The effect of aluminum content on the corrosion behavior of Fe-Al alloys in reducing environments at 700 °C

Abstract

The high-temperature corrosion behavior of monolithic Fe-Al alloys, with 0 to 20 wt pct Al, was investigated at 700 ЊC in a reducing atmosphere ( p(S 2 ) ϭ 10 Ϫ4 atm, p(O 2 ) ϭ 10 Ϫ25 atm) for up to 100 hours. Postexposure characterization of the corrosion reaction products consisted of surface and cross-sectional microscopy, in combination with energy dispersive spectroscopy, electron probe microanalysis, and quantitative image analysis. From the kinetic data, three stages of corrosion behavior (i.e., inhibition, breakdown, and steady state) were found with the observance and/or duration of each stage directly related to the aluminum content of the alloy. The first stage, labeled the inhibition stage, was characterized by low weight gains and the absence of rapid degradation of the alloy. Typically observed for compositions with 10 to 20 wt pct Al, protection was afforded due to the development of a thin, continuous alumina scale. For alloys with 7.5 wt pct Al, the ability to maintain the initially formed alumina scale was not observed, resulting in the breakdown stage. Localized corrosion product nodules, containing iron sulfide (Fe 1Ϫx S) and the spinel-type tau phase (FeAl 2 S 4 ), developed through the alumina scale due to sulfur short-circuit diffusion. These growths were accompanied by relatively high corrosion rates. Further decreasing the aluminum content to 5 wt pct and below lead to the formation of a continuous sulfide scale whose growth was controlled by iron and sulfur diffusion through the previously formed product. The alloy wastage rates in the steady-state stage were relatively high when compared to the previous two regions.

Evaluation of the Corrosion Resistance of Fe–Al–Cr Alloys in Simulated Low NO x Environments

“…Similar minimal weight gains for FeAlCr alloys in sulfur-bearing environments have been observed by other researchers [10,[18][19][20][21]. Banovic et al [10] exposed iron aluminum alloys to a 1%H 2 S-Ar atmosphere at 700°C for up to 100 h and found that alloys with more than 10 wt% aluminum exhibited weight gains of less than 1 mg/cm 2 .…”

“…This concentration is also close to the critical composition required to form an external Al 2 O 3 layer [8][9][10]. It has been shown that additions of chromium reduce this critical concentration of aluminum [11] in oxidizing conditions; however, other researchers have reported detrimental effects of adding chromium to iron aluminum alloys [12] with respect to sulfidation resistance.…”

“…9 free of any large iron sulfide type corrosion product, indicating that the scale on this alloy was more protective and stable. The relationship of decreased corrosion rates with increased aluminum concentration has been well established for iron aluminum alloys [8][9][10][24][25][26] and is generally attributed to the simultaneous increased likelihood of forming Al 2 O 3 and suppression of other base metal corrosion products with increased Al concentration. The results presented here indicate that for an iron alloy with 5 wt% chromium, a critical aluminum concentration of between 9.2 and 11.5 wt% (the actual alloy compositions in this study) is required to suppress the formation of iron sulfide at 500°C in the mixed oxidizing/sulfidizing environment.…”

Internal Oxidation–Nitridation of Ferritic Fe(Al) Alloys in Air

“…This phenomenon has been further studied as a function of temperature because of the potential enhanced degradation of iron-aluminide coatings, [3][4][5][6][7][8]17] particularly at lower temperatures (B700°C) where interdiffusion with the substrate is less of a life-limiting concern [18]. Figure 12 shows the mass gain for Fe-19Al-2Cr and Fe-19Al-2Cr-2Ti during 500 h cycles in air at 700, 800, 900 and 1,000°C.…”

“…up to *19 at.% or 10 wt.%) alloys which are being investigated as compositions for weld-overlay coatings for fossil energy systems [3][4][5]. Similar compositions develop when a short-duration, Al vapor deposition process is used on martensitic steels [6].…”