Effects of minor additions and impurities on oxidation behaviour of FeCrAl alloys. Development of novel surface coatings compositions

Abstract

In the present work the effects of single or combined minor additions of Zr, Hf, Ti and C on the oxidation behaviour of Y-containing, FeCrAl alloys have been studied. For this purpose high-purity, model alloys with single or multiple minor alloying additions were used. The results of long term discontinuous oxidation tests and detailed kinetics studies using thermogravimetry were complemented with extensive microstructural characterisation of the formed alumina scales using SEM and STEM. Hence, the oxidation kinetics and scale spallation rates and failure modes were correlated with the oxide composition and microstructure. The results demonstrate that the frequently reported positive effect of Zr, Hf and Ti on the lifetime oxidation behaviour of FeCrAl alloys can only be fully exploited if the concentrations of the above elements are carefully adjusted and the interaction with typical alloy impurities, such as carbon, is considered.

Lifetime‐determining spalling mechanisms of NiCoCrAlRE / EB‐PVD zirconia TBC systems

“…A decrease of oxidation rate at a low dose of Hf and increase at higher Hf concentrations have been also reported for cast FeNi 45 Cr 25 Al 5 alloys [5] and accordingly for FeCrAl-0.03 % Hf [8]. [7] stated the oxidation rate to decelerate at concentrations 0.3 % Hf.…”

“…The term "adhesive mode of spallation" was introduced for scales that mainly spalled at the metal/TGO interface, whereas a "cohesive mode of spallation" was attributed to prevailing spallation within the TGO [8]. These terms are also preferred in the following.…”

“…The bigger grain size of the TGOcompared to, e. g., Zr-modified alloys -can be hypothesized to originate from incorporation of far less nuclei provided by containing monoclinic HfO 2 , which is much less soluble in alumina than the tetragonal HfO 2 polymorph that is just forming at 1422 C [32]. A reduced incorporation of hafnia in alumina scales compared to zirconia has been also reported [8].…”

Lifetime extension of FeCrAlRE alloys in air: Potential roles of an enhanced Al-reservoir and surface pre-treatment

“…800 -950 8C) surface protection could be provided best by pre-oxidation to preform an a-Al 2 O 3 scale prior to service, as has been examined previously [22]. A novel, controlled gas annealing treatment has been developed by the SMILER project [23], which minimised depletion of the alloy Al prior to service. Assessment of the effectiveness of gas annealing upon the long term oxidation behaviour of FeCrAlRE alloys, in air, has been undertaken in the current study.…”

High-temperature oxidation of FeCrAl alloy with alumina–silica–ceria coatings deposited by sol–gel method

“…The protective character of a-Al 2 O 3 scale and its adherence to the substrate can be improved through the use of reactive elements (RE), e.g., Zr, Hf, Y, Ce, La, and other rare earth elements) [1,[8][9][10][11][12][13]. The required RE concentration in the scale depends on several factors such as the chemical composition of the alloy, the amount and kind of impurities (e.g., sulfur) the latter contains, the kind of reactive element and the oxidation conditions (temperature, time, environment).…”