Growth of Nodular Corrosion Products on Fe Al Alloys in Various High-Temperature Gaseous Environments

Transcription

1 Oxidation of Metals, Vol. 54, Nos. 3/4, 2000 Growth of Nodular Corrosion Products on Fe Al Alloys in Various High-Temperature Gaseous Environments S. W. Banovic,* J. N. DuPont,* and A. R. Marder* Received January 6, 2000; revised March 1, 2000 The mechanisms for nodular corrosion-product development were investigated in various high-temperature gaseous environments. Fe Al alloys, with 5 20 wt.% Al, were exposed in both oxidizing and sulfidizing [p(s 2 )G10 4 atm, p(o 2 )=10 25 atm] atmospheres at 700 C for times up to 100 hr. The corrosion kinetics were monitored by the use of a thermogravimetric balance and the morphological development through light-optical and scanning-electron microscopies, energy-dispersive spectroscopy, electron-probe microanalysis, and quantitative-image analysis. Under both conditions, the elimination of nodule formation was observed by increasing the aluminum content of the alloy, above 5 and 7.5 wt.% Al for oxidizing and sulfidizing environments, respectively, which promoted the growth and maintenance of a continuous surface scale of alumina. For those alloys that were observed to develop nodular corrosion products, their morphological appearance was similar in nature regardless of the corroding species. The nodules typically consisted of an outer iron-rich product, either sulfide or oxide, that was randomly dispersed across an alumina scale. Samples from the oxidizing atmosphere displayed a single growth-rate time constant from the kinetics data, suggesting that the nodule growth mechanism was by the simultaneous or codevelopment of two different (Fe and Al) oxides from the onset of exposure. Measurement of nodule planar diameter and depth of penetration into the alloy indicated that growth occurred through diffusional processes. Kinetics data from the development of sulfide nodules in the reducing environment revealed a different type of mechanism. Multiple growth-rate time constants were found due to the localized mechanical failure of an initially formed surface scale. At early times in the sulfidizing *Department of Materials Science and Engineering, Lehigh University, Bethlehem, Pennsylvania X $ Plenum Publishing Corporation

2 340 Banovic, DuPont, and Marder atmosphere, a low corrosion rate was recorded as a continuous-alumina scale afforded protection from excessive product development. However, with the mechanical failure of the scale, sulfur was able to attack the underlying substrate through a short-circuit diffusion mechanism that resulted in rapid weight gains from nonprotective, iron sulfide growth. The sulfide morphologies observed were very complex as continued growth of the nodule did not solely depend upon the diffusing species through the previously formed corrosion products, but also, continued mechanical failure of the oxide scale. It is suggested that the difference in development mechanisms between the two environments may lie in the relative growth rates of the nonprotective, Fe-base corrosion products formed. KEY WORDS: Fe Al alloys; oxidation; sulfidation; nodular growth; corrosion mechanism. INTRODUCTION Development of corrosion-resistant alloys for use in high-temperature industrial applications has been to vary the composition through the addition of critical elements, such as Al, Cr, and Mo. The concept of alloying is to reduce the rate of corrosion either by promoting the formation of a continuous surface product with a slow reaction rate or by improving the growth of an existing, protective scale. Many studies 1 20 have found decreased corrosion rates with the addition of aluminum to iron or ironbase alloys. The increased corrosion resistance during high-temperature exposure in both oxidizing 1,2,4,6 8,14 18 and oxidizing sulfidizing 13 15,17 20 environments was attributed to the selective oxidation of the aluminum that resulted in the formation of a thin, yet passive, δ-alumina scale (for exposure temperatures at or below 800 C). In sulfidizing environments, the protective scale was found to be aluminum sulfide. 10 However, depending upon the alloy composition, exposure temperature, and partial pressures of the corroding species in the gas, these multicomponent materials may form deleterious reaction-product morphologies due to other less-protective phases, i.e., iron sulfide, also being thermodynamically stable. One such type of morphology is the development of fast-growing, corrosion-product nodules dispersed across thin scales of a slower-growing phase (e.g., alumina) on the alloy surface. These localized growths can undermine the protectiveness of the passive surface scale, either oxide or sulfide, leading to higher weight gains and subsequent faster degradation of the alloy. From the literature, two mechanisms concerning nodule nucleation and growth on Fe-base alloys have been proposed: (1) disruption or mechanical failure in an initially formed passive layer with subsequent short-circuit diffusion 4,9,21 and (2) the codevelopment of two different scale phases from the onset of exposure. 4,8,22,23

