Mechanical Properties of Oxide Scales on Mild Steel at 800 to 1000°C

In the hot-rolling process of steels the oxide scales play a key role with regard to surface quality of the sheet. Therefore, a quantitative knowledge of the mechanical properties of oxide scales at rolling temperature can provide a significant improvement of sheet quality. In the present paper the mechanical properties of the oxide scales formed on mild steel were investigated in 4-point bend tests at 800, 900, and 1000°C in dry air, humid air (7–19.5 vol.% H₂O) and laboratory air at different deformation rates. It turns out that the environment has a significant influence on scale thickness and structure as well as on adhesion of the oxide scales. The mechanical measurements show plastic-creep deformation of the oxide scales. Water vapor did not have any significant effect on the creep properties of the oxide scales. In the measurements the secondary-creep-stress values of the oxide were determined as a function of the strain rate and plotted into an Ashby map for FeO. This plot can serve for an extrapolation of the data for even higher strain rates as relevant for the hot-rolling process.     

Effect of Annealing Atmosphere on Scale Formation and Pickling Performance of 410 SS

The compositions and microstructures of scales formed on the surface of 410 stainless steel after annealing at 850°C in dry air, moist air, N₂ and 10% H₂+N₂ atmospheres were investigated. The pickling performances in 25 wt.% H₂SO₄ solution of hot-rolled and annealed 410 SS were also evaluated. The experimental results indicate that the scale formed in the moist air was the most difficult to remove, while descaling was easy when the stainless steel was annealed in a reducing atmosphere (10% H₂+N₂). The composition and microstructure of the oxides formed during annealing strongly affected the descaling performance. Both the stability of the scale in pickling solution and the effectiveness of descaling could be evaluated and predicted by measuring the change of electrochemical potential during acid pickling. The reactivation time determined from the potential-decay curve was found strongly dependent on the atmosphere in which 410 SS was annealed.     

The characterization by raman spectroscopy of oxide scales formed on a 20Cr25NiNb stabilized stainless steel

Laser Raman spectroscopy (LRS) has been used to examine the composition of 0.3–2 μm thick scales formed on the 20Cr25NiNb stabilized stainless steel during oxidation, at 600–950°C, in CO₂ and in CO₂ + 4%CO + 350 vpm CH₄ + 300 vpm H₂O + 400 vpm H₂. All the scales contained iron rich spinel oxides, except in carbon dioxide at 600°C, where Fe₂O₃ predominated. This difference was responsible for the greater attack of the stainless steel at 600°C in carbon dioxide than in the mixed gas environment. Increasingly at ≥800°C the spinel composition was modified by manganese incorporation and Cr₂O₃ was also a major component of the scales formed in both environments. The principal scale constituents identified by LRS accord both with thermodynamic predictions and current understanding of scale development on the 20/25/Nb stainless steel.     

The Effect of Water Vapour on the Corrosion of Sandvik Sanergy HT Under Dual Atmosphere Conditions

Interconnects of PCFC and SOFC are exposed to dual atmosphere conditions; fuel (e.g. H₂) on the anode side and air on the cathode side. Sandvik Sanergy HT has been investigated under simulated fuel cell conditions at high temperatures. The microstructure and composition of the oxide scales formed at the cathode side (air) were significantly influenced by dual atmosphere conditions. The main effect was a substantial increase of Fe in the oxide scales by the formation of Fe rich nodules accompanied by localized metal loss. The size and number of the nodules increased when introducing water vapour on the air side of the samples. It is suggested that the preferred localization of nodule formation is given by the surface finish as a result of fabrication (e.g. grooves and scratches). By increasing the reaction temperature or duration of the exposures, the effect of dual atmosphere conditions became less pronounced. Alterations to the oxidation mechanism as a consequence of dual atmosphere environments are discussed with basis in the effect of hydrogen permeation through the interconnect alloy. Samples PVD coated with a double layer of metallic Ce and Co were also tested under single and dual atmosphere conditions. No significant change was found in the oxidation behaviour by dual atmosphere exposures.     

