Oxidized barrier thin film from plasma grown polysiloxane coating over austenitic stainless steel

Polysiloxane films were deposited on an AISI316L stainless steel by remote plasma assisted chemical vapour deposition from 1.1.3.3-tetramethyldisiloxane. Thicknesses up to 5 μm were developed at a maximum growth rate of 400 nm min^− 1. The organosilicon films were then treated by remote plasma assisted oxidation and thermal treatment under air.The remote plasma assisted oxidation modified the surface composition of the coatings but left the bulk properties unchanged. The thermal treatment allowed a full mineralization of the coatings. Below 450 °C the mineralization was partial and oxygen could not reach the coating core. This oxygen diffusion limitation was removed only for temperature of circa 600 °C. The final surface composition appeared to be SiO_1.8 which might represent a structure close to amorphous silica. After the most severe thermal treatment (600 °C), neither desquamation nor cracking of the deposit occurred. The final composite is a steel substrate covered by a coherent layer of silica glass material. The steel underneath remained non-oxidized which testified of a strong diffusion barrier role vs. O₂.     

Oxidation kinetics of an Fe-31.8Mn-6.09Al-1.60Si-0.40C alloy at temperatures from 600 to 900 °C

An Fe-31.8Mn-6.09Al-1.60Si-0.40C alloy was subjected to thermogravimetric tests, in atmospheres of pure oxygen and synthetic air. The alloy showed good oxidation resistance, especially at 600 and 700 °C in air and at 600 °C in oxygen. Owing to autocorrelation identified in the results, kinetic data were analyzed by the statistical method of Cochrane and Orcutt. Parabolic kinetic behaviour was observed for the Fe-Mn-Al-Si-C alloy oxidation process in air at 600, 700 and 800 °C and in pure oxygen at 900 °C.     

Thermal treatment effect on structure, electrical conductivity and transient photoconductivity behavior of thiourea modified TiO2 sol-gel thin films

Thiourea modified nanocrystalline titanium dioxide (TiO₂) thin films were prepared by sol-gel route and were thermally treated at five different temperatures (400, 500, 600, 800 and 1000 °C). The films were studied using GIXRD, PIGE and UV-vis spectroscopy. It was observed that the anatase to rutile phase transformation of TiO₂ was inhibited by the thiourea modification. The transmittance of the modified films appeared reduced which was attributed both to the modification of TiO₂ with thiourea and the light scattering in the films. The dark conductivity and the transient photoconductivity of the modified TiO₂ sol-gel thin films were studied in vacuum and in air. The environment does not influence significantly the dark conductivity, because of the almost equivalent competition between oxygen and water adsorption. The photoconductivity reaches high values for all samples in both environments, with the sample treated at 500 °C to present the highest value. The larger values in vacuum can be attributed to the reduced amount of adsorbed oxygen at the surface, which acts as electron scavenger.     

The initial oxidation of iron at 200° and 300° C and the effect of surface sulfur

The growth of thin (0–200 Å) oxide films on iron at 200 and 300 °C has been studied as a function of time and oxygen partial pressure using proton-induced X-ray emission (PIXE) and Auger electron spectroscopy (AES). Oxidation was found to be initially retarded by sulfur which had segregated onto the iron surfaces during preoxidation annealing, but only if the iron surface contained the maximum or near-maximum sulfur coverage (ca. one-half monolayer). During and immediately following the oxygen-sulfur interaction, oxide buildup appeared to be limited by a surface reaction (adsorption, ionization, or dissociation). For most of the oxidation period, pressure-dependent logarithmic oxide growth was observed at 200°C, and pressure-independent parabolic oxide growth at 300°C. Interpretation of the data indicated that oxide growth at 200°C may be limited by quantum mechanical tunneling of electronic species through the previously formed oxide film, and oxide growth at 300°C may be limited by ionic diffusion through the previously formed oxide film. Comparison of AES and PIXE data indicated that the oxide films formed at 200°C were uniform in thickness over the surface of the metal, whereas films formed at 300°C had relatively thin areas where sulfur had remained.     