3 Growth of Nodular Corrosion Products on Fe Al Alloys 341 Nodular formations on the surface of iron aluminum alloys typically follow kinetics similar to one of those shown schematically in Fig. 1. For the mechanism based upon mechanical failure of the scale, a continuous and passive layer forms on the surface, leading to initial protective behavior, as indicated by low reaction rates at early times. Upon loss of protection, due to failure, the corrosion rates increase as faster-growing, nonprotective phases form, resulting in multiple growth-rate time constants (n). Quan and Young 21 found these type of results by studying the corrosion behavior of an Fe 4.5 wt.% Mn 8.8 wt.% Al alloy in a sulfidizing gas [10 8 Fp(S 2 )F10 4 atm] at C. The metallographic study revealed that protection from extreme corrosion was afforded by a thin external scale of MnS that initially formed upon exposure under moderate conditions [p(s 2 ) 10 7 atm at 700 C]. However, when reaction conditions were severe [p(s 2 ) 10 7 atm at 900 C], protection was not found to last, as mechanical breakdown of a thicker MnS scale led to direct contact of the underlying alloy with the gas. Growth of the iron-sulfide nodules was attributed to a µm layer depleted in Al and Mn beneath the scale that did not alllow for reestablishment of the MnS scale once failure had occurred. Fig. 1. Schematic showing typical kinetics curves from the formation of nodules on the surface of Fe Al alloys. Note that the axes are logarithmic and that the slope of the lines represents the growth-rate time constant, n.

4 342 Banovic, DuPont, and Marder Examination of the nodule cross-section revealed the remains of the original protective scale between an outer (Fe, Mn) 1Ax S scale and an inner mixed layer of (Fe, Mn) 1Ax S and (Fe, Mn)Al 2 S 4. The mechanism for nodular development according to codevelopment of two different scale phases from the onset of exposure was found for experiments conducted in oxidizing atmospheres. 4,8,22,23 These formations typically consisted of iron oxide nodules dispersed across an aluminum oxide surface scale. The formation of the nodules could be eliminated by alloying with appropriate amounts of aluminum (Table I), with the critical aluminum content tending to decrease with an increase in temperature. This is due to the higher mobility (diffusion) of aluminum in the alloy at higher temperatures that enables it to reach the surface and form maintain a protective scale. In their model for scale growth, Tomaszewicz and Wallwork 8 proposed that, for alloys having an aluminum content below the critical level, nucleation and growth of the nodules began at the outset of exposure and continued growth of both phases occurred via diffusional processes. This idea of oxide codevelopment was confirmed by continuous weight-gain data that showed a single rate constant at early stages of exposure and the lack of an initial time period for protective scale growth, as schematically shown in Fig. 1. Through microstructural analysis, remnants of any original passive scale were not observed between the inner and outer oxide scales. Corrosion in mixed atmospheres of oxygen and sulfur has also produced nodular formations on Fe Al alloys. 24,25 These growths typically consisted of iron sulfide nodules on top of a surface scale of alumina. However, no interpretation for the growth was given, nor was a study found determining the mechanism of nodular formations from these environments. As many high-temperature applications typically experience these types of mixed atmospheres, it is important to have an understanding of this mechanism, as accelerated degradation of components by corrosion is a major concern. Therefore, it was the focus of this work to investigate the development of nodular corrosion products from both oxidizing and oxidizing sulfidizing Table I. Aluminum Content and Minimum Temperature for Eliminating Nodular Growth on Iron Aluminum Alloys in Oxidizing Atmospheres Reference Temperature ( C) wt.% Al Boggs Tomaszewicz and Wallwork Tomaszewicz and Wallwork Wallwork and McGirr Prescott and Graham Hagel

5 Growth of Nodular Corrosion Products on Fe Al Alloys 343 environments using an integrated experimental approach involving kinetics studies, postexposure metallographic examination, and chemical analyses of the reaction scales. The results from the two reaction conditions are first discussed in a general corrosion-characteristic section, followed by a discussion of the mechanism for nodular formation. EXPERIMENTAL PROCEDURE The Fe Al alloys used during this research were produced at Oak Ridge National Laboratory (Oak Ridge, Tennessee) by argon-arc melting high-purity Fe (99.99%) and Al (99.99%) followed by drop casting into a water-cooled copper mold. Substrates, with dimensions of 1cmB1cmB2 mm, were sectioned from the bulk using a high-speed diamond saw, followed by grinding of the surface to 600 grit. Specimens were prepared immediately before insertion into the balance so as to minimize the formation of a surface scale that may develop during storage. Other researchers observed an early transient or induction period in the kinetics data related to the breakdown of a preformed oxide scale on metallographically prepared samples that were in storage at room temperature. 3,26 29 By freshly abrading the samples before the test, this can be minimized, as well as standardized. The samples were ultrasonically cleaned in soapy water, rinsed, and ultrasonically cleaned in methanol. A Netzsch STA 409 high-temperature, thermogravimetric balance was used to measure weight gain as a function of time. Samples were isothermally held at 700 C for various times up to 100 hr. Two different gas types were used in order to produce reducing and oxidizing environments. For the former, an argon-base gas containing 0.1%H 2 1.0%H 2 S 5 ppm O 2 (by volume) was used to maintain a partial pressure of sulfur [p(s 2 )] and oxygen [p(o 2 )] at 10 4 and atm, respectively, at temperature. The p(o 2 ) was determined using a solid-state oxygen detector and the p(s 2 ) was calculated using the SolGasMix program. 30 According to superimposed thermostability diagrams for iron and aluminum, the location of the testing environment was found to lie in a region of aluminum oxide and iron sulfide (X in Fig. 2). The oxidizing atmosphere was produced by flowing zero-grade air into the chamber. Postexposure characterization of the corroded surfaces was conducted using a JEOL 6300F scanning-electron microscope with an Oxford (Link) energy-dispersive spectroscopy (EDS) system capable of detecting light elements. Polished cross sections were obtained by mounting in cold-setting epoxy with subsequent grinding procedures to 1200 grit with silicon-carbide papers. A final polishing step was conducted using 1.0 µm diamond paste on a low-nap cloth. Further polishing with any type of colloidal alumina or

