FeS Layer Research Articles

The sulfidation of two FeCe alloys in H 2H 2S mixtures at 600–800 °C

The sulfidation behavior of two FeCe alloys containing approximately 15 and 30 wt% cerium has been studied at 600–800 °C in H 2H 2S mixtures providing a sulfur pressure of 10 −8 atm. The alloys corrode more slowly than pure iron, but more rapidly than pure cerium. The sulfidation rates generally decrease with time and tend to become parabolic after an initial stage intermediate between linear and parabolic, except for Fe15Ce which corrodes linearly after an initial quasi-parabolic behavior. Both alloys produce complex scales, containing an outermost layer of practically pure iron sulfide and an inner complex layer where the sulfides of both alloy components are simultaneously present. Finally, a thin region of internal sulfidation of cerium is also generally present in contact with the alloy which is not depleted in cerium. Cerium is not able to diffuse out of the alloy consumption region, where it forms a cerium sulfide mixed with iron sulfide. The iron sulfide forms a continuous network which allows the growth of the external FeS layer, even though at rates reduced with respect to pure iron. Thus, a cerium/content up to 30 wt% is not sufficient to prevent the sulfidation of pure iron. These results as well as the details of the microstructure of the scales grown on the two alloys are interpreted by taking into account the limited solubility of cerium in the base metal and the presence of intermetallic compounds in the alloys.

The sulfidation of Fe-15 wt%Y and Fe-30 wt%Y in H2H2S mixtures at 600–800 °C

The corrosion at 600–800 °C of two iron-based alloys containing 15 and 30 wt% yttrium in H2H2S mixtures under 10−8 atm S2 was studied to establish whether yttrium additions can improve the sulfidation resistance of pure Fe. The presence of yttrium is beneficial, but not as much as expected. The corrosion rate of iron is considerably reduced by the yttrium addition, but the two alloys still corrode much more rapidly than pure yttrium. The sulfidation kinetics of pure yttrium are rather irregular, while the corrosion rate of pure iron and the two alloys tends to become parabolic after an initial period of variable length. Under all conditions the two alloys form an external FeS layer, overlying an intermediate region of complex composition containing a mixture of the compounds of the two metals and an innermost region of internal attack. Iron can still diffuse through the intermediate region to form the outer FeS layer with non-negligible rate. The scaling behavior of these alloys is discussed by taking into account the presence of intermetallic FeY compounds and the limited solubility of yttrium in iron.

Sporadic metal layers in the upper mesosphere

One of the most interesting current problems in the aeronomy of the mesosphere is the origin of sporadic Na (Nas) and sporadic Fe (Fes) layers. These are very thin (ca. 1 km), dense layers of atomic Na and Fe which often form explosively in a matter of minutes, may extend horizontally for several thousand kilometres, persist for several hours, and then disappear rapidly. While the typical concentrations of atomic Na and Fe in the upper mesosphere are respectively, 4 × 103 and 104 cm–3, maximum densities exceeding 105 cm–3 have been observed for both species during sporadic layer events. Incoherent scatter radar and lidar observations at Arecibo have shown conclusively that many Nas layers are linked to the formation of sporadic E (Es) layers, while rocket and lidar observations at Andoya have shown a similar link between Fes and Es. Recent lidar observations at Urbana and Hawaii have revealed the simultaneous formation of Nas, Fes, and Ca+ layers and the occurrence of substantial temperature enhancements and wind shears during some Nas events. We present new observations of Nas obtained with a Na wind/temperature (W/T) lidar during the recent ALOHA-93 (1993 airborne lidar and observations of the Hawaiian airglow) Campaign in Hawaii. These observations are used to characterize the wind and temperature structure associated with several of the major Nas events observed during ALOHA-93.

