Oxidation, carburisation and metal dusting of 304 stainless steel in CO/CO2 and CO/H2/H2O gas mixtures

The oxidation and carburisation behaviour of 304 stainless steel was studied during thermal cycling in CO/CO₂ at 700 °C, and also in CO/H₂/H₂O at 680 °C. Thermal cycling caused repeated scale separation which accelerated chromium depletion from the alloy subsurface regions. The CO/CO₂ gas, with a_C=7 and , caused internal precipitation of oxides and carbides, some surface damage, but no dusting. In contrast, the CO/H₂/H₂O gas, with a_C = 19 and caused rapid graphite deposition and metal dusting. This was accompanied by internal oxidation and carburisation. The internal oxide was identified as spinel, which forms in the short term, but not at long reaction time. Its formation produced a significant volume expansion, which disrupted the material and resulted in surface damage in both gas atmospheres. In CO/H₂/H₂O, however, direct graphite deposition and metal disintegration into dust was the main reaction. The very different reaction morphologies produced by the two gas mixtures are discussed in terms of competing gas-alloy reaction steps.     

Subsurface microstructural changes in a cast heat resisting alloy caused by high temperature corrosion

A cast HP ModNb alloy (Fe-25Cr-35Ni-1Nb, wt.%) was oxidised and carburised in CO-CO₂ corresponding to a_C = 0.1 and p_O2 = 3 × 10⁻¹⁶ atm at 1080 °C. Formation of an external, chromium-rich oxide scale led to depletion of this metal in a deep alloy subsurface zone. Within that zone, secondary chromium-rich carbides dissolved, primary carbides oxidised, solute silicon and aluminium internally oxidised, and extensive porosity developed. Pore volumes correspond to the difference between metal loss by scaling and metal displacement by internal oxidation, assuming the scale-metal interface to be fixed. The pores are concluded to be Kirkendall voids.     

Carburization of W- and Re-rich Ni-based alloys in impure helium at 1000 °C

The surface and microstructure stability of experimental W- and Re-rich Ni-based alloys in an impure-helium environment containing only CO and CO₂ as impurities (ppm level) have been investigated at 1000 °C. All the alloys carburized during 50 h of exposure, and, depending on the alloy composition, different carbides of the type M₆C, M₇C₃ and M₂₃C₆ formed on the alloy surface, in grain interiors and at grain boundaries. Microprobe analysis and Calphad-based calculations indicated that the chromium carbides (particularly Cr₂₃C₆) were enriched by rhenium. Extended exposure (225 h) led to the disappearance of surface transient carbides and the growth of surface oxide Cr₂O₃ occurred.     

Corrosion of a stainless steel and nickel-based alloys in high temperature supercritical carbon dioxide environment

Corrosion of four alloys has been studied in supercritical carbon dioxide at 650 °C and 20 MPa, specifically AL-6XN stainless steel and three nickel-based alloys, PE-16, Haynes 230, and Alloy 625. The tests were performed for exposure durations of up to 3000 h with samples being removed for analyses at 500 h intervals. The corrosion performance of the alloys was evaluated by weight change measurements, and the surface oxide layers were characterized by scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. Weight gain measurements showed that the Al-6XN stainless steel exhibited the least corrosion resistance while the weight gains were nearly similar for the other alloys. The oxide layer in AL-6XN stainless steel was composed of large equiaxed grained outer layer of Fe₃O₄ (magnetite) and an inner layer of FeCr₂O₄. Oxide spallation was observed in this stainless steel even after 500 h exposure. In all alloys, Cr-rich oxides phases of Cr₂O₃ and Cr_1.4Fe_0.7O₃ were identified as the protective layers. In alloy PE-16 a thin layer of aluminum oxide formed that promoted the corrosion resistance of the alloy. Cr₂O₃ was identified as the main protective oxide layer in nickel base alloys Haynes 230 and 625.     

