ICMCTF2018 Session A1-2: Coatings to Resist High Temperature Oxidation, Corrosion, and Fouling

Increasing temperatures in aero gas turbines is resulting in oxidation and hot corrosion attack of turbine disks. Since disks are sensitive to low cycle fatigue (LCF), any environmental attack, and especially hot corrosion pitting, can seriously degrade the life of the disk. Application of metallic coatings is one means of protecting disk alloys from this environmental attack. However, since LCF is sensitive to surface conditions, pre- and post-coat processing can have a significant effect on the LCF life of coated bars.

Various pre-and post-coating conditions were examined with a Ni-45wt.% Cr coating applied by High-power impulse magnetron sputtering (HiPiMS) to a Ni-based disk alloy. Initial pre-coat surface conditions examined a grit blasted surface. Later pre-coat surface treatments involved wet blasting, two shot peening levels (8N-200% and 16N-200%) and a highly polished surface. Post-coat treatments involved two shot peening levels (4N-100% and 16N-200%) and a post-coat diffusion anneal in high purity Ar or a low PO₂ environment to encourage chromia scale formation. Half of the coated and uncoated bars were LCF tested at 760°C without further environmental exposure to evaluate the pre- and post-coat treatments. The second half of samples were either oxidized for 500 hours at 760°C in air, hot corroded for 50 hours at 760°C in air using a Na₂SO₄-MgSO₄ salt, or subjected to both exposures. Results of the LCF testing and post-test characterization of the various coatings will be presented and future research directions discussed.

For nuclear safety issues, there is an international effort to develop innovative “Enhanced Accident Tolerant Fuels” (EATF) materials. EATF cladding tubes are of particular interest because these components constitute the first barrier against radioactive fission species dispersal in case of accidental scenario such as LOCA (Loss of Coolant Accident). One of the EATF development objectives is thus to improve drastically the high-temperature steam oxidation resistance of the cladding tubes in LOCA situation (temperature up to 1200°C, water steam environment…). Let us recall that the actual nuclear fuel cladding tubes are made from Zr-based alloys (1cm in diameter, 0.6mm thick and more than 4m long).

Several alternatives for outer wall protection have been proposed worldwide but there is currently no solution for the inner wall protection. Upon LOCA transients, the fuel cladding may experience ballooning and burst due to the inner pressure and the high temperature achieved. Then, its internal surface can be exposed to steam environment at high temperature, leading to fast inner oxidation and other secondary degradation effects, which induce significant embrittlement and degradation of the claddings.

In order to resist to this harsh environment, Cr-based coatings deposited by low pressure DLI-MOCVD (Direct Liquid Injection of MetalOrganic precursors) have been investigated. These hard coatings could also be of use in high-temperature oxidative and corrosive environment such as in aeronautics and other industries to protect components with 3D complex geometries.

The process has been optimized to ensure a thickness uniformity along the tube through experimental studies coupled with numerical modeling of the reaction mechanism and the whole deposition reactor. Thanks to a suitable chemistry of the Cr precursor, bis(ethylbenzene)chromium, different coatings were deposited including: metal Cr, chromium carbides Cr_xC_y and mixed carbides Cr_xSi_zC_y.

The high-temperature behavior of these Cr-based coatings under oxidative atmospheres has been studied: HT XRD, TGA and various oxidation tests including pure steam environment followed by water quenching at room temperature to be representative of LOCA situations. Amorphous Cr_xC_y coatings showed the most promising properties. The results are discussed in terms of oxidation mechanisms and protection of the fuel claddings inner surface deduced from fine characterizations of the samples before and after the tests.

With increasing operating temperatures and more severe environments in today’s gas turbine engines, there is a need for a localized chromium diffusion coating for protection against type II hot corrosion and stress corrosion cracking. Conventional pack and vapor phase chromizing process have their limitations of applying such a local chrome coating. Masking for these processes is costly and often unsatisfactory. Therefore, pack and vapor phase chromizing process typically coat the entire part surface and then require undesirable post-coating treatment to remove excess chrome coating from locations where it is unwanted.

This presentation will discuss a new slurry based process that produces a localized chromium diffusion coating on the selected regions of gas turbine components in a controlled and accurate manner. No post-chromizing treatments such as machining or chemical treatment are required to remove excess chrome coating in comparison to conventional process. The resultant coating chemistry and microstructure meet the OEMs’ specifications. Other advantages of this new process include capability to coat large parts, and achieve uniform coating on parts having complex geometries such as shank and fir-tree root of turbine blades. This new slurry based process has been in production at Praxair Surface Technologies since 2015 yielding robust consistency in coating thickness and chemistry.

Based on this localized chrome coating technology, a platinum-modified chrome (PtCr) coating is under development. The addition of platinum can further enhance the resistance of chrome coating to type II hot corrosion attack. The PtCr coating process and its compatibility with other coating processes will be discussed.

There has been considerable interest in the use of magnesium and magnesium alloys because of their outstanding properties, although their low corrosion resistance inhibits widespread applications. Therefore, many studies have been carried out to investigate the corrosion behavior of magnesium and magnesium alloys. Various coating techniques have also been proposed to improve their surface properties and corrosion resistance.

In the present work,thin films fabricated by immersion in solution, deionized water and NaOH solution were studied on AZ31 magnesium alloy subjected to heat treatment. The films were grown by active reactions at the interface of the magnesium alloy and the solution due to the high temperature of the substrate. Surface morphologies showed that the surface of the magnesium substrates was covered by dense films formed in the initial stages of immersion. The results revealed that the surface structures were different from those of films grown by simply immersing in aqueous solution. Cross-sectional results showed that the films formed in NaOH solution with heat treatment were denser than those grown by DI water immersion. X-ray photoelectron spectroscopy results indicated that the films fabricated by immersion in solution with heat treatment were mostly composed of MgO and Mg(OH)₂ in both the outer and inner layers. Potentiodynamic polarization measurements revealed that the thin films could improve the surface properties, acting as passive layers.

To ensure energy for mobility in a post-fossil future, power-to-liquids converting electric power from renewable energy sources into liquid fuels is one of the approaches that have attracted attention. The steam electrolysis used to split water to hydrogen and oxygen is run under pressure up to 30 bar at temperatures around 850°C in order to achieve a high efficiency of the process. The impact of such severe conditions on the interconnector materials and coatings designed to retain the evaporating chromium is a crucial issue and needs detailed understanding to ensure a reliable operation.

Coatings of the type La-Sr-Mn oxide (LSM) were deposited on the interconnector material Crofer 22 APU by thermal spraying and by roll coating, the latter as a cost-efficient method. The samples were exposed in laboratory test autoclaves to 850°C and 30 bar in 100% water vapor up to 1000 h and in pure dry oxygen up to 3000 h, both representing the opposite extremes of the possible process atmospheres.

The LSM coating shows in water vapor a grain coarsening after 1000 h as well as a separation of grains with higher La content accumulated in the surface area and grains containing Mn in the interface area. Cr is detected very weakly in the areas with high La contents. In water vapor the oxide scales on the interconnector material with LSM coating are notably thinner than on the uncoated material. However, the LSM coating deposited by roll coating decomposes with the time to a coarse-grained lanthanum-rich phase in the surface area as well as in vicinity of the oxide scale and to a manganese-rich phase concentrated between them. The thermally sprayed LSM coating is more efficient in leading to thinner oxide scales, probably to its higher density. Additionally, it shows a less pronounced separation into the two phases.