Influence of solid-state CaS-CaO-CaSO4 deposits on corrosion of high-temperature alloys in simulated FBC environments

This study addresses questions concerning the likelihood of sulfidation attack of heat-exchanger alloys beneath deposits of sulfur-sorbent material in fluidized-bed combustors. Alloy specimens were exposed at 900°C in calcium sulfate-calcium oxide and calcium sulfide-calcium oxide mixtures, in environments in which the oxygen partial pressures were fixed at values corresponding to the equilibrium values for each solids mixture, using controlled ratios of CO and CO₂. The only source of sulfur in these systems was the calcium sulfate or sulfide. Sulfidation attack of nickel-base alloys occurred in both mixtures, the calcium sulfide-calcium oxide mixture being the more aggressive. Iron-base alloys were less susceptible to attack, although susceptibility increased with increasing nickel content. FeCrAlY-type alloys were resistant to attack. Comparison with corrosion behavior under conditions in which the oxygen and sulfur partial pressures were the same as those used here, but in which the sulfur source was in the gas phase, indicates that the form of the sulfidation attack is similar but that its progress is much slower under solid deposits.     

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.     

Degradation of zirconium-containing nickel-based alloys in a hydrogen-hydrogen sulfide environment

The modes of degradation experienced by a Ni-15Cr and three Ni-15Cr-xZr alloys (where x=1, 5, or 10 wt. %) after exposure to a 1% H₂S/H₂ gas mixture at 1000° C have been examined, with particular reference to the morphological features. The addition of Zr at about the 1% level has been shown to have a beneficial effect, in terms of reduced weight gain relative to the Ni-15Cr control, explained by a thinner more protective external CrS scale, and a decreased tendency for CrS subscale formation (internal sulfidation). Zr additions beyond 1%, up to 5 and 10%, become increasingly detrimental principally on account of extensive internal ZrH_x formation, associated with the selective attack of the increasing amounts ofNi₅Zr in the alloy microstructures. The mode of action of Zr at about the 1% level in these alloys is considered in the light of earlier studies concerning the performance of similar alloys in O/S atmospheres.     

Corrosion of metals and alloys in mixed gas environments at elevated temperatures

The attack of nickel, cobalt, iron, and alloys of these metals containing chromium and aluminum, by gases containing sulfur-oxygen, carbon-oxygen, and nitrogen-oxygen has been studied at temperatures of 600 and 900°C. The degradation of these metals and alloys was characterized by using standard analytical techniques with emphasis on optical metallography. Three types of accelerated degradation were identified for the attack of alloys by gases containing another oxidant in addition to oxygen. One type of degradation occurred because of the formation of reaction products composed of mixtures of phases involving both of the oxidants. Another type resulted from the reaction of second oxidant phases with oxygen. The third form of degradation involved the development of less protective phases due to thermodynamic instabilities. Thermodynamic stability diagrams are used to help account for the effects produced by different elements in the alloys.     

Microcrack generation and its healing in the oxide scale formed on Fe-Cr alloys

Fe -base alloys containing 5, 10, and 20 wt. % Cr were oxidized in a stream of O₂ at 750 and 900°C up to 264 hr. A sulfur decoration method was applied to detect the cracks generated in the scale during oxidation. This method revealed frequent crack generation and its healing in the scale. In the case of low-Cr alloys, the cracks are filled up with newly formed Fe-rich oxide but may be regenerated during further oxidation. Cracks are generated in the scale on an Fe-20Cr alloy also, although this alloy is not so severely attacked because of rapid healing.     

The effect of preoxidation of some Ni,Fe, and Co-Base alloys on subsequent sulfidation at 982°C in sulfur vapor

The effect of preoxidation was studied on the subsequent sulfidation in sulfur vapor at a pressure of 0.1 atm at 982°C on numerous iron, nickel, and cobalt-base alloys which were either chromia or alumina formers. In general, alumina films were much more protective than chromia films, but the efficacy of preoxidation in reducing sulfidation rates depended more upon perfection of the films and whether cracking and/or spatting occurred. Increasing oxidefilm thickness had a beneficial effect until either penetration of the films by sulfur or cracking occurred, after which sulfidation rates were sometimes greater than for nonpreoxidized samples. The enhanced sulfidation rates are attributed to sulfidation of a solute-depleted substrate, the solute having been selectively removed by oxide formation. One alloy, MA 956, containing 0.5 Y₂O₃ as fine dispersions which normally provide spatting resistance, still exhibited extensive cracking and spalling of the oxide and was not much better than alloys without dispersoids or reactive-metal additions. The use of preoxidation to reduce sulfidation rates is not viable under the extreme conditions used. Preoxidation is conceptually a good method for inhibiting sulfidation at lower temperatures and much lower sulfur pressures.     