The Oxidation of Two Fe-Y Alloys Under Low Oxygen Pressures at 600-800°C

The corrosion of pure yttrium and of two Fe-basealloys containing approximately 15 and 30 wt.% Y wasstudied at 600-800°C in H₂-CO₂mixtures providing an equilibrium oxygen pressure of10^-24 atm at 600°C and 10^-20 atm at 700 and800°C. The corrosion of yttrium under these lowoxygen pressures resulted in the growth ofY₂O₃ scales and presented twoapproximately parabolic stages at 800°C, while at 600-700°C it was faster andnonprotective. The corrosion of the two alloys followedapproximately the parabolic rate law, except for Fe-15Yat 600°C which oxidized nearly linearly. At 600 and700°C, when the gas-phase oxygen pressure was in thefield of stability of iron oxide, the alloys formed alsoa thin external Fe₃O₄ layer, whileat 800°C, when the oxygen pressure was below thestability of FeO, a thin outermost layer of pure iron wasobserved to form. Under all conditions a region ofinternal oxidation formed in the alloy, in which theyttrium-rich phases were transformed into a mixture ofiron metal and oxides, which included double Fe-Yoxides as well as Y₂O₃. Themicrostructure of the internal-oxidation region followedclosely that of the original alloys, which moreover didnot undergo any yttrium depletion. These results are examinedby taking into account the low solubility of yttrium iniron and the presence of intermetallic compounds in thealloys.     

Oxidation kinetics,breakaway oxidation,and inversion phenomenon in 9Cr-1Mo steels

The oxidation behavior of 9Cr-1Mo ferrritic steels has been studied in air, oxygen, and steam at 1 atm pressure at various temperatures. Long-term experiments in air were carried out from 500–800°C by measuring the weight gains by interrupting the experiment at regular intervals of time. Short-term experiments in oxygen from 500–950°C and in air at 900 and 950°C were carried out by continuous recording of weight gain versus time in a continuousrecording thermogravimetric balance. Short-term experiments in steam were carried out using a special atmosphere furnace attached to the thermogravimetric balance. In air/oxygen, the weight gains at 700°C were lower than those at 600°C, while in steam, the weight gains at 800°C were lower than those at 700°C. This inversion phenomenon was observed for all the three steels viz. 9Cr-1Mo (high Si), 9Cr-1Mo (low Si), and 9Cr-1Mo-Nb steel. Examination of the oxide scales was carried out using SEM/EDAX, AES/ESCA, and X-ray diffraction techniques, and a mechanism is proposed for the occurrence of the inversion phenomenon.     

High-temperature oxidation of titanium diboride of different purity

Oxidation in air of TiB₂ powders of two different types in the temperature range from 100 to 900°C as well as of compact samples produced from them (at 600–1200°C) was studied by DTA, thermogravimetry, X-ray analysis, AES, EPMA, and SEM methods. The admixture elements in powders were found to prevent oxygen from enriching the diboride surface. In compact TiB₂ samples impurities form diffusion barriers during oxidation at the matrix/scale and scale/gas interfaces as well as facilitate sintering of the scale being formed.     

The Corrosion of Co-15 wt.% Y at 600-800°C in Sulfidizing-Oxidizing Atmospheres

The corrosion of Co-15 wt.% Y has been studiedat 600-800°C inH₂-H₂S-CO₂ mixturesproviding a sulfur pressure of 10^-8 atm at600-800°C and of 10^-7 atm at 800°Cand an oxygen pressure of 10^-24 atm at 600°C and of10^-20 atm at 700-800°C. The corrosionrates in such sulfidizing-oxidizing atmospheres werecompared with those of pure cobalt and yttrium. Theaddition of yttrium to cobalt is only slightly beneficial, sincefor a yttrium content of 15 wt.% the corrosion rate isreduced quite significantly with respect to pure cobaltat 800°C under 10^-7 atm S₂,only to a limited extent at 600°C, and even slightlyincreased at 700°C. Moreover, the alloy corrodesconsiderably more rapidly than pure yttrium at800°C, when the latter behaves protectively. At 600 and 700°C, yttrium exhibitedbreakaway behavior, while the alloy corroded morerapidly than yttrium at short times, but more slowly atlong times. Under all conditions, except at 800°Cunder 10^-8 atm S₂, the alloy formsan external layer of cobalt sulfide overlying anintermediate region of very complex compositioncontaining a mixture of the compounds of the two metalsand an innermost region of internal attack containing compoundsof yttrium with both oxygen and sulfur. Thus, cobalt canstill diffuse through the intermediate region to formthe outer cobalt-sulfide layer at nonnegligible rates. The scaling behavior of the Co-15% Yalloy is discussed by taking into account the limitedsolubility of yttrium in cobalt as well as the presenceof an intermetallic Co-Y compound in thealloy.     

