Sensor Coatings for High-Temperature Applications

Metallic plant components at high temperatures may suffer from high temperature corrosion. To minimize this type of attack Al and/or Cr-rich metallic or intermetallic coatings are applied. At high temperatures these coatings form continuous Al₂O₃ and/or Cr₂O₃ scales, which serve as a barrier between the environment and the underlying metal. Long operating periods deplete  the coating in Al and Cr due to progressive scale formation and interdiffusion between the coating and the substrate. The decrease of their concentration under a critical value results in accelerated corrosion and rapid breakdown of the component. Still, the measurement of this depletion process is performed only by destructive methods.

To provide a tool for a non-destructive depletion measurement, high temperature protective coatings made of a ferromagnetic material are being developed which simultaneously serve as a reservoir phase/depletion sensor. The application to the substrate surface occurs via pack cementation (one- or two-step) or by a CVD-process. In the course of operation the formation of a protective oxide scale depletes the reservoir in the coating and the measured magnetic signals decrease, thus allowing non-destructive detection of the depletion. The substrate must be non-magnetic, therefore this concept focuses on Cr/Ni-austenitic steels and Ni-base alloys.

Because of its superb properties, such as soft ferromagnetic character, a Curie temperature as high as 1000K (sensor) and a high Al- and Cr-content (reservoir), Cr-doped AlN i.e. Al_1-xCr_xN (x=2-7 at.%) meets the requirements for a perfect reservoir phase/depletion sensor.

Samples of Alloy 800, Alloy 602 CA and Alloy 80A were coated with Al_1-xCr_xN (x = 2-3 at.%) by pack cementation and CVD. Magnetic measurements show soft ferromagnetic behaviour of the coatings at temperatures up to 300°C. The studies of coated samples exposed to oxidation conditions (air, 600°C) for up to 1000h show changes in the magnetic moment of the coating with time of high temperature exposure. Characterisation of the degradation kinetics and the corresponding change in the coatings morphology is presented in the paper.

A prototype of a suitable contact-free magnetic measuring sensor for on-field measurement of the depletion has already been developed [1], and is currently optimised for protective coatings.

[1] I. Teliban, S. Chemnitz, C. Thede, C. Bechtold, W.J. Quadakkers, M. Schütze, E.Quandt, Rev. Sci. Instr., 80, 115106, 2009