The Oxide Scales Formed on a Dispersion-Strengthened Powder Metallurgical FeCrAl Alloy at 900 °C in O2 and in O2 + H2O

Abstract

Early oxide scale growth on an oxide dispersion strengthened rapidly solidified powder FeCrAl material, Kanthal Ò APMT, was investigated at 900°C in an O 2 ? N 2 and an O 2 ? H 2 O ? N 2 environment for up to 168 h. Gravimetry was used to follow oxide growth and the oxide scale was examined with XRD. Scale morphology was investigated in detail with SEM/EDX, TEM/EDX/CBED. The alloy rapidly formed a protective two-layered a-alumina scale containing oxide nodules. Between the top and bottom alumina layers there was a zone containing chromia-rich particles 5-20 nm in diameter, corresponding to the original sample surface. The alumina scale mainly grew inward after 1 h of oxidation. Alumina scale growth at 900°C was initially somewhat faster in an O 2 ? H 2 O ? N 2 environment than in an O 2 ? N 2 environment.

Cyclic Corrosion and Chlorination of an FeCrAl Alloy in the Presence of KCl

“…1). A TEM investigation by Hellström et al confirmed the presence of a two-layered a-Al 2 O 3 scale after oxidation at 900°C, the inward growing alumina layer was however not continuous over the entire surface [18]. Analysis using XRD showed that a-Al 2 O 3 was present on all pre-oxidized samples.…”

“…95 % N 2 . The chosen pre-oxidation conditions were studied more in detail by Hellström et al [18,19]. A short description of the obtained alumina scales can be found below.…”

Hot corrosion Type II of FeCr‐based model alloys for boiler and heat exchanger applications

“…Materials used for boilers and heat exchangers in coal‐fired power plants suffer from fireside corrosion, a mechanism proposed to be caused by the accumulation of molten salts . The propagation mechanism of fireside corrosion was shown recently by Lutz et al to be analogous to hot corrosion Type II. Hot corrosion has often been investigated on nickel‐ and cobalt‐based alloys for turbine engine applications where Na 2 SO 4 deposits from the combustion environment result in accelerated failure.…”

“…As the steam temperatures in coal‐fired power plants are increased to achieve higher efficiencies, the surface temperatures of the boiler and heat exchanger increase towards 700°C . The temperature dependency of hot corrosion Type II on both nickel‐ and iron‐based alloys is generally assumed to follow a bell‐shaped curve and the maximum metal loss is typically observed in the regime around 700°C but can be offset by SO 2 , the alkali content or the alloy composition . As the increasing temperatures reach the regime of maximum metal loss, the investigation of commercial alloys and their resistance against hot corrosion Type II remains an ongoing topic …”

“…Therefore, the acidic solubility also decreases towards the salt‐oxide interface and the negative solubility criterion is not satisfied. This is observed in the dissolution of pure metals when no new salt is supplied, whereas in the dissolution of iron‐, nickel‐, and cobalt‐based alloys a synergetic effect is proposed to fulfill this criterion . When two (or more) oxides are dissolved simultaneously, one could dissolve as an acidic solute by reacting with SO 3 , whereas the other could dissolve as a basic solute by reacting with Na 2 O.…”