Behavior of oxide scales on 12Cr-1Mo steel during thermal cycling

Soot-blower operation leads to thermal-cyclic-oxidation conditions of heat-resistant steels in conventional power stations. The consequence may be failure of the protective oxide scales and increased corrosive attack. The behavior of protective oxide scales on 12Cr-1Mo steel was investigated under isothermal conditions at 650°C and under thermal cycling conditions between 650 and 300°C (200°C). The tests were performed in air, air + 0.5%SO₂, simulating the fire side, and Ar-5% H₂-50% H₂O, simulating the steam side. Complete heat-exchanger tubes were used as specimens. The main instrument for the detection of scale failure was acoustic-emission analysis. In air and air + 0.5% SO₂ the M₂O₃ scales with M = Fe, Cr were very thin and did not show significant failure either during isothermal or during cyclic oxidation. The thicker scales formed in Ar-5% H₂-50% H₂O, consisting of several partial layers, failed even during isothermal oxidation due to geometrically-induced growth stresses in the scale. Thus, in the thermal-cycling cooling periods there was only very little additional scale cracking. The scale behavior can be explained consistently by applying the existing quantitative models.     

Degradation of the Tensile Properties of an Au-Coated 316L Stainless Steel Bipolar Plate in a Fuel Cell Stack

Cost reduction for fuel cell stainless steel bipolar plate (BPP)’s Au-coating requires in depth understanding of its corrosion behavior. To this end, this paper explores the degradation of the tensile properties of an Au-coated 316L stainless steel bipolar plate in a real fuel cell stack. 4 BPPs were randomly chosen and removed from a stack that had run for 1600 h and, along with 2 pristine BPPs, were subsequently tested for surface morphology and tensile properties. Results suggest that (1) Pristine BPPs have initial pinhole flaws on the surface, whereas corroded BPPs have punctate (on O₂ side) and continual (on H₂ side) corrosions. (2) The tensile ultimate elongation for a BPP significantly decreases after corrosion on both O₂ and H₂ sides. (3) The degradation of tensile properties is attributed to primary cell effect.

     

Small-angle neutron scattering study of the wet and dry high-temperature oxidation of alumina- and chromia-forming stainless steels

Foils of T347 and an alumina-forming austenitic (AFA) stainless steel were oxidized at 800 °C in dry air, air with 10% H₂O, and air with 10% D₂O. Significant changes in the small angle neutron scattering (SANS) signal were observed for the T347 stainless steel as a function of oxidation time in dry air whereas the AFA alloy showed no significant scattering changes resulting from oxidation. For both alloys, similar scattering was observed in dry and wet air (H₂O and D₂O) exposure, indicating that watervapour did not result in significant H/D retention or induce significant morphological changes in the oxide scales.     

Oxidation of Austenitic Stainless Steel Boiler Tubing in Carbon Dioxide

Abstract

The long-term oxidation characteristics of austenitic stainless steel boiler tubing in CO_2? vol.-% CO at up to 600 psig and at 650° and 700° have been studied. Weight gams, metallography and microprobe analysis indicated that heavily cold-worked material showed the best oxidation resistance owing to the formatiOn of a Cr₂O₃. protective scale. Oxidation of material heat-treated in nitrogen at the lower temperatures. resulted in the formation of a rather less resistant ‘spinel’ type oxide, while the material heat-treated in nitrogen at the higher temperature suffered severe pitting attack. Heat treatment in air at all temperatures provided a fairly resistant oxide film during subsequent oxidation in CO₂. Hence heat treatment in air is preferable to heat treatment in nitrogen, and leaving the heat-treatment scale on surfaces which are subsequently to be exposed to CO₂-based environment is recommended. Type 316 stamless steel showed rather better oxidation resistance than Type 347 stainless steel.     

Evaluation of ferritic stainless steel for use as metal interconnects for solid oxide fuel cells

