Scale Spallation Research Articles

The Isothermal and Cyclic Oxidation Behavior of a Titanium Aluminide Alloy at Elevated Temperature

The isothermal and cyclic oxidation behavior of Ti-47Al-2Mn-2Nb with 0.8 vol.% TiB2 particle-reinforced alloy was investigated in air between 700 and 1000 °C. In the study, the kinetics of isothermal and cyclic oxidation were performed by using a continuous thermogravimetric method which permits mass change measurement under oxidation conditions. The oxide scales and substrates were characterized by scanning electron microscopy with energy-dispersive x-ray analysis and x-ray diffraction. At 700 and 800 °C, the alloy showed an excellent oxidation resistance under isothermal and cyclic conditions. After exposure to air above 800 °C, the outer scale of the alloy was dominated by a fast-growing TiO2 layer. Under the coarse-grained TiO2 layer was the Al2O3-rich scale, which was fine-grained. At 900 and 1000 °C, the extent of oxidation increased clearly. The oxidation rate follows a parabolic law at 700 and 800 °C. However, the alloy, upon isothermal oxidation at 900 °C, can be divided into several stages. During the cyclic oxidation at 900 and 1000 °C, partial scale spallation takes place, leading to a stepwise mass change.

Determination of mechanical adhesion energy of thermal oxide scales on AISI 430Ti alloy using tensile test

The present paper deals with mechanical adhesion energy determination of thermal oxide scales on metallic alloys. AISI 430Ti alloy (Fe–18Cr–0·4Ti–0·5Si, wt-%) was isothermally oxidised at 900°C in Ar–20%O2 atmosphere to grow thermal oxide scales of various thickness. These oxide/metal systems were subjected to room temperature in situ tensile test in the SEM chamber allowing continuous observation of surface failure during the test. Characteristic observations were oxide transverse cracking and scale spallation. A theoretical model was developed to quantify stress and strain evolution in oxide during the test, allowing to derive adhesion energy from strain and stress values at spallation onset. It was found that oxide scales on AISI 430Ti exhibited high mechanical adhesion energy, decreasing from 163 to 38 J m−2 with increasing oxide thickness in the range from 0·3 to 2·2 μm. Excellent agreement was observed with adhesion energy values obtained using the inverted blister test on the same alloy.

High-temperature oxidation of FGH96 P/M superalloy

High temperature oxidation behaviors of FGH96 P/M superalloy have been studied in air at temperatures ranging from 600 to 1000°C. By means of isothermal oxidation testing. X-ray diffraction, SEM (scanning electron microscopy), and EDS (energy dispersive X-ray spectroscopy) analyses, the oxidation kinetics as well as the composition and morphology of scales were investigated. Thermodynamic calculations were used to explain the oxidation mechanism. The results showed that as the oxidation temperature increased, the oxidation rate, the scale thickness, and scale spallation increased. FGH96 P/M superalloy exhibits good oxidation resistance at temperature below 800°C. The oxidation kinetics follows an approximately parabolic rate law, and the oxide layer was mainly composed of Cr 2 O 3 , TiO 2 , and a little amount of NiCr 2 O 4 . The oxidation is controlled by the transmission of chromium, titanium, and oxygen through the oxide scale.

Preparation and high temperature oxidation behavior of refractory disilicide coatings for γ-TiAl intermetallic compounds

Novel refractory disilicide layers were applied to γ-TiAl to enhance oxidation resistance at 1050 °C. NbSi 2 and MoSi 2 layers were prepared by joining thin Nb and Mo foils to γ-TiAl surfaces, and siliconizing the combinations (Nb/γ-TiAl, and Mo/γ-TiAl) using molten salts. The coatings and their oxidation behavior were characterized using X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy techniques. Isothermal oxidation tests showed that the oxidation resistance of uncoated γ-TiAl at 1050 °C in air was insufficient, and scale spallation occurred. NbSi 2 coatings were formed and adhered firmly to the γ-TiAl substrate, whereas Mo film detached from the substrate surface causing failure of the MoSi 2 coatings. Oxidation of the NbSi 2-coated γ-TiAl (NbSi 2/Nb/γ-TiAl) at 1050 °C in air showed improved oxidation resistance at exposure times up to 100 h. Microstructural and compositional developments of the coating at prolonged time were discussed. The NbSi 2 coatings provided sufficient oxidation resistance for γ-TiAl at 1050 °C in air, and have potential use in high temperature applications.