Corrosion behavior of engineering alloys in synthetic wastewater

The corrosion behavior of 1018, 410, and 800 steels exposed to synthetic wastewater have been studied using linear polarization resistance, cyclic potentiodynamic curves (CPCs), electrochemical noise (EN), and electrochemical impedance spectroscopy (EIS) tests. The conditions were: a biochemical oxygen demand of 776 ppm; a chemical oxygen demand of 1293 ppm; a pH of 8; and a cell temperature of 25 °C. From the CPC and EN results, no localized corrosion was found for the stainless steels. However, small indications of a possible localized corrosion process were detected for the 1018 steel. The EIS results revealed that different corrosion mechanisms occurred in the carbon steel compared with the stainless steels. The results show that the corrosion mechanism strongly depends on the type of steel. Overall, the 1018 steel exhibited the highest corrosion rate, followed by the 410 alloy. The highest corrosion resistance was achieved by the 800 alloy. In addition, scanning electron microscopy analyses were carried out to explain the experimental findings.     

Corrosion behaviour of various steels and alloys in a simulated coal liquefaction environment

The corrosion behaviour of Fe-Cr, Ni-Cr and Fe-Cr-Ni alloys was studied in simulated liquefaction environments over the temperature range of 623 to 723 K. Corrosion of alloys in these environments was controlled by both sulfidation and oxidation. In the slurry, the oxygen potential was apparently higher and the corrosion rate was smaller in comparison with those in the gas phase in the solvent only environment. An oxygen-rich brown coal caused much smaller corrosion than a sub-bituminous coal. With regard to the effect of the alloy compositions, chromium content controlled the corrosion rate. The Fe-Cr-Ni austenitic alloys were the most resistant. The Fe-Cr alloys with high chromium content were fairly resistant, although they may be relatively sensitive to the variation of the environments such as temperature and H₂S content. Ni-Cr alloys of less than 20% Cr may have high corrosion rate due to their fast sulfidation. The structure of the scales formed on the alloys can be reasonably explained by the phase stability diagram of Metal-O-S system.     

High temperature corrosion of Ni-20Cr and Ni-15Cr-8Fe alloys in sulfur dioxide

Two nickel-base alloys, Ni-20Cr and Ni-15Cr-8Fe, in the form of wire specimens, have been exposed to 100 mbar of sulfur dioxide between 550 and 850°C. For Ni-20Cr, an outer Cr₂O₃ layer formed only at the beginning of the reaction, but very quickly Ni₃S₂ grew preferentially at the exterior by outward diffusion of nickel. The reaction rate is regulated by an external interfacial process. A barrier effect was noted near 645°C associated with the formation of NiCr₂O₄; a new acceleration takes place above 680°C. The external growth of Ni₃S₂ is attributed to the low radius of curvature of the samples. For Ni-15Cr-8Fe, the reaction mechanism is rather similar, except that no barrier effect occurred. A protective Cr₂O₃ layer formed above 800°C in both cases.     

Corrosion of ferritic steels (Fe-Cr-Al) in sulfur vapor and H2S/H2 mixtures

The high-temperature corrosion (700–1000°C) kinetics and processes in sulfur vapor and hydrogen sulfide were investigated for ferritic steels containing Cr and Al. Observation and analysis of the reaction curves obtained allowed the various corrosion steps to be clarified. Formation of compact, protective layers, at either the alloy-sulfide or at the sulfide-gas interface, occurred in the early stages of reaction and subsequently, most of the layers formed were microcrystalline and porous. The influence of the system parameters, i.e., temperature, pressure of the sulfidizing agent, and alloy composition, is reported in this paper.     

Initiation of Metal-Dusting Pits and a Method to Mitigate Metal-Dusting Corrosion

Initiation of metal dusting was studied by scanning-electron microscopy and Raman spectroscopy. A copper-indicator method was developed to identify locations that are easily attacked by metal dusting. The effect of surface scratches on metal dusting was investigated. Alloy 800 specimens from a hydrogen-reformer plant were analyzed. The alloy developed a nonprotective oxide scale in which the major phase was spinel with a high Fe content. The initiation of metal-dusting pits occurs when channels form through the oxide scale for the transfer of carbon. The channels can be blocked by short exposure of the metal-dusted alloy surface to an oxidizing environment. Metal-dusting corrosion could be mitigated by selection of alloys with long incubation time and by minimizing and/or slowing the pit-growth rate using an intermediate oxidation treatment at an appropriate temperature and in an appropriate environment for a relatively short time.     