Current oscillations in austenitic stainless steel induced by the presence of chloride ions

The conditions for current oscillations in austenitic stainless steel (AISI Type 303) in 1M H₂SO₄ containing Cl^? ions are given. The periodic oscillations are produced in a close potential range determined at the active-passive transition region. It requires a non-homogeneous distribution of inclusions and carbides at the metal surface and a concentration range of Cl^? ion where active and passive areas coexist on the metal surface. The constraints at the surface determining these two regions are related to the Cl^? ion competitive adsorption and the local accumulation of corrosion products. Electrochemical data and SEM observations are correlated.     

Effect of copper on metal dusting of austenitic stainless steels

Three steels, 304SS, 310SS and 800H, were alloyed with 5%, 10%, and 20% (by weight) copper, and then exposed to 68%CO-31%H₂-1%H₂O gas at 680 °C (a_C = 19 and p_O2=5.4×10^-25 atm) under thermal cycling conditions. Kinetic measurements showed that copper-free alloys all dusted, with 304SS experiencing the greatest metal wastage. Copper additions did not have any effect on metal wastage of 304SS, but reduced the attack on 310SS and 800H markedly at levels of 5% and 10%. However, increasing the copper content to 20% produced large copper-rich precipitates which accelerated dusting by promoting internal graphitisation.Dusting was associated with surface coking. When pitting occurred, on copper-free alloys and on copper containing 304SS, large coke structures grew above the pits. Internal grain boundary carburisation always took place, and intragranular carbides also precipitated when dusting occurred. A lamellar surface layer of internally precipitated spinel and austenite also developed in association with dusting. The copper effect is discussed in terms of its alloy solubility and its known beneficial effect in Ni-Cu binaries.     

Factors Affecting Chromium Carbide Precipitate Dissolution During Alloy Oxidation

Ferrous alloys containing significant volumefractions of chromium carbides were formulated so as tocontain an overall chromium level of 15% (by weight) buta nominal metal matrix chromium concentration of only 11%. Their oxidation at 850°C inpure oxygen led to either protectiveCr₂O₃ scale formation accompaniedby subsurface carbide dissolution or rapid growth ofiron-rich oxide scales associated with rapid alloy surface recession, which engulfedthe carbides before they could dissolve. Carbide sizewas important in austenitic alloys: an as-castFe-15Cr-0.5C alloy contained relatively coarse carbides and failed to form aCr₂O₃ scale, whereas the samealloy when hot-forged to produce very fine carbidesoxidized protectively. In ferritic alloys, however, evencoarse carbides dissolved sufficiently rapidly to provide the chromium flux necessary to formand maintain the growth of a Cr₂O₃scale, a result attributed to the high diffusivity ofthe ferrite phase. Small additions of silicon to theas-cast Fe-15Cr-0.5C alloy rendered it ferritic and led toprotective Cr₂O₃ growth. However,when the silicon-containing alloy was made austenitic(by the addition of nickel), it still formed aprotective Cr₂O₃ scale, showing that the principal function of silicon was inmodifying the scale-alloy interface.     

Assessing the tribocorrosion performance of Ti–6Al–4V, 316 stainless steel and Monel K500 alloys in artificial seawater

The electrochemical and tribocorrosion behaviors of Ti–6Al–4V, 316 stainless steel and Monel K500 alloys sliding against Al₂O₃ in artificial seawater are investigated in this paper. It can be observed that the open circuit potential drops down to more negative values due to the removal of passive film. And a rapid dissolution occurs in the wear track compared with the unworn area. The wear loss polarized in cathodic potential is lower than that in anodic potential and open circuit potential conditions, because the material deterioration is enhanced by corrosive attack. The wear volumes of 316 stainless steel are much higher than the ones measured for Ti–6Al–4V and Monel K500 alloys. Friction coefficients are significant large in cathodic polarization compared with anodic polarization for all alloys. Moreover, the 316 stainless steel exhibits large friction coefficients compared with Ti–6Al–4V and Monel K500 alloys.     

