Corrosion Behavior of Y–Al Alloys in a H2/H2S/H2O Gas Mixture at 800–950°C

The corrosion behavior of pure Y and two Y–Al alloys containing 5 and 10 wt.% Al was studied over the temperature range 800–950°C in a H₂/H₂S/H₂O gas mixture. Both alloys had the two-phase structure of Y+Y₂Al. With the exception of Y–10Al, for which a kinetics inversion was observed between 800°C and higher temperatures (T 850°C), the parabolic rate constants generally increased with increasing temperature, but decreased with increasing Al content. The scales formed on pure Y and the Y–Al alloys were single but heterophasic, consisting of mostly Y₂O₃ and minor Y₂O₂S. XRD results showed no evidence of Al₂O₃ and pure sulfides. The formation of Y₂O₃ and Y₂O₂S on Y–10Al at 800°C resulted in a subsurface phase transformation from Y+Y₂Al to YAl₂ and broke the structural integrity of the scale, being responsible for the fast corrosion rate.     

Formation of Z-Ti50Al30O20 in the sub-oxide zones of γ-TiAl-based alloys during oxidation at 1000°C

The oxidation behaviour of γ-TiAl and α₂-Ti₃Al+γ-TiAl alloys was studied at 1000°C in pure oxygen up to 250 h. All alloy coupons were polished to a 1 μm finish prior to oxidation. A series of γ-Ti₄₈Al₅₂ coupons polished to a 600-grit finish were also prepared. The type of scale which formed during oxidation and the consequent subsurface changes in the alloy were found to depend on the presence and amount of α₂ in the starting alloy microstructure, surface finish, and oxidation time. The γ-alloy with a 600-grit finish formed a protective Al₂O₃-rich scale during the early stages of oxidation, whereas the same alloy with a 1 μm finish formed a less protective Al₂O₃+TiO₂ scale. In the former case, a continuous subsurface layer of Z-Ti₅₀Al₃₀O₂₀ formed from γ by essentially equal amounts of oxygen enrichment and aluminium depletion; essentially no titanium diffusion was required. With continued oxidation, the continuous Z-layer destabilized at the Z/γ interface into a mixture of α₂(O)+Z. The presence of α₂(O)+Z in the subsurface zone always corresponded to the growth of a non-protective Al₂O₃+TiO₂ scale, suggesting that α₂(O) precipitation initiates the breakdown of the initially formed Al₂O₃-rich scale. The subsurface zone in the γ alloy with a 1 μm finish varied, containing either internal Al₂O₃ and no Z phase or α₂(O)+Z. The α₂+γ alloys formed a non-protective Al₂O₃+TiO₂ scale and subsurface zone of α₂(O)+Z at all the times studied. The discontinuous distribution of γ in the two-phase alloys prevented formation of a continuous Z layer. The Z phase was determined to be metastable on the basis of equilibration experiments with Ti–Al–O alloys.     

The oxidation of a Co–15 wt% yttrium alloy in 1 atm O2 at 600–800°C

The oxidation of a cobalt-based alloy containing 15 wt% yttrium (Y) as well as of pure yttrium has been studied in 1 atm of pure oxygen at 600–800°C. The alloy always corrodes faster than pure cobalt and follows the parabolic rate law only approximately, forming an external scale of cobalt oxide which in its inner section contains particles of yttrium oxide. Beneath the external scales, a region of internal oxidation of yttrium having a depth much larger than predicted using the known solubility and diffusivity of oxygen in pure cobalt is also present. This anomalous depth is attributed to enhanced oxygen diffusion in the alloy along preferential paths associated with the localized internal oxidation of yttrium. No yttrium depletion is observed in the alloy beneath the front of internal oxidation. The scaling behavior of this alloy is interpreted by taking into account the limited solubility of yttrium in cobalt and the presence of the intermetallic compound Co₁₇ Y₂.     

The Corrosion of Titanium Aluminides in H2/H2S/H2O Atmospheres at 800–1000°C

The corrosion behavior of three Ti–Al intermetallics containing 20, 30, and 40 wt.% Al was studied over the temperature range 800–1000°C in a H₂/H₂S/H₂O gas mixture. Ti–20Al and Ti–40Al alloys had the single-phase structure of Ti₃Al and TiAl, respectively, while Ti–30Al was a two-phase mixture of Ti₃Al+TiAl. The corrosion kinetics followed the parabolic rate law in all cases, regardless of temperature and alloy composition. The parabolic rate constants increased with increasing temperature, but decreased with increasing Al content. The Ti–40Al alloy exhibited the best corrosion resistance among all alloys studied. The scales formed on Ti–Al intermetallics were heterophasic and duplex, consisting of an outer-scale layer of pure -TiO₂ and an inner layer of -TiO₂ with minor amounts of -Al₂O₃ and Ti_l-xS. The amount of -Al₂O₃, which increased with increasing Al content, is responsible for the reduction of the corrosion rates as compared with those of pure Ti oxides.     