The oxidation-sulfidation of FeNb alloys at 600–800°C in H 2-H 2S-CO 2 mixtures

The corrosion of the two pure metals and of two Fe-Nb alloys containing 15 and 30 wt% Nb has been studied at 600–800°C in H 2-H 2S-CO 2 gas mixtures providing a sulfur pressure of 10 −8 atm and oxygen pressures of 10 −24 atm at 600°C and 10 −20 atm at 700–800°C. Both iron and the two alloys corroded according to an approximate parabolic rate law, with an instantaneous rate constant increasing with time. The rate constants for corrosion decreased with an increase in the Nb content of the alloys under constant temperature but increased with temperature for the same alloy. In all cases the rate constants were slightly lower than those for the corrosion in H 2-H 2S mixtures under the same temperature and sulfur pressure, but remained still quite high as compared to the rate of corrosion of pure niobium due to the lack of formation of a protective layer of niobium compounds. The scales were duplex, presenting an outer layer of pure FeS and an inner complex layer containing a mixture of FeS, double sulfide, niobium oxides and some double oxide. Islands of the Nb-rich Fe 2Nb phase remained partly unsulfidized in the inner scale region for the most concentrated alloy. The corrosion behaviour observed is a result of the low solubility of niobium in iron and of the two-phase nature of the alloys.

Sporadic Fe and E layers at polar, middle, and low latitudes

First observations of sporadic iron layers (FeS) at low and middle southern latitudes are reported. These and new measurements from northern polar latitudes are combined to study the latitudinal dependence of FeS layer properties and their correlation with simultaneously observed sporadic E layers (ES). We also compare the individual properties of FeS and sporadic sodium layers (NaS): (1) The overall appearance rate for FeS is much higher than for NaS. (2) FeS layers do not show the marked minimum in appearance. (3) Both the normal and the sporadic Fe layers are considerably more dynamic than in the case of Na layers. (4) Concerning the shape, FeS layers are typically broader, slower in growing and longer lasting than NaS layers. (5) The dependence of appearance rate of sporadic metal layers versus, local time and latitude shows a complex pattern. (6) The correlation with ES appears to be weaker for FeS than for NaS.

The effects of manganese additions on the sulfidation behavior of an Fe-28Mo alloy

Iron-based alloys containing ca 28 a/o (atom per cent) molybdenum and up to 32 a/o manganese were sulfidized in flowing H 2/H 2S gases corresponding to p s 2 = 4 Pa at temperatures of 700–1000°C. Parabolic sulfidation kinetics were invariably observed. Two-layered scales—an outer FeS layer and an inner molybdenum-containing layer—developed on binary Fe-27Mo. Manganese additions to Fe-28Mo resulted in the formation of an additional intermediate Mn(Fe)S-containing layer. The overall sulfidation rate was decreased by manganese additions of 24–32 a/o at all temperatures and by additions of 10–18 a/o at higher temperatures, but was increased by manganese additions of 10–18 a/o at lower temperatures. This behavior is due to the combination of the improved protection provided by the Mn(Fe)S-containing layer, and the increased metal flux through theFe xMO 6S 8−z phase in the inner layer.

Formation mechanisms for low‐altitude Es and their relationship with neutral Fe Layers: Results from the METAL Campaign

The METAL campaign was a multi‐instrument campaign conducted in September‐October 1991 that was designed to investigate the relationship between neutral and ionized metallic layers in the high‐latitude lower ionosphere. Measurements included electron density profiles and electric fields from the EISCAT UHF incoherent scatter radar, ionosonde measurements of Es layers, neutral Fe profiles from lidar and rocket observations of winds (by chaff release), and measurements of plasma density and ion composition (by mass spectrometer). Es layers were observed on eight occasions. The measurements are here compared with simulations of Es formation by tidal wind shear and electric fields. The main features of the Es layers are found to be consistent with formation by tidal wind shear alone in one case and by the combined action of wind shear and electric fields in the remaining cases. On three occasions, layers were seen to descend rapidly, in one case to below 90‐km altitude, and the rate of descent was found to be consistent with the expected action of the observed electric fields. Sporadic neutral Fe layers were observed on seven occasions, including four cases in close correlation with Es. However, the neutral layers were generally not at exactly the same altitude as the Es, nor did they follow the same altitude changes. The results indicate that both wind shears and electric fields are important for the formation of thin layers and the vertical transport of metal ions in the high‐latitude lower thermosphere. They show that neutral Fes layers, although they may be formed in close association with Es layers, can persist and migrate independently of Es layers. This latter observation suggests that partial reionization of neutral layers by intense auroral particle precipitation may explain some observations of multiple Es layers.

