ICMCTF2015 Session A1-1: Coatings to Resist High Temperature Oxidation, Corrosion and Fouling

Steam tubes in WTE plants have short life-time, due to intense corrosion processes. In general, pure metal-based alloy weld overlays are in use. The goal of this study is to develop cost-effective anti-corrosion coatings. Recently, coating structures consisting of a metal-based alloy (Inconel 625) as bond-coat, followed by a ceramic top-coat, both deposited by air-plasma-spray method (APS), are in discussion. One drawback of the ceramic coating is the high porosity, which gives way for corrosive gas species, attacking the underlaying steel tube. To diminish this destructive process an innovative chemical densification process is applied to the ceramic coating. Based on a solvothermal mobilization and recrystallization of the ceramic material, herein yttria-stabilized zirconia (YSZ), the porosity can be reduced by half. This leads to a better adhesion of the ceramic top-coat onto the bond-coat and enables the ceramic coating to act as a protective gas-barrier.

This solvothermal effect was found for the first time on coated test-probes, mounted in a WTE plant [1]. A successful reproduction was done in lab scale experiments under simulated WTE plant conditions [2]: temperature 600-800°C, atmosphere of 98 vol.% N2 and 2 vol.% HCl together with an equimolar mixture of potassium and zinc chlorides and sulfates, added into the gas flow. In recent experiments, O2 is added to the atmosphere, giving a solvothermal medium of O2, HCl, Cl2, H2O and SO2. These conditions enable a mobilization of the Zirconium. The recrystallization is triggered by changing partial pressures of the involved species within the ceramic topcoat.

[1] D. Bendix et al., Mater. Corros., 2008, 59, 389.

[2] P.J. Masset et al., ECS Trans., 2013, 50, 109.

Protective metallic coatings enhance the oxidation and corrosion resistance of the underlying high temperature materials. Some of the widely used types of coatings are MCrAlY (M = Ni, Co) overlay coatings and nickel aluminide (NiAl) diffusion coatings which ensure the growth of a slowly growing adherent alumina scale and thus protect the underlying substrate from rapid oxidation attack. Aluminium from the bondcoat is lost to the external oxide layer on the coating surface and to the substrate by interdiffusion resulting in dissolution of the ß-NiAl phase in the coating. Coating life generally corresponds to the operating life of the components and is usually measured in terms of the depletion of the ß-NiAl phase as it serves as an Al-reservoir for the growth of the protective alumina scale.

The performance of a coated material depends on the compatibility of a given type of a coating with its base material. Evaluation of the material’s high temperature behaviour requires extensive experimental testing. A reduction in the time and effort can be complemented by computational methods. A new computational approach to model the microstructural evolution in coating systems and thereby evaluate their lifetime was undertaken in the present study for typical coatings on Ni-based alloys and superalloys.

Microstructural development in the alloys was modelled by considering simultaneously occurring oxidation and interdiffusion processes. Using available thermodynamic and kinetic data for all occurring phases from Thermo-Calc the current work differs from contemporary modelling methodologies. Element concentrations and phase distribution were obtained by scanning electron microscopy (SEM). Phases were identified by energy/wavelength dispersive X-ray spectroscopy (EDX/WDX) and electron backscatter diffraction (EBSD). Good agreement was found between the measured and computed phase fractions and phase distributions after specific time intervals. The computational approach assists in estimating the lifetime of the coating and provides a tool to predict microstructural changes in coating systems as a function of alloy/coating composition, time and temperature.

Diffusion aluminide and Thermal Barrier Coatings (TBC) are widely applied on jet engine turbine blades and vanes operating under the most severe conditions. The ceramic layers are often applied using EB-PVD method on Pt-modified aluminide coatings (bond coats) providing excellent oxidation and high temperature protection. The beneficial effect attributed to Pt is improvement of oxide scale adherence during cyclic and isothermal oxidation as well as mitigation of the detrimental effects of sulfur impurities on scale adherence and inhibition of void formation at the scale-metal interface. While the Pt-modified aluminide coatings have been widely studied in the literature other solutions are being sought for due to high costs of Pt. The paper presents the results of research conducted on partial replacement of Pt with Pd indicating that the modification of aluminide coatings with Pd or both these elements provides very good oxidation resistance along with significant cost reduction, due to the fact that Pd is two times cheaper than Pt.

The Pd/Pt modified aluminide coatings demonstrate cyclic oxidation resistance comparable to this of Pt modified aluminide coatings. However, under service conditions these bond coatings undergo a number of commonly reported degradation mechanisms that limit their durability and eventually lead to spallation of the YSZ top coat. These include i.a. surface roughening, also referred to as rumpling, β→γ’ and martensitic transformations due to Al depletion, void formation and formation of secondary oxides. The macroscopic degradation analysis of TBC systems is aided by a non-destructive 3D optical scanning method which allows prediction of YSZ spallation both on samples and real turbine components.

Special effort has been done in order to investigate the microstructure of the Thermally Grown Oxide (TGO) formed during oxidation heat treatment prior to YSZ deposition and the phenomena occurring at the interface between the YSZ, TGO, and the bond coat. Microstructure evolution of the TGO is described and related to the conditions of the pre-oxidation treatment and the chemistry of the bond-coating. During thermal exposure the TGO grows by inward oxygen diffusion forming distinctive columnar grains. Several segregation phenomena can be found in the TGO that include i.a. the Reactive Element Effect involving the presence of Hf, Zr and Y on α-alumina grain boundaries that reduce the growth rate of Al₂O₃ and simultaneously the extent of vacancy injection to the interface with the bond coating.

