated temperatures—II. Alloys rich in chromium

The oxidation behaviour of two Na₂SO₄-coated, chromia-forming, iron-based alloys at 900°C has been studied thermogravimetrically, and the reaction products examined in detail by metallographic and E.P.M.A. techniques. Na₃SO₄ coatings markedly enhance the oxidation rates of both alloys and result in the formation of thick, compact, stratified scales. On the basis of subsequent experiments designed to characterize the singular roles of sodium oxide and sulphur, and in the absence of scale fluxing, it is postulated that the formation of sulphides in the alloy substrate and the mechanical failure of scale are responsible for the enhanced oxidation. Sodium chromate, a feature of the hot corrosion reactions of Na₂SO₄-coated chromia-forming, Ni- and Co-based binary alloys, is shown not to be a by-product of the corrosion reaction of equivalent iron-based alloys. Instead salt/scale reactions result in the formation of sodium-iron oxide, which is capable of assisting in the corrosion reaction, albeit in a minor way. The role of NaCl additions during Na₂SO₄ induced corrosion is also examined.     

Ultra-High-Temperature Oxidation and Thermal Stability of Ti<Subscript>2</Subscript>AlC in Air at 1600–1800 °C

The oxidation resistance and thermal stability of Ti₂AlC at 1600–1800 °C in air were studied by using induction heating method. The results showed that Ti₂AlC could survive with relatively low oxidation rate at temperatures up to 1650 °C for a short period of time due to the formation of an Al₂O₃ inner layer with certain protectiveness. However, at 1700 and 1800 °C, severe oxidation of Ti₂AlC happened, the entire Al₂O₃ inner layer no longer existed, and the whole oxide scale became porous, cracked and voluminous. In the oxidation processes, the Ti₂AlC substrate decomposed to TiC_x at 1700 °C, and transformed to Ti₃AlC₂ due to the reaction with TiC_x at 1800 °C, indicating that the massive consumption of Al in Ti₂AlC exceeded its deficiency tolerance.     

Transition metal- chalcogen systems XI: the platinum- selenium phase diagram

The platinum-selenium phase diagram was investigated by differential thermal analysis, metallography, and X-ray powder diffraction methods. The two previously known intermediate phases, Pt₅Se₄ and PtSe₂, both melt congruently: Pt₅Se₄ at ∼1070 °C and PtSe₂ (under its own vapor pressure) at 1245 ± 10 °C. Pt₅Se₄ forms a eutectic with Pt at 1065 ± 2 °C and ∼42 at % Se and another one with PtSe₂ at 1067 ± 2 °C and somewhat below 45 at.% Se. On cooling, Pt and PtSe₂ form a metastable eutectic at 1037 ± 2 °C and ∼43 at % Se. Between PtSe₂ and Se, a degenerate eutectic was found at 221 °C, which also indicates negligible solubility of platinum in solid selenium.     

Oxidation Mechanisms of Copper and Nickel Coated Carbon Fibers

Differential-Thermal Analysis (DTA) and X-ray diffraction analysis were applied to determine the mechanisms of high-temperature oxidation of copper- and nickel-coated carbon fibers. Both kinds of coatings were deposited by electroless plating onto the fiber surface. The as-deposited copper film was crystalline, whereas the nickel coating consisted of an amorphous Ni–P alloy. Coated fibers were heated from room temperature to 900 °C in air at 10 °C min^?1. For the copper coating, the main oxidation product formed at low temperatures was Cu₂O, while at higher temperatures was CuO. The crystallization of Ni–P took place at 280–360 °C with the formation of Ni and Ni₃P. The final compounds were NiO, Ni₂P and Ni₃(PO₄)₂. After complete oxidation of the carbon fibers, copper and nickel-oxidized microtubes were obtained. Besides, while copper reduced the temperature of the fiber oxidation, nickel coatings increased the minimum temperature needed for this reaction.     

High temperature corrosion of cobalt in SO2

The reaction of cobalt in SO₂ has been studied in the temperature range 800–1000°C and at SO₂ pressures from 10 to 760 torr. Reaction kinetics have been studied by thermogravimetry, while the reacted specimens have been characterized by means of optical metallography, scanning electron microscopy, and electron microprobe analysis. The reaction involves formation of cobalt oxide (CoO), cobalt sulphides, and probably cobalt sulphate. The latter compound is formed at the lower temperature due to the presence of oxygen impurities in the SO₂. The relative importance of formation of the different reaction products is a function of temperature and the partial pressure of SO₂ (and O₂). At sufficiently high temperatures and reduced SO₂ pressure, CoO is the only reaction product. Reaction kinetics vary with reaction conditions. The amount of reaction goes through a maximum at about 920°C at 1 atm.SO₂. The reaction mechanism is interpreted in terms of the stability diagram of the Co-O-system.     