Structure and seasonal variability of the nighttime mesospheric Fe layer at midlatitudes

Lidar measurements of mesospheric Fe were conducted for 325 h during 75 nights at Urbana, Ill. (40°N, 88°W), in fall 1989 and from spring 1991 through summer 1992. The Fe layer abundance and root‐mean‐square (RMS) width have strong annual variations, with minima in summer. The abundance varied from 3.5 × 109 to 25 × 109 cm−2 with a mean of 10.6 × 109 cm−2, and the RMS width varied from 2.3 to 5.3 km with a mean of 3.4 km. The centroid height of the Fe layer has a strong semiannual variation, with minima at the solstices. The centroid varies from 86.0 to 90.3 km and has a mean of 88.1 km. Sporadic Fe (Fes) layers were present about 27% of the total observation time. The Fe measurements are compared with the extensive Na layer observations obtained during the past decade at Urbana and with common volume observations made simultaneously on 24 nights with a Na temperature lidar. The mean Fe column abundance is approximately twice the mean Na column abundance. The Fe layer centroid height is also on average nearly 4 km lower and the RMS width is approximately 24% narrower than the corresponding Na layer parameters. A chemical model of the mesospheric Fe layer is described and compared to various experimental results. The reaction of Fe with O3 to form FeO on the bottom side of the layer and the subsequent reaction of FeO with CO2 to form FeCO3 appear to be the dominant chemical sinks for Fe. The temperature dependency of the latter reaction may explain the annual variation in the column abundance. The lidar observations and the chemical model calculations suggest that the expected cooling of the mesopause region by approximately 10 K due to the doubling of CO2 and other greenhouse gases during the next century may reduce the mean Fe abundance by as much as 45% and the mean Na abundance by 55%.

Evidence for substantial seasonal variations in the structure of the mesospheric Fe layer

Lidar measurements of mesospheric Fe were conducted at Urbana, IL (40°N, 88°W) during the fall of 1989 and the spring through fall of 1991. The Fe layer abundance, centroid height and rms width varied substantially during the observation period with minimum values for all three parameters in summer. The abundance varied from 3.3×109 ‐ 25×109 cm−2, the centroid height from 86.7 – 90.4 km and the rms width from 2.1 – 3.9 km. Sporadic Fe (Fes) layers were also frequently observed. The Fe measurements are compared with the extensive Na layer observations obtained during the past decade at Urbana. The mean Fe column abundance is more than twice the average Na column abundance. The Fe layer is also on average approximately 3.6 km lower and 32 % narrower than the Na layer. The seasonal behavior of the Fe layer is discussed in light of our preliminary temperature measurements of the mesopause region as well as a simple chemical model.