Effect of carbide volume fraction on the oxidation of austenitic Fe‐Cr‐C alloys

A series of Fe‐15Cr‐(2‐3)Mo alloys (compositions in weight percent) was produced with different carbon concentrations, to control the distribution of chromium between matrix metal and M₂₃C₆ precipitates. The alloys were oxidized in the austenitic state at 850°C in pure oxygen, with and without a pre‐oxidation treatment at low oxygen potential, where no iron oxide could form. Protective, chromia‐rich scaling took place if the chromium concentration at the metal‐scale interface was high enough. This concentration was controlled by the original alloy matrix chromium concentration, and whether or not a high diffusivity ferrite zone developed at the surface by decarburization. Ferrite zone formation was assisted by pre‐oxidation at low oxygen potentials. The value of the carbides as suppliers of additional chromium was demonstrated by comparison with the oxidation performance of carbide‐free alloys of corresponding matrix chromium levels. However, because dissolution of the coarse carbides could be slow, alloys with high volume fractions of large carbides were unsuccessful.     

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.     

An anodic behaviour study of an analogical sintered system of austenitic stainless steel in H2SO4 solution

The anodic behaviour of sintered austenitic stainless steel was investigated on an analogical system using wrought plate stainless steel of Type 316. Potentiodynamic polarization measurements were carried out on wrought plate stainless steel of Type 316 in an analogical concentration cell using a special method. The effect of carbides precipitation on the anodic behaviour was investigated on carburized 316 wrought stainless steel containing an average carbon concentration of 0.06 and 0.15 (wt. %). The carburization treatment results in an increase in the critical current density although a clear active passive transition is observed. The anodic behaviour of sintered stainless steel in H₂SO₄ solution is mainly affected by the existence of interconnected open pores. The local intergranular sensitivity to the corrosive environment is increased due to carbide precipitation.     

TA15钛合金与304不锈钢的电子束焊接

对TA15钛合金和304不锈钢的电子束焊接进行了研究,对接头显微组织、相组成和显微硬度进行了分析.结果表明,TA15与304不锈钢电子束焊接性较差,在较小的热应力下即在焊缝内产生大量裂纹.焊缝内生成连续分布的化合相,主要包括TiFe2,TiFe,Cr2Ti等,脆性化合物的产生是裂纹形成的根本原因.焊缝区内显微硬度明显高于母材,且TiFe2的硬度高于TiFe相,贯穿裂纹在TiFe2相富集的区域产生.二者的直接电子束焊接难以实现,需要添加中间层以改善焊缝的冶金条件,改变化合物的种类和分布,从而实现可靠连接.     

Wear behaviour of an FeAl intermetallic alloy containing carbon and titanium

FeAl based alloys with carbon and titanium additions were prepared using arc induction melting and their effect on wear behaviour was investigated using ball-on-disk technique. The experimental results showed that carbon addition to FeAl alloys results in formation of perovskite-type Fe₃AlC_0.5 carbide phase and graphite. Addition of Ti promotes the formation of TiC and Fe₃AlC_0.5 and prevents the formation of graphite in the alloy. Hardness and wear resistance of FeAl based alloys increase with increase in the volume fraction of carbides. The FeAl alloys containing Ti exhibited low wear rate and coefficient of friction. Examination of wear tracks revealed micro ploughing at a lower load of 5N. Thin surface flakes with traces of their detachment were observed at a higher load of 10N. It was also observed that presence of graphite in localized regions reduce the wear resistance of the alloy. The results are correlated with observed microstructure and hardness.     

The effect of elevated temperature refinery service on the chemistry of M23C6 carbides in 9Cr‐1Mo steel

In a previous study the influence of sulphur‐bearing refinery environment on the corrosion behaviour of commercial 9Cr‐1Mo steel has been investigated [1]. It has been stated that the progress of high temperature sulphide corrosion can be followed by internal carburization of this steel due to a release of carbon from the sulphur‐attacked carbides and diffusion of carbon into the steel interior, accompanied by formation of new carbides, or growth of the existing M₂₃C₆ carbides. In the study presenting this it has been revealed that at the rapid, intergranular progress of sulphide corrosion in the presence of hydrogen, the growth/formation of M₂₃C₆ carbides, which is most intensive near the corrosion‐attacked surface, results in an increased chromium and molybdenum content close to this surface, and in increased Cr/Fe and Mo/Fe ratios in M₂₃C₆ carbides, which was established by means of EDS analyses. Just beneath the surface, at a depth of less than about 0.2 mm, a steep depletion of the steel in the alloy elements measured by EPM technique can prove the reduced Cr/Fe and Mo/Fe ratios in the carbides.     

