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A new technique for measuring the tensile properties of surface coatings at high temperatures is described. It is shown to be capable of measuring the properties of growing surface scales on metals and has been applied initially to the measurement of fracture strains of growing surface oxides on mild steel at high temperatures. The fracture strains determined ranged from 1.1×10−4 to 2.3×10−4 over the temperature range 600–900°C.

The effect of water and water vapor on the lifetime of Ni-based superalloy samples coated with a typical thermal barrier coating system—β-(Ni,Pt)Al bond coat and yttria stabilized zirconia (YSZ) top coat deposited by electron beam physical vapor deposition (EB-PVD) was studied. Samples were thermally cycled to 1,150 °C and subjected to a water-drop test in order to elucidate the effect of water vapor on thermal barrier coating (TBC) spallation. It was shown that the addition of water promotes spallation of TBC samples after a given number of cycles at 1,150 °C. This threshold was found to be equal to 170 cycles for the present system. Systems based on β-NiAl bond coat or on Pt-rich γ/γ′ bond coat were also sensitive to the water-drop test. Moreover, it was shown that water vapor in ambient air after minutes or hours at room temperature, promotes also TBC spallation once the critical number of cycles has been reached. This desktop spalling (DTS) can be prevented by locking up the cycled samples in a dry atmosphere box. These results for TBC systems confirm and document Smialek’s theory about DTS and moisture induced delayed spalling (MIDS) being the same phenomenon. Finally, the mechanisms implying hydrogen embrittlement or surface tension modifications are discussed.

Three kinds of hot rolled steel slabs, viz. high strength steel, bake hardened steel and low carbon steel, were oxidized isothermally between 1100 and 1250 °C for up to 2 hr in 1 atm of air and an 85%N2–10%CO2–5%O2 gas mixture. The steels were oxidized in a similar fashion in both the atmospheres. The oxidation process followed an initial linear rate law, which then gradually transformed to a nearly parabolic rate law. Thick, porous and nonadherent scales formed rapidly, due to the high oxidation temperature. The scales formed consisted of Fe2O3,(Fe2O3+Fe3O4), (Fe3O4+Fe2O3 +FeO) and (FeO+Fe3O4) from the outer surface. The presence of supersaturated oxygen beneath the scale resulted in grain boundary oxidation and the formation of internal oxide precipitates.

The kinetics of oxidation of copper powders in oxygen and in dry and humid air was investigated using thermogravimetric analysis (TGA). The extent of oxidation grew linearly with time until the weight-based thickness of the oxide film reached 0.13–1.22 nm, depending on the temperature. Between 30 and 90°C there was little difference between the kinetic curves observed in air and in oxygen, respectively. Higher humidity of the air resulted in an increased oxidation rate. Following the initial linear segment, the oxidation kinetics could be best described in terms of a logarithmic rate law between 30 and 45°C and in terms of a power law between 60 and 90°C. The activation energy for the initial linear stage was (44±2) kJ and for the subsequent oxidation (102±12) kJ. Delayed increases in oxidation rate were observed with a ca. 0.1-μm powder around 100°C, with a ca. 1-μm powder around 320°C, and with a < 10μm powder around 360°C. A three-stage model consisting of an initial linear stage, parabolic growth culminating in cracking of the oxide film, and subsequent re-start of the parabolic growth, gave good agreement with the experimental data. Whenever the powder is relatively uniform and the distribution of film-cracking times among the powder grains is narrow, e.g., within 23% of the median cracking time, an increase in the oxidation rate of the entire sample can be observed.

Under CO2 exposure at an intermediate temperature, typically 550 °C, 9Cr–1Mo steel forms a duplex oxide scale made of an outer magnetite layer and an almost-as-thick inner Fe–Cr rich spinel oxide layer. It is proposed that the inner Fe–Cr spinel layer grows according to a mechanism involving void formation at the oxide/metal interface. The driving force for pore formation is the outward magnetite growth: iron vacancies are injected at the oxide/metal interface then condense into voids. The fresh metallic surface made available is then oxidized by CO2, which diffuses fast through the scale. The physical aspects, the integrity and the nature of the scale are shown to be very dependent on the oxygen potential existing in the environment.

