Scale Growth Kinetics Research Articles

The influence of laboratory test procedures on scale growth kinetics and microstructure during steam oxidation testing

The influence of laboratory test procedures on scale growth kinetics and microstructure during steam oxidation testing

The influence of laboratory test procedures on scale growth kinetics and microstructure during steam oxidation testing

Laboratory exposures cannot reproduce all the features present in service conditions. The experimentalist is faced with the conflict between increasing the complexity of laboratory tests to replicate service more closely and keeping testing costs low by maintaining a simple procedure. The influence of various experimental parameters, which can be controlled in the laboratory, on the steam oxidation response of materials is reviewed and recommendations for best practice are proposed.

Numerical Model for Oxide Scale Growth with Explicit Treatment of Vacancy Fluxes

In the framework of research on behaviour of nuclear waste containers, to evaluate the effects of possible evolution of experimental conditions, as well as evolution of parameters controlling oxidation rate during long-term interim storage, a numerical model has been developed in order to take into account non-stationary states. To anticipate effects like cold working of the metal on the scale growth kinetics and risks of scale detachment by over saturation of vacancies at the metal/oxide interface in the course of scale growth, the model is based on the calculation of chemical species, but also vacancies profiles evolution in the oxide and the metal following a simple time integration. An original numerical treatment is proposed to easily describe elimination of vacancies by introducing sink strength in the metal. The first calculations are presented and discussed.

Isothermal Oxidation Behaviour of a Hot-Work Tool Steel

Isothermal oxidation behaviour of a hot-work tool steel (X38CrMoV5) was investigated at 600degreesC and 700degreesC in dry and wet air. Growth kinetics were determined by using TGA and oxide scales were characterised by means of SEM (EDS, X-ray mapping) and XRD examinations. Moreover, as the microstructural properties of the studied hot-work steel strongly depend on the carbides precipitates formed during its heat treatment, these carbides were extracted from the X38CrMoV5 matrix and their oxidation behaviour in dry and wet air was also studied. Oxidation behaviour of X38CrMoV5 is very sensitive to the presence of water vapour : a large increase of the scale growth kinetics was observed as soon as the water vapour partial pressure exceeds a value of 9 mbar. Microstructural characterisations showed that scales grown in wet air are porous and sometimes cracked and deformed. They are composed of an external iron-rich oxides scale (hematite alpha-Fe2O3), an internal oxide scale enriched in Cr (spinel oxides (Fe,Cr)(3)O-4) and a narrow zone of internal oxidation. Whereas no significant influence of the water vapour partial pressure has been observed on the oxide scale microstructure (composition, morphology), texture of the superficial hematite scales becomes more pronounced when p(H2O) increases from 9 to 310 mbar. Preferential orientation of alpha-Fe2O3 scales is also favoured by increasing thickness of oxides. On the other hand, the oxidation behaviour of carbide precipitates is rather complex and strongly affected by the presence of water vapour in air.

Effect of reactive element oxide coating on the high temperature oxidation behaviour of FeCrAl alloys

The influence of yttrium oxide coating (processed by the sol–gel method) on the oxidation behaviour of a commercial FeCrAl alloy (Kanthal A1) has been investigated during isothermal exposures in air at 1373K. The scale growth kinetics of the uncoated alloy obey a parabolic rate law during the whole oxidation test, whereas the kinetic curves of the Y-coated specimen exhibit an initial transient stage for the first few hours, followed by a parabolic regime. The yttrium sol–gel coating deposited on the bare alloy does not provide the beneficial effect usually ascribed to reactive elements. No oxidation rate improvement of the coated alloy is observed, the parabolic rate constant values are strictly identical for both specimens. In situ X-ray diffraction reveals a marked influence of the reactive element on the composition of the oxide scale. The oxide layer formed on the yttrium-coated specimen comprised, in addition to α-alumina which is the main oxide also identified on the bare specimen, the presence of yttrium aluminates (YAlO3, Y3Al5O12) located in the outermost part of the layer.

