High-temperature sulfidation behavior of iron-based alloys in hydrogen sulfide-hydrogen gas mixtures

Sulfidation of Fe-Cr binary alloys, Fe-Cr-Al ternary alloys, and commercial stainless steels has been carried out at 540°C in a H₂S-H₂ gas mixture of 0.7 vol.% H₂S and a total pressure of 1 atm. Sample exposure time was from 3 to 5 days. Gravimetric, metallographic, electron microprobe, and x-ray studies were made on a few selected alloys. Sulfidation rates of Fe-Cr-Al alloys were generally slower than corresponding Fe-Cr binary alloys and in many cases were orders of magnitude slower. Scales were extensively dual-layered and showed Al and Cr enrichment at the alloy-scale interface. Tests on the commercial stainless steels confirmed the results obtained with the laboratoryprepared Fe-Cr-Al alloys.Supported by The American Petroleum Institute.     

Investigation of the high-temperature oxidation of Fe-Cr alloys by means of the isotope18O

The mechanism of oxidation at 1000°C of Fe-Cr alloys has been investigated. The samples were oxidized in two stages: first in an atmosphere of ¹⁶ O and subsequently in ¹⁶ O. Prior to each stage the samples had been marked with platinum wire markers. The concentration of ¹⁸ O in the scale was determined by means of two nuclear reactions: ¹⁸ O(,n) ²¹ Ne, and ¹⁸ O(p,n) ¹⁸ F independently. The results show that the dissociative mechanism of scale growth on Fe-Cr alloys is similar to that found earlier for Cu-Zn type alloys for which measurements were made using the radioactive sulfur isotope,³⁵ S.     

The oxidation of Fe-Cr alloys containing an oxide dispersion or reactive metal additions

The oxidation of an iron-16% chromium alloy containing a dispersion of yttria particles and of iron-16 to 18% chromium alloys containing small additions of yttrium or zirconium has been studied at 1100 and 1200°C in 100 Torr oxygen. The yttria-containing alloy exhibited the excellent oxidation resistance usually associated with oxide-dispersion-containing alloys, having a thin, adherent, virtually iron-free scale which resisted the breakaway rapid oxidation behavior commonly found in Fe-Cr alloys in this composition range. Of the alloying additions intended to form a fine oxide dispersion during oxidation, only zirconium affected the oxidation behavior in a beneficial way, the scale on the yttrium-containing alloy being possibly less protective than that on the equivalent binary alloy.Supported by Battelle, Columbus Laboratories, Columbus, Ohio.     

High-temperature oxidation of Fe-Cr alloys in wet oxygen

Fe-Cr binary alloys have been oxidized in a stream of oxygen containing different amounts of water vapor at 900–1000°C to study the effects of water vapor. The Fe-Cr alloys exhibit an initial protective behavior due to formation of a Cr-rich scale and followed by a nonprotective breakaway oxidation due to formation of iron-rich scale. The appearance of the breakaway oxidation was very sensitive to the water vapor content in the atmosphere. The higher the water vapor content, the earlier the breakaway oxidation takes place. Increasing the oxidizing temperature or decreasing the Cr content in the alloys facilitate an earlier breakaway oxidation. The breakaway oxidation was inhibited effectively by surface-applied CeO₂ particles before oxidation. The oxide scales were examined and analyzed by optical metallography, X-ray diffraction, SEM, and EPMA. A mechanism of the effects of water vapor has been proposed.     

Oxygen Solubility and Oxide Phases in the Fe-Cr-O System

The solubility of oxygen in molten Fe-Cr alloys has been determined at 1550°, 1600°, and 1650°C for alloys containing up to about 50 pet Cr and found to decrease as chromium increases to 6 pet and then to increase gradually. Phase relations in the Fe-Cr-O system at steelmaking temperatures have been evaluated and two previously unreported oxides have been identified.     

