ICMCTF2008 Session A2: Coatings for Use in Harsh Environments

Materials envisioned for application in the harsh high-temperature environments associated with advanced energy systems must show remarkable resistance to chemical degradation or be protected by coatings with this attribute. In either case, the surface layers that develop during exposure in such environments must exhibit long-term stability while acting as an effective permeation barrier to reactive species. The ability to form such stable layers depends on a number of chemical and physical factors as influenced by material parameters, (e.g., composition and structure) as well as by the conditions set by the environment. For coatings, there are additional considerations associated with mechanical and chemical compatibility with the substrate on which they are applied and with the effects of a particular synthesis technique on structure and composition. All these issues will be discussed in terms of corrosion-resistant coating design for extended lifetimes in reactive, high-temperature environments. From this, speculation on concepts for, and routes to, smart and/or highly resistant coatings that rely on present knowledge as well as anticipated advances in our understanding of reaction dynamics and materials behavior under very aggressive environmental conditions will be made.

Work sponsored by the U.S. Department of Energy, Division of Materials Sciences and Engineering and Office of Fossil Energy, Advanced Research Materials Program.

Chlorine gas is widely encountered in chemical industries, as well e.g. as in waste incinerators and plastic/polymer decomposition mills. The corrosion resistance of materials at high temperatures in oxidising atmospheres is usually ensured by the formation of a slow growing protective surface oxide scale. Under chlorine-based atmospheres, the process of scale formation may be considerably affected and the presence of chlorine usually impedes the formation of a long term protective dense oxide scale. A previous study has already been performed in oxidizing-chloridizing atmospheres and has led to the new concept of material quasi-stability diagrams [1- 2]. This study showed that nickel-based alloys offer the best resistance against chlorine attack in general, among the commercial alloys.The quasi-stability diagrams predict that molybdenum should have a positive behaviour in “reducing”-chloridizing atmospheres, whereas aluminium has a positive behaviour in “oxidizing”-chloridizing atmospheres. According to these thermodynamic calculations and first experimental results, several NiAlMo alloys were produced and investigated, and the most successful alloy was selected to be used as a coating for conventional steels against chlorine corrosion. A coating of approximatively 300 µm thickness was thermally sprayed on Armco Iron by means of Atmospheric Plasma Spraying (APS). This work presents the corrosion behaviour of a NiAlMo APS-coating under chlorine-based atmospheres in “reducing”-chloridizing atmospheres and in “oxidising”-chloridizing atmospheres at 800°C. In addition, metallographic characterisation as well as EPMA measurements of the coating cross sections were carried out before and after the corrosion tests. The coating revealed a surprising high resistance against chlorine corrosion which was mainly due to the presence of á-molybdenum phase.

[1] R. Bender, M. Schütze, Mater. Corros., 54 (2003) 567.

[2] R. Bender, M. Schütze, Mater. Corros., 54 (2003) 652.

High temperature applications demand materials that have a variety of properties such as high strength, toughness, creep resistance, fatigue resistance, as well as resistance to degradation by their interaction with the environment. All potential metallic materials are unstable in many high temperatures environments without the presence of a protective oxide or coating on the component surface. High temperature alloys derive their resistance to degradation by forming and maintaining a continuous protective oxide surface scale layer that is slow-growing, very stable, and adherent.

Turbine engines for both Naval aircraft and ships are subjected to the corrosive environment of the sea to differing degrees. Higher operating temperatures also are leading to new degradation modes of coatings and materials. Fuel contaminants or the lack of contaminant from alternative synthetic fuels may also strongly influence coatings and materials which can adversely affect the life in these engines. This presentation will dwell on some past results of materials testing and offer some view on future direction into materials research in high temperature materials in aggressive environments.

The objective of this research project is to improve the corrosion resistance of stainless steel parts to corrosive media (chlorine- and fluorine-based gas) using metallic thin films deposited by Physical Vapour Deposition. In the present work, sputter nickel alloys thin films have been considered to improve the corrosion resistance of stainless steel in fluorinated gas. Silver has been chosen as the alloy element due to its supposed capability to reduce the fluorine diffusion speed in the passive layer of NiF₂. Indeed, Atkinson¹ carried out a study of the influence of certain doping agents on this diffusion speed by numerical simulation. This study has shown that the smallest monovalent ions such as Li, Na and Ag could obstruct the migration of fluorine in the passive layer. For practical reasons, it has been planed to study the corrosion resistance of Ni-Ag films in fluorinated media. Ni-Ag coatings have then been deposited by co-sputtering in various compositions (Ni_x-Ag_1-x with 0≤x≤1) on stainless steel substrates. The different films have been observed by Scanning Electron Microscopy (SEM) and characterized by Energy Dispersive X-ray Analysis (EDX) before and after accelerated corrosion tests in fluorinated medium. Then the corrosion resistance of the coatings has been linked to the coating stoechiometry. The doping concentration of silver into the passive NiF₂ layer has also been analyzed by Auger spectroscopy in order to be correlated to the corrosion resistance properties of the Ni-Ag coatings.

¹K. Atkinson, PhD - Imperial College of Science, Technology and Medicine, London, 2002.