Simulating steam oxidation of high temperature plant under laboratory conditions: practice and interpretation of data

Specimens of ferritic and martensitic steels have been exposed to steam atmospheres at 550 °C using three different experimental procedures. Slight differences in scale growth kinetics were observed but the major differences were in the morphology of the oxide scales that were formed. The microstructures of the scales have been compared with those formed on similar materials during service.

High temperature oxidation of high purity nickel: oxide scale morphology and growth kinetics

The oxidation of high purity nickel was studied between 600 and 1200°C for scale thickness between 1 and 30 µm. At or above 1100° C, the scale growth kinetics are strictly parabolic. The scales are then compact with columnar and facetted NiO grains. A more complex behaviour is observed below 1000°C: (i) for test temperatures between 1000 and 800°C, mass gain curves cannot be fitted to a parabola, (ii) different scale morphologies and microstructures are observed depending on scale thickness and temperature, (iii) a duplex scale is formed below 800°C. In addition to the possible effect of grain-boundary diffusion, the departure of growth kinetics from simple pure parabolic kinetics could be also related to the complex scale microstructure and its large evolution during scale growth. In situ oxidation of nickel specimens in an environmental SEM equipped with a hot stage specimen holder permitted to follow the morphological evolution of NiO scales. In situ grown NiO scales show the same microstructural features as observed on Ni specimens oxidised for longer duration in pure oxygen at atmospheric pressure.

High temperature oxidation of high purity nickel: oxide scale morphology and growth kinetics

The oxidation of high purity nickel was studied between 600 and 1200°C for scale thickness between 1 and 30 µm. At or above 1100° C, the scale growth kinetics are strictly parabolic. The scales are then compact with columnar and facetted NiO grains. A more complex behaviour is observed below 1000°C: (i) for test temperatures between 1000 and 800°C, mass gain curves cannot be fitted to a parabola, (ii) different scale morphologies and microstructures are observed depending on scale thickness and temperature, (iii) a duplex scale is formed below 800°C. In addition to the possible effect of grain-boundary diffusion, the departure of growth kinetics from simple pure parabolic kinetics could be also related to the complex scale microstructure and its large evolution during scale growth. In situ oxidation of nickel specimens in an environmental SEM equipped with a hot stage specimen holder permitted to follow the morphological evolution of NiO scales. In situ grown NiO scales show the same microstructural features as observed on Ni specimens oxidised for longer duration in pure oxygen at atmospheric pressure.

Oxidation kinetics of sputtered Ni-5Cr-5Al nanocrystalline coating at 900°c and 1000°c

Isothermal oxidation of sputtered Ni-5Cr-5Al (at%) nanocrystalline coatings were investigated at 900°C and 1000°C in air. Both of them showed low oxidation rates. The oxidation kinetics of sputtered Ni-5Cr-5Al coating was much slower than that predicted by parabolic kinetics of scale growth by lattice or volume diffusion. The much smaller oxide grains could be formed on sputtered coating thereby higher density of grain boundary could be existed in the oxide scale than that formed on cast alloy. Taking the short circuit diffusion through grain boundaries into account, an oxidation kinetic with fourth power law was derived. The excellent results in agreement with the experimental data were obtained.

Role of surface chemistry on the nature of passive oxide film growth on Fe–Cr (low and high) steels at high temperatures

High temperature materials degradation or protection of Fe–Cr alloys are often related to the nature of their oxide scale formation. Breakdown of passive oxide films leads to localized corrosion. Various alloying elements are often incorporated in these alloys to prevent high temperature oxidation. The addition of selected alloying elements is cumbersome and not always cost effective. In this article, we investigate the role of rare earth oxide coatings on high temperature corrosion prevention of both low and high Cr steel. An in situ high temperature oxidation setup has been built to study the oxidation kinetics of both coated and uncoated low and high Cr steels under ambient pressure and dry air. Reduction in scale growth kinetics is observed in the presence of coating. While scanning electron microscopy and x-ray diffraction are employed to study the structure and morphology of the oxide films, x-ray photoelectron spectroscopy and Auger electron spectroscopy are used to study the surface chemistry of the oxide layer. This article relates some of these data to explain the nature of scale growth kinetics (linear or logarithmic or parabolic) observed in both low and high Cr steels.