The oxidation behavior of Fe-Cr alloys containing HFO2-dispersed phase

The present investigation examines the high-temperature oxidation behavior of Fe-Cr ferritic alloys containing 1.0% Hf which has been convened into an oxide dispersion. The oxide dispersions were produced by an internal oxidation treatment using a 50-50 Cr-Cr₂O₃ powder mixture in a sealed quartz capsule at 1100°C: the samples were not in direct contact with the powders. The effect of the dispersed oxide was spectacular under isothermal oxidation conditions, but it had almost no effect during thermal cycling conditions. Unlike the TiO₂-containing Fe-Cr ferritic alloys, virtually no particle coarsening was observed. The absence of the coarse particles caused poor thermal cycling behavior, whereas in TiO₂-containing ferritic alloys, coarse particles acted as oxide pegs giving a keying-on effect.     

合金元素对铸造Fe-Cr铁素体不锈钢耐浓硫酸腐蚀性能的影响

采用电化学技术、浸泡腐蚀及能谱分析等研究了合金元素对铸造Fe-Cr铁素体不锈钢耐浓硫酸腐蚀行为的影响。结果表明:随铬含量的增加,Fe-Cr合金的耐浓硫酸腐蚀性能增强,单一的铬合金化不能使Fe-Cr合金在60℃,98%H2SO4中自钝化;钼能促进Fe-Cr25-Mo合金的钝化和自钝化,随钼含量的增加,Fe-Cr25-Mo合金的耐浓硫酸腐蚀性能增强;辅助合金元素镍、铜可促进Fe-Cr25Mo2合金的钝化和自钝化,而钛、铌的影响不大。     

Accumulation and annealing of radiation defects under low-temperature electron and neutron irradiation of ODS steel and Fe-Cr alloys

The processes of accumulation and annealing of radiation defects at low-temperature (77 K) electron and neutron irradiation and their effect on the physicomechanical properties of Fe-Cr alloys and oxide dispersion strengthened (ODS) steel have been studied. It has been shown that the behavior of radiation defects in ODS steel and Fe-Cr alloys is qualitatively similar. Above 250 K, radiation-induced processes of the solid solution decomposition become conspicuous. These processes are much less pronounced in ODS steel because of specific features of its microstructure. Processes related to the overlapping of displacement cascades under neutron irradiation have been considered. It has been shown that, in this case, it is the increase in the size of vacancy clusters, rather than the growth of their concentration, that is prevailing. Possible mechanisms of the radiation hardening of the ODS steel and the Fe-13Cr alloy upon irradiation and subsequent annealing have been discussed.     

A review of the diffusion path concept and its application to the high-temperature oxidation of binary alloys

Ternary diffusion theory, particularly the concept of diffusion paths on a ternary phase diagram, is reviewed in terms of its application to the problem of binary alloy oxidation. To illustrate this applicability, the oxidation behavior of Fe-Ni, Fe-Cr. and Ni-Cr alloys at 1000°C is examined in detail.     

The High temperature oxidation of iron-chromium alloys containing 0.1 wt. % cerium or cerium oxide

The effects of 0.1 wt% Ce in both metal and oxide form on the high temperature oxidation of Fe-Cr (Cr = 10, 12, 14, 16, 18, 20) have been investigated at 1000°C in a 0.1315 bar O₂/He mixture at a total pressure of 1 bar. The presence of Ce and CeO₂ markedly affects the oxidation characteristics of the tested alloys, causing rapid initial coverage with protective oxide, decreased overall rates of oxidation, modifications in scale morphologies and enhanced scale adhesion. There is a possibility of a “trade-off” between the amount of Cr in the Fe-Cr materials and the presence of Ce of CeO₂, since such additions have been shown to decrease the amount of Cr necessary to form a protective Cr₂O₃ scale on the alloy surface. The mechanisms by which the Ce or CeO₂ improve the oxidation behaviour of Fe-Cr alloys are discussed.     

Modeling of crevice solution chemistry on the initiation stage of crevice corrosion in Fe-Cr alloys

A theoretical model has been developed to evaluate transport processes and chemical reactions in crevices. The model is general in form, allowing applications to a variety of systems; even including 4 component stainless steel (e.g. alloy 316L). The model has been applied to recent experimental data and other simulated data on crevice corrosion in Fe-Cr alloys. The model considers the time evolution of the solution chemistry and electro-chemistry within crevices, and uses the method of finite elements to solve the complex set of mass-conservation equations describing the system. Activity is corrected in this model because relatively high current densities of Fe-Cr alloys cause high ionic strength in the crevice solution. The results of the model are a little different from those of Sharland model, but both simulated results are reasonable compared with the experimental results.     

