Continuous Chromia Layer Research Articles

Corrosion Behaviors of Ni–Cr Alloys in O2, H2O and H2O + O2 Gases at 700°C and the Effect of Temperature

Pure nickel and four binary Ni–Cr (5, 10, 20 and 30 wt%) alloys were exposed to Ar–20O2, Ar–20O2–20H2O and Ar–20H2O (vol.%) at 700°C. In dry and wet O2, three Ni–Cr alloys (5, 10 and 20 wt%) formed a three-layered structure: an external NiO layer, an inner oxide layer of NiO + Cr2O3 and an internal oxidation zone (IOZ) with Cr2O3 dispersed. The Ni–30Cr alloy formed a continuous chromia layer, with locally outer NiO islands. In pure water vapor, all alloys underwent internal oxidation, nickel metal expulsion and NiO formation. Compared with the results at 650°C, increasing the temperature to 700°C did not alter the morphology of the oxide scale, but enhanced the scale growth kinetics. A significant difference between these two temperatures was that the strong retarding effect of water vapor on NiO formation in oxygen at 650°C became insignificant at 700°C. However, in water vapor only condition, NiO formation remained much slower at both temperatures. The effects of water vapor and oxygen partial pressure on NiO formation of different alloys and the effect of temperature on scale growth kinetics and morphology were discussed.

A high-resolution study of the different surface state effects on the corrosion behaviors of a ferritic steel and an austenitic steel in supercritical water

The corrosion behaviors of two iron-based steels in 600 ºC deaerated supercritical water (SCW) were investigated to study the effect of surface finishing. For the austenitic stainless steel (Fe-19Cr-9Ni), grinding significantly enhanced the oxidation resistance by favoring the formation of a thin and continuous chromia layer. This is because that grinding induced ultra-fined grain layer with a high dislocation density promotes the outward Cr diffusion, thus favors the transition from internal to external oxidation. For the ferritic steel (Fe-9Cr), the oxidation rate remained at a high level regardless of the surface condition mainly because the Cr diffusivity is already high enough but the Cr content is too low to promote the internal-external oxidation transition.

Effects of cold work on the corrosion behavior of Alloy 800H exposed to aerated supercritical water

• Cold work reduces the corrosion resistance of Alloy 800H in supercritical water containing 200 ppb oxygen. • At initial corrosion stage, the corrosion behavior of as-received and cold-worked materials is similar. • The formation of a continuous chromia layer reduces the oxygen partial pressure at the oxide scale-matrix interface, which contributes to the later internal oxidation and selective oxidation of the elements. • With the increase of exposure time, the microstructure of the cold worked materials becomes more complicated and is further subdivided into Fe 3 O 4 /Ni x Fe 3- x O 4 /Ni x Fe y Cr 3- x - y O 4 /Cr 2 O 3 / NiO/Fe x Cr 2- x O 4 +pure Ni multi-layer structure. • Through observation of the oxidation process and the calculation of thermodynamic equilibria, the oxide scale differences were explained through the hypothesis that diffusion barriers were created by the prevailing oxides. Revealing the corrosion mechanism of materials in supercritical water (SCW) is a key issue for the development of a supercritical water reactor (SCWR). Considering cold work is inevitable in the construction of nuclear reactors and its effect on the corrosion behavior of materials in SCW is still unclear, the corrosion behavior of solution-annealed (as-received) and 30% cold-worked Alloy 800H were studied at 600 °C in aerated SCW for 1500 h. The microstructure of the oxide scale was studied by a variety of characterization techniques. The results indicate that cold work increases the corrosion rate of the materials, promotes selective oxidation and internal oxidation, and this was attributed to the high diffusion rate of different ions along “short-circuit paths”. The oxide scale composition and structure of the cold-worked material, which is subdivided to Fe 3 O 4 /Ni x Fe 3- x O 4 /Ni x Fe y Cr 3- x - y O 4 /Cr 2 O 3 /NiO/ Fe x Cr 2- x O 4 + pure Ni multi-layer structure, are much more complicated than the as-received material after 1500 h exposure. Combined with the thermo-dynamic equilibrium calculation, the oxidation processes of the as-received and cold-worked materials were analyzed to reveal the effect of cold work on the corrosion behavior of Alloy 800H.

