Observation of oscillating reaction rates during the isothermal oxidation of ferritic steels

Oscillating reaction rates have been observed in the steam oxidation of 21/4Cr-1MoNb and 9Cr-1Mo ferritic steels at 500–550°C. Changes in reaction rate are associated with the formation of a laminated, inner-oxide layer, made up of bands of fine and coarse-grain spinel oxide. The lowest reaction rates occur during growth of the fine-grain oxide. Coarse-grain oxide generally contains the same levels of Cr, Mo, and Si as the steel (after allowing for loss of Fe to the outer layer), while the fine-grain material contains three times these levels. Ni builds up in the metal and is present in the oxide as metallic particles (mostly associated with fine-grain oxide). A mechanism is proposed in which the highest reaction rates are controlled by diffusion of Fe ions through the oxide layer (as in normal parabolic oxidation) and the lowest rates by diffusion of Fe through the Ni-rich layer in the metal.     

Oxidation kinetics,breakaway oxidation,and inversion phenomenon in 9Cr-1Mo steels

The oxidation behavior of 9Cr-1Mo ferrritic steels has been studied in air, oxygen, and steam at 1 atm pressure at various temperatures. Long-term experiments in air were carried out from 500–800°C by measuring the weight gains by interrupting the experiment at regular intervals of time. Short-term experiments in oxygen from 500–950°C and in air at 900 and 950°C were carried out by continuous recording of weight gain versus time in a continuousrecording thermogravimetric balance. Short-term experiments in steam were carried out using a special atmosphere furnace attached to the thermogravimetric balance. In air/oxygen, the weight gains at 700°C were lower than those at 600°C, while in steam, the weight gains at 800°C were lower than those at 700°C. This inversion phenomenon was observed for all the three steels viz. 9Cr-1Mo (high Si), 9Cr-1Mo (low Si), and 9Cr-1Mo-Nb steel. Examination of the oxide scales was carried out using SEM/EDAX, AES/ESCA, and X-ray diffraction techniques, and a mechanism is proposed for the occurrence of the inversion phenomenon.     

Morphology and Buckling of the Oxide Scale after Fe–9Cr Steel Oxidation in Water Vapor Environment

Oxidation of Metals - Under water vapor exposure at 550–560 °C, Fe–9Cr ferritic–martensitic steels form a triplex oxide scale made of an outer magnetite layer...     

Oxidation Behavior of ODS Fe–Cr Alloys

Four experimental oxide dispersion strengthened (ODS)Fe-(13–14 at. %)Cr ferritic alloys were exposed for up to 10,000 hr at 700–1100 °C in air and in air with 10vol.% water vapor. Their performance has been compared to other commercial ODS and stainless steel alloys. At 700–800°C, the reaction rates in air were very low for all of the ODS Fe–Cr alloys compared to stainless steels. At 900°C, a Y₂O₃ dispersion showed a distinct benefit in improving oxidation resistance compared to an Al₂O₃ dispersion or no addition in the stainless steels. However, for the Fe-13 %Cr alloy, breakaway oxidation occurred after 7,000 hr at 900°C in air. Exposures in 10 % water vapor at 800 and 900°C and in air at 1000 and 1100°C showed increased attack for this class of alloys. Because of the relatively low Cr reservoirs in these alloys, their maximum operating temperature in air will be below 900°C.     

Corrosion and deposition of magnetite in ferritic steel steam generator tubes under acid chloride conditions