Nano‐ and Microcomposite Al2O3‐Y2O3 Coatings Produced by Sol‐Gel Process

Nano‐ and microcomposite Al2O3‐Y2O3 coatings have been deposited on Fe‐Cr‐Mo alloy substrates using sol‐gel composite coating technology. The processes include dipping samples in a sol‐gel solution dispersed with fine ceramic powders, which is prepared by high‐energy ball milling. High‐resolution microscopy (FE‐SEM) analyses show that the coating is composed of composite particle clusters with an average diameter of 1 µm, and the coating is relatively dense without cracking during drying and sintering stages. XRD analyses show that the oxide coating is mainly composed of α‐Al2O3 and γ‐Al2O3. The oxidation tests performed in air at 600°C show that the coatings possess much improved resistance to high temperature oxidation and scale spallation. It is believed that the nano‐structured composite particles and reactive elements are integrated into the coatings, which played an important role in improving the aggregation of nanoparticles and the production of cracks.

Corrosion of Three Commercial Steels Under ZnCl2 –KCl Deposits in a Reducing Atmosphere Containing HCl and H2S at 400–500°C

The corrosion of three commercial steels in a reducing atmosphere containing HCl plus H2S in the presence of ZnCl2-KCl deposits has been investigated at 400-500 degrees C and compared with the corrosion of the same materials in a similar gas mixture free from H2S. The presence of H2S in the gas accelerated the corrosion of the three commercial steels beneath ZnCl2-KCl deposits. All materials suffered from severe corrosion with partial detachment and spalling of the external scales. Degradation of the steels resulted from the penetration of chlorine-containing species through the scale formed initially down to the metal matrix, because chorine-rich species were detected close to the alloy/scale interface. Although the corrosion resistance generally increased with increasing Cr content, even the high-Cr stainless steel SS304 is still unable to provide good corrosion resistance against the ZnCl2-KCl deposits in the presence of H2S due to the bad adherence of the scales to the alloy. The mechanisms of attack are discussed on the basis of thermodynamic considerations and of the active-oxidation model.

Improvement in oxidation resistance of a Ni 3Al-based superalloy IC6 by rhenium-based diffusion barrier coatings

The oxidation behavior of a Ni 3Al-based superalloy IC6 coated with a duplex Re–Cr–Ni–Mo diffusion barrier layer and an Al reservoir layer was investigated in air at 1423 K for up to 1080 ks. The diffusion barrier layer was formed by electroplating Re(Ni) and Ni films on the alloy, followed by Cr pack cementation at 1573 K, and as a result, forms a continuous inner Re–Cr–Ni–Mo diffusion barrier layer and an outer Ni(Cr,Mo,Al) layer. Then a Ni film was electroplated on the Ni(Cr,Mo,Al) layer, followed by Al-pack cementation at 1273 K for 18 ks, to form an Al reservoir layer with a duplex Ni 2Al 3 and γ-Ni(Cr,Mo,Al) layers. After oxidation at 1423 K in air for 1080 ks, the Al reservoir layer changed to a γ-Ni–4Cr–5Mo–12Al (all in at%) layer, on which a protective α-Al 2O 3 scale formed. The Re–Cr(Mo)–Ni layer was stable and effectively retarded the interdiffusion between the Al reservoir layer and the alloy, as a result, the depth of the microstructural change zone of the alloy was less than 15 μm. In contrast, the bare and the coated IC6 superalloy only with an Al reservoir layer were significantly oxidized, accompanied by serious spallation of oxide scales. After oxidation at 1423 K for 1080 ks, the depth of the microstructural change zone of the alloy was about 200 μm for the bare and coated alloy only with an Al reservoir layer. These results indicate that the oxidation resistance of IC6 superalloy can be effectively improved by coating with a Re–Cr–Ni–Mo diffusion barrier layer and an Al reservoir layer.

