Corrosion behavior of Fe-20Cr and Ni-20Cr alloys in Ar-H2O-HBr gas mixtures at 1000 K

Discontinuous mass-change measurements and corrosion-product analyses were made for Fe-20Cr and Ni-20Cr alloys after exposing them to Ar-H ₂ O-HBr gas mixtures at 1000 K for 24 hours. Predominantly chromia scales formed on both alloys. Upon cooling, the scales remained adherent to the Fe-20Cr alloy but spalled extensively from Ni-20Cr samples. After tests in HBr-rich gas mixtures, bromine-rich corrosion products were found underneath chromia scales on both alloys while nickel evaporation was observed from Ni-20Cr samples. Preoxidation of the Ni-20Cr alloy prior to exposure to Ar-H ₂ O-HBr gas mixtures increased chromia scale adherence but did not prevent nickel loss from the alloy. Chromia scales formed on the Fe-20Cr alloy were more protective due to the absence of iron oxides in the scale. Large NiO crystals formed over the Ni-20Cr alloy decreased chromia-scale adherence and increased nickel loss from the alloy due to the low stability of NiO in HBr-containing gas mixtures.     

Effect of water vapor on high-temperature oxidation of FeCr alloys

The suppression of protective chromia scale formation in water vapor containing service environments limits in many cases the upper application temperature of high-Cr martensitic and ferritic steels. The present paper discusses the mechanisms which are responsible for this technologically important effect, using results of oxidation tests with two types of FeCr model alloys in Ar-O₂, Ar-O₂-H₂O, and Ar(-H₂)-H₂O mixtures. The data shows that in atmospheres with a high ratio of water vapor to oxygen, Cr exhibits a higher tendency to become internally oxidized than in dry Ar-O₂, or e.g. air. Contrary to previous studies which showed the presence of water vapor to affect transport processes in the scale and/or to enhance formation of volatile Cr species, the present results thus reveal that the presence of water vapor also affects the transport processes in the alloy, likely by incorporation of hydrogen.     

The oxidation of Ni-20 wt. % Cr-2ThO2

The oxidation of TD NiCr (Ni-20 wt. % Cr-2ThO₂) has been investigated between 900 and 1200°C, and the oxidation behavior is compared with Ni-30 wt. % Cr and Co-35 wt. % Cr alloys. All alloys develop Cr₂O₃ scales but the weight changes obtained for the NiCr and CoCr alloys show an increase with time whereas above 1000° C the TD NiC shows a progressive loss in weight from the evaporation of CrO₃ from the scale, and the reaction products appear to be formed mainly at the alloy-scale interface. However, no mechanism for its formation has been established.     

Effect of water vapour on cyclic oxidation of Fe–Cr alloys

Model Fe–Cr alloys containing 9, 17 or 25 wt% Cr were subjected to repeated 1 h cycles of exposure at 700 °C to flowing gas mixtures of Ar‐20O₂, Ar‐20O₂‐5H₂O and Ar‐5O₂‐20H₂O (all in volume %) for up to 400 cycles. The kinetics and morphological development of these reactions were compared with those found during isothermal exposure to the same gases. Under isothermal conditions, all alloys developed thin protective chromium‐rich scales in dry oxygen. Addition of 5% H₂O induced breakaway for Fe‐9Cr within 48 h, but had little effect on higher chromium alloys. Isothermal chromia scale growth on Fe‐17Cr and Fe‐25Cr was accelerated by the addition of 20% H₂O, but breakaway did not result. Under cyclic conditions in dry oxygen, Fe‐9Cr quickly entered breakaway, oxidising according to fast, linear kinetics, but the higher chromium alloys exhibited protective behaviour. When 5% H₂O was added to the oxygen, the 17% Cr alloy also underwent fast breakaway oxidation, but Fe‐25Cr continued to be protected by a chromia scale. In the 20% H₂O gas, all alloys failed under cyclic conditions, producing thick, iron‐rich oxide scales. The synergistic effects of water vapour and temperature cycling are discussed in terms of alloy chromium depletion and the effects of H₂O(g) on oxide transport properties.     

