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.     

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.     

Corrosion of metals and alloys in sulfate melts at 750°C

The corrosion of Ni, Co, Ni-10Cr, Co-21Cr, and IN738 was studied at 750°C in the presence of molten sulfate mixtures (Na₂SO₄-Li₂SO₄ and Na₂SO₄-CoSO₄) and in an atmosphere consisting of O₂+0.12% SO₂-SO₃. The corrosion was observed to be similar for both Na₂SO₄-Li₂SO₄ and Na₂SO₄-CoSO₄ melts. The corrosion of Ni and Co tookplace by the formation of a mixed oxide plus sulfide scale, very similar to the corrosion in SO₂ or SO₃ alone. The initial stage for the corrosion of Ni-10Cr involved the formation of a thick NiO+Ni₃S₂ duplex scale, and Cr sulfide was formed during the later stages. A pitting type of morphology was observed for both Co-21 Cr and IN738. The pit was Cr sulfide at the beginning, and subsequently the sulfides oxidized to Cr₂O₃. A base-metal oxide layer was present above the pit, and this was observed to be formed very early in the corrosion process. A mechanism is proposed to explain this. In general, the formation of sulfides appears to be the primary mode of degradation in mixed sulfate melts.     

Breakdown of chromium oxide scales in sulfur-containing environments at elevated temperatures

Structural Fe-Cr-Ni alloys may be rapidly degraded in oxygen-sulfur mixed-gas environments at elevated temperatures unless protective oxide scales can be formed and maintained. The breakaway corrosion process was examined in model alloys of Fe-25wt.% Cr-20wt.% Ni with and without Nb and Zr additions. Oxide scales were preformed in S-free environments and subsequently exposed to oxygen-sulfur mixed-gas atmospheres. Preformed scales were found to delay the onset of breakaway corrosion. The beneficial effects of refractory metal additions were achieved via formation of a barrier layer at the Cr₂O₃ alloy interface.     

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.     

Corrosion mechanism of mechanically alloyed Mg50Ni50 and Mg45Cu5Ni50 alloys

As the loss of active material Mg may affect electrode‘s discharge capacity and the cycling stability, a more refined mechanism study on cycling capacity degradation should be made. The present investigation is based on the supposition that the capacity degradation of the binary MgSONi50 alloy and ternary Mg45CuSNi50 alloy electrodes is solely due to the corrosion of Mg, the active hydrogen storage element. That means amount of capacity degradation is determined by the corrosion current time, which is also the time of operation. The corrosion current Jcoxt dependance on cycling time was deduced. A mathematic relation between the cycling capacity retention CN/C1( % ) and the duration of operation was also deduced. The data calculated from the equations deduced agree well with those of the expeximemt result. The loss of the active hyckogen-absorhing element Mg is proved to be the main cause for cycling capacity deterioration in the present investigation.     

Corrosion of nickel-base and iron-base alloys in a simulated fluidized-bed coal-combustion environment

The modes of initiation and propagation of corrosion attack on a series of high-temperature alloys were studied in synthetic gas mixtures at 900°C. The gas mixtures were intended to simulate the oxygen and sulfur partial pressures experienced in reducing zones in a coal-fired fluidized-bed combustor and comprised mixtures of CO, CO₂, and SO₂. The alloys studied were candidates for in-bed heat exchanger tubing for an air-heater cycle operating at 843°C and 300–500 psig and so ranged from type 300-series stainless steels to nickel-base alloys. With the exception of two FeCrAlY alloys and types 304 and 347 stainless steels, it was found that sulfidation corrosion could be initiated on all the alloys within 0.25 hr; the rate of propagation of the corrosive attack depended on the flux of SO₂ in the environment and on the nickel content of the alloys. The presence of iron in the alloys appeared to slow the initiation of sulfidation, by forming a continuous iron oxide layer. The effects of various alloying additions are discussed, and a schematic model for the initiation of sulfidation is proposed.     

Corrosion behavior of magnesium and magnesium alloys

The automotive industry has crossed the threshold from using magnesium alloys in interior applications such as instrument panels and steering wheels to unprotected environment such as oil pan, cylinder head and wheels. The expanding territory of magnesium leads to new challenges.mainly environmental degradation of the alloys used and how they can be protected. The present critical review is aimed at understanding the corrosion behavior of magnesium and magnesium alloys in industrial and marine environments, and the effect of microstructure, additive elements and inhibitors on the corrosion mechanism.     