Interconnect development for planar solid oxide fuel cells is considered a vital technical area requiring focused research to meet the performance and cost goals. A commercial ferritic stainless steel composition for oxidation resistance properties was investigated by measuring the weight gain due to air exposure at fuel cell operating temperature. A surface treatment process was found to produce a dense, adherent scale and to reduce the oxide scale growth rate significantly. A process was also identified for coating the surface of the alloy to reduce the in-plane resistance and potentially to inhibit chromium oxide evaporation. The combination of treatments provided a very low resistance through the scale. The resistance measured was as low as 10 mΩ-cm² air. The resistance value was stable over several thermal cycles. The treated samples were exposed to a variety of atmospheres that were relevant in fuel cell operation to evaluate changes in scale morphology. Analysis of the scale after such exposure showed the presence of a stable composition. When exposed to a dual atmosphere (air and hydrogen on opposite sides of the metal sheet), however, the scale composition contained a mixture of phases. Additional process modifications are planned to reduce the effect of dual-atmosphere exposure. This paper was presented at the Fuel Cells: Materials, Processing, and Manufacturing Technologies Symposium sponsored by the Energy/Utilities Industrial Sector & Ground Transportation Industrial Sector and the Specialty Materials Critical Technologies Sector at the ASM International Materials Solutions Conference, October 13–15, 2003, in Pittsburgh, PA. The symposium was organized by P. Singh, Pacific Northwest National Laboratory. S.C. Deevi, Philip Morris USA, T. Armstrong, Oak Ridge National Laboratory, and T. Dubois, U.S. Army CECOM.     

High-Temperature Corrosion Studies of HVOF-Sprayed Cr3C2-NiCr Coating on SAE-347H Boiler Steel

Cr₃C₂-NiCr coating was deposited on SAE-347H boiler steel by high velocity oxy fuel (HVOF) spray process. Subsequently, high-temperature corrosion behavior of the bare and coated boiler steel was investigated at 700 °C for 50 cycles in Na₂SO₄-82Fe₂(SO₄)₃ molten salt, as well as air environments. Weight-change measurements after each cycle were made to establish the kinetics of corrosion. X-ray diffraction, field emission-scanning electron microscopy/energy dispersive spectroscopy, and x-ray mapping analyses were performed on the exposed samples to analyze the oxidation products. The bare 347H steel suffered accelerated oxidation during exposure at 700 °C in the air as well as the molten salt environment in comparison with its respective coated counterparts. The HVOF-spray Cr₃C₂-NiCr coating was found to be successful in maintaining its adherence in both the environments. The surface oxide scales were also found to be intact. The formation of chromium rich oxide scale might have contributed for the better hot corrosion/oxidation resistance in the coated steel.     

The Oxidation of Ferritic Stainless Steels in Simulated Solid-Oxide Fuel-Cell Atmospheres

The cyclic oxidation of a variety of chromia-forming ferritic stainless steels has been studied in the temperature range 700–900°C in atmospheres relevant to solid-oxide fuel-cell operation. The most detrimental environment at 800°C and 900°C was found to be air with 10% water vapor. This resulted in excessive oxide spallation or rapid scale growth. Impurities in the alloys, particularly Al and Si, were found to have a significant effect on the oxidation behavior. Oxide growth was slow at 700°C but the higher-Cr-content alloys were observed to form sigma-phase at this temperature. The sigma phase formation was accelerated by higher silicon contents, and remarkably, by the presence of water vapor in the exposure environment. Alloys containing Mn were observed to form an outer layer of MnCr₂O₄ over the chromia scale. The potential for this overlayer to suppress reactive evaporation of the chromia scales has been analyzed.     

The effectiveness of aluminizing in protecting iron-base alloys in sulphidizing gases at high temperature

Following earlier research which has shown that Al₂O₃ scales are more effective than Cr₂O₃ scales in protecting iron-nickel-base alloys against sulphur-containing gases, several commercial steels, 310 stainless, 314 stainless, and 321 stainless, and an experimental ferritic steel, FeCrAlHf, have been pack aluminized to develop aluminide coatings for applications in mixed-gas environments of high sulphur and low oxygen potential. Results are presented for long-term exposures to H₂/1.6% H₂O/1.1% H₂S at H₂S at 750°C and 1000°C under thermal-cycling conditions. Short-term tests in this environment at 750° C led to considerable sulphidation of the uncoated, Cr₂O₃-forming alloys. The aluminized alloys were much more resistant to sulphidation than the uncoated materials, with relatively little degradation being observed after 200 hr at 750°C. Even at 1000°C, with the exception of the ferritic steel, the coated systems showed reasonable degradation resistance for 500 hr. Eventual sulphidation resulted from back diffusion of aluminum into the substrate and dilution of the coating surface in this element until an Al₂O₃ scale was unable to reform and base metal sulphides could develop. The composition of the substrate was important in determining the rate of aluminium depletion from the coating, with interdiffusion being faster in ferrite-rich matrices than the austenite matrices. Thus, the higher nickel-containing alloys developed the most effective coating systems for use in such environments. The structures of the various aluminized systems are presented and their mechanisms of protection and breakdown are discussed and correlated with their performances under these high-temperature conditions.     