Smearing-type wear behavior of Al62Cu25.5Fe12.5 quasicrystal abrasive on soft metals

The abrasive polishing behavior of Al₆₂Cu_25.5Fe_12.5 quasicrystal on Cu, Al and austenite stainless steel alloys were investigated, to compare with commonly used hard abrasives such as diamond, alumina and silica. The quasicrystal abrasive showed a dominating smearing-type wear mechanism, in sharp contrast to all the other three abrasives, as reflected by large indent size shrinking with respect to surface removal depth. The quasicrystal abrasive polishing, producing a flattened surface with minor depth removal, may open new application fields where low-wearing and fine surface finishing are demanded.     

Behaviour of ferritic stainless steels subjected to dry biogas atmospheres at high temperatures

The objective of this study is to understand the high temperature corrosion behaviour of the ferritic stainless steel type AISI 441 (18CrTiNb), a candidate for SOFC interconnectors, under dry synthetic fermentation biogas (CH₄ + CO₂ mixtures), possibly used at the anode side of the cell. Thermodynamic analysis showed that, in such mixtures, the partial pressure of oxygen lies in the range of 10^?23 to 10^?20 bar for temperature between 700 and 900 °C and that the formation of solid carbon may take place in several conditions. XRD results confirmed the formation of Cr₂O₃ and Mn‐Cr spinel, with a mixture of internal carbides. In this temperature range, kinetic experiments showed linear mass change. Comparing with the linear rate constants of 441 oxidised in pure CO₂, corrosion in biogas was larger and increased with increasing the methane content in the biogas. The surface morphology of the corroded specimens showed a dense oxide scale at temperatures less than 800 °C, serving as an efficient barrier to carbon penetration. However, when the temperature reaches 900 °C, cracks and pores appear in the oxide scale, carbon can precipitate and diffuse easier than at 800 °C and may lead to internal carbide formation. In such biogas atmospheres, 800 °C seems the maximum operating temperature of devices containing this ferritic stainless steel.     

Effects of aluminum on the oxidation of 25Cr-35Ni cast steels

The oxidation kinetics and morphological development during reaction of two cast austenitic steels at 1000°C in pure dry oxygen at 20 kPa are reported. Both steels contained approximately 25 wt.% Cr and 35 wt.% Ni and, in addition, one steel contained 3.3 wt. %. Both steels oxidized to form external scales consisting mainly of Cr₂O₃ with a thin outer layer of manganese rich spinel. Scale growth kinetics were parabolic, and somewhat faster rates were observed for the aluminum bearing steel. In both steels, deep internal oxidation occurred at the site of primary (interdendritic) carbides. The kinetics of this process were parabolic, and rate control was attributed to oxygen diffusion along the interface between internal oxide and matrix metal. In the aluminum-free steel, interdendritic carbides were converted to chromium rich oxide, but when aluminum was present, a sheath of aluminum rich oxide formed around the carbides. In this latter case, the rate of interdendritic penetration was somewhat slower. The aluminum bearing steel also formed large numbers of rod-shaped Al₂O₃ precipitates within the austenitic dendrites. Deepening of the Al₂O₃ precipitate zone also proceeded according to parabolic kinetics at a rate consistent with rate control by diffusion of oxygen along the oxide-alloy interfaces.     

General and local corrosion of as-cast 25cr-20ni steel in molten sulphates: The influence of Si and W additions

The corrosion of as-cast austenitic stainless steels 25Cr-20Ni (AISI 310) in alkaline sulphate melts was studied at 700°C. General corrosion occurred on the austenitic matrix and local corrosion in the interdendritic zones. Local corrosion depended on the type of solidification precipitates (carbides). A Cr-depleted zone was found around M₇C₃ type carbides. Electrochemical investigations with alloys and sintered carbides showed anodic behaviour of this Cr-depleted zone in relation to the austenitic matrix or M₇C₃ carbides. The influence of silicon and tungsten on general and local corrosion is discussed.