Performance of Al‐rich oxidation resistant coatings for Fe‐base alloys

The oxidation resistance of Al‐rich coatings made by chemical vapor deposition and pack cementation was examined on representative ferritic‐martensitic (FM, e.g. Grade 91, Fe‐9Cr‐1Mo) and austenitic steel substrates at 650°‐800 °C. To evaluate the potential benefits and problems with these alumina‐forming coatings, oxidation exposures were conducted in a humid air environment where the uncoated substrates experience rapid oxidation, similar to steam. Exposure temperatures were increased to accelerate failure by oxidation and interdiffusion of Al into the substrate. The difference in the coefficient of thermal expansion (CTE) between coating and substrate was found to cause cracking and coating failure during rapid thermal cycling on thicker coatings with Fe‐Al intermetallic phases. Therefore, thinner coatings with less Al and a ferritic Fe(Al) structure were evaluated more extensively and tested to failure at 700° and 800 °C on FM steels. The remaining Al content at failure was measured and used to improve a previously developed coating lifetime model. At 700° and 800 °C, thin coated austenitic specimens continue to exhibit protective behavior at more than double the lifetime of a similar coating on FM steel. The longer lifetime was attributed to the ferritic coating‐austenitic substrate phase boundary inhibiting Al interdiffusion.     

Effect of pressure and impurities on oxidation in supercritical CO2

Both indirect‐ and direct‐fired supercritical CO₂ cycles for high‐efficiency power generation are expected to have impurities that may greatly alter the compatibility of Fe‐ and Ni‐based structural alloys in these environments. Recent work has attempted to quantify reaction rates at 750°C in simulated laboratory environments with controlled impurity levels at ambient pressure, as well as under supercritical conditions (30 MPa). With low impurity levels in research and industrial‐grade CO ₂, pressure appeared to have only a limited effect on oxide thickness and internal oxidation and reaction products were similar to those formed in laboratory air. However, a direct‐fired simulation at 750°C/30 MPa in CO ₂ + 1%O ₂ + 0.25%H ₂O has found an increased mass gain and characterization after 2,500‐hr exposures have found thicker reaction products, especially for Fe‐based alloys. At these impurity levels, pressure may have a significant effect on the role of impurities.     

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 O₂, 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.     

The development of alumina-forming austenitic stainless steels for high-temperature structural use

A new family of alumina-forming austenitic stainless steels is under development at Oak Ridge National Laboratory for structural use in aggressive oxidizing environments at 600–900°C. Data obtained to date indicate the potential to achieve superior oxidation resistance compared to conventional Cr₂O₃-forming iron-and nickel-based heat-resistant alloys, with creep strength comparable to state-of-the-art advanced austenitic stainless steels. A preliminary assessment also indicated that the newly developed alloys are amenable to welding. Details of the alloy design approach and composition-microstructure-property relationships are presented. Author’s Note: Part of this research summary is based on a recent review paper (see Reference 1) and findings first reported in References 2–7.     

Transplacental induction of lymphomas in Swiss mice by carbendazim and sodium nitrite

Swiss mice at different stages of pregnancy were treated intragastrically with the pesticide Carbendazim (MBC, BCM, methyl-2-benzimidazole carbamate) together with sodium nitrite. Lymphomas developed in 33.3% of young mice whose mothers were treated in the first week of pregnancy, in 53.3% of whose mothers were treated during the second week, and in 38.8% of those born of mothers treated during the third week. Treatment during the whole period of pregnancy yielded on an average 70.0% malignancy in offspring. However, administration of Carbendazim by itself did not produce lymphomas in the first generation. In lymphomas induced by in vivo-formed n-nitroso compounds, A- and C-type oncornavirus particles were observed with the electron microscope.     

Effect of H2O and CO2 on the Oxidation Behavior and Durability at High Temperature of ODS-FeCrAl

Cyclic oxidation testing was conducted on alloy MA956 and two different batches of alloy PM2000 at 1,100 and 1,200 °C in different atmospheres rich in O₂, H₂O and CO₂. Compared to 1 h cycles in dry O₂, exposure in air + 10 vol.% H₂O resulted in an increase of the oxidation rate and a decrease of the time to breakaway for all alloys at 1,200 °C, and a faster consumption of Al in the MA956 alloy. One hour cyclic testing in 49.25 % CO₂ + 50 % H₂O + 0.75 % O₂ had a smaller effect on the oxidation rate but led to increased formation of voids in alloy MA956, which had an impact on the alloy creep resistance. At 1,100 °C, exposure in 50 % CO₂ + 50 % H₂O resulted in significant oxide spallation compared with oxidation in air, but this was not the case when 0.75 % O₂ was added to the CO₂/H₂O mixture as a buffer. The control of impurity levels drastically improved the oxidation resistance of PM2000.     

Effect of Experimental Procedures on the Cyclic,Hot-Corrosion Behavior of NiCoCrAlY-Type Bondcoat Alloys

A simplified test procedure was established to assess the hot-corrosion behavior of MCrAlY-type nickel-base alloys under the influence of molten sodium sulfate as well as sodium sulfate/potassium sulfate salt blends. Salt-coated specimens were exposed to 1-hr thermal cycles at 950°C in flowing oxygen for up to 500 cycles. Mass-change data of the specimens revealed a significant dependence of the corrosion attack not only on the average contaminant flux rate, as expected, but also on the initial amount of salt deposited during each recoating cycle. Furthermore, deposit removal before salt recoating markedly influenced the corrosion attack of the alloys. This was apparently related to changes in salt chemistry by the dissolution of elements such as Cr from the alloy, which can shift the basicity of the salt and thus affect the extent of attack. Substituting Na for K in sodium sulfate/potassium sulfate salt blends generally resulted in decreased attack. For K-containing salt deposits, increasingthe gross amount of alkali compared to sulfur resulted in increased sample weight losses due to scale spallation. In contrast, decreasing the amount of sulfur in such deposits which contained exclusively Na as the alkali resulted in a significantly reduced corrosion attack compared to stoichiometric sodium sulfate.