Comparison of Short-Term Oxidation Behavior of Model and Commercial Chromia-Forming Ferritic Stainless Steels in Dry and Wet Air

A high-purity Fe–20Cr and commercial type 430 ferritic stainless steel were exposed at 700 and 800?°C in dry air and air with 10% water vapor (wet air) and characterized by SEM, XRD, STEM, SIMS, and EPMA. The Fe–20Cr alloy formed a fast growing Fe-rich oxide scale at 700?°C in wet air after 24?h exposure, but formed a thin chromia scale at 700?°C in dry air and at 800?°C in both dry air and wet air. In contrast, thin spinel?+?chromia base scales with a discontinuous silica subscale were formed on 430 stainless steel under all conditions studied. Extensive void formation was observed at the alloy–oxide interface for the Fe–20Cr in both dry and wet conditions, but not for the 430 stainless steel. The Fe–20Cr alloy was found to exhibit a greater relative extent of subsurface Cr depletion than the 430 stainless steel, despite the former’s higher Cr content. Depletion of Cr in the Fe–20Cr after 24?h exposure was also greater at 700?°C than 800?°C. The relative differences in oxidation behavior are discussed in terms of the coarse alloy grain size of the high-purity Fe–20Cr material, and the effects of Mn, Si, and C on the oxide scale formed on the 430 stainless steel.     

Corrosion Inhibition of 304 Stainless Steel by Nano-Sized Ti/Silicone Coatings in an Environment Containing NaCl and Water Vapor at 400–600°C

The corrosion behavior of 304 stainless steel (SS) and its corrosion inhibition by brushing nano-sized Ti/silicone coatings on its surface in an environment containing a solid NaCl deposit and water vapor at 400–600°C was studied. Results indicated that water vapor or NaCl, especially water vapor plus NaCl accelerated the corrosion of the steel markedly. The corrosion scales of the uncoated steel had a duplex structure at 400–500°C and internal oxidation occurred for the uncoated steel at 600°C in an environment containing NaCl and water vapor. The corrosion of the 304SS was inhibited efficiently by the coatings at 400–500°C, and the coated steel suffered corrosion to some extent and most of the coatings were destroyed at 600°C. X-ray diffraction (XRD) indicated that the corrosion products of the uncoated steel were mainly Fe₂O₃, Cr₂O₃, NiO or Na₂CrO₄, and the coatings consisted mainly of TiO₂ and SiO₂ after exposure at 400–500°C. The good corrosion resistance of the nano-sized Ti/silicon coatings was attributed to the formation of SiO₂, and TiO₂ that resulted from the decomposition of the organic components in the coating and fast oxidation of nano-Ti powder respectively during the experiments, TiO₂ mixed together with SiO₂ and formed a new coating on the steel surface that played an important role in the protection of the steel.     

Corrosion of AISI 430 ferritic stainless steel in (N<Subscript>2</Subscript>/3.1%H<Subscript>2</Subscript>O/2.42%H<Subscript>2</Subscript>S)-mixed gas at 600–800 °C

The AISI 430 ferritic stainless steel with a composition of Fe-16.5Cr-0.5Mn-0.6Si-0.06C in wt% was corroded at 600, 700 and 800 °C for up to 30 h in 1 atm of (N₂/3.1%H₂O/2.42%H₂S)-mixed gas. It displayed poor corrosion resistance because of H₂O/H₂S. The hydrogen dissolution in the scales owing to H₂O/H₂S, and the predominant formation of sulfides owing to H₂S made the scale highly susceptible to cracking and spallation. The sulfur potential in the mixed gas was so high that sulfides formed predominantly. Sulfides with fast growth rates always overgrew oxides to constitute the main scale. Iron corroded to FeS via the outward diffusion of Fe²⁺ ions to form the outer scale, which led to the formation of Kirkendall voids in the scale. Chromium corroded to (Fe, Cr)-mixed sulfides or Cr₂S₃ in the inner scale. The scales grew fast, and were highly fragile, porous, and susceptible to spallation.     

