Hot corrosion mechanisms of arc-evaporated Ti1-xAlxN coatings

Abstract

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 Ti1-xAlxN as an interesting candidate as protective PVD coatings for extending the lifetime of highly-stressed material components in hot-corrosion environments. Ti1-xAlxN 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 SOx-rich atmosphere for a maximum of 30 h according to the HTHC and LTHC conditions, and subsequently analysed and evaluated for their applicability using a set of high-resolution characterization techniques.