ICMCTF 2022 Session A1-1-MoM: Coatings to Resist High-temperature Oxidation, Corrosion, and Fouling I

A key challenge for systems cooled by molten lead is that the cladding materials must resist the harsh environmental conditions that they are exposed to; namely, chemical corrosion by molten lead-based coolant; oxidative corrosion at temperatures above 650°C; and neutron irradiation at doses up to 250 dpa. Austenitic and ferritic-martensitic (FM) steels have been identified as good candidates for cladding materials in molten lead environments. Nevertheless, these steels are susceptible to corrosion in oxidised molten lead and, therefore, their properties need to be improvedeither by alloying with corrosion protective elements or by depositing a corrosion protective barrier coating on the surface of a steel. The ability of alloying elements, such as aluminium and chromium, to form protective oxides is well known. Thus, the aim of this work was to investigate the deposition of protective barrier layers consisting of Al, Al₂O₃ and FeCrAl on austenitic stainless steel type 316L coupons. The coatings were deposited by mid-frequency pulsed DC unbalanced magnetron sputtering in reactive (Al₂O₃) and non-reactive modes (Al, FeCrAl). Individual layers were investigated, as well as multi-layer FeCrAl/Al/AlOx coating stacks. Coatings were characterised by SEM and EDX and then subjected to a corrosive environment of molten lead at elevated temperatures (up to 800^oC) and varying oxygen contents to investigate and compare the corrosion resistance of the deposited layers to allow the best performing corrosion barrier coatings for HLMC FRs applications to be nominated and further developed.

Refractory metals and their alloys are interesting materials to be used in high temperature applications. They are characterized by a very high melting point, typically beyond 2000°C, and their good mechanical properties at high temperatures and are therefore promising candidates for novel high-temperature materials. Besides their low oxidation resistance at target temperatures, already at intermediate temperatures, they can be prone to two different attack mechanisms: Pesting and hot corrosion.

Pesting, also referred to as catastrophic oxidation, describes the formation of volatile oxide species at intermediate temperatures and leads to the total disintegration of metal bulk material into powder. Besides pesting another particularly severe corrosion attack is induced by deposits on the surface of compounds, which usually are the result of impurities in combustion atmospheres. In turbine environments sulphates and alkali metal saltsare known to induce hot corrosion.To counteract both attack mechanisms, the formation of protective oxide scales is necessary. As α-Al₂O₃ is commonly known to be very resistant in oxidizing as well as hot corrosion environments, this work aims to make the formation of such an oxide scale possible by the application of Al-coatings.

By means of pack cementation, an in situ chemical vapor deposition process, Al-coatings (with thicknesses up to 60 μm) were successfully applied on four different refractory metals (Mo, Nb, Ta and W) and two recently developed Mo-Si-Ti alloys (eutectic Mo-20.0Si-52.8Ti and eutectoid Mo-21.0-34.0Ti) [1]. Using thermogravimetric analysis for 100 h in synthetic air at intermediate temperatures (700 °C and 900 °C), the oxide formation was investigated and the improvement of the oxidation behavior correlated to Al₂O₃ scale formation was confirmed. The formation of these scales results in a substantially decreased oxide growth rate in comparison to the uncoated substrates. Post exposure, Al reservoirs (intermetallic phases) were still present below the formed oxide scale, making healing of the scale possible in the case of crack formation. The applied layers, the formed intermetallic phases and the formed oxide scales were analyzed before and after exposure to the oxidizing atmospheres using optical microscopy, XRD, SEM, EDX and EPMA.

Hot corrosion is a common phenomenon observed in gas turbine engines, coal gasification plants and waste incinerators. It occurs in high-temperature settings, where a sulfur-rich atmosphere reacts with salt impurities such as Na, Mg, Cl or V, and form high-melting sulfate-salts, that then deposit and adhere on machining component surfaces. There, the salt deposit elicits an accelerated degradation of the material through the formation of non-protective porous oxide scales. Depending on the temperature range, two distinctively different corrosion mechanisms can emerge. At temperatures below the melting point of the salt deposit (~600-850 °C), low-temperature hot corrosion dominates as mechanisms, whereas at temperatures above the melting point, high-temperature hot corrosion predominates (~850-950 °C). For all of the above mentioned fields of application, Ni-, Co-, and Fe-based superalloys have proven to be a reliable choice of material, due to their superior mechanical properties at high temperatures, as well as good oxidation resistance in air. However, if exposed to hot corrosion conditions, their overall oxidation resistant qualities diminish drastically.

From this perspective, this contribution showcases Ti_1-xAl_xN as an interesting candidate as protective PVD coatings for extending the lifetime of highly-stressed material components in hot-corrosion environments. Ti_1-xAl_xN coatings with varying metal content ratios were arc-evaporated on Ni-based superalloy substrates and tested in an in-house built hot-corrosion testing rig. By applying a sulphate-salt mixture from the alkali and alkaline earth metal group, the samples were corroded in a SO_x-rich atmosphere for a maximum of 30 h according to the HTHC and LTHC conditions, and subsequently analyzed and evaluated for their applicability using a set of high-resolution characterization techniques.

Keywords: Corrosion Resistance; PVD coatings; Diffusion Pathways; Hot Corrosion; HTHC; LTHC;

The integrity of hafnium diboride, a refractory and hard metallic ceramic, is compromised at high temperatures (900-1000°C) when air is present due to oxidation of the boron sublattice to form liquid boron oxide, which evaporates. One potential solution is to deposit a Hf_1-xAl_xB₂ alloy film, with the expectation that Al can oxidize to form a protective Al_xO_y overcoat.

We use low temperature chemical vapor deposition to deposit the alloys, as described elsewhere in this conference. This approach has the advantage that Al will be distributed throughout the material; by contrast, in conventional hot-pressing of separate HfB₂ and AlB₂ powders the phases may not mix intimately. We previously showed that as-deposited HfB₂ is amorphous and crystallizes upon annealing at temperatures ≥ 600°C. For the alloy, we hypothesize that excess aluminum will be liberated upon crystallization, and will diffuse to the surface and form a passivating oxide.

We show that the oxidation product is strongly temperature dependent. Upon annealing at 700°C for 1 hour, a mixture of hafnium, aluminum, and boron oxides form. But this mixed oxide is not protective, and complete oxidation of films occurs when heated to higher temperatures. At annealing temperatures between 800°C and 900°C, the oxides of boron and aluminum react to form acicular aluminum borate, which is spread uniformly on the surface of the film. TEM cross sections reveal hafnium oxide particles embedded in aluminum borate needles; SEM shows surface roughening; STEM-EDS identifies the composition of the reaction products; and SAED confirms the crystal structures of the products. We conclude that aluminum is not a viable alloying element for the formation of a passivating oxide on diboride thin films. Based on phase diagrams and oxide literature, we suggest using alloying elements like chromium, which forms a passivating oxide but does not react with the oxides of boron and hafnium.