The corrosion behavior of Fe-Al alloys in H2/H2S/H2O atmospheres at 700–900°C

The corrosion behavior of five Fe-Al binary alloys containing up to 40 at. % Al was studied over the temperature range of 700–900°C in a H₂/H₂S/H₂O mixture with varying sulfur partial pressures of 10^–7–10^–5 atm. and oxygen partial pressures of 10^–24–10^–2° atm. The corrosion kinetics followed the parabolic rate law in all cases, regardless of temperature and alloy composition. The parabolic rate constants decreased with increasing Al content. The scales formed on Fe-5 and –10 at.% Al were duplex, consisting of an outer layer of iron sulfide (FeS or Fe_1–xS) and an inner complex scale of FeAl₂S₄ and FeS. Alloys having intermediate Al contents (Fe-18 and –28 at.% Al) formed scales that consisted of mostly iron sulfide and Al₂O₃ as well as minor a amount of FeAl₂S₄. The amount of Al₂O₃ increased with increasing Al content. The Fe 40 at.% Al formed only Al₂O₃ at 700°C, while most Al₂O₃ and some FeS were detected at T800°C. The formation of Al₂O₃ was responsible for the reduction of the corrosion rates.     

Corrosion of Fe-(4.8, 9.2, 14.3) Wt%Al alloys at 700 and 800 °C in N2/H2O/H2S gases

Fe-(4.8, 9.2, 14.3)wt%Al alloys were corroded at 700 and 800 °C for up to 70 h in 1 atm of N₂/H₂O and N₂/H₂O/H₂S gases. Oxidation prevailed in N₂/H₂O gases. Fe-(4.8, 9.2)Al alloys formed a duplex scale that consisted of an outer iron oxide layer and an inner (Fe, Al, O)-mixed layer. The Fe-14.3Al alloy formed a thin layer consisting of α-Al₂O₃. Sulfidation dominated in N₂/H₂O/H₂S gases, resulting in rapid corrosion. Fe-(4.8, 9.2)Al alloys formed a duplex scale that consisted of an outer FeS layer and an inner (Fe, Al, S, O)-mixed layer. The high growth rate of FeS impeded the formation of a continuous, protective aluminium-rich oxide. The Fe-14.3Al alloy formed a thin layer consisting of α-Al₂O₃ that was incorporated with a bit of sulfur.     

Roles of Nb on the oxidation characteristics and oxide scale evolution of Fe-25Cr-35Ni-2.5Al-XNb alloys at 1000°C and 1100°C

The oxidation characteristics of Fe-25Cr-35Ni-2.5Al-_XNb (0, 0.6, and 1.2 wt%) alumina-forming austenitic alloys at 1000°C and 1100°C in air were investigated. Results show that Nb has an important effect on the high-temperature oxidation resistance. A bilayer oxide scale with a Cr₂O₃-rich outer layer and Al₂O₃-rich internal layer forms on the surface of the Nb-free alloy and exhibits a poor oxidation resistance at 1000°C and 1100°C. With Nb addition, both the 0.6Nb-addition and 1.2Nb-addition alloys exhibit better oxidation resistance at 1000°C. Because of the third element effect, Nb addition reduces the critical Al content and forms a single external protective Al₂O₃ scale, which greatly improves the oxidation resistance. After oxidation at 1100°C, niobium oxides (mainly Nb₂O₅) are formed on the surface of the 1.2Nb-addition alloy and destroy the integrity of the Al₂O₃ scale, which causes the formation of Cr-rich oxide nodules and eventually develops to be a loose bilayer oxide scale with NiCr₂O₄, Cr₂O₃, and Fe₂O₃ outer layers and Al₂O₃ inner layer.     

Improved oxidation and hot corrosion resistance of Ta-doped NiAlY alloy at 750°C

The oxidation and hot corrosion behaviors of the NiCrAlY, NiAlY, and Ni–xTa–Al–Y alloys (x = 1, 3, 5, and 10 wt%) were investigated at 750°C. The doped Ta promoted the formation of the protective α-Al₂O₃ scales. The NiTaAlY alloys exhibited an improved oxidation resistance compared with the NiAlY alloy. Under the NaCl-induced hot corrosion test, the addition of Ta reduced the consumption of Al and inhibited the internal corrosion of the alloys. The Ni–xTa–Al–Y alloys (x = 1, 3, 5, and 10 wt%) showed better resistance to the NaCl-induced hot corrosion. Moreover, the hot corrosion mechanism of the tested alloys was also discussed.     

