The effect of gas composition and contaminants on the lifetime of surface-treated FeCrAlRE alloys

Abstract

The degradation of a variety of alumina-forming Fe20Cr5Al based alloys has been investigated at temperatures ranging from 1100 8C to 1300 8C for up to 4000 h (100 h/cycle) in different oxidising environments such as laboratory air, air þ 10 vol% H 2 O, air þ 60 vol% H 2 O and simulated automotive exhaust gas. Seven model alloys with controlled levels of impurities such as P, S and C and carefully controlled levels of additional elements (Y, Zr, Ti, Hf, Si, La, etc.) and two different commercial alloys (Aluchrom YHfAl and Kanthal APMT) were chosen for this study.The investigation showed that the model alloys containing La, Y þ Si and Y with added C, and the commercial alloy APMT usually had the lowest initial oxidation growth rates, whereas model alloys containing Y plus Zr, and the commercial alloy YHfAl had the higher oxidation rates regardless of the different oxidising environments. Scale spallation was more prevalent in the case of alloys with low oxide growth rates but changing the levels of water vapour in the oxidising atmospheres had only a minor influence on the degree of spallation or the oxide morphology. The scales formed on the alloys containing La, Y þ Si and high C spalled in an adhesive manner (at the scale/metal interface), whereas scales formed on alloys containing Y plus Hf, Zr or Ti cracked and spalled in a cohesive manner (within the scale). Inhomogeneities in the distribution of the alloying additions led to greater changes in the oxide morphologies than any difference in oxidising atmosphere, but the crystallographic textures of all the oxides were similar.This pilot study enabled us to rank the alloys according to their resistance to spallation and also to determine the influence of minor elements when added as tightly controlled single or multiple element additions. Some of these alloys were then used as mechanically weak PVD coatings on a strong (APMT) substrate, and further oxidation experiments confirmed that the coatings then oxidised in a similar way to the bulk alloys.

Alumina Scale Formation on a Powder Metallurgical FeCrAl Alloy (Kanthal APMT) at 900–1,100 °C in Dry O2 and in O2 + H2O

“…The nominal chemical composition of this alloy is given in Table 1 [31,32]. Kanthal APMT is a rapidly solidified powder (RSP) material with RE oxide particles dispersed in the FeCrAl matrix.…”

Microstructural development of α alumina scales developed on FeCrAl alloys

“…There are notable 'hot spots' where chains of REE particles link the outer surface of the scale with the interior. TEM work on the investigation of Y and Hf segregation 24,25 showed that Hf and Y segregate at the GB of the alumina scale in the region near scale/metal interface as elements. In the outer layer of the scale near scale/gas interface Hf and Y were found to precipitate as Y rich particles or Hf rich particles and in some cases as Y-Hf rich particles (oxide particles).…”

Influence of KCl and HCl on a laser clad FeCrAl alloy: In-Situ SEM and controlled environment High temperature corrosion

“…Other work has shown that a thin alumina layer will form at temperatures of 600 o C on a FeCrAl alloy, however this is most likely transient and so not stable and therefore non-protective [3]. The formation of stable alumina is beneficial as it forms dense, thermodynamically stable protective layer [5].…”