Abstract

Corrosion of a tube made up of 9Cr–1Mo, 2·25Cr–1Mo, and 2·25Cr–1Mo–Nb ferritic steels and containing artificial defects has been investigated under realistic steam generator conditions (355°C, 17·6 MPa) with acid chloride fault water chemistry (2 mg/kg HCl). Four regions of corrosion and magnetite deposition behaviour were observed on the tube surface. In non-heat flux regions, magnetite deposition was affected by mass transfer and probably also by surface potential. In low heatflux regions (<660 kW m^?2) observed increases in the rates of magnetite deposition and corrosion wereprobably due to rises in the degree of iron supersaturation and HCl concentration, brought about by boiling. Enhanced HCl concentrations in the normal heat flux region (660 kW m^?2) prevented magnetite deposition and caused an increase in corrosion of the three steels. Increases in corrosion and magnetite deposition were also observed at the weld between the 9Cr–1Mo and 2·25Cr–1Mo steels. In defects, accelerated corrosion was seen only in the 9Cr–1Mo steel and was confined to the top 0·15 mm. It is concluded that the corrosion behaviour observed in this work is determined by the residence time and concentration of solutions of HCl on tube and defect surfaces.     

Performance of Al‐rich oxidation resistant coatings for Fe‐base alloys

The oxidation resistance of Al‐rich coatings made by chemical vapor deposition and pack cementation was examined on representative ferritic‐martensitic (FM, e.g. Grade 91, Fe‐9Cr‐1Mo) and austenitic steel substrates at 650°‐800 °C. To evaluate the potential benefits and problems with these alumina‐forming coatings, oxidation exposures were conducted in a humid air environment where the uncoated substrates experience rapid oxidation, similar to steam. Exposure temperatures were increased to accelerate failure by oxidation and interdiffusion of Al into the substrate. The difference in the coefficient of thermal expansion (CTE) between coating and substrate was found to cause cracking and coating failure during rapid thermal cycling on thicker coatings with Fe‐Al intermetallic phases. Therefore, thinner coatings with less Al and a ferritic Fe(Al) structure were evaluated more extensively and tested to failure at 700° and 800 °C on FM steels. The remaining Al content at failure was measured and used to improve a previously developed coating lifetime model. At 700° and 800 °C, thin coated austenitic specimens continue to exhibit protective behavior at more than double the lifetime of a similar coating on FM steel. The longer lifetime was attributed to the ferritic coating‐austenitic substrate phase boundary inhibiting Al interdiffusion.     

The Sulfidation Behavior of Several Commercial Ferritic and Austenitic Steels

The sulfidation behavior of C-steel, 1Cr-0,5Mosteel, 12Cr-1Mo-0.25V steel, 18Cr-10Ni-Ti steel, thebinary alloys Fe-20Cr, Fe-25Cr, Fe-30Cr, and pure Cr wasinvestigated between 400 and 700°C in a94Ar-5H₂-1H₂S gas mixture. All steels sulfidize according tocomplex kinetics which, after a period with decreasingrate, can be approximated by a linear rate law. Thescale of the three ferritic steels consists of two layers, an outer outward-growing one of FeSwith traces of dissolved Cr and an inner, inward-growingone, which contains in addition to Fe the alloyingelements Cr and Mn. Most of the outer FeS layer is separated from the inner layer and can be splitinto several partial layers, the number increasing withincreasing sulfidation time and temperature. The scaleon the austenitic 18Cr-10Ni-Ti steel differs insofar as that of the ferritic steels as theouter FeS layer contains some Ni and that a third layerof the spinel FeCr₂S₄ is formedbetween the outer and the inner layer. This intermediatelayer is responsible for the lower sulfidation rate of this materialcompared with that of the ferritic steels. The scale ofthe binary Fe-Cr alloys is similar to that of theaustenitic steel. From AE-measurements it can be deduced that the separation of the outer FeSlayer occurs during isothermal sulfidation and isaccompanied by an increase in the AE event rate. Theseparation is a consequence of the formation and growth of pores in the region close to the inner/outerlayer interface and the development of compressivegrowth stresses in the outer FeS layer. While detachmentof the FeS layer on the ferritic steels was already observed at 400°C, the austenitic steelshowed a similar separation of the FeS layer only at600°C. The detached FeS layer is obviously rathergas tight. Differences in the sulfur partial pressure ofthe bulk gas and the gas in the cavity between theinner and separated outer layer lead to a reduction ofFeS at the inner surface of the detached FeS layer. TheFe ions and electrons, produced by this reaction, diffuse outward, forming new FeS on the outerFeS surface. This process not only shifts the detachedFeS layer continuously away from the core of thespecimen but offers also the possibility of healing cracks in the separated FeS layer. This scaledetachment does not stop scale growth. After scaleseparation the total sulfidation reaction consists of atleast seven partial reactions: phase-boundary reaction at the outer surface, diffusion of iron ionsand electrons outwards in the detached FeS layer,formation of H₂S at the inner surface of thedetached layer, gas diffusion in the cavity, formationof FeS on top of the porous inner layer, gas diffusionin the channels of the porous inner layer, FeS formationat the metal/scale interface. When the new FeS layer ontop of the porous inner layer exceeds a critical thickness, the detachment of the FeSlayer from the inner porous layer repeats. This processcan take place several times, leading to an outer FeSpartial scale, split into several layers, which are separated by relatively large cavities andkept together only locally by FeS bridges. The overallreaction rate is controlled by the phase-boundaryreaction at the outermost FeS surface.     