High-temperature sulfidation of Fe-Nb alloys

The sulfidation behavior of Fe-Nb alloys containing up to 30 w/o Nb was studied over the range of 600–900°C in 0.01 aim. S2 vapor. All alloys were two-phase, consisting of an Fe-rich solid solution and Fe2Nb, and followed the parabolic rate law at all temperatures. Scales consisted of two layers-an outer layer of FeS and an inner, complex layer which contained some FeS, FeNb2S4 (possibly some FeNb3S6), NbS2, and intermetallic particles which were either completely or only partially sulfidized. Platinum markers were located always at the interface between the two layers, which corresponded to the original metal surface. Activation energies were 18±3 kcal/mol in close agreement with the 19.8 reported for pure iron. The sulfidation rate decreased markedly with increasing Nb content of the alloys. The decrease is attributed to increasing amounts of Fe2Nb with increasing Nb, the net effect being that the diffusion path for outward iron diffusion through the inner layer is reduced as the Nb content increases. An analysis of the structure of NbS2 reveals that it is easily intercalated with Fe between loosely bonded layers of S-Nb-S. The S-Nb-S layers are covalently bonded which results in very low diffusivities of either S or Nb in pure NbS2. Although intercalated Fe tends to change the Van der Waal's type bonding between layers to more ionic or covalent, Fe diffuses readily between the layers in NbS2. Intercalation of Fe also increases the concentration of sulfur defects in NbS2, which in turn increases the diffusivity of sulfur. Nb was observed to be immobile. Thus, it is thought that either outward iron diffusion or inward sulfur diffusion in the inner layer is the rate-controlling step, in spite of the close agreement of activation energies with that of the sulfidation of pure iron.

Kinetics and mechanism of the sulfidation of Fe-Mo alloys

Iron-molybdenum alloys containing up to 40 wt.% molybdenum were exposed to sulfur vapor at a partial pressure of 0.01 atm at temperatures of 600–900°C. Sulfidation kinetics were measured over periods of up to 8 hr using a quartz-spring thermogravimetric method. The sulfidation kinetics of all alloys studied obeyed the parabolic rate law. The sulfidation rate of iron was found to be reduced by factors of 60 at 900°C and 120 at 600°C by the addition of 40 wt.% molybdenum. Duplex sulfide scales formed on all alloys at all temperatures, the scales consisting of an inner layer of mostly MoS2 and an outer layer of FeS. Platinum markers were located at the interface between the outer and inner scales, showing that outward iron diffusion and inward sulfur diffusion through the inner layer occurred. The improved sulfidation resistance was attributed to the formation of the MoS2, which acted as a partially protective barrier to the diffusion of the reacting species. Sulfidation activation energies were found to range from 24.3 to 28.5 kcal mole for the alloys compared to 20.6 kcal/mole, for pure iron. The rate-controlling step was outward iron diffusion through the outer iron sulfide layer.

Sulfidation properties of Fe‐6.1 at.% Mo alloy at 973–1273 K

AbstractSulfidation of an Fe‐6.1 at% Mo alloy was investigated in H2S‐H2 atmospheres, 10−4 ⩽ Ps2 ⩽ 102Pa, at 973‐1273 K. The reaction kinetics are parabolic except at 1273 K as liquid sulfide formation leads to catastrophic corrosion. This solid‐liquid transformation between Fe2Mo2S4 and Mo2S3 occurs at 1214 ± 9 K. At 1073 K and Ps2 = 10−4Pa, growth of a duplex Mo2S3/FeMo2S4 scale offers high resistance to sulfidation. At 973, 1073 and 1173 K, 10−2 ⩽ Ps2 ⩽ 102Pa, parabolic sulfidation kinetics of the same magnitude as for pure iron describe growth of a duplex scale composed of an inner (FeMo2S4 + Mo2S3) layer and at an outer FeS layer. Marker measurements indicated that growth of the inner two‐phase layer was supported by inward migration of sulfur and that growth of the outer FeS layer resulted from outward migration of iron.

Sulfidation properties of Fe-Al alloys at 1173 K in H2S-H2 atmospheres

The sulfidation kinetics and morphological development of reaction products are reported for Fe-9 and 18 at.% Al alloys exposed at 1173 K to H2S-H2 atmospheres at sulfur pressures in the range 10−1−103 Pa. The Fe-9 Al alloy sulfidized parabolically at\({\text{P}}_{{\text{S}}_{\text{2}} } = 10^{ - 1} \) Pa giving rise to a duplex scale composed of an outer Al-doped FeS layer and an inner FeS + FeAl2S4 lamellar layer and to an internal sulfidation zone containing Al2S3 precipitates. The Fe-18 Al alloy which was sulfidized at\(10^2 {\text{Pa}} \geqslant {\text{P}}_{S_2 } \geqslant 10\).