Cyclic Oxidation and Hot Corrosion Behavior of Y/Cr-Modified Aluminide Coatings Prepared by a Hybrid Slurry/Pack Cementation Process

Y/Cr-modified aluminide coatings were prepared on a Ni-base superalloy K417G using a hybrid slurry/pack cementation process. The coatings consisted of a NiAl layer with dissolved Cr and Y. The microstructures and high temperature corrosion behavior of the coatings were characterized using SEM/EDS, XRD, EPMA and SIMS. Cyclic oxidation tests at 1000 °C for 200 h were carried out in air. The results indicated that specimens coated by either the Y/Cr-modified aluminide coatings or the simple aluminide coatings exhibited better oxidation resistances than the cast alloy. The Y/Cr-modified aluminide coatings possessed lower oxidation rates and better degradation resistance than the simple aluminide coatings during the oxidation tests. Furthermore, the alumina scales formed on the Y/Cr-modified aluminide coatings were considerably more adherent than those on the simple aluminide coatings during the thermal cycling. The hot corrosion tests consisted of applying a 25 wt% K₂SO₄ +75 wt% Na₂SO₄ salt mixture to the specimens and exposing at 900 °C. The Y/Cr-modified aluminide coatings showed the longest service life compared with the cast alloy and aluminide coatings, which suffered significant sulfur attack. After 200 h, the Y/Cr-modified aluminide coatings were still protective.     

Besonderheiten im Korrosionsverhalten hochchrom- und molybdänhaltiger Legierungen in heißer 92, 5%iger Schwefelsäure

Peculiarities in the corrosion behaviour of high chromium and molybdenum containing alloys in hot 92.5% sulfuric acid In laboratory tests at temperatures above 50°C unusual high corrosion rates of passivating stainless steels and nickel alloys containing more than 26% Cr were observed in 92.5% sulphuric acid. In order to investigate the cause of this phenomenon further corrosion tests and additional chemical analyses were performed. The H₂SO₄ concentration tested displays a relative maximum of the electrical conductivity, the reason being a stronger dissociation of the sulfuric acid. Electrochemical investigations revealed an enhanced activity of the cathodic reactions which lead to higher corrosion rates. The cathodic reactions are strongly dependend on alloy constitution with special emphasis on the contents of Cr, Ni and Mo. Mo containing stainless steel show potential oscillations (of the open circuit potential) between ?50 and +550 mV_H. These alloys corrode under development of SO₂ (reduction of H₂SO₄ molecules) and formation of several sulfur compounds with different oxidation numbers (6+ and 2?). Alloys with chromium contents above 26% develop additionally hydrogen gas due to a lower hydrogen overvoltage of these alloys. With increasing nickel content the overvoltage for the reduction reaction of H₂SO₄ molecules will be lowered. This fact results in an elevation of the exchange current density for the Alloy NiCr45 and therefore to the highest corrosion rate observed. Alloy B-2 shows the best resistance, i.e. very low corrosion rates. Obviously high levels of molybdenum can compensate the influence of nickel on the overvoltage of the reduction reaction or even hinder the cathodic reaction.     

Transient Oxidation of Cu-5at.%Ni(001): Temperature Dependent Sequential Oxide Formation

The transient oxidation stage of a model metal alloy thin film was characterized with in situ ultra-high vacuum (UHV) transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and analytical high-resolution TEM. We observed the formations of nanosized NiO and Cu₂O islands when Cu-5at.%Ni(001) was exposed to oxygen partial pressure, $ {\text{pO}}_{ 2} = 1 \times 10^{ - 4} \,{\text{Torr}} $ and various temperatures in situ. At 350 °C epitaxial Cu₂O islands formed initially and then NiO islands appeared on the surface of the Cu₂O island, whereas at 550 °C NiO appeared first. XPS and TEM revealed a sequential formation of NiO and then Cu₂O islands at 550 °C. The temperature-dependent oxide selection may be due to an increase of the diffusivity of Ni in Cu with increasing temperature.