Oxidation and volatilisation behaviour of a type 430 stainless steel coated by Mn-Co oxide by slurry method with pre-oxidation for SOFC interconnect application

An AISI 430 stainless steel was coated by Mn-Co spinel using a slurry method. Pre-oxidation before the coating helped reduce the oxidation rate of the coated steel at 800 °C in O2-5%H2O, relating to the formation of the continuous chromia layer which inhibited the outward diffusion of iron to the coating layer. It also helped reduce the mass flux of Cr loss due to the volatilisation. The combined molecular and Knudsen diffusion of the volatile species through the coating layer was suggested to explain the reduced volatilisation rate of the coated sample with pre-oxidation.

Corrosion behaviour of Ni-Cr alloys in mixed oxidising gases at 650 °C

Binary Ni-Cr alloys containing 5−30 wt% Cr were exposed to Ar-20O2, Ar-20O2-20H2O, Ar-20CO2, Ar-20CO2-20H2O and Ar-20H2O (vol.%) at 650 °C. In dry/wet O2, and dry CO2, 5−25 Cr alloys formed a three-layered structure: an external NiO layer, an inner oxide layer and an internal oxidation zone. The Ni-30Cr alloy formed a continuous chromia layer, with local outer NiO islands in dry/wet O2, but a continuous Ni-rich outer layer in dry CO2. In wet CO2/water vapour alone, all alloys underwent internal oxidation, nickel metal expulsion and NiO formation. Gas composition effects on the Cr concentration required for chromia formation are discussed.

Chromium Diffusion Coating on an ODS Ferritic-Martensitic Steel and Its Oxidation Behavior in Air and Steam Environments

A chromium diffusion coating was applied on an oxide dispersion strengthened ferritic-martensitic (ODS-FM) steel to improve oxidation resistance at high temperature. By carrying out physical vapor deposition followed by inter-diffusion heat treatment, a thin Cr-rich carbide layer was produced on the ODS-FM steel. Both the as-received and surface-modified specimens were oxidation-tested at 650 °C in air and steam environments for 500 h. The surface-modified specimens showed improved oxidation resistance in both environments. In an air environment, both conditions exhibited a thin and continuous chromia layer, but the formation of Cr2O3 and (Mn, Cr)3O4 nodules resulted in greater weight gain for the as-received specimen. In a steam environment, weight gain increased for both the as-received and surface-modified specimen. Especially, the as-received specimen showed much greater weight gain with the formation of a thick oxide layer consisted of outer Fe-rich oxide and inner (Fe, Cr, Mn) oxide layers. On the other hand, a thin and continuous chromia layer was formed for the surface-modified specimen, which resulted in much less weight gain in a steam environment.

Corrosion behaviour of Ni-Cr alloys in wet CO2 atmosphere at 700 and 800 °C

The corrosion behaviour of binary Ni-Cr alloys with 5–30 wt% Cr was studied in Ar-20CO2-20H2O at 700 and 800 °C. Samples were electropolished before exposure to avoid the effect of surface cold work on alloy diffusion. At 700 °C, all alloys underwent both outer NiO formation and internal oxidation. At 800 °C, the alloys containing 5–20 wt% Cr developed non-protective scales, presenting high weight gain kinetics. However, the Ni-25Cr and Ni-30Cr alloys formed a continuous chromia layer, resulting in much reduced corrosion rates. The effects of water vapour on NiO growth, chromia formation and carburization are discussed.

Corrosion and Carburization Behaviour of Ni-xCr 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.

The effect of Mn1.5Co1.5O4 coatings on the development of near surface microstructure for Haynes 230 oxidized at 800 °C in air

The effect of reactively sintered Mn1.5Co1.5O4 (MCO) coatings on the development of surface microstructure for Haynes 230 (H230) oxidized in air at 800°C has been studied using a combination of thermo-gravimetric analysis and electron microscopy techniques. The bare alloy exhibits a parabolic rate constant of 8.8×10−9mg2cm−4s−1, and forms a two-layer oxide scale with a continuous chromia layer and a thinner discontinuous MnCr2O4 overlayer. For the MCO-coated H230, the reduction step of the reactive sintering process converts the MCO coating to a mixture of Co and MnO with a thin Cr-rich oxide layer at the interface with the alloy substrate. Following the re-oxidation step, there is a 200nm chromia layer and a 400nm cubic spinel reaction layer (RL) between the alloy and the MCO. These layers thicken to 800nm and 1.2μm, respectively after 1000h oxidation. These observations are compared to our previous studies of MCO-coated Crofer 22 APU, and the implications for long-term SOFC performance are discussed.