6 344 Banovic, DuPont, and Marder Fig. 2. Superimposed thermostability diagrams for Fe and Al at 700 C calculated using Ref 30. The solid lines represent the coexistence lines between the iron-base phases, while the dashed line is for the aluminum-containing phases. The exposure atmosphere for the reducing conditions is denoted by X. silica was avoided in order to eliminate any possibilities of contamination or pull-out of the scale. In addition, the use of 200-proof, dehydrated ethanol was used for both lubrication and cleansing solutions during preparation of samples from the reducing environment, in order to avoid degradation of the corrosion products from water. 31 Cross-sectional scale thicknesses were measured on both light-optical micrographs (LOM) and secondary-electron images (SEI) using a Donsanto digitizing pad interfaced with a Nikon Optiphot microscope. A minimum of 20 lengths were taken per layer per sample on various planes. Quantitativechemical information was obtained using a JEOL 733 electron-probe microanalyzer (EPMA) equipped with wavelength-dispersive spectrometers (WDS). The accelerating voltage and probe current were 20 kv and 50 na, respectively. K α X-ray lines were analyzed and counts converted to weight percentages using a ϕ (ρz) correction scheme. 32 A fracturing technique using liquid nitrogen was also employed to view the scales in cross section as significant rounding occurred on the edges of the polished samples. By notching the back side (approximately 4 5 ofthe thickness of the substrate) with a low-speed diamond saw and submersing the specimen for a minimum of 3 min in liquid nitrogen, the samples easily broke. Cross-sectional micrographs of these samples were also taken using the JEOL 6300F.

7 Growth of Nodular Corrosion Products on Fe Al Alloys 345 RESULTS Corrosion Kinetics The effect of alloy composition can be observed readily in the decreasing weight changes with increasing aluminum content in both atmospheres. Fig. 3. Plot of weight gain vs. time for (a) reducing atmosphere and (b) oxidizing atmosphere at 700 C. Figure 3 displays the kinetics results obtained at 700 C for the longest exposure time for each alloy in the given environment. For the reducing gas (Fig.

8 346 Banovic, DuPont, and Marder 3a), increasing the aluminum content above 7.5 wt.% Al resulted in relatively little change in weight. When compared to the reducing environments, the kinetics data for the samples exposed to the oxidizing atmosphere showed significantly lower weight changes (Fig. 3b). However, the similar trend of decreased corrosion rates with increased aluminium content of the alloy was observed. In all cases, shorter exposures followed their respective weight-gain curves with good reproducibility. Morphologies from Sulfidation Environments Correlating with the kinetics data, the relative amount of scale development was found to decrease with increasing aluminum content. Figure 4 shows this effect through the low-magnification, scanning-electron micrographs of the samples exposed to the oxidizing-sulfidizing atmospheres. The 5 wt.% Al alloy grew thick, continuous scales that were bilayered in nature (Fig. 5). EPMA analysis showed that the outer scale consisted of irregularly shaped, iron sulfide (Fe 1Ax S) plates. The inner scale was found to be composed of two phases. The dark phase in Fig. 5b and c were τ-plates (FeAl 2 S 4, a spinel-type compound), while the light particles in the same micrographs were iron sulfide. Figure 5c also shows porosity (black) in this inner layer. EPMA traces into the substrate did not reveal depletion of either Fe or Al or the ingress of sulfur into the alloy, within the detection limits of the equipment (approximately 1 µm). Samples above 7.5 wt.% Al were devoid of thick, continuous, corrosion product on the surface (Fig. 4d). Upon removal from the specimen chamber, these samples were tan, blue, or purple in color to the naked eye. Higher magnification of the sample surfaces revealed continuous coverage by a granular scale with small growths emerging from it (Fig. 6). Note that the preparation marks from the 600 grit paper can be seen in the background (arrows) signifying the absence of thick scale growth in this region (Fig. 6a). The size of the platelets growing on the surface inhibited direct analysis of individual particles; however, a cluster of platelets were analyzed with EDS, indicating the presence of iron, aluminum, and sulfur, which would be consistent with a sulfide phase. Analysis of the granular background scale (Fig. 6b) by EDS indicated high counts of Al and O. As shown by micrographs of fractured cross sections in these areas (Fig. 6c), the scale appeared to be uniform and was roughly 100 nm thick after 50 hr exposure. Aluminum contents between 5 and 15 wt.% Al were found to develop nodular formations on their surfaces. For the 10 and 12.5 wt.% Al samples, the shorter time exposures (2 hr) were free of the thick corrosion product, similar to the 20 wt.% Al sample shown in Fig. 4d. However, for longer exposures, some development was found at the corners and edges (Fig. 4c),