Anomalous temperature dependence of oxidation kinetics during steam oxidation of ferritic steels in the temperature range 550-650 °C

The oxidation behavior of a number of selected ferritic steels in a simulated steam environment at temperatures between 550 and 650 °C was studied. In the prevailing test gas, some of the studied 9-12% Cr steels tended to exhibit an anomalous temperature dependence of the oxidation behavior. This means, that the oxidation rates do not steadily increase with increasing temperature. At higher temperatures, some of the studied steels tend to form a very thin and protective oxide scale whereas at lower temperature rapidly growing, less-protective oxides are being developed. The anomalous temperature dependence is related to differences in chromium distribution in the inner part of the oxide scale. The effect is observed for steels with intermediate-Cr contents (∼10-12%) whereas steels with either lower or higher Cr contents exhibit an increasing oxidation rate with increasing temperature.     

The Effect of Water Vapor on the Oxidation Behavior of CVD Iron-Aluminide Coatings

The oxidation behavior of iron-aluminide coatings, Fe₃Al or (Fe,Ni)₃Al, produced by chemical-vapor deposition (CVD) was studied in the temperature range of 700–800°C in air + 10 vol.% H₂O. A typical ferritic steel, Fe–9Cr–1Mo, and an austenitic stainless steel, 304L, were coated. For both substrates, the as-deposited coating consisted of a thin (<5μm), Al-rich outer layer above a thicker (30–50 μm), lower-Al-content inner layer. In addition to coated and uncoated Fe–9Cr–1Mo and 304L, cast Fe–Al model alloys with similar Al contents (13–20 at.%) to the CVD coatings were included in the oxidation exposures for comparison. The specimens were cycled to 1000 1 hr cycles at 700°C and 500 1 hr cycles at 800°C, respectively. The CVD coating specimens showed excellent performance in the water-vapor environment at both temperatures, while the uncoated alloys were severely attacked. These results suggest that an aluminide coating can substantially improve resistance to water-vapor attack under these conditions.     

High Temperature Corrosion of Cast Alloys in Exhaust Environments. II—Cast Stainless Steels

This paper describes in detail the oxidation of two cast stainless steels in synthetic diesel and gasoline exhaust gases. One alloy was ferritic (Fe18Cr1.4Nb2.1Mn0.32C) and one austenitic (Fe20Cr9Ni1.9Nb2.7W0.47C). Polished sections were exposed, mostly for 50 h, at temperatures between 650 and 1,050 °C. The oxidation product was characterized by means of SEM/EDX, AES, and XRD. Inter-dendritic non-Cr carbides initiated thick oxides. The ferritic steel formed a rather thin and adherent oxide scale at all temperatures. It consisted of (Mn, Cr) oxide on top of Cr₂O₃ and, starting at 850 °C, a thin silica film at the metal–oxide interface. Chromium depletion triggered dissolution of carbides providing Cr to the oxide. Water vapor did not accelerate the attack since the outer (Mn, Cr) spinel oxide reduced the Cr evaporation. The austenitic grade was very sensitive to water vapor. Chromium segregation directed pitting to the dendrites up to 950 °C whereas uniform catastrophic oxidation occurred at 1,050 °C.     