Oxidation resistant coatings in combination with thermal barrier coatings on γ-TiAl alloys for high temperature applications

Titanium aluminide alloys based on γ-TiAl are considered of growing interest for high temperature applications due to their attractive properties. To extend the service temperatures above 750 °C, the oxidation behaviour has to be improved predominantly by protective layers. In the present study environmental and thermal protection coatings on gamma titanium aluminides were investigated. Nitride and metallic overlay coatings based on Ti–Al–Cr–Y–N and Ti–Al–Cr, respectively, were produced by magnetron sputtering techniques. Thermal barrier coatings (TBCs) of partially yttria stabilized zirconia were deposited onto Ti–45Al–8Nb, either pre-oxidized or coated with protective layers, applying electron beam physical vapour deposition (EB-PVD). Cyclic oxidation tests were performed at 900 °C and 950 °C in air. The nitride coating exhibited poor oxidation resistance when exposed at 900 °C providing no protection for γ-TiAl. The oxidation behaviour of the Ti–Al–Cr coating was reasonable at both exposure temperatures. During prolonged exposure the coating was depleted in chromium, resulting in the breakdown of the protective alumina scale. EB-PVD zirconia coatings deposited on γ-TiAl exhibited promising lifetime, particularly when specimens were coated with Ti–Al–Cr. The adherence of the TBC on the thermally grown oxide scales was excellent; failure observed was associated with spallation of the oxide scale. At 950 °C, TBCs on specimens coated with Ti–Al–Cr spalled after less than 200 thermal cycles caused by severe oxidation of γ-TiAl and reactions between the zirconia coatings and the thermally grown oxides.

The effect of water vapor on the oxidation behavior of Ni–Pt–Al coatings and alloys

Turbines fired with hydrogen or syngas from coal gasification will have significantly higher water vapor contents in the combustion gas than natural gas fired turbines. The effect of increased water vapor on alumina-forming coatings and model alloys was investigated at 1100 °C in furnace cyclic testing. Increasing the water vapor content from 10% to 50 vol.% increased the amount of scale spallation on undoped alumina-forming alloys. Compared to dry O 2, increased spallation was observed for β and γ/γ' phase coatings on the substrates of alloys 142 and N5. In all cases, the addition of water vapor appeared to reduce the formation of alumina whiskers and ridges at the scale–gas interface, but did not significantly change the alumina growth rate. The addition of water vapor may have a detrimental effect on the selective oxidation of Al in γ/γ' alloys and coatings.

Implications of steam oxidation for the service life of high-strength martensitic steel components in high-temperature plant

The significance of the formation of thick oxide scales during exposure of Cr steels in steam-containing environments on the service life of components is discussed. The quantitative effects of the loss of load-bearing cross-section on the creep rupture life are presented. For tubes of 6 mm wall thickness, the reduction in rupture life due to material loss will be significant after comparatively short exposure times of 10 000 h at 600 °C. Potentially much more damaging is, however, the thermal insulation effect of the porous, thick oxide scales, which leads to overheating of heat transfer tubes. The higher metal temperatures will then lead to early failure by creep. The scale present after 10 000 h exposure in steam may be sufficient to cause a temperature increase of 50 K which will reduce the rupture time for a constant stress by a factor of 30. Oxide scale spalling can also lead to overheating due to the restriction of the flow inside tubes. The design should ensure that spalled oxide cannot accumulate inside the tubes.

Oxidation Behavior of In Situ-Synthesized (TiB+TiC)/Ti6242 Composites

The oxidation behaviors of TiB and TiC particle-reinforced, titanium-matrix composites (TMCs) were studied in air at 550–650°C, The oxidation kinetics follow approximately a parabolic rate law. The oxidation rates, which were lower than those of Ti6242, decrease gradually as oxidation proceeds. The oxide scales formed on TMCs were predominantly rutile and α-Al2O3. No B2O3 and other oxides were observed within the oxide scale. The in situ-synthesized TiB and TiC reinforcements can increase the oxidation resistance of TMCs. The oxide scales that formed exhibited excellent spallation resistance under all testing conditions. No scale cracking or spallation could be observed, implying that growth and thermal stresses generated during heating and cooling have been effectively released. The mechanisms of the decrease in oxidation rate and the improvement on spallation resistance are discussed based on microstructure studies.

Mechanisms of steam oxidation in high strength martensitic steels

The oxidation resistance of 9–12% chromium steels in steam-containing environments simulating the service conditions of steam power plant has been investigated for exposure times ranging from 1 h up to 10,000 h. The time-dependent changes in the complex structures of the scales were determined using optical microscopy. Different oxidation mechanisms were observed for different temperature regimes, exposure times and exposure conditions. The cracking and spallation of scales was correlated with the type, morphology and growth of pores and voids in the scale and could be influenced by the steel microstructure. For some steels, the steam oxidation resistance increased with increasing exposure temperature. The oxidation rates only slightly depend on the exact water vapour content in the test gas but can be affected by the gas flow rates.