9 Growth of Nodular Corrosion Products on Fe Al Alloys 347 Fig. 4. Low-magnification, scanning-electron micrographs showing the corners of sulfidized samples. The aluminum contents and exposure times are (a) 5 wt.% Al after 50 hr; (b) 7.5 wt.% Al after 25 hr; (c) 10 wt.% Al after 100 hr; and (d) 20 wt.% Al after 100 hr.

10 348 Banovic, DuPont, and Marder Fig. 4. Continued.

11 Growth of Nodular Corrosion Products on Fe Al Alloys 349 Fig. 5. Scanning-electron micrographs of polished cross-section showing the Fe 5 wt.% Al alloy after 50 hr of exposure in the sulfidizing atmosphere. (a) Bilayered scale with thick outer iron-sulfide plates; (b) inner layer; and (c) higher magnification of inner scale indicating the various constituents of the layer.

12 350 Banovic, DuPont, and Marder Fig. 5. Continued. while the 7.5 wt.% Al sample had a higher density of the formations found dispersed across the surfaces (Fig. 4b). The nodules had a similar appearance regardless of the exposure time and their planar diameter size was observed to be related to this variable (Fig. 7a). With the growth-rate time constant (n) observed to be 0.95, it was found that growth of the nodules, parallel to the surface, was linear with time. It did not appear that the substrate grain boundaries played a major role in the location of the nodules as they were well dispersed across the sample face (Fig. 4b). In the nodulefree areas, a granular surface scale, which was similar to the one found on the higher aluminum alloys, was present, (Fig. 6). Cross-sectional analysis (Fig. 8), showed that the nodules consisted of similar phases, as seen in the thick-scale growths. The overall appearance had a lenticular shape (Fig. 8a), with further analysis revealing an outer scale of thick iron sulfide (Fe 1Ax S) plates with various growth directions. The inner scale was also composed of τ plates and iron sulfide particles that developed, normal to the surface, with an n-value of 0.63 (Fig. 7b). In the substrate directly below the nodules, EPMA analysis did not indicate the presence of sulfur or the depletion of either iron or aluminum. The same can be said for the alloy located below the thin scale. Higher magnification at the inner outer scale interface

13 Growth of Nodular Corrosion Products on Fe Al Alloys 351 Fig. 6. Characteristic scanning-electron micrographs of the granular scale found on high aluminum alloys in the sulfidizing atmosphere. (a) and (b) show the surface morphology with the arrows in (a) indicating the original 600-grit preparation lines. A fracture cross section is found in (c). Sample shown is Fe 12.5 wt.% Al after 50-hr exposure.

14 352 Banovic, DuPont, and Marder Fig. 6. Continued. revealed the remnants of a thin-layer phase (Fig. 8b). This phase was observed to span the entire length of the nodule cross section, ending at the alloy surface. Morphologies from Oxidation Environments The morphology of the scales formed in the oxidation environment was similar in nature to that in the reducing atmosphere where increasing the aluminum content led to decreasing corrosion product (Figs. 9 and 10). For samples with greater than 5 wt.% Al, a thin, granular scale was observed to blanket the surface (Fig. 10a and b). Again, the samples had the appearance of being tan, blue, or purple after exposure, with EDS analysis indicating high counts of Al and O. Fractured cross sections (Fig. 10c), revealed that the thickness was also on the order of 100 nm after 50 hr exposure, similar to that found in the mixed-gas environment. EPMA traces into the substrate did not reveal the penetration of oxygen below the surface scale. For the 5 wt.% Al sample, nodule formations of oxide product were observed (Fig. 9b). Again, their appearance was similar regardless of time, with the lateral growth rate having a time exponent of 0.52 (Fig. 7a). The substrate grain