A comparison of the oxidation behaviours of Al2O3 formers and Cr2O3 formers at 700 °C – Oxide solid solutions acting as a template for nucleation

The oxidation behaviour of a number of ferritic iron based commercial steels and model alloys containing 6 and 9 wt% Cr and 0–2.5 wt% Al have been studied at 700 °C. The oxidation time ranged from 5 min to 500 h and the atmosphere consisted of flowing dry synthetic air. The oxide layers formed were analysed by SEM, GI-XRD and ToF-SIMS. The material without Al formed a (Cr,Fe)₂O₃ film with an Fe enrichment in the outer part of the layer. The Al containing alloys showed more complex oxidation behaviour. The oxidation started initially by formation of (Cr,Fe)₂O₃ with an Cr enriched inner part. With time Al was oxidized and dissolved in the inner Cr rich part of the oxide. This process continued until it eventually was transformed into α-Al₂O₃ with minute amount of Fe in the outer and Cr in the inner part of the oxide. The thickness of all oxide films ranged from 20 to 400 nm apart from the material that contained 9% Cr and no Al, which experienced breakaway oxidation after 500 h at 700 °C. This means that materials alloyed with small amounts of Al must also be considered to be protective at 700 °C, as the thicknesses of the Al₂O₃ oxides was comparable with the ones not containing Al, and as they do not experience breakaway corrosion.     

Corrosion of low-temperature nitrided molybdenum-bearing stainless steels

Austenitic stainless steels with up to 6.1 wt.% Mo were nitrided at 425 °C and examined in 0.1 M Na₂SO₄ without and with chlorides at pH 3.0 and 6.5. Nitrided steels exhibited an increased resistance to pitting, but at pH 3.0 they had a decreased resistance to general corrosion. After corrosion at pH 3.0 surface films contained chromium nitrides and oxides of Mo, Cr and Fe. It is proposed that the improved pitting resistance of nitrided steels is associated with the initially accelerated dissolution which leads to the accumulation of corrosion resistant CrN and of oxidised steel components.     

Simultaneous Aluminizing and Chromizing of Steels to Form (Fe,Cr)3Al Coatings

A cementation pack involving halide activatorsand elemental Al and Cr powders has been used to achievethe codeposition and diffusion of aluminum and chromiuminto low-alloy steels. A two-step treatment at 925°C and 1150°C yields dense anduniform ferrite coatings of about 400-m thickness,with surface compositions of approximatelyFe₃Al plus several percent Cr. The two stepheating schedule prevents the formation of a blocking chromium carbide atthe substrate surface. An attempt was made to add atrace of Ce to the Al + Cr content of the coating byintroducing Ce oxide into the pack, but there is no evidence that this doping was achieved. Uponcyclic oxidation in air at 700°C, the coated steelexhibits a negligible 0.085 mg/cm² weightgain for 1900 one-hour cycles. Virtually no attack wasobserved on coated steels tested in a simulated boileratmosphere at 500°C for 500 hr. But coatings with asurface composition of only 8 wt.% Al and 6 wt.% Crsuffered limited sulfidation attack in the simulated boiler atmosphere at temperatures higher than500°C for 1000 hr.     

High temperature corrosion of nanoceria coated 9Cr-1Mo ferritic steel in air and steam

In this article, air and steam oxidation resistance of bare and nanoceria coated 9Cr-1Mo ferritic steel at 650 °C for 500 h is reported. The kinetics of the oxidation process and the changes in the morphology and the chemistry for both the specimens were analyzed by backscattered electron and EPMA mapping. The nanoceria coated 9Cr-1Mo ferritic steel showed better oxidation resistance than uncoated steel under similar conditions. The thickness of the oxide scales formed on the coated samples was significantly reduced as compared to the uncoated substrates. The effectiveness of such a thin and simple nanocoating to improve the oxidation resistance of 9Cr-1Mo ferritic steel compared to the pristine is attributed to the change in the migration of cations from outward to inward diffusion of oxygen as the major mechanism.     