A MCrAlY (M = Ni,Co) modified with Si and Hf was deposited on three commercial Ni-base alloys: CMSX-4, CM 247 LC and PW1483. Duplicate specimens were also prepared on which additionally yttria-stabilized zirconia (YSZ) was deposited. All samples were tested in a cyclic rig at 1100°C in laboratory air in order to evaluate the oxide scale adhesion formed on the bond coating. The effect of substrate was investigated by studying in detail the microstructures and morphology of the oxide scales formed in specimens containing three different substrates using transmission electron microscopy (TEM). The effect of the YSZ presence on scale formation and morphology was also investigated. Additionally, after oxidation exposure in-situ TEM compression testing on the alumina scale formed on MCrAlY bond coating will be performed for quantitative in-situ deformation experiments. These tests will be used to evaluate the differences in mechanical behavior of the oxide scales between the three different substrates.

Ni-base superalloys are widely used in the gas turbine technology as construction materials for single-crystal blades and vanes. The excellent creep resistance of these materials is achieved by a specific γ/γ´-microstructure as well as solid solution strengthening by refractory metals such as W, Ta, Re. The single-crystal blades are usually coated with (Ni,Co)CrAlY overlay coatings to provide a better oxidation/corrosion resistance. The MCrAlY coatings are known to interact both with the corrosive environment (oxidation-induced Al-depletion) and the alloy substrate (interdiffusion), which are the two main chemical degradation modes of such coating systems. The chemical lifetime prediction is one of the fundamental issues in the gas turbine technology. In general practice, the lifetime estimates are based on extensive experimental testing. A reasonable alternative to this tedious and time consuming practice is the CALPHAD-based thermodynamic-kinetic modelling.

In the present study the interaction between a commercial (Ni,Co)CrAlY coating with model single-crystal Ni-Cr-Al-X (X = Co, Ta, W) alloys was investigated. In all tested coating systems the formation of secondary β-NiAl was observed in the alloy substrate. Scanning electron microscopy (SEM) and electron microprobe analysis (EPMA) provided local element concentrations in a multiphase microstructure.

The extent and the character of the microstructural changes are dependent on the substrate chemistry. The effect of the alloying elements in the substrate on the interdiffusion processes and β-NiAl depletion was evaluated by employing a thermodynamic-kinetic model. The model was able to explain the observed microstructural changes in the investigated coating systems. A good agreement was observed between the measured and calculated phase fractions and positions of the interfaces between the different multiphase zones.

Ni-base single-crystal superalloys with plasma sprayed thermal barrier coatings are commonly used for high-temperature, high-load applications in gas turbine blades. The thermal barrier coating often consists of a MCrAlY (M=Ni,Co) bond coat for oxidation protection and a ceramic top coat for thermal insulation. During operation interdiffusion between the superalloy substrate and the metallic bond coat takes place. On the one hand, interdiffusion is needed for good adhesion of the bond coat. On the other hand, interdiffusion induced loss of critical elements in the bond coat and the resulting deterioration of the superalloy microstructure are undesirable.

In this work, the interdiffusion is investigated and quantified for three distinct crystal orientations of the Ni-base single-crystal superalloy CMSX-4, namely [001], [100], and [110]. The substrate surfaces are subjected to different pretreatment processes and then coated with a bond coat using vacuum plasma spraying. Three different MCrAlY bond coats are used to study the influence of the chemical composition on the interdiffusion behavior: a Ni-base, Al-rich bond coat, a Ni-base, Al-rich bond coat with Re additions, and a Co-base bond coat with reduced Al content. After an initial diffusion bonding process at 1140 °C for 4 h and 870 °C for 16 h, heat treatment is conducted isothermally at 1050 °C in air for up to 1000 h.

SEM investigations show that the microstructures of the interdiffusion zones at the beginning of thermal treatment are similar to the bond coat microstructures. This can be observed best for the Re-containing bond coat which shows precipitation of Cr-rich phases in the bulk coating and the interdiffusion zone that are consistent with thermodynamic simulations. During thermal treatment the interdiffusion zones evolve in different ways, depending on the chemical composition of the bond coat. The Co-base bond coat forms a slightly thicker interdiffusion zone than the Ni-base bond coats. However, the choice of surface pretreatment is found to be the main reason for different sizes of interdiffusion zone and secondary reaction zone. The impact of the crystal orientation zone is negligible for both zones.

Inconel 625 coatings are used in the chemical and energy industries, among others, for the protection of construction elements working in difficult corrosive conditions. The necessity of reducing emissions from combustion in power boilers (reducing the amount of NOx) leads to the introduction of ammonia, which is very chemically aggressive, to the fuel mixture in the combustion chamber. In this paper, laser clad and thermal sprayed (HVOF) layers of fine grain powders (15 µm) were examined. Long-term tests of corrosion resistance of boiler steel P265GH covered with the coatings were conducted.

The tests were performed at 450^oC for 1000 hours in a complex corrosive atmosphere containing: N₂ + 10% CO + 0,2 % HCl + 0,08% SO₂ + ammonia (contents in the mixture within 2÷5 ppm). This type of environment is considered as a model for tests of high temperature corrosion of materials for service in power boilers with low-emission combustion. The structure of the coatings examined was characterized before and after the corrosion tests (LM, SEM, XRD). The tests revealed that in the corrosive environment analyzed, the laser clad coating provides better protective properties for steel.