15 Growth of Nodular Corrosion Products on Fe Al Alloys 353 Fig. 7. Kinetics plots showing (a) the growth of the nodule planar diameter and (b) the development of cross-sectional scale thickness with time for the various conditions. boundaries again did not appear to have an influence on the nodule dispersion, as they more readily formed along the 600-grit preparation lines. In nodule-free areas, the scale appearance was the same as previously seen for the higher aluminum samples. Cross-sectional analysis revealed that the nodules were multilayered (Fig. 11), with similar morphologies to that previously seen in the literature. 8 Through EPMA analysis, the lighter, outer layer was found to be Fe 2 O 3, while the darker gray phase at the inner outer

16 354 Banovic, DuPont, and Marder Fig. 8. Scanning-electron micrographs of polished cross section showing the 7.5 wt.% Al alloy after 50-hr exposure in the reducing atmosphere. (a) Lenticular shape of nodule; (b) remnants of an original surface scale (arrows) between the inner and outer scale.

17 Growth of Nodular Corrosion Products on Fe Al Alloys 355 Fig. 9. Scanning-electron micrographs showing the 5 wt.% Al sample after 50 hr in an oxidizing environment. (a) Low magnification of the corner; (b) nodule development on the surface.

18 356 Banovic, DuPont, and Marder Fig. 10. Characteristic scanning-electron micrograph of the granular scale found on high aluminum alloys in the oxidizing environment. (a) Low magnification of the corner; (b) higher magnification of the surface; and (c) fractured cross section (10 wt.% Al after 50- hr exposure).

19 Growth of Nodular Corrosion Products on Fe Al Alloys 357 Fig. 10. Continued. scale interface was Fe 3 O 4. The scale protruding into the substrate appeared to be homogeneous; however, its composition (Table II) resided in a multiphase region (Fe 2 O 3 Fe 3 O 4 Al 2 O 3 ) on the Fe Al O ternary diagram (Fig. 12). Higher magnification of the internal-oxidation layer can be found in Fig. 11b. While the precipitates were too fine to analyze using microprobe techniques, a scan of the area revealed that the composition (Table II) resided in the two-phase region of Al 2 O 3 and α-iron (substrate), as shown in Fig. 12. Thickness measurements of the overall growth of the layers (shown in Fig. 7b), revealed a parabolic relationship with time (ng0.49). DISCUSSION General Corrosion Characteristics In general, the morphologies of corrosion products formed due to hightemperature exposure were similar in the two atmospheres. Given a high enough aluminum content, a protective scale was able to form in both systems that resulted in low corrosion rates and negligible corrosion-product development. While the thin surface scale could not be positively identified in either atmosphere through the techniques presently used, enough data

20 358 Banovic, DuPont, and Marder Fig. 11. Polished cross-section showing the 5 wt.% Al alloy after 20 hr exposure in the oxidizing atmosphere. (a) Light-optical micrograph of a nodule; (b) scanning-electron micrograph of the internal oxidation zone.

21 Growth of Nodular Corrosion Products on Fe Al Alloys 359 Table II. EPMA Data from the 5 wt.% Al Sample Exposed in the Oxidizing Environment Location on sample Fe (wt.%) Al (wt.%) O (wt.%) Outer light gray phase 69.9J J0.3 Outer dark gray phase 72.3J J0.1 Mixed scale protruding into substrate 66.4J J J0.5 Internal oxidation layer 91.2J J J0.2 exist to assume it is an aluminum oxide, most likely gamma. This conclusion can be drawn from the EDS analysis, the temperature regime in which it has formed, the low weight gain observed, and the color of the scales (Hagel 1 also reported tan-, blue-, and purple-colored scales for gamma formation). In addition, many others researchers working on similar alloys have also found protection due to γ -alumina scales in both oxidizing and mixed oxidation sulfidation environments. 1,4,13,15,16,18,22,33 Other surfacecharacterization techniques, such as grazing-incidence X-ray diffraction (GIXRD) and backscatter-electron Kikuchi patterns (BEKP), were used in an attempt to identify the scales. Unfortunately, results from these analyses could not confirm the crystal structure. However, it can be seen that the formation of a continuous and protective scale was promoted by increasing the aluminum content of the alloy above 7.5 wt.% in the reducing environment and 5 wt.% in the oxidizing atmosphere. The result obtained for the oxidation conditions appears to be in agreement with the aluminum content necessary (nearing 7.5 wt.%) for eliminating nodular growth at 700 C (Table I). When the aluminum content was under that required for full surface coverage by alumina, thick corrosion products were found to develop either in the form of nodules or a continuous surface scale (the latter for sulfidation only). The growth of these products was diffusion controlled (Fig. 7b), predominantly through the outward movement of Fe cations, to form an external layer of iron-rich scale (Fe 2 O 3,Fe 3 O 4, or Fe 1Ax S), and the inward diffusion of sulfur or oxygen, as indicated by the inner scale development. Other studies 8 have found that these nodular growths tend to form at highly stressed areas, such as corners and edges or at regions of crystal misfit, such as substrate grain boundaries or triple points; these latter areas typically allow for enhanced diffusion. While some of the development can be found located at the corners and edges, the nodules across the face of the sample were either found randomly distributed or well aligned with preparation marks on the surface. It appears that the rougher surface finish of these samples (600 grit as opposed to polished specimens for other studies) may have eliminated any effect of the substrate grain boundaries to promote nodule formation.