Effects of alloy heat treatment on oxidation kinetics and scale morphology for Crofer 22 APU

The effect of alloy heat treatment on the oxidation kinetics and oxide scale microstructure of Crofer 22 APU has been studied. Parabolic oxidation rate constants were measured for the as-received alloy and after pre-oxidation heat treatment in argon at 1050 °C for 1 and 4 h. The oxide scale microstructure was investigated using scanning electron microscopy, focused ion beam milling and transmission electron microscopy. It was found that the alloy forms a two-layer scale with a continuous chromia layer and a discontinuous MnCr2O4 overlayer. Two forms of internal oxides were also formed: subscale pockets of spinel and isolated TiOx precipitates in the underlying alloy. The pre-oxidation heat treatment had a profound effect on the grain size and morphology of the Cr2O3 and MnCr2O4 layers in the scale. The heat-treated samples exhibit a 3.5× lower parabolic oxidation rate constant than the as-received Crofer 22 APU. This improvement in oxidation resistance is attributed to the dramatic differences in the morphology of the oxide scale that forms during the earliest stages of oxidation (<5 h). The implications of these findings for oxidation mechanisms and long-term SOFC performance are discussed.

Effect of Phosphoric Acid Treatment on Isothermal High Temperature Oxidation Behaviour of AISI 304 Stainless Steel at 800°C

Phosphoric acid treatment is used as a way to improve the high temperature oxidation resistance of a chromia-forming AISI 304 steel. Chromia-forming steels are excellent candidates to resist to high temperature oxidizing atmospheres because of the formation of protective oxide scales. The oxide scale growth mechanisms are studied by exposing phosphoric acid-treated and untreated 304 steel samples to high temperature conditions in air. The analyses were carried out by means of thermogravimetry, and in situ X-ray diffraction (XRD). The experimental results show that the phosphoric acid treatment does not have a beneficial effect on cyclic high temperature oxidation (up to 70h of the oxidation test) of AISI 304 steel because of growth of a layer mainly formed by external cation diffusion which grows very quickly. The isothermal high temperature oxidation of this steel at 800°C in air shows a very fast initial iron oxidation towards the external interface, allowing to chromium element to be more available to the internal interface to form a continuous chromia layer, thus causing the establishment of a parabolic oxidation regime and leading to a beneficial reduction of the oxidation rate (after 70h of the oxidation test).

Effect of superficially applied ZrO 2 inhibitor on the high temperature corrosion performance of some Fe-, Co- and Ni-base superalloys

High temperature corrosion is an acute form of corrosion occurring at elevated temperature in the presence of an oxidizing gas and is associated with a thin electrolytic deposit (salt or ash) on alloy. Inhibitors and fuel additives have been used with varying success to combat oil ash corrosion. In this paper, the effect of an oxide additive namely ZrO 2 on the hot corrosion behaviour of some superalloys, viz. Superfer 800H (alloy A), Superco 605 (alloy B) and Superni 75 (alloy C) has been investigated in an Na 2SO 4–60%V 2O 5 environment at 900 °C for 50 cycles. Each cycle consisted of 1 h heating in a Silicon Carbide Tube furnace followed by 20 min cooling in ambient air. Weight change measurements after each cycle were taken by an electronic balance having an accuracy of 0.01 mg. XRD, SEM and EPMA analyses of the exposed specimens were carried out to characterize the oxide scales. In the Na 2SO 4–60%V 2O 5 environment, the corrosion rate for the Co-base alloy was found to be highest, whereas that for the Ni-base Superni 75 a lowest. Whereas, with ZrO 2 superficial coating, the overall weight gains got reduced for the alloys B and C, however the inhibitor was marginally effective in the alloy A. A thick scale was observed in the latter case, which was rich in Cr, Ni, Fe and V. Absence of protective continuous chromia layer and presence of less protective NiO was probably the main reason for more corrosion rate in this case.