Combustion chemical vapour deposition of Al2O3 films: Effect of temperature on structure, morphology and adhesion

Alumina films were synthesized on Si(100) substrates at different temperatures in the range of 600 to 900 °C using open atmosphere combustion chemical vapour deposition (C-CVD) technique. A custom made premixed-diffusion type burner with an extra coaxial oxygen inlet close to the burner mouth enabled variation of deposition temperature from 600 to 900 °C in steps of 100 (± 10) °C. The presence of γ- and θ-alumina phases were observed in films synthesized in the temperature range of 600-800 °C, whereas at 900 °C single phase θ-alumina films were obtained. Adherent coatings were obtained at temperatures ≥ 700 °C. The grain size and roughness of the films increased with deposition temperature. The films underwent two types of adhesion failures, a continuous ductile perforation and a tensile type hertzian crack due to the presence of interfacial oxide layer, during scratch test. The presence of SiO₂ interfacial layer between substrate and film was discerned from ellipsometric studies.     

Iridium coating prepared on rhenium substrate by electrodeposition in molten salt in the air atmosphere

Compact and smooth iridium coating was obtained on the surface of the rhenium substrate by electrodeposition from molten salts of chlorides at the temperature of 600 °C in the air. The deposition rate is about 25 μm/h at cathodic current density of 25 mA/cm². The iridium coating has a columnar structure with preferential growth orientation of <111>. The coating/substrate interface exhibits excellent adherence with no evidence of delamination, cracks, or other defects. No obvious diffusion layer is found within the as-deposited coating. The microhardness of the coating is 442 kg/mm² and the bond strength between the coating and substrate is higher than 16 MPa.     

Corrosion Behavior of a Sputtered K38G Nanocrystalline Coating with a Solid NaCl Deposit in Wet Oxygen at 600 to 700°C

A nanocrystalline coating of K38G alloy was obtained by means of magnetron sputtering. The corrosion behavior of cast K38G alloy and its sputtered nanocrystalline coating with a solid NaCl deposit in dry air or wet oxygen at 600 and 700°C have been studied and compared. The results indicated that the mass gain of the coating is higher than that of the cast alloy at 600 and 700°C. Therefore, nanocrystallization reduced the corrosion resistance at these temperatures. Furthermore, the mass gains of the coating and the cast alloy with a NaCl deposit in wet oxygen were less than that with a NaCl deposit in dry air. The relevant corrosion mechanisms are discussed.     

Oxidation of iron at 400-600 °C in dry and wet O2

The oxidation of iron in dry and wet O₂ at 400-600 °C has been re-investigated using gravimetry, SEM/EDX, XRD and FIB. In the presence of O₂, water vapour accelerates iron oxidation at 500 and 600 °C. At 400 and 500 °C the magnetite layer is duplex and exposure to water vapour results in the formation of blades on top of a fine-grained hematite layer. At 600 °C it results in a surface without needles and blades. The increased oxidation rate at 500 and 600 °C is attributed to a smaller grain size in the hematite layer resulting in faster ion transport.     