Effect of internal oxidation pretreatments and Si contamination on oxide-scale growth and spalling

Internal oxidation pretreatments carried out in quartz capsule with a Rhines pack were found to have a profound effect on the subsequent oxidation behavior of alloys. Specimens of Co-15 wt.% Cr, Co-25 wt.% Cr, Ni-25 wt.% Cr, and Ni-25 wt.% Cr-1 wt.% Al were tested at 1100°C after pre-oxidation treatments. Even without the development of internal oxide particles, pretreated binary CoCr and NiCr alloys oxidized with significantly lower rates. Selective oxidation of chromium was observed on the non-Cr₂O₃-forming Co-base alloys, whereas on the Cr₂O₃-forming Ni-base alloys, elimination of base-metal oxide, reduction in the Cr₂O₃ growth rate, and better scale adhesion were found. These effects were more apparent with pre-oxidation temperatures greater than 1000°C and with longer pretreatment times. Contaimination of Si from the quartz is believed to be the cause.     

Temperature and gas composition dependence of internal oxidation kinetics of an Fe?10%Cr alloy in water vapour containing environments

Recently it was proposed, that the hampered formation of external protective chromia scales on FeCr‐alloys in water vapour containing, low‐pO₂ gases is correlated with enhanced internal oxidation of chromium. In the present study the internal oxidation kinetics of Fe? 10Cr (in mass%) during isothermal oxidation in Ar? H₂? H₂O mixtures at temperatures in the range 800–1050 °C has been investigated. It was found that the tendency for Cr to become internally oxidized decreased with decreasing temperature. At the higher test temperatures the internal oxide precipitates consisted of Fe/Cr‐spinel. With decreasing temperature the precipitates near the oxidation front gradually exhibited increasing amounts of chromia. At 900 °C the oxidation morphology in the Ar? H₂ base gas mixture changed from exclusive internal oxidation of Cr at a water vapour content of 2% towards a combined internal Cr oxidation and external Fe‐oxide formation at higher water vapour partial pressures.     

Relation between impurities and oxide-scale growth mechanisms on Ni-34Cr and Ni-20Cr alloys. II. Influence of sulphur

The oxidation of presulphidized Ni-Cr alloys has been studied by taking into account the influence of the two distinct oxidation mechanisms described in part I of this article. Sulphur enters the Cr₂O₃ scale (in Ni-34Cr alloys) mainly as S^2– species, which at high temperatures increases the V_Cr content, and hence the oxidation kinetics. Sulphur is randomly distributed in the scale, except at the inner oxide-alloy interface, where intergranular microsulphides are analyzed in the oxide-scale zone. In the case of NiO, NiCr₂O₄, Cr₂O₃ oxide multilayers (in a Ni-20Cr alloy), sulphur in the S^2– state is distributed in the oxide layers or at Si-precipitate interfaces. Such a distribution leads to crack formation, especially during cooling.     

Water Vapour Effects on Fe–Cr Alloy Oxidation

Isothermal oxidation at 700 °C of binary Fe–Cr alloys containing 9, 17 and 25 wt% chromium was measured using continuous thermogravimetric analysis. All alloys developed thin, protective chromia scales in Ar–20O₂ (vol%). Chromia scale growth on the 17 and 25 Cr alloys was faster in Ar–20O₂–5H₂O and Ar–5O₂–20H₂O. In these gases, the Fe–9Cr failed to form a chromia scale and suffered rapid breakaway oxidation, growing iron-rich oxides instead. A low oxygen potential gas, Ar–10H₂–5H₂O caused chromia scaling on Fe–17Cr and Fe–25Cr, but internal oxidation of Fe–9Cr. Application of Wagner’s criterion for sustaining external scale growth is shown to account satisfactorily for these observations.     

