Depletion sensor for protective high temperature coatings

Materials and Corrosion 62 (2011), 706-712

In order to minimise corrosion at high temperatures metallic or intermetallic Al- and/or Cr-rich protective coatings are applied to metallic alloys. Protection against corrosion is achieved by the formation of a continuous Al₂O₃ and/or Cr₂O₃ layer. Progressive scale formation, as well as interdiffusion between the coating and the substrate during long operating stages depletes the scale-forming elements, Al and Cr. The decrease of their concentration below a critical value is followed by accelerated corrosion and rapid breakdown of the component. Non-destructive depletion measurement is not possible, because of the absence of suitable materials that serve simultaneously as depletion sensors and reservoir phases. In a novel development, protective high temperature coatings containing a magnetic phase which at the same time acts as a reservoir phase are used as a depletion sensor. The alloy surface is coated with the magnetic substance either by reactive magnetron co-sputtering or by using pack cementation. In the course of operation, the formation of a protective oxide scale depletes the reservoir and the measured magnetic signal decreases. Measurement of the change of the coating's magnetic signal enables in situ assessment and non-destructive detection of depletion. In order to avoid perturbances in the magnetic signal coming from the coating the metallic substrate must be non-magnetic. Therefore this concept is restricted to Cr/Ni-austenitic steels and Ni-base alloys. Doping of AlN with transition metals (Al_1 − xMe_xN, Me = Cr, Co, Mn, x  = 2–7 at%) makes it a suitable magnetic reservoir phase, i.e. Al- and/or Cr-containing. Furthermore, it is ferromagnetic, has a high Curie temperature and is magnetically soft. Samples of Alloy 800 and Alloy 602 CA were coated with Al_1 − xCr_xN (x  = 2 and 3 at%) using both pack cementation and PVD. Measurements of the magnetic moments of the coatings at temperatures up to 300 °C show very soft ferromagnetic behaviour. Coatings on different substrates with Al_1 − xMn_xN, Al_1 − xCo_xN (x = 2–7 at%) and Al_1 − xCr_xN with higher Cr contents (x  = 4–7%) are underway. Investigations of the magnetic properties of the coatings at temperatures up to the Curie point are also in progress.