Internal-oxide-band formation during oxidation of Ag-Mg alloys

Ag-3a/oMg was oxidized in air over the range of 400–900°C. Internal-oxide bands of MgO formed approximately parallel to the surface, the first band appearing at some finite, but irregular depth below the surface. The region between the surface and the first band appeared to be free of precipitates, but TEM showed that very small clusters, about 50 Å in diameter, formed in the PFZ, causing significant hardness (greater than 300 VHN). The clusters contain more oxygen than that corresponding to stoichiometric MgO. The hardness between oxide bands was also high, but not as high as in the PFZ. The kinetics of thickening of the internal-band region followed the parabolic rate law between 400 and 700° C, with departures from the parabolic law occurring at higher temperatures. The activation energy for the parabolic rate constants was 19.4 Kcal/mol, a value less than the total for oxygen diffusion and oxygen dissolution. The reaction front was planar and parallel to the surface prior to band formation at temperatures of 400–600° C. Nucleation of the first band resulted in nonplanar and nonparallel oxide. Little or no correlation existed between grain boundaries and oxide formation. Nodules of virtually pure silver formed on the surface initially at grain boundaries and subsequently within grains. Nodule formation is attributed to stress-enhanced (resulting from strains associated with precipitation) diffusion of silver to the surface via dislocation pipes. Internal-band formation is discussed in terms of prior data in the literature and various models. It is thought that stress effects (induced by precipitation), nucleation, and clustering of oxygen with Mg play significant roles in causing internal-band formation.     

The oxidation of Co−Nb alloys under low oxygen pressures at 600–800°C

The oxidation of two Co–Nb alloys containing 15 and 30 wt.% Nb has been studied at 600–800° C in H₂–CO₂ mixtures providing an oxygen pressure of 10^–24 atm at 600°C and 10^–20 atm at 700 and 800°C, below the dissociation pressure of cobalt oxide. At 600 and 700°C both alloys showed only a region of internal oxidation composed, of a mixture of alpha cobalt and of niobium oxides (NbO₂ and Nb₂O₅) and at 700°C also the double oxide CoNb₂O₆, which formed from the Nb-rich Co₃Nb phase. No Nb-depleted layer formed in the alloy at the interface with the region of internal oxidation at these temperatures. Upon oxidation at 800°C a transition between internal and external oxidation of niobium was observed, especially for Co–30Nb. This corrosion mode is associated with the development of a single-phase, Nb-depleted region at the surface of the alloy. The corrosion mechanism of these alloys is examined with special reference to the effect of the low solubility of niobium in cobalt and to the relation between the microstructures of the alloys and of the scales.     

Study of the initial oxidation of a Ni-20Cr alloy in the temperature range 550–830°C: Influence of mechanical deformation

The influence of a uniaxial deformation on the initial oxidation rate of a Ni-20Cr polycrystalline alloy was studied between 550 and 830°C at 10^–4 Pa of oxygen. At 550, 650, and 750°C, it was shown that the defects induced by a mechanical predeformation accelerate chromium-oxide formation on the alloy surface. This homogeneous oxide film is characteristic of the low-pressure-oxidation conditions. The film appeared on the sample after various incubation periods for the experimental conditions used. Microanalyses (scanning Auger microanalyses, composition profiles) reveal that growth of this oxide is heterogeneous and that it needs preferential orientations with the substrate. Furthermore, near chromium-oxide areas, an incorporation of oxygen at concentrations higher than the solubility was noted. At 830°C, this chromium oxide does not grow, because the superficial defects disappear by thermal annealing and the oxygen incorporation does not apparently depend on mechanical deformation.     

The Role of Oxygen Uptake and Scale Formation on the Embrittlement of Vanadium Alloys

Vanadium alloys are of interest in fusion energy systems, however, their environmental durability is a major concern. Specimens of V–4Cr–4Ti were exposed to air and oxygen (10⁵,Pa), low pressure (10^–3–10^–6 Pa) oxygen and high purity He environments (10⁵–10¹ Pa) it at 500–700°C in order to characterize the surface oxide, determine oxidation kinetics and quantify effects on mechanical properties at 25 and 600°C. At low oxygen pressures (P_{O_2}.10^–5 Pa), linear reaction kinetics were measured for exposures up to 2000 hr and the data were used to develop a mathematical expression for the oxidation rate as a function of temperature and oxygen pressure. At higher pressures, linear–parabolic reaction kinetics were associated with high oxygen uptake and the formation of an external oxide layer. Room-temperature and 600°it C tensile ductility was reduced by these exposures, but specimens which formed an external oxide were found to retain some tensile ductility after exposure. However, similar specimens with an external oxide that were subsequently annealed for 2000 hr at 700°C became severely embrittled demonstrating that a surface oxide will not prevent degradation of this refractory alloy. Exposures in He were performed to determine the effect of total gas pressure on oxygen uptake.Dedicated to the Memory of Jackson H. DeVan.     

