ICMCTF2013 Session A1-2: Coatings to Resist High Temperature Oxidation, Corrosion and Fouling

Cast NiCrAl alloys with additions of Y, La, Hf and Ti were evaluated at 1100°C in wet (10 and 50% H2O) and dry air in order to optimize such dopants for superalloy bond coatings. The results suggest that the typical Y addition in most coatings could be replaced by La. Also, scale adhesion in cyclic testing was improved with the co-addition of Hf with La or Y. Ti was added to investigate its incorporation in coatings on superalloys containing significant Ti additions. Particularly with co- doped alloys, the addition of Ti had little effect. Water vapor increased spallation, especially for the least adherent alloys, such as Y,Ti-doped NiCrAl, water vapor increased scale spallation. For the co- doped compositions with Hf, water vapor had a limited effect on scale adhesion on the alumina growth rate in isothermal exposures. In addition to specimen mass change, beta phase depletion in the substrate was evaluated. Analytical transmission electron microscopy showed that Y, La and Hf co- segregated to the alumina scale grain boundaries and formed dopant rich oxide precipitates in the scale.

Research sponsored by the U. S. Department of Energy, Office of Fossil Energy, Coal and Power R&D.

The diffusion behavior of Pt deposited on pure nickel by the electroplating process was examined by the analysis of Pt concentration profiles against the distance in pure nickel specimens for various diffusion times (2, 4, 6, 8, 10, 12 and16 hours) at 1323K. Platinum diffusion coefficient was calculated by the use of the finite differences method. In solving the equations, it was accepted , that for a c(x) function, it is possible to approximate its derivative (slope/tangent) at a certain point by: the slope of the arc described as the difference of functions at the end of the calculation area or by the slope of the arc described as a reverse difference of functions – close to the samples surface;or by the slope of AB arc given as a central difference of functions – inside the sample. The platinum diffusion coefficient varied from 2.28x10^-14 (for 2 hours) to 8.96x10^-14(for 16 hours). The thickness of the diffusion zone is proportional to the a power function of the diffusion time. The exponent m of the power function is 0.4. As the m coefficient is slightly smaller than 0.5, we may expect, that the volume diffusion predominantly controls the layer growth and the boundary diffusion lightly contributes to the rate- controlling process.

The study concerns comparison of Pt and Pd/Pt-modified aluminide bond coatings for

EB-PVD TBCs on N5 superalloy in terms of microstructural evolution during cyclic oxidation tests.

The coatings were deposited by Pt and Pd+Pt electroplating, followed by high activity vapor phase aluminizing at 1050°C and pre-oxidation heat treatment in order to form α-alumina TGO. The 7wt.% yttria stabilized zirconia TBC was deposited using electron beam physical vapor deposition. Cyclic oxidation tests of the coatings were performed at 1100°C in 1h cycles in laboratory air. The microstructures of the coatings in the as-deposited state as well as after cyclic oxidation tests were studied using SEM, EDS and EBSD. The evolution and phenomena occurring at the interface between the bond coating, thermally grown oxide and TBC during high temperature exposure were studied in detail using high resolution CTEM and S/TEM. The samples for S/TEM analysis were prepared using FIB (Focused Ion Beam). The study is mostly focused on the growth of TGO during high temperature exposure and the overall performance of the Pd/Pt-modified aluminide coatings as a cost effective alternative to Pt-modified aluminide coatings commonly used as EB-PVD bond coats.

Modern heat resistant ferritic-martensitic steels are of high interest as superheater materials in fossil fuel power plants. They have much better heat transfer behavior and a lower coefficient of thermal expansion, as well as lower costs in comparison to austenitic steels and nickel base alloys. Modern 9% Cr-steels have sufficient creep strength up to 650°C, however their corrosion resistance particularly in H₂O containing environments needs further improvement. The inner side of the superheater tubes is exposed to steam, which leads to increased corrosive attack.

Results pertaining to the enrichment of chromium and manganese in the metal subsurface region of P91 and P92 using a diffusion process (pack cementation) without altering the bulk phase will be presented. The diffusion treatment developed is based on thermodynamic considerations for the design of the coating process. High temperature oxidation exposure of uncoated and coated samples at 650°C in argon with 50% H₂O, a gas which was shown before to yield the same oxidation behavior such as pure steam oxidation, shows the improved behavior. The paper discusses the coating process parameters and the oxidation behavior.

The fireside environment in power boilers may be quite corrosive, especially in boilers firing biomass and waste, where the fireside environment is characterized by a combination of high levels of reactive alkali (NaCl and KCl), HCl and relatively low SO₂ concentrations. The corrosion problems shorten the lifetime of the waterwall and the steam superheater, limiting the maximum steam temperature. This paper presents new results on the mechanism behind the accelerated high temperature corrosion suffered by stainless steel and low alloyed steel in the presence of alkali chlorides, O₂ and H₂O. We also discuss different measures to mitigate the corrosion problems in real boilers. Thus, sulphur can be added to the fuel in order to convert the alkali chlorides to the corresponding alkali sulphates, which are far less corrosive. Another opportunity to increase the lifetime of the boiler is to apply coatings or use alloys that are optimized with respect to chloride-induced corrosion. This work is mainly based on laboratory investigations of corrosion but will also make comparisons with corrosion experiments in real boilers.