Electrochemical and Sulfide Stress Corrosion Cracking Behaviors of Tubing Steels in a H2S/CO2 Annular Environment

The electrochemical and sulfide stress corrosion cracking (SSCC) behaviors of 13Cr stainless steel and P110 steel were investigated in a simulated acidic annular environment with low-temperature and high-pressure H₂S/CO₂ using electrochemical methods, U-bend immersion tests, and scanning electron microscopy. In the solution containing high pressure CO₂, 13Cr, and P110 steels exhibited general corrosion and severe pitting, respectively. Compared with sweet corrosion, additional H₂S in the solution enhanced the corrosion of 13Cr steel but inhibited the corrosion of P110 steel. By contrast, in a solution containing 4 MPa CO₂ and different $ {P}_{{{\text{H}}_{ 2} {\text{S}}}}$ (0-0.3 MPa), the susceptibility of both 13Cr stainless steel and P110 steel toward SSCC was significantly promoted by increases in H₂S partial pressure. The 13Cr stainless steel exhibited higher susceptibility toward SSCC than P110 steel under a H₂S/CO₂ environment but lower susceptibility under a pure CO₂ environment.     

H2S 分压对 13Cr 不锈钢在 CO2注气井环空环境中应力腐蚀行为的影响简

利用高压下的电化学实验及U型弯浸泡实验结合微观分析手段,研究了13Cr不锈钢在不同H2S分压下CO2注气井环空环境模拟液中的电化学特征及应力腐蚀规律。结果表明:油套管钢的刺漏现象以及环境中硫酸盐还原菌的存在使得环空环境成为复杂的高压H2S-CO2-Cl-环境,13Cr不锈钢在该种环境下具有明显的应力腐蚀敏感性。随着H2S分压的升高,13Cr不锈钢击破电位下降,应力腐蚀敏感性增强,这主要因为H2S分压的增大对不锈钢表面膜(钝化膜及腐蚀产物膜)的破坏作用加强。当H2S分压达到0.20 MPa时,13Cr不锈钢发生明显的应力腐蚀,断口表现为由沿晶应力腐蚀裂纹(IGSCC)和穿晶应力腐蚀裂纹(TGSCC)组成的混合断口,应力腐蚀受阳极溶解和氢致开裂共同控制。     

Corrosion and Interdiffusion in a Ni/Fe–Cr–Al Couple Used for the Anode Side of Multi-Layered Interconnector for SOFC Applications

Under operating conditions in solid-oxide fuel cells (SOFC), metallic-interconnect plates form oxide scales (e.g., Cr₂O₃, Al₂O₃) on their surface, which have an electrically insulating effect and increase the contact resistance between the interconnect and the electrodes. In order to ensure high electrical conductivity between the electrodes and the interconnects, the formation of oxide scales on the interconnect surface must be prevented or minimized. The present work shows possibilities for improved contacts at the anodic side in solid-oxide fuel cells by plating an Fe–Cr–5.7Al interconnect with a Ni foil. Contact-resistance measurements and microscopic studies show the electrical behavior and corrosion of materials used at 800°C under an Ar/4 vol.% H₂/3 vol.% H₂O atmosphere. The results reveal that the interconnects coated with nickel exhibit low aging rates in the investigations performed and are thus suitable for use on the anode side.     

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.     

Hot corrosion behavior of plasma and HVOF sprayed Co- and Ni-based coatings at 900°C

High temperature Co- and Ni-based metallic coatings were applied on samples of 310 stainless steel using plasma spray and high velocity oxy fuel (HVOF) methods. The samples were sprayed with Na₂SO₄, V₂O₅ and NaCl salt solutions and exposed in air at 900°C to cyclic conditions with an aim to simulate an environment typically encountered in oil refinery operation. Extent of hot corrosion damage was assessed using gravimetric measurements while microstructure of the coatings was examined using scanning electron microscopy coupled with energy dispersive X-ray spectroscopy. X-ray diffraction was used to determine phase constitution. The aim of this study was to evaluate and compare the performance of specific coatings in hot corrosive environments. Experimental results indicate that the presence of V₂O₅ and NaCl serves to enhance hot corrosion while Co-based coatings perform better than Ni-based coating.