Effects of mode-I stresses on the oxidation and failure mechanisms of Ni-20Cr and Ni-15Cr-8Fe alloys in sulfur dioxide

Two alloys, Ni-20Cr and Ni-15Cr-8Fe, as wire specimens, were exposed to sulfur dioxide between 325 and 800°C, with applied external stresses (mode I). Their mechanical properties have been investigated, and the variation of their radius has been precised by conductivity measurements. For the Ni-20Cr alloy, below 550°C, the failure process combines cracking and corrosion: Cr₂S₃ crystals formed at the tip of the cracks facilitate their propagation. Above 550°C, no barrier effect is observed, and intergranular corrosion takes place. For the Ni-15Cr-8Fe alloy, mode-I stresses bolster intergranular corrosion.     

Investigations of the degradation of high-temperature alloys in a potentially oxidizing-chloridizing gas mixture

The degradation of high-temperature alloys in argon-5.5% oxygen-0.96% hydrogen chloride-0.86% sulfur dioxide at 900°C under isothermal and thermal cycling conditions has been investigated. All the alloys showed reasonable resistance under isothermal conditions, although the Al₂O₂ ***-forming material, alloy 214, gave the lowest amount of corrosion, consistent with Al₂O₃ being a more effective barrier than Cr₂O₃ to inward penetration of chlorine or sulfur-containing species from the environment. Significant internal corrosion was observed for some alloys. Degradation of all the alloys was much more severe under thermal cycling conditions because of the failure of the protective scales. In all cases, formation of volatile chlorine-containing compounds was observed. Degradation of the alloys resulted from the penetration of chlorine-containing species through the initially formed oxide scale and formation of chlorides or, possibly, oxychlorides at the alloy-scale interface or in the subjacent alloy. The sulfur dioxide did not play any obvious role in the process.     

Oxidation resistance of eight heat-resistant alloys at 870°, 980°, 1095°, and 1150°C

Oxidation testing of heat-resistant alloys is described. The testing procedure utilized weight-gain measurements using one specimen, which was withdrawn and weighed at intervals of 1 week, for 10–18 weeks. The specimen was placed in a porcelain cup during exposure and covered upon cooling to retain spalled oxide. Weight gain was used to determine the kinetics of oxidation and was extrapolated to 3000 hr. The specimen was withdrawn at the end of the exposure, weighed, cathodically descaled, and reweighed. The ratio of oxygen ions to metal ions was determined for each alloy and test temperature. This ratio approaches the stoichemetric ratio for Fe₃O₄ or Cr₂O₃. The ratio for each test is used to convert weight gain to weight loss. The amount of adherent oxide was determined as well as the total amount of oxide, leading to an expression for oxide adherency. The oxidation resistance of Fe-Cr-Ni alloys increased with increasing Cr and Ni, with Cr being the most critical element. Additions of Si, Al, or Ce were shown to extend the usefulness of Fe-Cr-Ni alloys.     

Sulfidation properties of austenitic Fe-Mn and Fe-Mn-Al alloys under low sulfur vapor pressure (8 Pa) in the temperature range 873–1173 K

The sulfidation properties of austenitic Fe-Mn and Fe-Mn-Al alloys containing small amounts of carbon have been characterized with respect to the sulfidation kinetics, scale morphological development, structures, and composition of the sulfide phases. The alloys contained 21–40 wt. % Mn and 2.5–8 wt.% Al. The sulfide phase was monosulfide of manganese and iron containing the other metallic elements in solid solution. Two regimes of sulfidation categorized by slow and fast reaction rates were exhibited by all alloys when sulfidized in sulfur vapor at = 8 Pa and over the temperature range 873–1173 K. In the slow regime, a compact duplex -Mn(Fe)S/Fe(Mn)S scale evolved by a classical parabolic law associated with metal diffusion in scale. A porous microcrystalline mixed scale of the above sulfides evolved in the regime of rapid sulfidation by quasilinear kinetics associated with sulfur ingress through the porous scale.     

Corrosion of iron-, nickel-, and cobalt-base alloys in atmospheres containing carbon and oxygen

The corrosion of iron-, nickel-, and cobalt-base alloys has been studied in atmospheres containing carbon and oxygen in the temperature range 894–1366 K. It was observed that preformed Cr₂O₃ films are not effective barriers to carbon transport in atmospheres in which the oxide is not stable but that stable, growing Cr₂O₃ films are excellent barriers to carbon penetration. The presence of Fe-containing oxides on Fe-Ni-Cr and Fe-Cr alloys cause the scales to be permeable to carbon. This phenomenon was found to be sensitive to alloy surface preparation. Carbon transport through oxide scales may occur by two mechanisms: diffusion or molecular transport through physical defects. The present work has evidence of the latter but cannot rule out the former in cases where the carbon activity is sufficiently large. In gases containing CO and CO₂ in which Cr carbide is stable Cr₂O₃ was found to form at the carbide-alloy interface by oxygen transport through the carbide. In A-CH₄ Fe-Ni-Cr were found to undergo graphitization attack. The results were consistent with the formation and subsequent decomposition of metastable carbides, as proposed by Hochmann.     