Studies concerning the effect of nitrogen on the oxidation behavior of TiAl-based intermetallics at 900�C

The oxidation behavior of the titanium aluminides Ti-50Al and Ti-48Al-5Nb has been investigated in Ar+20%O2 and in air at 900°C. Thermogravimetric studies in combination with structural analyses using optical metallography, SEM/EDX and X-ray diffraction show a marked influence of nitrogen on the composition and growth rate of the oxide scales. For a more detailed study concerning the effect of nitrogen on the scale-growth kinetics, thermogravimetrical analyses were carried out during which the gas atmosphere was changed from air to Ar−O2, and vice versa, without intermediate cooling of the specimen. The results show, that nitrogen adversely affects the formation of the initially formed alumina scale and that it enhances the growth rate of the rapidly growing Ti-rich oxide. This effect was observed in both alloys investigated, although the thermogravimetric results at first sight indicated an opposite effect for the Nb-containing alloy. This apparent contradiction is caused by internal oxidation which occurs in this alloy during exposure in Ar−O2.

On-line measurement of parameters in atomization process of metals : J. Guangming (Research Inst. of Electric Light Source Materials, Nanjing, China.) PM Technology, vol 12, no 1, 1994, 43–49. (In Chinese)

Isothermal oxidation kinetic studies on electrolytic grade iron as well as Fe-Cr alloys using different linear heating rates have been carried out. Mass changes of the samples were measured gravimetrically. Some of the iron and iron-chromium alloy samples were given a pretreatment with a solution that resulted in improved scale adherence. Fraction of reaction against temperature plots have been analysed in terms of available non-isothermal kinetic equations utilizing both the differential and integral forms and activation energies have been evaluated. Isothermal rate constant values evaluated from non-isothermal kinetic equations match fairly well with those obtained isothermally, especially for the iron system. The activation energy and the rate constants have been found to change with heating rates, indicating the influence of initial oxide grain structure and morphology on the kinetics of scale growth.

Atmosphere generation for sintering by dissociation of methanol : Y. Jialin. (Dongxin No 3 Plant for Electro-Carbon, Zigong City, Sichuan, China) PM Technology, vol 12, no 1, 1994, 23–25. (In Chinese)

Isothermal oxidation kinetic studies on electrolytic grade iron as well as Fe-Cr alloys using different linear heating rates have been carried out. Mass changes of the samples were measured gravimetrically. Some of the iron and iron-chromium alloy samples were given a pretreatment with a solution that resulted in improved scale adherence. Fraction of reaction against temperature plots have been analysed in terms of available non-isothermal kinetic equations utilizing both the differential and integral forms and activation energies have been evaluated. Isothermal rate constant values evaluated from non-isothermal kinetic equations match fairly well with those obtained isothermally, especially for the iron system. The activation energy and the rate constants have been found to change with heating rates, indicating the influence of initial oxide grain structure and morphology on the kinetics of scale growth.

Computer simulation of diffusion during liquid phase sintering : R.M. Spriggs, Z.S. Nokolic (Alfred University, New York, USA) Science of Sintering, vol 26, no 1, 1994, 1–9

Isothermal oxidation kinetic studies on electrolytic grade iron as well as Fe-Cr alloys using different linear heating rates have been carried out. Mass changes of the samples were measured gravimetrically. Some of the iron and iron-chromium alloy samples were given a pretreatment with a solution that resulted in improved scale adherence. Fraction of reaction against temperature plots have been analysed in terms of available non-isothermal kinetic equations utilizing both the differential and integral forms and activation energies have been evaluated. Isothermal rate constant values evaluated from non-isothermal kinetic equations match fairly well with those obtained isothermally, especially for the iron system. The activation energy and the rate constants have been found to change with heating rates, indicating the influence of initial oxide grain structure and morphology on the kinetics of scale growth.