Reduction of nitrite in aged heat transfer media at elevated temperatures

Nitrite is part of many commercially available inhibitor formulations used for corrosion protection in water or water/glycol based heat transfer media in contact with carbon steel. In a heat transfer system of the OMV‐AG used to heat crude oil to lower its viscosity, the nitrite concentration was found to decrease in about three months from 300 ppm to 10 ppm. The system is operated at a maximum temperature of 110°C and 6 bar and an ethylene glycol/water mixture is used as the heat transfer media. An oxidation to nitrate was assumed, but the nitrate concentration was found to be constant. An analysis of the vapor phase present in the system showed next to significant amounts of hydrogen dinitrogen oxide. A possible microbiological infection of the system was investigated, but no bacteria were found. To find the reason for the reduction of nitrite, a laboratory investigation was performed. Stability of nitrite at elevated temperatures and pressure was tested in water and water/glycol mixtures. It was found that nitrite is instable in glycol solutions at temperatures above 120°C. The degradation reaction in aged and unused MEG/water mixtures was compared. An accelerated reduction reaction was found in aged heat transfer media.     

Corrosion properties of high silicon iron-based alloys in nitric acid

The effect of copper and rare-earth elements on corrosion behavior of ~iigh silicon iron-based alloys in nitric acid was studied by means of static and loading current corrosion experiments. The anodic polarization curve was also made to discuss the corrosion mechanism. The examination on alloy microstructure and SEM corrosion pattern showed that when silicon content reached 14.5%, the Fe3Si phase appeared and the primary structure of the iron-base alloy was ferrite. When adding 4.57% copper in the iron alloy, its corrosion resistance in static diluted sulfuric acid was improved while its corrosion resistance and electrochemical corrosion properties in the nitric acid were decreased. In contrast, the addition of rare earth elements could improve the corrosion properties in all above conditions including in static diluted sulfuric acid and in nitric acid.     

铜合金及其复合材料在氯化钠盐雾中的腐蚀行为

通过氯化钠盐雾腐蚀实验,结合X射线衍射(XRD)和电子探针(EPMA)分析,研究了铜合金及其复合材料在氯化钠盐雾中的腐蚀行为.结果表明:在实验初期材料的腐蚀速度随时间的延长而降低,约96 h以后腐蚀速度基本不再变化;BZn15-17与纯镍复合材料的耐蚀性最好;实验初期和后期的腐蚀产物组成基本相同,2种白铜合金的主要腐蚀产物都是CuCl2@2H2O,黄铜合金的主要腐蚀产物为CuCl,其次为CuCl2@2H2O和(CuZn)2(OH)3Cl;腐蚀形貌变化较大.     

Corrosion studies of single crystals of iron-gallium alloys in aqueous environments

Iron-gallium (Fe-Ga) alloys have attracted lot of attention for use in sensor and actuator applications due to an excellent combination of large low-field magnetostriction, high mechanical strength, good ductility and low cost. This paper reports the very first measurement of the corrosion behavior of single crystals of Fe-Ga alloys. The corrosion behaviors of single crystals of Fe-15 at.% Ga, Fe-20 at.% Ga and Fe-27.5 at.% Ga alloys were studied in acidic (0.1 M HCl), basic (0.1 M NaOH) and simulated seawater (3.5 wt.% NaCl) environments. Dependence of corrosion rates on the crystal orientation is also examined. The polarization resistance technique was used for measuring the corrosion potentials and corrosion rates. The corrosion rate of single crystals of Fe-Ga alloys in 0.1 M HCl aqueous solution is higher than that in 3.5 wt.% NaCl aqueous solutions. The corrosion rate of single crystals of Fe-Ga alloys is least in 0.1 M NaOH aqueous solutions.     

Corrosion and electrochemical behavior ofAZ31D magnesium alloys in sodium chloride

The corrosion and electrochemical behavior of extruded AZ31D magnesium alloys in NaCl solution were investigated using SEM, XRD and electrochemical method. It is found that AZ31D is susceptive to Cl^- ion, and the open circuit potential shifts to more negative values with increasing chloride concentration. Pitting occurs at corrosion potential and corrosion area enlarges with enhanced polarization. Tafel slopes of the cathode branches in different testing solution are almost the same. Cl^- concentration affects cathode course slightly. High frequency capacitive loops shrink with the increase of Cl^- concentration. Corrosion initiates from the grain boundary and spreads to entire surface with time.     