AES and SEM studies of oxide and sulfide scales formed on sputter-deposited 304 stainless steel at 800 and 950 °C

High temperature (800–950 °C) corrosion studies of fine-grained sputter-deposited 304 stainless steel have been conducted at low pressures (<10−2 Pa) in an Auger electron spectroscopy (AES) UHV chamber and compared with studies performed in an environment of mixed gases containing O and S. In situ surface analysis performed at low gas pressures has provided a detailed examination of the diffusion processes that occur during alloy corrosion. Oxide scales formed at 10−2 Pa O2 on sputter-deposited fine-grained 304 stainless steel have been found to closely resemble those formed in mixed-gas environments at atmospheric pressure, suggesting that the same mechanisms are active in both situations. During initial oxidation at 10−2 Pa of Y-doped samples, a silicon oxide was observed by AES to form and then to be covered by a growing chromium oxide. This relates well to the silicon-rich oxide subscale found on similar samples following 24 h in the mixed-gas environment and suggests that the presence of Y affects Si mobility. In addition, the oxide scales on some samples exposed to the mixed gas were observed to be covered by FeS. In the AES system, exposure of oxidized samples to H2S caused a sulfur layer to build on the surface which drew iron onto the oxide-scale surface faster than sulfur diffused into the oxide. This result implies that an FeS layer can form on the surface at an earlier stage of attack than previously thought and can be a significant aspect of the corrosion process.

Diffusion-limited sulfidation of wustite

The sulfidation of wustite in H2S−H2O−H2−Ar atmospheres has been studied at temperatures of 700, 800, and 900°C with thermogravimetric techniques. Polycrystalline wustite wafers were equilibrated in a flowing H2O−H2−Ar gas stream and then sulfidizedin situ. During an initial transient stage a protective layer of FeS formed on the sample, and an intermediate layer of Fe3O4 formed between the FeO and FeS layers. Subsequently, the reaction followed a parabolic rate law. The parabolic rate constant varied from 0.22×10−2 mg2 cm−4 min−1 at 700°C to 6.5×10−2 mg2 cm−4 min−1 at 900°C. The reaction rate was limited by the diffusion of iron through the intermediate Fe3O4 layer which grew concurrently with the FeS layer and at the expense of the FeO core. After the FeO core was completely converted to Fe3O4, the process entered a passive stage during which no further mass changes could be detected.

Studies of rolling element lubrication and fatigue life in a reduced pressure environment : D. W. Reichard, R. J. Parker and E. V. Zaretsky, ASLE Trans., 11 (3) (1968) 275–281; 6 figs., 1 table, 6 refs.

In this study, the FeS/ferroalloy composite coating with the upper FeS coating and the lower ferroalloy welding coating was prepared on the 1045 steel by a compound technology—metal-arc inert-gas (MIG) welding and low temperature ion sulfurizing. The surface morphologies, phase structure and mechanical properties of the FeS/ferroalloy composite coating have been characterized by surface analysis methods such as scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and nano-indentation tester. The SEM images showed that the surface of the FeS/ferroalloy composite coating was homogeneous and smooth, and the thickness of the FeS layer obtained from ion sulfurizing was about 3 μm. The EDS analysis results proved that the light gray belt zone in the cross-section morphology was the FeS layer. The XRD patterns implied the FeS/ferroalloy composite coating was composed of FeS, α-Fe and Fe–C solid solution. The coated disc slided against an AISI 52100 ball under dry friction condition. The results showed that the FeS/ferroalloy composite coating exhibited low friction coefficient and high wear-resistance.