Oxidation protection of 20Cr-25Ni-Nb stainless steel at 1300°C

Enhanced environmental protection of chromia-forming advanced metallic alloys at normal operating temperature, typically 900°C, may be provided by two well-established approaches—incorporation of reactive elements into the protective oxide scale or an amorphous ceramic coating acting as a diffusion barrier. The continued effectiveness of such approaches, namely by cerium and yttrium ion implantation and with a vapor deposited amorphous silica coating, in reducing oxidation of 20Cr-25Ni-Nb stainless steel in a carbon-dioxide-based environment has been examined during 0.5 and 1 hr transients to 1300°C. The influence of pre-oxidation of the ion-implanted, silica-coated, and uncoated steel for extended periods in the same environment at 825–900°C has also been established.     

The Corrosion Behavior of Sulfidation-Resistant Fe–Mo–Al Alloys in H2/H2S Atmospheres at 900°C

The corrosion behavior of Fe–22Mo–10Al (a/o, atom %),Fe–20.5Mo–15.7Al, and Fe–10Mo–19Al was examined inflowing H₂/H₂S gases of 4 Pa sulfur partial pressureat 900°C. Al₂O₃ was stable on all the alloys inthe atmospheres investigated. Fe–22Mo–10Al andFe–20.5Mo–15.7Al reacted slowly, following the parabolic ratelaw. Multilayered reaction products were formed on these alloys and it isuncertain which layer(s) provided the protection. Fe–10Mo–19Alreacted even more slowly, exhibiting two-stage parabolic kinetics. Duringthe early stage of this alloy's reaction, a preferential reaction zone,consisting of an oxide mixture, possibly Al₂O₃+FeAl₂O₄,and nonreacting Fe₃Mo₂, provided the protection. Duringthe later reaction stage, the formation of a continuous, externalAl₂O₃ layer further decreased the alloy reaction rate.     

Solubility of sulfur in pure Cr2O3 at 1000°C

Sulfur solubility in pure Cr₂O₃ was measured over a large range of oxygen (10^–6–10^–12 atm) and sulfur (10^–2 -10^–16 atm) partial pressures at 1273 K. Different methods of analysis were used; it was found that the neutronactivation technique produced the most reliable and reproducible results. The results obtained showed that the limiting solubility of sulfur in Cr₂O₃ varied between 16–93 ppm for the range of oxygen and sulfur pressures used in this study. The solubility was found to vary, depending on the combined effect of and . For a given , the amount dissolved increases with an increase of . An empirical equation, was found to best fit the experimental results and indicates a stronger influence of than of . A specific dissolution mechanism connot be formulated from this equation. However, a number of possibilities have been proposed, but none of these specific mechanisms seems to fit the experimental data.     

Scale formation on γ-TiAl during oxidation at 800 and 900°C in air and in He + 20% O2

The oxide scale formation on -TiAl at 800 and 900°C was studied using high temperature X-ray diffraction as anin situ-method. The experiments were performed in air and in He with 20 vol.% O₂. The formation of alumina in the form of -Al₂O₃ and of TiO₂ in the form of rutile was observed in both atmospheres and the formation of TiN was detected in air. Depending on the atmosphere the diffraction peaks of two different additional phases were detected, which do not exist in any data base nor in the Ti-Al-O phase diagram. One of them, the Z-phase, appears in He with 20 vol.% O₂ and the other, the X-phase, in air. The Zphase was also found at room temperature after oxidation at 900°C in air. The growth of both phases, X and Z, starts immediately with the oxidation process and follows the parabolic rate law.     

Long-time oxidation of iron at 1100°C in air as a function of time

Long-time oxidation experiments, 180 hr, 360 hr, and 660 hr, were carried out on AREMA iron in ambient air at 1100°C. Fe samples, 3 mm in diameter and 39 mm in length, were completely oxidized. Weight-gain measurements, electron microanalysis, and X-ray diffraction (XRD) analysis were used as experimental techniques. The completely oxidized samples remained cylindrical, compact, and solid. The content of hematite in the scale mass increased exponentially from 88.1% (180 hr) to 88.4% (660 hr), with the limit value 100% at t . No wustite and no pure Fe traces have been found in completely oxidized samples.     

The Oxidation of a Directionally Solidified Ni-Al-Cr3C2 Alloy at 1100 and 1200°C in Oxygen