Element diffusion during fabrication of EB-PVD NiAl coating and its 1100 °C isothermal oxidation behavior (II)

A two-layer NiAl coating can be deposited by electron beam physical vapour deposition (EB-PVD) on Ni-based superalloy, and the isothermal oxidation test at 1100 °C in air was conducted to evaluate the oxidation behavior of the coating. The dominant phase changed from β-phase to γ′-phase with the oxidation going on. Oxide ridges can be detected on the surface of the as-processed coating, oxide scales on the coating surface can be divided into two categories including the primary scales and the secondary scales formed only after the spallation of the primary scales. The primary scales form to connect the isolated oxide ridges, while the secondary scale's peak clipping effect may cause the surface morphology more and more even.

Oxidation of Nb–Si–Cr–Al in situ composites with Mo, Ti and Hf additions

The effects of Mo, Ti and/or Hf additions on the isothermal oxidation behaviour of Nb–Si–Cr–Al-based in situ composites in static air at 800 °C and 1200 °C were studied for the as-cast and heat-treated materials. After heat treatment at 1500 °C, the microstructures of all the alloys consisted only of the niobium solid solution (Nb ss) and Nb 5Si 3 phases. The addition of Ti and the decrease of the Mo concentration to 2 at.% improved the oxidation resistance of the alloys at 800 °C dramatically. The Hf addition had no significant effect on the oxidation behaviour of Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf (at.%) at 800 °C. The oxidation resistance of the alloys was found to be sensitive to the volume fraction of Nb ss. The coarsening and the increase of the volume fraction of the Nb ss phase in the heat-treated alloys were mainly responsible for the degradation of their oxidation resistance. Preferential attack of the Nb ss phase was observed in all the alloys. Pesting oxidation behaviour was exhibited at 800 °C by the Nb–18Si–5Al–5Cr–5Mo (as-cast), Nb–24Ti–18Si–5Al–5Cr–5Mo (as-cast), Nb–24Ti–18Si–5Al–5Cr–2Mo (heat-treated) and Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf (heat-treated) alloys. Pesting of the alloys occurred due to a combination of the presence of elements that oxidised rapidly and the restricted deformation capability of the scales. Spallation of oxide scales during cooling was observed for the as-cast and heat-treated Nb–24Ti–18Si–5Al–5Cr–2Mo and Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf alloys at 1200 °C. In the diffusion zones formed in the alloys without Hf addition, the 5–3 silicide was not oxidised. The TiNbO 4, TiNb 2O 7, Ti 2Nb 10O 29 and silicon oxide were present in the scales formed on the Nb–24Ti–18Si–5Al–5Cr–2Mo and Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf alloys at 1200 °C and Hf oxide was also present in the latter alloy. At 1200 °C, in the Ti containing alloys, large Ti oxide particles formed at the Nb ss and 5–3 silicide interface and fine Ti oxide particles formed inside the Nb ss. Needle-like Hf oxide formed inside the 5–3 silicide in the heat-treated Hf containing alloy, which resulted in the degradation of its oxidation resistance. No protective oxide scale was formed on any of the alloys studied in this work.

Al 2O 3-Y 2O 3 Nano- and Micro-Composite Coatings on Fe-9Cr-Mo Alloy

Al 2O 3-Y 2O 3 nano- and micro-composite coatings were deposited on Fe-9Cr-Mo substrates by using sol-gel composite coating technology. The processing includes dipping samples in a sol-gel solution dispersed with fine ceramic powders, which are prepared by high-energy ball milling. High-resolution microscopy (FE-SEM) analyses show that the coating is composed of composite particle clusters with an average diameter of 1μm, and the coating is relatively dense without cracking during drying and sintering stages. XRD analyses show that the oxide coating is mainly composed of α-Al 2O 3 and γ-Al 2O 3. The oxidation tests performed at 600 °C in air show that the coatings are provided with much improved resistance against high temperature oxidation and scale spallation. It is indicated that nano-structured composite particles and reactive elements are integrated into the coatings, which plays an important role in preventing agglomeration of nano-particles and initiation of cracks.