Characterization of the oxidation and oxidation-sulfidation of chromium niobium alloys

Materials in mixed corrosion environments, such as those in coal gasification processes or other energy conversion systems, may be exposed to both oxygen and sulfur. Alloys that exhibit simultaneous oxidation and sulfidation resistance are sought for these applications. Alloys containing sufficient Al or Cr readily form a protective oxide scale but generally have high sulfidation rates. Refractory metals, such as Nb or Mo, form protective sulfides but oxidize at catastrophic rates. Materials containing additives of one of these groups have been studied for their complex corrosion resistance. A binary alloy containing components from each group (Cr and Nb) has been investigated in mixed oxidants .Isothermal thermogravimetric measurements give overall scale growth kinetics but do not give details for complex scales or substrates. Multiple layers, different growth regimes and preferential corrosion can complicate interpretation. The environmental SEM with a hot stage attachment allows the observation of scale development at temperature.

Chromia Scale Growth in Alloy Oxidation and the Reactive Element Effect

An analysis for the kinetics of scale growth involving simultaneous cation and anion diffusion, and for anion diffusion with blocked cation diffusion, is presented and compared to experimental data in the literature. The significant reduction in scaling kinetics for the growth of chromia on pure Cr and on Fe‐, Ni‐, or Co‐base alloys is attributed to the elimination of cation diffusion by the blocking of the cationic reaction step at the metal/scale interface. Large highly charged reactive element (RE) ions segregate at the metal/scale interface and pin the misfit dislocations whose climb otherwise serves to create interstitial cations (or annihilate vacancies). Then scale growth must proceed by oxygen diffusion over anion vacancies, corresponding to the lateral climb of misorientation dislocations (reaction at monoatomic steps) at the metal/ scale interface. This poisoned interface model provides an interpretation for the reactive element effect (REE) which is consistent with the four well‐known REE characteristics.

Oxidation of hafnium carbide and hafnium carbide with additions of tantalum and praseodymium

The oxidation behavior of HfC, HfC-25 wt. % TaC, and HfC-7 wt.% PrC2 has been studied between 1200–2200° C. Parabolic growth of the oxide layer has been observed for both HfC and HfC-TaC over the entire temperature range. A break in the temperature dependence of the oxidation kinetics occurs around 1600°C. At lower temperatures, the kinetics are limited by gaseous diffusion via pores in the oxide. Above 1800°C, gaseous diffusion through pores becomes less important as scale-growth kinetics are dominated by bulk (ambipolar) diffusion of oxygen and electrons through the oxide.

Non-isothermal oxidation kinetics of iron and iron-chromium alloys

Isothermal oxidation kinetic studies on electrolytic grade iron as well as Fe-Cr alloys using different linear heating rates have been carried out. Mass changes of the samples were measured gravimetrically. Some of the iron and iron-chromium alloy samples were given a pretreatment with a solution that resulted in improved scale adherence. Fraction of reaction against temperature plots have been analysed in terms of available non-isothermal kinetic equations utilizing both the differential and integral forms and activation energies have been evaluated. Isothermal rate constant values evaluated from non-isothermal kinetic equations match fairly well with those obtained isothermally, especially for the iron system. The activation energy and the rate constants have been found to change with heating rates, indicating the influence of initial oxide grain structure and morphology on the kinetics of scale growth.

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.

The effect of temperature on the kinetics of scale growth

AbstractThe temperature dependence of the kinetics of incrustations growth on heat‐exchange surfaces from solutions of NH4NO3 was examined. With increasing temperature the formal kinetic exponent of growth is decreasing and the kinetic constant of growth is increasing. The linear rate of growth of NH4NO3 incrustations is increasing with rising temperature within the whole range of measured supersaturations.