Role of Water Vapor in Chromia-Scale Growth at Low Oxygen Partial Pressure

The oxidation behavior of pure chromium and ODS-Cr alloys in Ar-H₂-H₂O and Ar-O₂-H₂O was studied at 1000°C. At high oxygen potentials, the addition of H₂O to the gas had negligible effect on the scaling behavior. However, at low oxygen potentials, when the p_H2O/p_H2 ratio was held constant, the oxidation rate increased with water partial pressure. Increasing values of p_H2O/p_H2 led to more rapid rates. At fixed p_H2O values, the rate increased with increasing p_H2. Compact scales were formed under all conditions. In addition Cr₂O₃ blades grew on the scale surface when pure chromium was reacted with H₂O/H₂ mixtures, but not in reaction with O₂/H₂O. These blades did not form when Y₂O₃ dispersion-strengthened material was reacted. A model, in which oxide growth was sustained by diffusion of chromium vacancies and adsorption of H₂O on oxide exposed to low oxygen-activity gas led to the formation of hydroxyl species, explained most of the complex effects of gas composition on scale growth and blade formation. However, it failed to account for the observed increase in scaling rate with p_H2 at fixed p_H2O. The latter effect is ascribed to alteration of an additional contribution to diffusion from chromium interstitials.     

High-temperature corrosion of titanium-, niobium-, and manganese-rich Fe-25Cr alloys in H2-H2O-H2S gas mixtures

The simultaneous sulfidation and oxidation of Fe-25Cr, Fe-25Cr-4.3Ti, Fe-25Cr-7.5Nb, and Fe-25Cr-9.0 Mn alloys were studied at 1023, 1123, and 1223 K, respectively, in H₂-H₂O -H₂S gas mixtures. The influences of titanium, niobium, and manganese on the transition from protective oxide formation to the formation of sulfide-rich corrosion products of Fe-25Cr alloys have been investigated. It has been found that additions of titanium and niobium can improve the scaling resistance of Fe-25Cr alloys against sulfidation in H₂ -H₂O -H₂S gas mixtures at high temperatures. However, the addition of manganese does not increase the resistance to sulfidation of Fe-25Cr alloy. The oxide Cr₂Ti₂O₇, which can suppress sulfide formation, formed on the Fe-25Cr-4.3Ti alloy. The addition of manganese to Fe-25Cr does not form more stable and protective oxides than Cr₂O₃ which formed on Fe-25Cr. Thermodynamic stability diagrams are used to explain the experimental results.     

Metal dusting of nickel-base alloys

Nickel-base alloys are generally less susceptible to metal dusting than steels and the attack is slower. Exposures in strongly carburizing CO-H₂-H₂O mixtures at 650°C and 750°C have shown, however, gradually increasing attack on the alloys with lower Cr-content. Alloy 600 and even 601 were gradually attacked by pitting, whereas for alloys with >25 % Cr the materials loss was negligible even after 10,000 h. For these alloys such as 602 CA and 690 the formation of a protective chromia scale is strongly favored compared to carbon ingress and metal dusting.     

Oxidation of Binary FeCr Alloys (Fe?C2.25Cr, Fe?C10Cr, Fe?C18Cr and Fe?C25Cr) in O2 and in O2?+?H2O Environment at 600???C

The oxidation behaviour of the binary alloys Fe?C2.25Cr, Fe?C10Cr, Fe?C18Cr and Fe?C25Cr (wt%) in dry and wet O₂ at 600???C is investigated by isothermal exposures of carefully polished samples for up to 168?h. The oxidized samples are investigated gravimetrically and the oxides formed are studied by X-ray diffraction. X-ray photoelectron spectroscopy is used for depth profiling of the thin oxides. The scale surface is imaged by SEM. Cross-sections through the scale are analyzed by SEM/EDX for imaging and for measuring the chemical composition. The oxidation behavior of the four FeCr alloys is intermediate between those of iron and chromium. Fe?C2.25Cr oxidizes in a way similar to iron in both environments, forming a poorly protective scale consisting of FeCr spinel at the bottom, magnetite in the middle and a hematite cap layer. In dry O₂, Fe?C10Cr, Fe?C18Cr and Fe?C25Cr form a thin and protective (Fe,Cr)₂O₃ oxide similar to the chromia film formed on pure chromium. In wet O₂, Fe?C10Cr, Fe?C18Cr and Fe?C25Cr initially form the same kind of protective oxide film as in dry conditions. After an incubation time that depends on alloy chromium content, all three alloys go into breakaway oxidation and form thick, poorly protective scales similar to those formed on Fe?C2.25Cr. Breakaway oxidation in wet O₂ is triggered by the evaporation of CrO₂(OH)₂ from the protective (Fe,Cr)₂O₃ oxide.     