The oxidation behavior of a directionallysolidified Ni-Al-Cr₃C₂ alloy ofover all composition Ni-12.3Cr-6.9Al-1.8C (wt.%) hasbeen investigated at 1100 and 1200°C under 1 atmoxygen. Experiments have also been carried out on specimens having the samecomposition but with a nonaligned structure. At1100°C, in both cases and unlike conventionalnickel-base superalloys with the same chromium andaluminum contents, aluminium was found to oxidize internallybeneath an external Cr₂O₃ scale.Although the volume fraction of the internalprecipitates was significant, they showed no tendency tocoalesce into a compact subsurface layer, but formed randomly distributed clustersin the alloy matrix. The kinetics of oxidation andmorphologies of the oxide scales were not substantiallyaffected either by thermal cycling or by the alloy microstructure. At the higher temperature,continuous Al₂O₃ scales formedbeneath thick layers of transient nickel andnickel-chromium-rich oxides; no internal precipitationof aluminum-rich oxides was observed. However, internal degradation of thedirectionally solidified specimens at 1200°C wasquite significant, due to in situ oxidation of primarycarbides. The multilayered scales formed at 1200°C spalled extensively on cooling as a consequenceof loss of contact, starting preferentially at theintersections of the Cr₂C₃ fiberswith the alloy-scale interface. The observed behaviorcan be attributed to a reduction in the availability of chromiumbecause of the multiphase structure of the alloy; this,in turn, resulted in an increase in the flux of oxygeninward, leading to internal oxidation of aluminum at 1100°C. The almost exclusive externaloxidation of aluminum becomes possible at 1200°C,probably because of an increase in the diffusivity ofaluminum in the alloy matrix.     

The influence of the surface ion implantation of aluminium and yttrium upon the oxidation behaviour of a Fe-15% Cr-4% Al Fecralloy® stainless steel,in air,at 1100°C

An increase in the concentration of aluminium in a 0.25 μm thick surface layer of a 15%Cr-4%Al yttrium free Fecralloy steel by the implantation of up to 10¹⁷ ions cm^?2 had no significant influence upon the oxidation behaviour of the steel, in air, at 1100°C. An alloy addition of 0.86% yttrium reduced the attack and oxide spallation for at least 3271 h, while the surface implantation of 3 × 10¹⁵ yttrium ions cm^?2, together with 10¹⁷ aluminium ions cm^?2 improved the oxidation behaviour of the steel for only a limited period (784 h). It is concluded that yttrium alloy additions in the Fecralloy steels exert their beneficial influence on the oxidation behaviour within the oxide film rather than the subscale.     

The Oxidation of Fe-2 and 5 at.% Y Alloys at 600-800°C in Air

The oxidation of Fe-Y alloys containing 2 and 5at.% Y and pure iron has been studied at 600-800°Cin air. The oxidation of pure iron follows the parabolicrate law at all temperatures. The oxidation of Fe-Y alloys at 600°C approximatelyfollows the parabolic rate law, but not at 700 and800°C, where the oxidation goes through severalstages with quite different rates. The oxide scales on Fe-2Y and Fe-5Y at 700 and 800°C arecomposed of external pure Fe oxides containingFe₂O₃,Fe₃O₄, and FeO, with FeO being themain oxide and an inner mixture of FeO andYFeO₃. The scales on Fe-2Y and Fe-5Y at 600°C consist ofFe₂O₃,Fe₃O₄, andY₂O₃, with a minor amount of FeO.Significant internal oxidation in both Fe-Y alloysoccurred at all temperatures. The Y-containing oxidesfollow the distribution of the original intermetalliccompound phase in the alloys. The effects of Y on theoxidation of pure Fe are discussed.     

Oxidation behavior of oxidation protective coatings for C/C-SiC composites at 1 500 °C

Porous carbon/carbon preforms were infiltrated with melted silicon to form C/C-SiC composites. Three-layer Si-Mo coating prepared by slurry painting and SiC/Si-Mo multilayer coating prepared by chemical vapor deposition(CVD) alternated with slurry painting were applied on C/C-SiC composites, respectively. The oxidation of three samples at 1 500 °C was compared. The results show that the C/C-SiC substrate is distorted quickly. Three-layer Si-Mo coating is out of service soon due to the formation of many bubbles on surface. The mass loss of coated sample is 0.76% after 1 h oxidation. The sample with SiC/Si-Mo multilayer coating gains mass even after 105 h oxidation. SiC/Si-Mo multilayer coating can provide longtime protection for C/C-SiC composites and has excellent thermal shock resistance. This is attributed to the combination of dense SiC layer and porous Si-Mo layer. Dense SiC layer plays the dual role of physical and chemical barrier, and resists the oxidation of porous Si-Mo layer. Porous Si-Mo layer improves the thermal shock resistance of the coating.     

In vitro biocompatibility of titanium-nickel alloy with titanium oxide film by H2O2 oxidation

Titanium oxide film with a graded interface to NiTi matrix was synthesized in situ on NiTi shape memory alloy（SMA） by oxidation in H2O2 solution. In vitro studies including contact angle measurement, hemolysis, MTT cytotoxicity and cell morphology tests were employed to investigate the biocompatibility of the H2O2-oxidized NiTi SMAs with this titanium oxide film. The results reveal that wettability, blood compatibility and fibroblasts compatibility of NiTi SMA are improved by the coating of titanium oxide film through H2O2 oxidation treatment.