PM stainless steel's resistance to scaling

In North America, conversion of automotive exhaust systems from carbon steel to stainless steel has been progressing steadily since the mid-1990s. In an all stainless steel exhaust system, PM stainless steel exhaust flanges and HEGO (hot exhaust gas oxygen sensor) bosses are frequently selected over wrought (stamped and formed) stainless steel components. This is because PM stainless steel provides significant advantages in performance, cost and design flexibility, over their wrought counterparts. The improved performance is primarily due to the fact that PM stainless steel is more resistant to spalling of oxide scales in application-specific cyclic oxidation tests, compared with wrought stainless steel…

Effect of Enamel Coating on Oxidation and Hot Corrosion Behaviors of Ti-24Al-14Nb-3V Alloy

The effect of enamel coating on the isothermal and cyclic oxidation at 900 °C in air and on the hot corrosion resistance of Ti-24Al-14Nb-3V in both 85% Na2SO4+15%K2SO4 and 15%NaCl+85% Na2SO4 molten mixed salts at 850 °C was investigated. The results indicated that Ti-24Al-14Nb-3V alloy exhibited poor oxidation resistance due to the formation of nonprotective Al2O3+TiO2+AlNbO4 scales and poor hot corrosion resistance due to the spallation of scales formed in molten Na2SO4+K2SO4 and NaCl+Na2SO4. Enamel coating suppressed the migration of oxygen and corrosive ions into the substrate to improve the oxidation and hot corrosion resistance of Ti-24Al-14Nb-3V alloy. However, the dissolution of oxides components of the coating into the molten salts degraded enamel coating and the degradation of the coating involved a process by which Cl− anion penetrated into the substrate through voids in the coating to accelerate corrosion of Ti-24Al-14Nb-3V alloy.

Quantitative Adhesion Energy Values of Chromia-Rich Thermal Oxides on Stainless Steels Determined by Blister and Tensile Tests

Adhesion energy values for thermal oxide scales cyclically grown at 850 and 950°C in air on ferritic and austenitic stainless steels were obtained using an inverted blister test and a tensile test working in the SEM chamber. The blister test used water pressure for debonding the metalscale interface, whereas the tensile test led to transverse compression generating scale spallation by buckling. Adhesion energy, defined by energy for interface crack propagation by unit area, was shown to be in the range 10 to 650 J.m–2 for the chromia-rich scales with thickness in the micrometer range. Ferritic grades gave less adherent scales than austenitic ones, and a great influence of titanium was evidenced, greatly increasing scale adhesion; niobium was less operative. Adhesion was well connected with nature and morphology of Ti and/or Nb-containing precipitates at the metal-scale interface.

Oxidation Behavior of Ru-Containing Ni-Base Single-Crystal Superalloys

The 4th and advanced generation Ni-base single crystal superalloys, which contain large amounts of refractory metals for strengthening and platinum group metals, e.g., Ru, for TCP-phase prevention, show excellent high-temperature strengths. However, these alloying elements seem to decrease high-temperature oxidation resistance. In this study, Ni-base superalloys with various amounts of Ta, Re and Ru were examined in isothermal and cyclic exposures at 1373K to investigate the effect on the oxide growth rate and resistance to scale spallation. Structures of the oxide for the alloys were analyzed by XRD, SEM and EDX, and the oxidation kinetics is discussed. Ru and Re were found to degrade the oxidation resistance by the vaporization of their oxide. Ta-rich oxide in the spinel layer affects to stabilize ruthenium and rhenium oxide in the scale and improve the oxidation resistance of Ru-containing Ni-base superalloys.

Thermal oxidation kinetics and oxide scale adhesion of Fe–15Cr alloys as a function of their silicon content

Three types of Fe–15Cr alloys, containing 0, 0.5 and 1 wt.% Si, were studied between 850 and 950 °C in terms of oxidation kinetics, oxide spallation and oxide adhesion energy. The oxidation kinetics in Ar–15% O 2 were found to be parabolic, exhibiting non-monotonic evolution of the parabolic rate constant with increasing Si content, with a minimum value for 0.5% Si. Chromia scale spallation on cooling was quantified as a function of oxide thickness, showing again non-monotonic behaviour, with no spallation for the Si-free alloy, up to a scale thickness of 2.7 μm, and maximum spallation for the 0.5% Si alloy. Room temperature tensile testing in a scanning electron microscope chamber was used to quantify oxide adhesion. Interfacial fracture energy values derived from these experiments were shown to be in the range 20–80 J m −2, with the respective values in good agreement with the observation of spallation during cooling.