The influence of the electrode potential on the corrosion behavior of a series of Ni-base superalloys has been investigated in a (mole %) 90Na2SO4-10K2SO4 melt at 1173 K. Acidic fluxing occurs at positive potentials and basic fluxing at negative potentials. A protective scale is formed in an intermediate (neutral) potential range on high chromium-containing alloys such as IN-738LC, IN-939, IN-597, and IN-657. The breakthrough potentials for acidic and basic fluxing depend on the composition of the alloy. Alloys with low chromium contents such as IN-100 and IN-713LC do not form stable protective scales at any potential. Numerous sulfide phases have been identified in the scale and subscale, depending on potential, severity of attack, and material composition. NaCrS2 only forms under basic fluxing conditions. Its presence can therefore be considered as an indication that basic fluxing conditions have existed.

Studies of the simultaneous creep and oxidation of Fe-1Si and Fe-4Si alloys at a constant tensile stress of 16 N· mm−2 at 973–1073 K have shown that scales formed at oxygen partial pressures of 20–1013 mbar were thicker by a factor of 2 than those formed on uncrept specimens. Scales on uncrept alloys comprised alternate layers of wustite and fayalite, whereas scales on crept alloys exhibited an additional external layer of magnetite. Only intergranular oxidation (fayalite) was observed in uncrept alloys, but crept alloys showed both intra- and intergranular oxidation (silica). Uniquely nodular scales were formed only on the Fe-4Si alloy on crept and uncrept specimens. Oxidized, uncrept Fe-1Si showed a fine-grained ferrite substrate which was absent in the crept alloy. It is believed that oxide growth stresses stimulated a recrystallization process.

Oxide dispersions were added to β-NiAlalloys using a powder-metallurgy technique. During 20,2-hr cycles at 1200°C, scale adhesion of theexternal alpha Al3O2 was improvedby the addition of Y2O3 and ZrO2. Relative to an undoped, castNiAl alloy, no improvement was observed forTiO2 and HfO2 additions andnegative effects were observed forAl2O3 andLa2O3 additions. The variouscation additions also had differing effects on the scale morphologyand isothermal growth rates at 1200°C. The effect ofthe dopants added as oxide dispersions was compared tosimilar alloy additions of the dopants to β-NiAl.

To develop satisfactory alloys without Cr or Ni for high-temperature application up to 1100†C, three alloys based on Fe-10%Al-Si with differing fourth (or fifth) element additions were oxidized in air at 1100°Cfor 24 hr. A low carbon, Fe-30Mn-10Al-Si alloy exhibited excellent high-temperature oxidation resistance. The total weight gain for 24 hr oxidation in air at 1100°C was only 1.03 mg/cm 2. After air oxidation for 6 days at 1100°C, no nodule formation or breakthrough oxidation occurred. Post-oxidation SEM and EDAX examination showed that a thin, compact, protective alumina scale formed on the alloy.

The thermally grown oxide on sputter-deposited NiAl–0.09Zr was studied using transmission electron microscopy (TEM) and transmission Kikuchi diffraction (TKD) to microstructurally assess its oxidation resistance. Sputter deposition resulted in a refined grain size of 0.3 μm that was compared to extruded NiAl–Zr with a grain size of approximately 25 μm. Thermogravimetric oxidation of the sputter-deposited material showed a shorter transient regime with lower mass gain than an extruded NiAl–0.1Zr alloy and improved spallation resistance through 50 h of isothermal oxidation at 1000 °C. After 5 h the thermally grown oxide on the sputter-deposited alloy exhibited a three-layer structure consisting of external θ-Al2O3 whiskers, intermediate equiaxed α-Al2O3 grains (< 100 nm) + ZrO2 precipitates and internal columnar α-Al2O3 grains, in contrast to the extruded alloy which showed sparse α growth. After 50 h of oxidation, the three-layer structure was retained, but the top θ-Al2O3 layer was transformed to α-Al2O3. TKD after 50 h showed the top and bottom oxide layers to be composed of high-misorientation α-Al2O3 grains approximately three times smaller than the extruded sample. Monoclinic and tetragonal ZrO2 precipitates were identified in the fine-grained middle region. These features show that grain refinement significantly increases Zr diffusion to the reacting surface, while simultaneously mitigating the effects of overdoping. This increased Zr diffusion is believed to have expedited the formation of a continuous α-Al2O3 layer, resulting in a shorter transient oxidation period.