22 360 Banovic, DuPont, and Marder Fig. 12. Fe Al O ternary phase diagram at 700 C (Ref. 40) indicating the compositions of the phases formed in the oxide nodules. Axes in weight percentages. The protection afforded by the thin alumina scale could also be observed through the lack of ingress of either gaseous species. Below the thin-scale regions, no oxygen or sulfur was found. However, with the formation of nodules, oxygen and sulfur were able to short circuit the protectiveness of the scale. This can clearly be observed in the alloys exposed to the oxidizing environments where internal oxidation was exclusively associated near a nodule (Fig. 11a). At no other place can this internal oxidation layer be found. Mechanisms of Nodular Formation While the similarity in scale morphology of the nodular corrosion products from the differing environments can be noted, their development, according to kinetics data, was measurably different. Figure 13 shows an example of weight gain plotted on logarithmic axes for two samples (5 and 10 wt.%) that developed nodules in the different exposure environments

23 Growth of Nodular Corrosion Products on Fe Al Alloys 361 Fig. 13. Kinetics data showing weight gain vs. time from Fig. 3, on logarithmic axes, for specified samples. (oxidizing and sulfidizing, respectively). By plotting the data in this manner, changes in the growth-rate time constant (n) can easily be observed as reactions that occur over a limited amount of time can have the form: 34 xgkt n CC (1) where x is the weight gain per area (or scale thickness), t is time, and k and C are constants. By creating a double-logarithmic plot, the n-value can be found by determining the slope of the line: log(x) Gn log(t)cc (2) Values for which n equals 1, 0.5, or 0.3 are said to follow linear, parabolic, or cubic growth rates, respectively. As seen, the differences in the shapes of the curves are readily apparent. In the oxidizing atmosphere, the 5 wt.% Al samples that formed nodules had kinetics curves similar to that found for the mechanism based upon codevelopment of different cation oxides 2,8,22,23 (Fig. 1). This shows a single rate constant from the onset of exposure indicating that a continuous, passive scale did not initially form to provide protection. Instead, it is believed that the iron and aluminum oxides formed concurrently from the inception of exposure.

24 362 Banovic, DuPont, and Marder At early times, the development of iron oxide and alumina nuclei on the surface of the Fe Al alloys would occur as they are both thermodynamically stable in the oxidizing gas. 8 Providing that the aluminum content of the alloy is high enough, above 5 wt.% Al in this study, alumina will develop a somewhat continuous layer, isolating the few particles of iron oxide and providing protection against further rapid degradation. This was indicated by the lack of thick iron oxide phases on the surface (Fig. 10) and relatively low corrosion rates (Fig. 3b) for alloys with 7.5 wt.% Al or greater. For very low-aluminum content alloys, it would be expected that the number and distribution of the iron oxide nuclei would be greater and allow for the Fe-base products to form a thick scale, as found in the literature for low-carbon steels. 2 Intermediate compositions have neither iron oxide nor alumina-forming continuous layers, but will continue to develop together with the nodule morphology (Figs. 9 and 11a). During its growth, the nodule consisted of a outer shell of Fe 2 O 3, with interior layers containing other iron and aluminum oxides (as shown in Fig. 11). Tomaszewicz and Wallwork 8 proposed that if the aluminum content is low, the nodules may continue to develop and coalesce to form a bulky, stratified scale of iron oxides. However, if the aluminum content is high enough, development of a continuous aluminum oxide healing layer may occur at the base of the nodule. This can limit the diffusion of iron atoms outward, thus halting nodule growth. 8 For the 5 wt.% Al sample in the oxidizing environment, the growth rate of the nodule diameters is parabolic (Fig. 7), indicating that a healing layer has not formed and that the surface will eventually be covered by iron oxide at longer times. In addition, the microscopy techniques used during this analysis did not reveal the formation of an alumina layer between any of the phase boundaries that may have been remnant of an initial continuous and passive scale. In the reducing environments, the shape of the kinetics curve and the remnants of the initial surface scale between the two corrosion layers aid in showing the sulfide nodule development as a scale failure followed by shortcircuit diffusion. In plotting the weight-gain data on a logarithmic scale (Fig. 13), the existence of two distinct regimes of reaction kinetics was clearly observed and can be well correlated to the amount of corrosion product on the surface of the sample. The initially low weight gains for the first few hours were due to formation of a continuous and protective scale that inhibited excessive scale growth, as indicated schematically in Fig. 14a. This inhibited sulfur diffusion into the alloy, as well as drastically reduced iron diffusion outward. It is important to note here that the growth of the small sulfide platelets on the surface, as seen in Fig. 6, did not affect the protectiveness of the alumina layer, as they were also found on the higher aluminum samples (20 wt.%) after long exposure times (100 hr). These platelets were apparently emanating from the alumina grain boundaries and not