还原性气氛中HCl和H2S导致Fe-Cr合金在700℃的加速失效

研究了Fe-8Cr,Fe-12Cr和Fe-18Cr合金在700℃含氯和两种硫含量还原性气氛中的腐蚀行为.气氛中H2S含量增加导致合金发生加速腐蚀,尤其造成Fe-18Cr合金表面氧化铬膜退化.合金的加速腐蚀与膜中生成的硫化物和氯化物密切相关.合金的腐蚀速率随其Cr含量的升高而降低.通过计算气氛中平衡时的氯势、氧势、硫势预测了合金与气氛可能发生的反应,并解释了腐蚀机制.     

Influence of a mixed Al2O3+Cr2O3 superficial coating on the non-isothermal oxidation behavior of Fe and Fe-Cr alloys

The non-isothermal oxidation behavior of electrolytic-grade iron and Fe-Cr alloys in dry air has been studied using linear heating rates of 6 K/min, 10 K/ min, and 15 K/min up to a final temperature of 1273–1473 K. Some of the iron and iron-chromium alloy samples were given a surface treatment by dipping them in an aqueous solution containing both Cr and Al ions before their oxidation studies. This pretreatment has resulted in improved oxidation resistance and scale adherence as depicted by no scale rupture even after a second thermal cycle. Mass changes were recorded gravimetrically, and scales have been characterized by SEM, EPMA, and x-ray diffraction analyses.     

Thermodynamisch-kinetische Deutung der Passivierungserscheinungen bei Fe-Cr-Legierungen

Thermodynamic-kinetic interpretation of the passivation phenomena with Fe? Cr alloys By superimposing two thermodynamic kinetic current output/potential curves corresponding to iron and chromium, respectively, it is possible to plot the selfpassivation curve for Fe-Cr alloys, which is similar to the potentiostatic polarisation curve. For the ideal Fe-Cr alloy, it is possible to draw conclusions concerning the Fe and Cr passivity zones and the composition of the oxide layers on the passivated Fe-Cr alloy at the Flade reference potential.     

High temperature interaction of Al/Si-modified Fe-Cr alloys with KCl

The corrosion behaviour of pure iron, pure chromium, and aluminium/silicon alloyed Fe-Cr materials was investigated at 650 °C in air accompanied by gaseous or solid KCl salt. The corrosion rates of these materials with KCl salt are high and they are strongly affected by the salt amount, the types of the alloying elements and the concentration of chromium. The dominant degradation mechanism for the chromia-forming alloys by KCl attack is the preferential formation of potassium chromate over the conventionally protective chromia, characterized by typical features of bubbles, cracks, volatility and severe spallation for the corrosion products. A detrimental effect of chromium is confirmed. Al-alloying addition to Fe-Cr alloys is beneficial by enhancing the corrosion resistance. Silicon is more effective in promoting the corrosion resistance of Fe-Cr alloys by forming a stable and dense oxide layer in the inner zone which suppresses the rapid growth of iron oxides.     

Silicon as an alloying element in ferrite stainless steels containing 8–13% Cr

A jumpwise improving of the passivation ability and pitting-resistance of the Fe-(8–13)% Cr-(0.32–2.7)% Si alloys upon surpassing the limit of ∼14–15 at % by the chromium and silicon summary concentration is found. It is suggested that the nature of the critical summary concentration C _Cr + C _Si is identical to that in Fe-Cr binary alloys; it is caused by the silicon building-in to the alloy’s crystal lattice and its substitution for chromium in the statistical proportion to its atomic part in alloys. When the summary concentration C _Cr + C _Si approaches ∼14–15 at %, each elementary cell in the Fe bcc lattice must necessarily contain either a Cr atom or the proportional number of Si atoms. The improved passivation ability and pitting-resistance of the Fe-Cr-Si alloy, compared with the Fe-Cr alloys, is explained by the fayalite formation at the alloy surface. Original Russian Text ? I.I. Reformatskaya, I.G. Rodionova, A.N. Podobaev, I.I. Ashcheulova, E.V. Trofimova, 2006, published in Zashchita Metallov, 2006, Vol. 42, No. 6, pp. 591–597.     

