Hot Corrosion of Na[sub 2]SO[sub 4]-Coated Ti[sub 3]AlC[sub 2] in Air at 700-1000°C

Abstract

Hot corrosion of layered machinable ceramic Ti 3 AlC 2 coated with 3.8 Ϯ 0.2 mg/cm 2 Na 2 SO 4 in air at 700-1000°C has been investigated by means of thermogravimetric analysis, X-ray diffraction, Raman spectroscopy, and scanning electron microscopy/ energy-dispersive spectroscopy. The hot corrosion of Ti 3 AlC 2 was slight at low temperatures of 700 and 800°C, while Ti 3 AlC 2 was severely attacked by fused sodium sulfate at 900 and 1000°C. No protective scales were observed on Ti 3 AlC 2 , and sulfur-rich layers were present at the Ti 3 AlC 2 substrate/scale interface. The linear hot corrosion kinetics at 800 and 900°C demonstrated that electrochemical reactions of Ti 3 AlC 2 with sodium sulfate at the substrate/scale interface dominated, while the parabolic kinetics at 1000°C implied that the rate-limiting step involved in the hot corrosion was the diffusion of hot corrosion medium through the scale formed. The hot corrosion behaviors were explained by a mechanism of electrochemistry coupled with basic dissolution-precipitation. Two-layered machinable ternary carbides, Ti 3 AlC 2 and Ti 2 AlC, in the Ti-Al-C system, are of interest as potential high-temperature structural materials due to their high specific strength and excellent high-temperature properties.1,2 Ti 3 AlC 2 and Ti 2 AlC exhibit good oxidation resistance due to the formation of a protective ␣-Al 2 O 3 layer at high temperatures in spite of the low aluminum content. 3,4 Moreover, in contrast to traditional carbide ceramics, they can be readily machined by both conventional tools originating from the relatively weak bonding between sheets of Ti 6 C octahedra and neighboring close-packed Al atoms, 5 and by electrodischarge machining because of its high electrical conductivity. 1,2As a potential high-temperature structural material, Ti 3 AlC 2 will always be exposed to the service circumstance in which hot corrosion occurs. Hot corrosion became a topic of popular interest in the late 1960s as gas turbine engines of military aircraft suffered severe corrosion during the Vietnam conflict when operating over seawater. 6 Metallographic inspection of failed parts often showed sulfides of nickel and chromium, so the mechanism was initially called sulfidation. However, studies by Goebel and Pettit 7 and by Bornstein and DeCrescente 8,9 showed that sulfide formation indeed resulted from the reaction of the metallic substrate with a thin film of fused salt of sodium sulfate base rather than with Na 2 SO 4 vapor. Because corrosion by a thin electrolyte film bears some common features with ''atomospheric corrosion'' by an aqueous film at room temperature, the phenomenon has been renamed ''hot corrosion''. While aqueous atomospheric corrosion is often controlled by the diffusion of dissolved oxygen in the water film, numerous measurements have shown that the soluble oxidant in hot corrosion is SO 3 (S 2 O 7 2Ϫ ) in the fused salt. [10][11][12][13] It is well acknowledged that the subject of hot corrosion can be divided into two subtypes, i.e., hightemperature ...

Hot corrosion and protection of Ti2AlC against Na2SO4 salt in air

“…1. Similar tri-stage characteristic was observed in the hot corrosion of Ti 3 AlC 2 at 900 • C. 18 At the first stage, the so-called gestation/or incubation period, the weight-changes increase at relatively slow rates, i.e. 1.2 and 8.0 mg cm −2 h −1 at 900 and 1000 • C, respectively.…”

“…The hot corrosion kinetics of Ti 3 AlC 2 displays a tri-stage characteristic at 900 • C, 18 which is similar to those of Ti 2 AlC. However, the linear weight-change rates of Ti 3 AlC 2 are much higher than those of Ti 2 AlC.…”

“…The most distinguished feature is that sulfur-rich area is present at the scale/Ti 2 AlC substrate interface for the specimens hot corroded at 900 and 1000 • C. The segregation of sulfur was also observed during the hot corrosion of Ti 3 AlC 2 . 18 The presence of sulfurrich bands at the substrate/scale interfaces demonstrates that chemical reaction of Ti 2 AlC with Na 2 SO 4 occurred during hot corrosion.…”

“…17 It is therefore important to understand the effects of the deposited salt on the hot corrosion behavior of Ti 2 AlC at high temperatures. Wang and Zhou 18 reported that the hot corrosion resistance of Ti 3 AlC 2 was not satisfied at temperatures higher than 800 • C. However, information on the hot corrosion behavior of Ti 2 AlC is sparse.…”

Corrosion behavior of Ti3AlC2 in molten KOH at 700 °C

“…The inner is a continuous alpha-Al 2 O 3 layer, while the outer is a rutile TiO 2 layer [3,5,10]. Hot corrosion of MAX phases in molten salts was also reported by literatures [13][14][15][16]. For instance, Cr 2 AlC exhibits exceptionally good hot corrosion resistance against molten Na 2 SO 4 salt due to the formation of a protective Al 2 O 3 -rich scale at 800-1300 ℃ in air [7].…”