25 Growth of Nodular Corrosion Products on Fe Al Alloys 363 Fig. 14. Schematic showing the scale development at various times for nodular formations. (a) Initial protection by granular surface scale of alumina; (b) mechanical failure of scale with sulfur short-circuiting the protective scale; and (c) formation and growth of nodules in failed regions. through the lattice of the scale itself. This did not appear to be deleterious to the integrity of the scale, as no sulfur was found to penetrate the alloy beneath these growths, and weight gains remained negligible. Therefore, it is presumed that a catastrophic event must take place in which the mechanical integrity of the scale degrades (Fig. 14b), through which sulfur can now permeate the protective layer by means of a short-circuit mechanism, i.e., microcracks. The local equilibrium, in terms of the partial pressures of oxygen and sulfur, in the alloy beneath the scale has now changed from that previously observed at the gas scale interface where alumina formation was promoted. Instead, fast-growing iron sulfide became thermodynamically

26 364 Banovic, DuPont, and Marder stable at the metal scale interface and began to develop (Fig. 14c), thus resulting in a dramatic increase in the rate exponent due to nodule formation. As the iron-sulfide products continued to develop at both the gas scale and the scale alloy interface, the initially formed alumina scale was imbedded between them (Fig. 8b). The ability of the alloy to resist nodule formation was found to be highly dependent upon aluminum content (Fig. 15). The 15 and 20 wt.% Al samples did not exhibit massive nodule formations for the extended times. These alloys are intermetallic compositions and have typically exhibited very long protective lifetimes in a number of environments and at higher temperatures ,17 20,24,25,35 However, as the aluminum content decreased, the time to nodule formation decreased from 5 hr for 12.5 wt.% Al, 2 hr for 10 wt.% Al, and relatively short time for 7.5 wt.% Al samples. The location of the nodules was also affected by the aluminum content. The 7.5 wt.% Al sample had widespread coverage by nodules across its surface because of Fig. 15. Time to breakdown stage as a function of aluminum content. The y axis is logarithmic scale. Arrows indicate that 15 and 20 wt.% Al samples did not have nodular growths after 100 hr.

27 Growth of Nodular Corrosion Products on Fe Al Alloys 365 the inability to heal the mechanical disruption of the passive scale. This inability was related to the low aluminum content of the alloy. However, for the 10 and 12.5 wt.% Al alloys, nodular growth occurred only at the corners and edges of the samples (Fig. 4c). Other studies 8 have also found that the nodular growths preferentially form at the corner and edges of the sample, as these areas typically allow for enhanced diffusion, possibly through some type of stress effect in the scale. Therefore, it may be the sample geometry that led to the loss of protection in these areas and the subsequent results may not be indicative of the protective nature of the scale for the 10 and 12.5 wt.% Al samples. As mentioned earlier, the formation of the alumina scale will deplete the alloy of Al near the surface due to selective oxidation. Characteristics of this depletion region depend upon a number of factors, including original alloy content, relative diffusivities in the alloy and scale, and exposure temperature. These three factors will dictate how quickly the aluminum is removed from the alloy, the extent of its depletion, and the ability to replenish and or maintain the area with further additions of aluminum from the bulk. As Quan and Young 21 observed studying the corrosion behavior of an Fe 4.5 wt.% Mn 8.8 wt.% Al alloy in a sulfidizing gas [10 8 Fp(S 2 )F10 4 atm] at C, a µm depletion layer beneath the MnS scale was attributed to the inability of the passive scale to heal at higher temperatures and iron sulfides began to develop. For high aluminum contents, above 15 and 20 wt.% Al in this study, the samples were able to maintain a sufficiently high surface aluminum activity to readily heal mechanical failures at this temperature. Thus, the formation of gross sulfide phases was not seen for these alloys over the 100-hr exposure. However, no aluminum-depletion region was observed beneath the initial oxide for the alloys that formed nodules, within the resolution limits of the equipment (1 µm). This may be expected at the relatively low temperature studied. Since diffusion of Al in alumina scales is expected to be much slower than in the alloy and the rate of aluminum consumption is very low due to the thinness of the scale, it could be expected that a depletion layer may not be found as Al from the bulk readily diffuses to the surface. However, with mechanical failure of the scale and ingress of sulfur, the fast-growing sulfide phases appear to be promoted at the scale alloy interface. Therefore, it may be that the initial surface activity of aluminum for the 7.5 wt.% Al samples was high enough to form a protective scale, but not to suppress formation of the sulfide phases below it upon scale failure. The morphologies of the sulfide phases were observed to be very complex as the corrosion product growth was no longer solely a uniform process of ionic diffusion (Fig. 7b), but contained an element of lateral growth across the surface (Fig. 7a). This was substantiated by comparing the