Oxidation behavior of Ni-Cr-1ThO2 alloys at 1000 and 1200°C

The oxidation behavior of Ni-13.5-33.7Cr-1ThO₂ alloys in flowing oxygen at 150 Torr was investigated in the temperature range 1000–1200°C. Gravimetric measurements of the oxidation kinetics have been combined with microstructural studies of the reacted samples in order to evaluate the reaction mechanisms. The oxide products formed on the alloys were a function of Cr content, sample surface preparation, reaction time, and temperature. The presence of ThO₂ appears to produce two effects during alloy oxidation. First, enhanced Cr diffusion to the alloy surface results in rapid formation of a Cr₂O₃ subscale beneath NiO on Ni-13.5Cr-1ThO₂ and selective oxidation of Cr for Ni-22.6Cr-1ThO₂. Second, the mechanism of formation of Cr₂O₃ is apparently different from that for simple Ni-Cr alloys, resulting in about an order of magnitude reduction in the Cr₂O₃ growth rate. The oxidationvaporization of Cr₂O₃ to CrO₃ becomes rate controlling for the higher Cr alloys after only a few hours of exposure at 1200°C.     

Corrosion and Carburization Behaviour of Ni-<Emphasis Type="Italic">x</Emphasis>Cr Binary Alloys in a High-Temperature Supercritical-Carbon Dioxide Environment

Pure Ni and Ni-xCr (x = 7, 14, 22 and 27 wt%) binary alloys were exposed to supercritical-carbon dioxide environment at 600 °C and 20 MPa for 200 h. For pure Ni, a thick NiO layer was formed on the surface. Meanwhile, for Ni-7Cr alloy, an inner oxide layer consisted of rather irregular chromia and NiO was formed below the outer NiO layer. When Cr content was greater than 14%, a continuous chromia layer was formed, resulting in much lower weight gain and oxide thickness. However, amorphous carbon layers had developed along the oxide–matrix interface when chromia was formed. The presence of the carbon layer was explained in view of the high C activity corresponding to the low equilibrium oxygen potential of chromia.     

Oxidation Behavior of Ni-3, 6 wt% Al Alloys at 800 °C in Atmospheres Containing Water Vapor

The oxidation behavior of Ni, Ni–3Al, and Ni–6Al alloys at 800 °C in air + H₂O was investigated. The oxidation kinetics of Ni and the alloys in air + H₂O were very similar, but the mass gains of Ni and each alloy were smaller in air + H₂O than in air. Oxidation products formed on Ni-3 and 6Al alloys consisted of an outer NiO scale and internal Al₂O₃ precipitates. The growth rates of both NiO and the internal oxidation zone were much smaller in air + H₂O. The NiO scale formed in air + H₂O was duplex in structure with outer porous and inner dense layers. The outer porous layer consisted of fine powder-like NiO particles. A thicker metallic Ni(Al) layer formed at the NiO/alloy interface in air + H₂O, caused by extrusion of Ni from the substrate due to volume changes accompanying the internal oxide formation. Formation of the metallic Ni layer appeared to be the reason for the similarity between the oxidation kinetics of both Ni and the alloys in air + H₂O.     