The oxidation behavior of Fe-20Cr alloy foils in a synthetic exhaust-gas atmosphere

Oxidation tests of rare-earth-modified and Ti-modified Fe–20Cr alloy foils, which are under consideration for catalytic converter supports, were performed in a synthetic exhaust-gas atmosphere (N₂+H₂O+CO₂) between 900°C and 650°C. Between 900°C and 750°C, the rare earths had no effect on oxide growth rates while Ti increased growth rates. Oxide growth rates for the rareearth alloys at 800°C and 750°C are much lower than those found in the literature for oxidation of Fe–Cr alloys or pure Cr in O₂-rich atmospheres. The slow growth rates for the rare-earth alloys agree with literature data for oxidation of stainless steels containing >20% Cr in wet atmospheres and are caused by growth of an oxide scale only one grain thick. At temperatures 700°C, Fe–20Cr alloys grow massive Fe oxides; however, this can be suppressed by adding rare earths or Ti. To ensure good oxide adherence, free sulfur must be eliminated in the alloy by tying it up with a reactive-element addition. Both Ti and the rare earths can be used to tie up S, but the rare earths are more effective. For converter applications, the optimum alloy composition may contain rare earths for good oxide adherence and a small amount of Ti to suppress growth of Fe-rich oxides.     

Steam oxidation resistance of new 12%Cr steels: Comparison with some other ferritic steels

The oxidation resistance in pure steam at the 600-650 °C temperature range of a newly developed 12%Cr steel has been investigated for long-term exposures (224 days = 5,376 h). The laboratory and industrial heats were tested in comparison with other ferritic 9-13% chromium steels. Corrosion rates were determined by direct measurements of mass losses obtained after a reducing descaling process. Weight loss and metallographic results confirm the good corrosion resistance in steam of the new steel and allow classing the tested steels in 2 families: one classical with average oxidation behaviour, “T91-type” and another one with low mass losses, varying very slightly with the temperature and the exposure time increasing. To have a better understanding of the observed phenomena, the possible influences of the main alloying elements (Cr, Si, Mn, Mo, W) of steels mentioned by different authors were reviewed and compared to the results obtained for the ten 9-13%Cr studied steels. It appears that the alloying elements cannot be considered separately: as a matter of fact they have not only a specific influence but also a joint influence on the steam corrosion behaviour of the 9-13%Cr ferritic steels.     

Codeposited chromium and silicon diffusion coatings for Fe-Base alloys via pack cementation

The simultaneous deposition of Cr and Si into plain carbon, low-alloy, and austenitic steels using a halide-activated pack-cementation process is described. Equilibrium partial pressures of gaseous species have been calculated using the STEPSOL computer program to aid in designing specific processes for codepositing the desired ratios of Cr and Si into a given alloy. The calculations indicate that NaCl-activated packs are chromizing, while NaF-activated packs deposit more Si with less Cr. The use of a dual activator (e.g., NaF+NaCl) allows for the deposition of both Cr and Si in the desired amounts. Single-phase ferritic coatings (150–250 microns thick) with a surface concentration of 20–35 wt.% Cr and 2–4% Si have been grown on AISI 1018, Fe-2.25 Cr-1.0Mo-0.15C, and Fe-0.5 Cr-0.5 Mo-0.2C steels using packs containing a 90 wt.% Cr-10Si binary source alloy, a NaF+NaCl activator, and a silica filler. Two-phase coatings (approximately 75 microns thick) containing 20–25 wt.% Cr and 2.0–2.4% Si have been obtained on 304 stainless steel using packs containing a 90 wt.% Cr-10Si binary source alloy, a NaF activator, and an alumina filler. The same pack chemistry allowed the diffusion of Cr and Si into the austenitic Incoloy 800 alloy without a phase change. A coated Fe-2.25 Cr-1.0 Mo-0.15 C coupon with a surface concentration of Fe-34 wt.% Cr-3Si was cyclically oxidized in air at 700°C for over four months and 47 cycles. The weight gain was very low (<0.2 mg/cm²) with no scale spalling detected. Coated coupons of AISI 1018 steel, and Fe-0.5 Cr-0.5 Mo-0.2C steel have shown excellent oxidation-sulfidation resistance in reducing, sulfur-containing atmospheres at temperatures from 400 to 700°C and in erosion and erosion-oxidation testing in air at 650 and 850°C.     