The corrosion of Ni3Al in a combustion gas with and without Na2SO4-NaCl deposits at 600–800°C

The corrosion behavior of Ni₃Al containing small additions of Ti, Zr, and B in combustion gases both with and without Na₂SO₄–NaCl deposits at 600–800°C has been studied for times up to four days. The corrosion of the saltfree Ni₃Al leads to the formation of very thin alumina scales at 600°C but of mixed NiO–Al₂O₃ scales containing also some sulfur compounds at higher temperatures, while the rate increases with temperature up to 800°C. The presence of the salt deposits considerably accelerates the corrosion rate, especially at 600 and 800°C. The duplex scales formed at 600°C are composed mostly of a mixture of NiO and unreacted salt in the outer layer and of alumina and aluminum sulfide with some nickel compounds in the inner layer. The scales grown at 700°C contain only one layer of complex composition, while those grown at 800°C are similar but have an additional outer layer containing similar amounts of nickel and aluminum. At 600 and 700°C NiSO₄ can be detected also in the salt layer. The samples corroded at 700°C and 800°C also show an Al-depleted zone containing titanium sulfide precipitates at the surface of the alloy. The hot corrosion of Ni₃Al involves a combination of various mechanisms, including fluxing of the oxide scale as well as mixed oxidation-sulfidation attack. At all temperatures Ni₃Al shows poor resistance to hotcorrosion attack as a result of the formation of large amounts of Ni compounds in the scales.     

Oxidation properties of Zr-Nb alloys at 500-600 °C under low oxygen potentials

Zr-Nb alloys with 1-10 wt.% Nb content were oxidized at 500-600 °C in the CO-CO₂ gas mixtures. The oxidation weight gain increased with Nb content and the kinetics except for Zr-1Nb alloy changed from cubic rate to linear one at 600 °C for a long period of time, 7 d. The cubic rate constant was almost insensitive to oxygen potential of oxidizing atmosphere. As the oxidation resistance deteriorated, the volume ratio of tetragonal to monoclinic zirconia phase and the relevant compressive stress in oxide film decreased with increase of Nb content. Before and after oxidation, Nb re-distribution could not be observed under the present experimental condition.     

High-Temperature Oxidation of an Aluminized NiCr Alloy formed by a Magnetron-Sputtered Al Diffusion Coating

Aluminium diffusion coatings were obtained on Ni–20Cr substrate by sputtering an aluminium film, followed by a two stage diffusion treatment in an argon inert gas atmosphere (first stage at 600°C, second at 900 or 1100°C). Aluminides obtained at 900°C and 1100°C are close to those obtained by pack cementation process with high aluminium activity. These diffusion coatings are able to develop alumina scales during isothermal oxidation at high temperatures, whereas the untreated substrate had a chromia-forming behaviour. The weight gain recorded at 1100°C on coated sample is then smaller than the one of uncoated NiCr at 950°C. Presence of chromium was detected in the diffusion coating and Cr-rich precipitates were observed at the diffusion coating/substrate interface. After oxidation at 900°C and 1100°C, only α-Al₂O₃ was revealed by XRD. An intermediate scale with a “whiskered” morphology could however be observed after 48 hr oxidation at 900°C. After 100 hr of oxidation at 1100°C, the Ni_xAl_y diffusion phases were no longer detectable and the upper part of the oxide scale spalled away during cooling. Large cavities appeared at the initial location of the diffusion coating/substrate interface.     

High-temperature oxidation of rhodium

The oxidation kinetics of Rh were measured in air at 1 atm. in the temperature range 600–1000°C. The oxidation weight gain proceeds logarithmically at the lower temperatures (600°C, 650°C) followed by a transition to power law behavior at the higher temperatures (800°C). The logarithmic growth kinetics result from thickening of a hexagonal Rh₂O₃ scale. The transition from logarithmic to power law growth kinetics occurs in the range 700–800°C, and reflects thickening of hexagonal and orthorhombic Rh₂O₃ scales. Above 800°C, the growth kinetics result from thickening of a predominately orthorhombic Rh₂O₃ scale. At 1000°C the oxide becomes volatile, leaving the metal surface exposed.