The hot corrosion of nickel-based ternary alloys and superalloys for gas turbine applications—I. Corrosion in SO2/O2 atmospheres

Hot corrosion studies of IN738 superalloy, Ni-22Cr control alloy as well as Ni-20Cr-X ternary alloys containing additions in w/w 3Al, 4Mo, 0.3Si and 5V in SO₂/O₂ equilibrated atmospheres were carried out. The investigations covered two SO₂/O₂ ratios of 2:1 and 1:4 at two temperatures of 700 and 900°C. In some cases irreproducible kinetic patterns pointed to the presence of Kirkendall voids opening between metal and scale and also to voids occurring as a result of sulphide instabilities and their subsequent conversion to oxides in the prevailing gas compositions and also to metal oxide formation in the dissociation pO₂ of NiO. The change in SO₂/O₂ ratio did not alter the corrosion rate, suggestive that the pSO₃ in the gas phase (necessary for the formation of NiSO₄) or pS₂ within the NiO layer is enough for Ni₃S₂ formation (the precursor of hot corrosion). Rates at 700°C were much greater than those at 900°C. Al, Si and V were beneficial at 700°C whereas at 900°C, V additions enhanced the corrosion rate of the control alloy due to the formation of a molten phase.     

Effect of Alloy Composition and Exposure Conditions on the Selective Oxidation Behavior of Ferritic Fe–Cr and Fe–Cr–X Alloys

Selective oxidation behavior of ferritic martensitic Fe–Cr base alloys, exposed in various atmospheres containing combinations of O₂, CO₂, and H₂O, were studied at various temperatures relevant to oxy-fuel combustion. This paper begins with a discussion of the required Cr content to form a continuous external chromia scale on a simple binary Fe–Cr alloy exposed in oxygen or air based on experiments and calculations using the classic Wagner model. Then, the effects of the exposure environment and Cr content on the selective oxidation of Fe–Cr alloys are evaluated. Finally, the effects produced by alloying additions of Si, commonly present in various groups of commercially available ferritic steels, are described. The discussion compares the oxide scale formation on simple binary and ternary Fe–Cr base model alloys with that on several commercially available ferritic steels.     

The influence of zirconium on the approach to steady-state scaling in a Ni-Cr alloy and the mechanism of inhibition of corrosion in an oxygen-sulphur environment

The corrosion behaviour of a binary Ni-15 Cr alloy and a ternary Ni-15 Cr'1 Zr alloy has been examined when exposed to a bioxidant O₂:SO₂ atmosphere at 850°C. The patterns of scaling exhibited by the two alloys, especially in the early stages of reaction, have been studied using optical and scanning electron microscopy and EDAX analysis. It has been established that the nucleation of Cr₂ O₃ on, and its subsequent growth over the sample surface was much more rapid with the ternary alloy than the binary material. Furthermore the steady-state scale formed on the ternary alloy was single-layered and contained no NiO, in contrast to the anticipated duplex-layered scale developed on the binary material. It is suggested that the pre-existing intermetallic network in the as-cast microstructure of the Ni-15 Cr-1 Zr alloy is a key factor in promoting the rapid formation of the thin protective layer of Cr₂O₃, free from NiO. These features are responsible for the reduced rate of corrosion of the Zr'bearing material, relative to that exhibited by the binary alloy. The observations are discussed in the light of the published literature concerning the effects of rare earth/reactive metal and inert oxide additions to chromia-forming alloy systems.     

Radiotracer studies of carbon permeation through oxide scales on commercial high temperature alloys and model alloys

Chromia- and alumina-forming commercial high temperature alloys and model alloys were preoxidized at 900 or 1000 °C in H₂-H₂O at a low oxygen potential. The oxide layers were characterized by different methods. The carbon permeation through the oxide layers was studied by exposing the preoxidized specimens to an atmosphere CO-CO₂-H₂-H₂O, tagged with radiocarbon, for long time. The carbon was detected by stepwise polishing and measuring the radioactivity. A slow carbon ingress occurs through chromia layers, differences in the protection by the oxide scale could be tested by the radiotracer method for the different alloys. The alumina layer on Fe-6 Al is not protective, but no carbon ingress could be detected for an alloy Fe-6Al-0.5Ti. Autoradiography, AES and X-ray structure analysis showed the presence of Ti(O,C) beneath the outer Al₂O₃-layer. The oxicarbide improves the nucleation and adherence of the Al₂O₃ and prohibits the carbon penetration. The results were confirmed by gravimetric experiments, after preoxidation samples were exposed to CO-CO₂-H₂-H₂O at high carbon activity (a_c = 1.02), carburization and graphite deposition were retarded or prohibited by dense and well adherent oxide layers.