Influence of H2O ( g ) on the Oxide Microstructure of the Stainless Steel 353MA at 900 ° C in Oxygen

This work investigates the impact of water vapor on the corrosion behavior of the austenitic Si-containing FeCrNi steel 353MA at through a detailed microstructural characterization of the oxide scales formed after in and in with 40% . The oxidized samples were investigated by focused ion beam and transmission electron microscopy in combination with energy dispersive X-ray analysis. The microstructural investigation showed that the oxide scales were affected by the presence of water vapor. However, there was no significant difference in scale thickness. In both atmospheres a continuous chromia [Cr-rich ] layer was present beneath the spinel oxides. The influence of water vapor on scale composition is attributed to chromia evaporation by the formation of . The ability of the alloy to maintain a continuous chromia layer in spite of chromia evaporation and to avoid breakaway oxidation is attributed to several factors. First, the supply of chromium to the scale by diffusion in the alloy must be rapid. Second, the presence of spinel oxides at the oxide/gas interface may decrease chromia evaporation. Third, the high ratio in the alloy is suggested to make it difficult to convert the protective chromia to poorly protective hematite.

Oxidation of a quaternary three-phase Cu–20Ni–20Cr–5Fe alloy at 700–900 °C in 1 atm of pure O 2

The oxidation of a quaternary Cu–Ni–Cr–Fe alloy containing approximately 20 at.% Ni, 20 at.% Cr and 5 at.% Fe, balance Cu (Cu–20Ni–20Cr–5Fe), was studied at 700–900 °C in 1 atm of pure oxygen. The alloy is composed of a mixture of three phases, where the lightest α phase with the largest Cu content forms the matrix, while the other two, much richer in Cr, form a dispersion of isolated particles. At variance with the ternary three-phase Cu–20Ni–20Cr alloy examined previously, which was unable to form protective chromia scales over the alloy surface even after an extended period of oxidation, the present alloy formed complex external scales containing mixtures of the oxides of the various components plus a deep internal region containing a mixture of alloy and oxide phases. With time, a very irregular and thin but essentially continuous chromia layer formed at the bottom of the mixed internal oxidation region, producing a gradual decrease of the oxidation rate. Thus, the addition of 5 at.% Fe to Cu–20Ni–20Cr alloy is able to decrease the critical Cr content required to form the most stable oxide and promotes the formation of a continuous chromia scale under a lower Cr content in spite of the simultaneous presence of three different phases.

Microstructural effects on the high temperature oxidation of two-phase Cu–Cr alloys in 1 atm O 2

The oxidation of a Cu–Cr alloy containing about 60 wt% Cr and of two Cu–Cr alloys containing about 40 wt% Cr was studied at 700 and 800 °C in 1 atm O 2. The 60 wt% Cr alloy was prepared by powder metallurgy (PM) and had a phase particle size of 50–150 μm. One of the two alloys containing about 40 wt% Cr was prepared by mechanical alloying (MA) and had a phase grain size ranging from 10–50 nm to 200–300 nm, depending on the location, while the other was prepared by magnetron sputtering (MS) and had a phase grain size around 5–10 nm. The most important difference between the oxidation behavior of the three alloys is the formation of an exclusive chromia scale on the surface of the Cu–40 wt% Cr alloy prepared by magnetron sputtering and of a continuous chromia layer beneath an outermost layer of copper oxides on the corresponding alloy prepared by mechanical alloying, while the Cu–60 wt% Cr alloy prepared by powder metallurgy formed complex scales composed mostly of CuO, Cu 2O with some Cu 2Cr 2O 4 and Cr 2O 3. Thus, the microstructure of two-phase binary alloys has a strong effect of their oxidation behavior. In particular, a decrease of the alloy grain size favors the exclusive external oxidation of the most reactive component, reducing the corresponding critical content in the alloy. This effect is attributed to the presence of larger concentrations of rapid diffusion paths for the migration of the components in the alloy as well as to a faster dissolution of the particles of the Cr-rich phase in the copper matrix.