Depletion sensor for protective high temperature coatings

AbstractIn 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 (Al1 − xMexN, 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 Al1 − xCrxN (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 Al1 − xMnxN, Al1 − xCoxN (x = 2–7 at%) and Al1 − xCrxN 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.