Weld-overlay iron-aluminide coatings for use in high-temperature oxidizing/sulfidizing environments

Abstract

The development of iron-aluminide weld overlay coatings for high-temperature oxidation and sulfidation resistance critically depends on the determination of the aluminum concentration range for which good corrosion behavior and weldability coexist. This study demonstrated that a sound weld overlay composition with about 30 at.% A1 has relatively low corrosion rates in a highly aggressive oxidizing/sulfidizing (H2S -H2 -H20 -Ar) environment at 800°C. Its corrosion resistance was superior to alloys with compositions like that of type 310 stainless steel and Fecralloy. The results with this overlay composition can be explained on the basis of what is known about the effects of variations in aluminum and chromium concentrations on high-temperature oxidationhlfidation from studies with bulk iron aluminides. While higher aluminum concentrations are even better for long-term corrosion resistance, the ability to reliably produce weld overlays of such compositions without hydrogen cracking is problematical and is the subject of continuing development.IRON ALUMINIDES have several attractive attributes that have prompted many alloy development studies of these systems (see, for example. refs. 1 and 2). Alloys based on Fe3A1 and FeAl have been shown to have good-to-exceptional resistance to degradation in high-temperature oxidizing, oxidizinglsulfidizing, sulfidizing, and oxidizing/chlorinating gases (1,3-9). Indeed, the oxidatiodsulfidation resistance of iron aluminides containing greater than about 18 -25% A1 (at.% unless noted otherwise) is maintained at temperatures that are well above those at which these alloys have adequate mechanical strength (for example, SOOOC). Because of this and possible cost considerations, it is anticipated that appropriate compositions of iron aluminides may find application as coatings or claddings on more conventional higher-strength materials which are generally less corrosion-resistant at high temperatures.In a previous paper (lo), initial results from 800°C exposures of weld-overlay iron-aluminide specimens to a highly aggressive (H2S -H, -H,O -Ar) environment were reported. The investigation involved the characterization of the thermodynamic stability and reaction kinetics of these coatings, but did not determine their ability to physically separate the corrosive species from susceptible substrate material. Under conditions that produced coatings with -35% AI, corrosion resistance equivalent to the best wrought iron aluminides was observed. A second set of weld deposits, with A1 concentrations e 2395, showed significantly less sulfidation resistance.A key issue in the development of appropriate weld deposit compositions is the determination of the optimal A1 concentration. Based solely on oxidation/sulfidation considerations, higher AI levels are preferable (8). However, the susceptibility to hydrogen-induced (cold) cracking of the weld overlays increases with the concentration of aluminum in the deposit (1 1,lZ). This type of cracking occurs due to the combined influence of st...

Investigation of Iron Aluminide Weld Overlays

“…This concept was studied and modeled by Quadakkers et al (1994a and1994b) and recently reviewed by Tortorelli and Natesan (1998). The time frame for protection has been found to be a fimction of the total amount of aluminum available for reaction (at the surface and in reserve) and the rate at which it is consumed.…”

“…Intermetallic iron aluminide alloys based on the ordered structures of F@l and FeAl show excellent resistance to oxidation and sulfidation in highly aggressive environments (McKamey, 1991;DeVan, 1993aDeVan, , 1993bTortorelli, 1995Tortorelli, , 1994% 1994b. This outstanding resistance has been attributed to the formation of a dense, adherent surface scale of aluminum oxide (A1203) which is stable under a wide range of environments.…”

Corrosion Behavior of Fe-Al-Cr Alloys in Sulfur- and Oxygen-Rich Environments in the Presence of Pyrite

“…Although weld overlays were the coatings of interest, cast alloys were used because it was shown previously that the hightemperature corrosion behavior of weld claddings could be explained by using cast alloys of equivalent composition. 13 It has been reported since that the corrosion behavior between weld overlays and cast alloys are similar due to the lack of microsegregation within the welds and both having coarse-grained microstructures. 14 Binary Fe-Al alloys, ternary Fe-Al-Cr alloys, and quaternary Fe-Al-Cr-Ti alloys that were used for this study are listed in Table 1.…”

The corrosion behavior of a sputtered micrograin film on Fe-5Cr-5Si alloy in H 2 -CO 2 -H 2 S mixture at 700 °C

“…For example, Bamba [13] and Guo [14] reported that the addition of Cr to Fe-Si alloy can induce the formation of SiO2 layer in pure oxidizing atmosphere. Second, we also could decrease the grain size of the alloy to improve the outward diffusion of the Cr and Si to achieve the selective oxidation of Si and form the protective scale [11,12]. In our previous study, the Fe-Si alloy exhibited poor corrosion resistance in the H2-CO2-H2S at 700 o C [3], which is not acceptable.…”

“…Dudziak et al [11] reported that multilayer CrAlYN/CrN coatings could induce a protective layer locally on Ti-45Al-8Nb in H2/H2S/H2O mixture. Besides, Nano-or micro-coating is reported to be able to improve the oxidation/sulphidation resistance of the alloy [12].…”