Materials for ultrasupercritical coal power plants—Boiler materials: Part 1

The efficiency of conventional boiler/steam turbine fossil power plants is a strong function of the steam temperature and pressure. Research to increase both has been pursued worldwide, since the energy crisis in the 1970s. The need to reduce CO₂ emission has recently provided an additional incentive to increase efficiency. Thus, steam temperatures of the most efficient fossil power plants are now in the 600 °C (1112 °F) range, which represents an increase of about 60 °C (108 °F) in 30 years. It is expected that steam temperatures will rise another 50 to 100 °C (90 to 180 °F) in the next 30 years. The main enabling technology is the development of stronger high-temperature materials, capable of operating under high stresses at ever-increasing temperatures. Recently, the EPRI performed a state-of-the-art review of materials technology for advanced boiler/steam turbine power plants (ultrasupercritical power plants). The results of the review show that with respect to boilers, high-strength ferritic 9–12Cr steels for use in thick section components are now commercially available for temperatures up to 620 °C (1150 °F). Initial data on two experimental 12Cr ferritic steels indicate that they may be capable of long-term service up to 650 °C (1112 °F), but more data are required to confirm this. For higher temperatures, austenitic steels and Ni-based alloys are needed. Advanced austenitic stainless steels for use as super and reheater tubing are available for service temperatures up to 650 °C (1112 °F) and possibly 700 °C (1292 °F). Ni-based superalloys would be needed for higher temperatures. None of these steels have been approved by the ASME Boiler Code Group so far. Higher-strength materials are needed for upper water walls of boilers with steam pressure above 24 MPa (3400 psi). A high-strength 2-1/2%Cr steel recently ASME code approved as T-23 is the preferred candidate material for this application. Field trials are in progress. This paper will present the results of the EPRI review in detail, relating to boiler material. Results relating to turbine materials are presented in a companion paper as Part 2.     

Etching of oxide scales on chromium ferritic steels

Abstract

The oxide scale formed on chromium ferritic steels (21/4–9% Cr) by reaction with water/steam in the temperature range 200–570°c consists of an inner Fe/Cr spinel layer and an outer layer of magnetite. These layers are often indistinguishablefrom one another when the scale is sectioned and viewed under an optical microscope. Possible etching techniques, able to reveal the two layers (often without affecting the metal surface), have been investigated. Successful techniques are listed and the mechanisms involved are discussed in relation to current knowledge of oxide dissolution and passivation of metal surfaces.     

Untersuchungen über den Einfluß der Wärmebehandlung auf die Korrosion titanstabilisierter ferritischer Chromstähle in siedender konzentrierter Salpetersäure

Investigations into the influence of the thermal treatment on the corrosion of titanium-stabilized ferritic chromium steels in boiling concentrated nitric acid Titanium-stabilized ferritic chromium steels with about 17% Cr are, after quenching from high temperature, susceptible to grain boundary corrosion in boiling nitric acid; this corrosion is attributable to the chemical dissolution of the titanium carbonitrides coherently segregated at the grain boundaries. This carbide corrosion can be reduced by heat treatment within the temperature range around 850°C. In this process, the segregated carbo-nitrides are formed-in (in-formation annealing). The influence of quenching (0.5–5 hours, 900–1200°C/W[L]) and subsequent in-formation annealing (1–30 hours, 850°C/W) on the corrosion rate in boiling 65% nitric acid has been investigated in detailed on two steels of types X 8 Cr Ti 17 and X 8 Cr Mo Ti 17. In addition, the corrosion behaviour of titanium-stabilized and unstabilized ferritic chromium steels with about 17 pC chromium have been compared with each other.