28 366 Banovic, DuPont, and Marder growth of the nodule diameters to the corrosion product penetration into the scale. Assuming that when the scale initially breaks down, sulfur was first allowed access to the underlying substrate. If growth of the nodules occurs exclusively through a diffusional process, then the lateral growth should be on the same order of magnitude as the perpendicular penetration similar to that observed for oxidation. From the growth-rate time constants calculated, this type of comparison cannot even be made as growth of the nodule diameter was linear (ng0.95) and the cross-sectional scale development was near parabolic (ng0.63). It can also be noted that as the sulfide products grow underneath the passive scale, differences in phase volume caused expansion beneath the outer FeS layer and the nodule was found to expand outward into the gas, giving it a lenticular shape. This may place additional stresses within the protective scale at the base of the nodule and lead to further failure until eventual coalescence of the iron sulfide to form a continuous scale. The difference between the two growth mechanisms described above may lie in the growth rates of the iron-base reaction products that form in the given environment. It is well established that the sulfidation rate of iron is nearly two orders of magnitude higher than that of oxidation These faster kinetics are due to the sulfide having a very large defect concentration and resulting higher iron diffusion coefficient. 39 Therefore, upon initial exposure to the mixed environment, if alumina cannot attain full coverage of the surface, its nuclei are quickly overgrown, and the formation of a continuous surface scale of sulfides can occur, as found for alloys with less than 7.5 wt.% Al. As the aluminum content increases, alumina can obtain full coverage of the surface and provide initial protection. The length of this protection was previously discussed to be dependent upon the aluminum content of the alloy (Fig. 15). The 7.5 wt.% alloys were found to be protective for extremely short times and the question may be raised as to whether they oxidized initially to form an alumina scale. The results, in terms of the shape of the kinetics curve and the remains of the initial scale between the outer and inner layer, strongly indicates that it did. The reason for its breakdown was most likely related to the lack of aluminum available to reestablish the scale after mechanical failure at short times. This led to short-circuit diffusion of sulfur to the alloy interior and eventual nodule formation. As the aluminum content increases further, the ability of the alumina layer to heal itself was shown by the longer periods of protection. When failure did occur in sulfidizing environments, as well as the oxidizing atmospheres, the nodules were located primarily on the corners and edges of the samples, which may suggest a limitation of the testing procedure in terms of the sample geometry. Failure in these locations may not be indicative of the protective nature of the scales formed on the higher aluminum alloys. For the case of oxidation, a different scenario can be found for the growth of

29 Growth of Nodular Corrosion Products on Fe Al Alloys 367 the surface nodules. Since the iron oxides grow at a much slower rate than the iron sulfides, rampant overgrowth of the alumina nuclei may not occur. Depending upon the aluminum content of the alloy, the iron oxide nuclei can either codevelop with the alumina scale into nodules (lower aluminum contents), or be completely isolated, effectively ending its growth (higher aluminum contents). CONCLUSIONS Exposure of Fe Al alloys to both oxidizing and sulfidizing environments at 700 C and examination of the morphological developments, the following conclusions can be drawn: (1) The nodule growth mechanism in the oxidizing atmosphere was observed to be the simultaneous or codevelopment of two different (Fe- and Al-rich) oxides from the onset of exposure. Kinetics data indicated a single growth-rate time constant from early times and that growth occurred through diffusional processes. (2) Localized mechanical failure of an initially formed scale layer led to the development of sulfide nodules in the reducing environment. The surface scale afforded protection at early times. However, with the mechanical failure, the corrosion species was able to attack the underlying substrate via a short-circuit mechanism. The morphologies observed were very complex as continued growth of the nodule did not solely depend upon the diffusing species through the previously formed corrosion products, but also, continued mechanical failure of the scale. (3) The difference between the two types of nodule growth mechanisms, i.e., codevelopment in pure oxidizing environments and mechanical failure short-circuit diffusion in atmospheres containing sulfur, may be due to the relative kinetics growth rates of the Febase corrosion products as compared to the alumina phase. (4) In both atmospheres, the elimination of nodule developments was observed by increasing the aluminum content of the alloy, which promoted the growth of a continuous surface scale of alumina. This critical content was above 5 wt.% Al in the oxidizing environment and greater than 7.5 wt.% Al in the sulfidizing atmosphere. ACKNOWLEDGMENTS This research was sponsored by the Fossil Energy Advanced Research and Technology Development (AR&TD) Materials Program, U.S. Department of Energy, under contract DE-AC05-96OR22464 with Lockheed Martin Energy Research Corporation. The authors wish to thank V. K. Sikka

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