Sulfidation-oxidation of advanced metallic materials in simulated low-Btu coal-gasifier environments

The corrosion behavior of structural alloys in complex multicomponent gas environments is of considerable interest for their effective utilization in coal conversion schemes. Little understanding of the degradation mechanisms of advanced high-temperature alloys in conditions typical of low-Btu coal gasifiers presently exists. Analysis of scale and subscale characteristics of several alloy types after exposure to the aggressive simulated low-Btu gasifier environments yielded a reaction model for these sulfidation-oxidation conditions. Initial competition between reactive metal oxide and base metal sulfide nuclei is followed by base metal sulfide overgrowth, chromium sulfide formation at the scale-metal interface, dissociation near voids in the subscale, and internal chromium sulfide precipitation. Additions of aluminum, titanium, and an oxide dispersion improve the sulfidation resistance by increasing the number of oxide nucleation sites and their growth kinetics on the surface in the crucial competition stage. Thermogravimetric tests carried out in three mechanistic regimes agreed with these hypotheses.     

Estimating the gas permeability of commercial volatile corrosion inhibitors at elevated temperatures with thermo-gravimetry

Volatile corrosion inhibitors (VCIs) are incorporated into packaging paper or film to protect metals against atmospheric corrosion. The vapour pressure determines the equilibrium concentration of a volatile corrosion inhibitor (VCI) in the surrounding atmosphere. However, the rate at which the VCI can be delivered across the air gap to a metal surface is determined by the gas permeability. This is the product of the vapour pressure and the diffusion coefficient: S_A = P_AD_AB. The gas permeability of commercial VCIs was estimated at elevated temperatures from vaporisation rates measured using a simple thermo-gravimetric method (TG).     

High-temperature corrosion and creep of Ni-Cr-Fe alloys in carburizing and oxidizing environments

The carburization of NiCr 32 20 and NiCrSi 60 16 has been studied in CH₄-H₂ mixtures in the temperature range 900–1100°C. The methods included thermogravimetric measurements and studies on reacted specimens by X-ray diffraction, metallographic, and chemical analysis. Upon carburization internal carbides M₇C₃ and M₂₃C₆ are formed (M=mainly Cr); the rate of carburization is determined by carbon diffusion in the Fe-Ni matrix with carbide precipitations. The effect of the alloying elements Ni and Si on the carburization resistance of austenitic alloys is explained. By the same methods the oxidation and carburization in CO-H₂O-H₂ mixtures have been studied. The important role of a stable chromium oxide layer for the carburization resistance was confirmed. Creep tests at 1000°C in a CO-H₂O-H₂ atmosphere where Cr₂O₃ is stable showed carburization occurring through cracks in the oxide layer. At high strain rates premature failure occurs by carburization, which is followed by internal oxidation and formation of cracks, voids, and holes.     

Corrosion resistance of dilute CuMg alloys at elevated temperature

In comparison with CuAl (Al: 0.2 and 0.5 wt.%) alloys, corrosion resistance (CR) of CuMg (Mg: 0.12 and 0.34 wt.%) alloys was studied at 673-1173 K in atmospheric O₂. All the samples were pre-annealed at 873 K in atmospheric H₂. The CR of CuMg alloys at 673-973 K is improved in contrast to a pure Cu but much poorer than that of CuAl alloys, while the improvement can hardly be observed for CuMg alloys at and above 1073 K, which is similar to CuAl alloys. The poorer CR of CuMg alloys compared with that of CuAl alloys at 673-973 K is largely attributed to the incorporation of Cu in the MgO surface layer and the low Pilling-Bedworth ratio of CuMg-O system smaller than unity, and the vanishing of CR for CuMg alloys at and above 1073 K is ascribed to the instability of the MgO layer at the Cu₂O/CuMg interface.     

Corrosion of Al-Sn-Bi alloys in alcohols at high temperatures. Part II: Effect of anodizing on corrosion

Nine kinds of Al alloys were anodized to form porous anodic oxide films, and then anodized specimens were immersed in 2-(2-(2-methoxyethoxy)ethoxy)ethanol (MEEE) and 2-(2-(2-butoxyethoxy)ethoxy)ethanol (BEEE) at 415 K. Al-1.0%Sn-1.0%Bi alloy was corroded severely in both BEEE and MEEE, whereas other eight alloys showed no corrosion. The corrosion proceeded under the anodic oxide films through cracks formed in the film. Cathodic polarization in Cu electroplating solution after corrosion suggested that the crack formation during immersion in hot MEEE is due to thermal expansion of the substrate and Sn and Bi containing particles included in the anodic oxide film.