EFFECT OF GRAIN BOUNDARY CARBIDE ON CREEP CRACK GROWTH IN Fe-Cr15-Ni25 ALLOY

Creep crack growth behavior of Fe-Cr15-Ni25 alloys with different grain boundary featureshas been studied.Cavities nucleate at triple junctions of grain boundaries for the single phasealloy and at grain boundary carbide for the alloy with grain boundary carbide.Grain bounda-ry carbide particles are obstacles to cavity growth and coalescence,and therefore increase thecreep crack growth resistance greatly.     

Simulation analysis of electrochemical nature of real passive films with artificial passivation films

Simulation experiments using oxide thin films with analogues composition and thickness of passive films on Fe-Cr alloys have been performed to reveal the function of alloying elements in passive films. The objective of the present paper is to review recent results of the simulation experiments using Fe₂O₃-Cr₂O₃ films. The thinning rate of the films was measured as a function of potential in 1 M HCl by in-situ ellipsometry. The potential-dependent change of the rate well simulated the potential-dependent dissolution behavior of Fe-Cr alloys. The composition of the films having resistance against given environment was obtained by the simulation experiments.     

用AES和EPMA研究Fe-Cr合金钝化膜和蚀孔内壁的组成特征

采用 AES 和 EPMA 对三种 Fe-Cr 合金在3.5%NaCl 溶液中的钝化膜以及点蚀孔的内壁进行了分析,由 Ar~+溅射得到了 Cr~+/Fe 比随深度变化的分布,考察了铬在钝化膜中的作用.通过实验和分析证明:Fe-Cr 合金表面钝化膜和点蚀孔内壁的 Cr/Fe 比都明显高于基体的 Cr/Fe 比,决定钝化膜性能的重要因素是 Cr/Fe 比,并且 Cr/Fe 比越高,钝化膜的保护性越好。     

The corrosion behavior of Fe-Cr alloys containing Co,Mn, and/or Ni and of a Co-base alloy in the presence of molten (Li,K)-carbonate

The corrosion behavior or commercial Fe ana Co base alloys and Fe-Cr model alloys with different contents of Co and/or Mn was investigated by continuous exposure tests in the presence of a thin carbonate film. All alloys studied form multi-layered corrosion scales consisting of outer Li containing oxides and inner Cr rich oxides, i.e. spinels or LiCrO₂. The LiCrO₂ is formed on alloys with high Cr contents (≤ 20 wt.%), whereas mixed (Fe,M)_3-x Cr_xO₄ spinels (M = Co, Mn, Ni) were found on alloys with lower Cr content (15–20 wt.%). Insoluble Cr containing oxides occur only in the inner layers of the corrosion scale, whereas on the surface of corroded specimens soluble chromates were detected. Alloys with Mn contents greater than 15 wt.% form Mn₂O₃ in the initial stages of the experiments, this oxide reacts with the melt and formation of Li₂MnO₃ takes place. In exposure tests up to 500 h Fe-Cr alloys with low contents of Mn and Co (10 wt.% Co or Mn) form iron rich oxides (LiFeO₂ and LiFe₅O₈) with varying amounts of dissolved Mn or Co. In the later corrosion stages outward diffusion of Mn and/or Co takes place and LiCoO, and Li₂MnO₃ are formed on top of LiFeO₂, whereby the concentration of Mn and/or Co in the inner layers (LiFeO₂ and spinel) decreases. The outer Li containing oxides LiFeO₂, LiCoO₂ and Li₂MnO₃ are nearly insoluble in the melt and when present at the surface protect the metallic material from further corrosive attack. Fe-Cr model alloys containing Co and Mn form multi-layered corrosion layers after 2000 h of exposure. These layers consist of four oxides in the following sequence from the metal-scale to the scale-melt interface: (Fe,Cr,Co,Mn)₃O₄ spinel, LiFeO₂, Li₂MnO₃ and LiCoO₂.