Corrosion of some pure metals in basaltic lava and simulated magmatic gas at 1150°C

The extraction of thermal energy from subterranean magma can be achieved by the use of a suitable heat exchanger extending into the molten rock. Although the engineering feasibility of this scheme has not been proven, engineering data, including materials compatibility information, will be required ultimately. The work summarized in this paper was designed to provide an understanding of the reaction mechanisms and modes of degradation of various metals so that the best generic types of alloys can be selected for structural components and instrumentation. Fifteen pure metals were studied. These included base metals such as iron, nickel, and cobalt; some precious metals: platinum, rhodium, and palladium (possible thermocouple or lead-wire materials); refractory metals: tungsten, molybdenum, tantalum, niobium, vanadium, and rhenium; plus other high melting metals such as titanium and zirconium. Samples were exposed to basaltic lava at 1150°C for periods of 24 and 96 hr. A cover gas was used to produce oxygen and sulfur fugacities corresponding to those of the gases dissolved in basaltic melts. The corrosion behavior can be classified into five categories: (A) no attack (Pt and Re); (B) slight oxidation (Cr and Mo); (C) heavy oxidation (W, Ta, Nb); (D) sulfidation (Fe, Ni, Co, Pd, Rh); and (E) reaction with lava constituents (V, Ti, Zr). Group (A) metals were inert for all practical purposes. Group (B) metals formed thin adherent oxides initially, under which sulfides eventually formed in the substrate. Attack was minimal. Group (C) metals exhibited extensive oxide formation and virtually no sulfidation. Some reaction between the base-metal oxides and those in the lava took place. Group (D) metals all formed liquid sulfides which penetrated the substrate grain boundaries. All of these metals except cobalt were completely degraded. Cobalt was only partially penetrated by the liquid sulfide formed. Group (E) metals formed silicates, oxides, mixed oxides, and dissolved oxygen in the metal which completely embrittled the metal substrate. A small amount of sulfidation occurred, but sulfidation played virtually no role in the corrosion of these metals. Extensive analyses of the reaction products by scanning electron microscopy, X-ray energy dispersive analysis, electron microprobe analysis, and metallography are presented for each metal. The products formed are discussed with reference to thermodynamic stability diagrams, and the reaction path concept is used to explain some of the corrosion product morphologies.This work supported by the United States Department of Energy under Contract AT(29-1)-789.on leave at Sandia Laboratories, Albuquerque, New Mexico.     

316L不锈钢在沉积盐碱混合物下的高温腐蚀

对316L不锈钢沉积4.2% NaOH、1.3% NaCl、4.4% KOH、3.7% KCl盐碱混合物后在450 ℃至900 ℃下8 h和1 min高温腐蚀行为进行了试验.探讨了316L不锈钢在沉积盐碱混合物下的高温腐蚀机理,分析了温度、腐蚀介质和氧化膜的稳定性等因素对316L不锈钢腐蚀的影响.结果表明,316L不锈钢在450 ℃至900 ℃的高温腐蚀环境下其腐蚀速率随着温度的上升而增加.     

Corrosion behavior of a superduplex stainless steel in chloride aqueous solution

Super duplex stainless steels (SDSS) have been widely used as structural materials for chemical plants (especially in those engaged in phosphoric acid production), in the hydrometallurgy industries, and as materials for offshore applications due to their excellent corrosion resistance in chloride environments, compared with other commercial types of ferritic stainless steels. These alloys also possess superior weldability and better mechanical properties than austenitic stainless steels. However, due to their two-phase structure, the nature of which is very dependent on their composition and thermal history, the behavior of SDSS regarding localized corrosion appears difficult to predict, especially in chloride environments. To improve their final properties, the effect of the partition of the alloying elements between the two phases, and the composition and microstructure of each phase are the key to understanding the localized corrosion phenomena of SDSS. This paper concerns the effects of the SDSS microstructure and heat treatment on the SDSS corrosion resistance in aqueous solutions, containing different amounts of NaCl at room temperature.     

Corrosion behavior of Mg and Mg-Zn alloys in simulated body fluid

The corrosion behavior of Mg and Mg-Zn in simulated body fluid was studied.The mass loss of pure Mg,Mg-Zn-Zr and Mg-Zn-Zr-Y in simulated body fluid was measured using photovoltaic scale meter.Corrosion rate was determined through electrochemical tests.Finally,the corrosion mechanism was tbermodynamically studied.The results show that the corrosion rate decreases with the lapse of time for both pure Mg and Mg alloys.The purer the alloy,the borer the corrosion resistance exhibits.The corrosion behavior of Mg alloy is improved by the addition of trace Y.