Thermodynamic characteristics and numerical modeling of internal nitridation of nickel base alloys

Kinetics and thermodynamics of the process of internal nitridation of various nickel‐base alloys have been investigated in oxygen‐free nitrogen atmospheres. Furthermore, the influence of formation and spalling of a protective oxide scale on the internal nitridation behavior of the alloys was studied by isothermal and cyclic oxidation tests in air. In general, nitridation kinetics of model nickel‐base alloys of the system Ni‐Cr‐Ti was found to obey a parabolic rate law indicating that the nitridation process is diffusion‐controlled. The temperature dependence of the nitridation rate constants is well described by an equation of the Arrhenius type. A thermodynamic calculation of the Ni‐Cr‐Ti‐Al‐N system was used to determine the nitrogen solubility in respective alloys as a function of temperature and alloy composition. The results show that a higher chromium content gives rise to an increase in the nitrogen solubility of Ni‐Cr‐Ti alloys leading to an increased nitridation rate in accordance with the experimental observations. From the calculated values for the nitrogen solubility, the diffusion coefficients of nitrogen were assessed using Wagner's classical theory of internal oxidation. A computer model of internal nitridation was developed that combines a commercial thermodynamic software (ChemApp) with a finite‐difference diffusion calculation. It was found that this model describes the internal nitridation process in reasonable agreement with the experimental results and allows to treat the case of simultaneous formation of different nitrides. The dependence of internal nitridation behavior on spalling and cracking of the oxide was incorporated into the simulation on the basis of simple assumptions showing that this calculation method successfully applies also to complex internal corrosion processes.     

Internal corrosion of engineering alloys: Experiment and computer simulation

High-temperature corrosion is generally known as a material degradation process that occurs at the surface of engineering components. In the case of internal corrosion, the corrosive species penetrates into the material by solid-state diffusion leading to the formation of internal precipitates, for instance, oxides (internal oxidation), nitrides (internal nitridation), and carbides (carburization). It is known from numerous publications and technical failure cases that internal corrosion results in a strong deterioration of the properties of a material (i.e., near-surface embrittlement or the dissolution of strengthening phases). The present article introduces the classic theory of internal oxidation and reviews some recent research on internal corrosion phenomena that are closely related to the failure mechanisms of thermally grown protective oxide scales on several commercial high-temperature alloys (e.g., single-crystalline and polycrystalline Ni-base alloys and Cr steels). The mechanisms and kinetics of internal corrosion processes are determined by the temperature, the local chemical composition of the material, the solubility and diffusivity of the corrosive species, as well as the mechanical loading conditions. These influence factors are taken into account by means of a computer model combining a numerical finite-difference approach to solve the diffusion differential equations with the thermodynamic tool ChemApp. Using several examples, it is shown that the model has been applied successfully to simulate the internal nitridation, carburization, and oxidation of high-temperature alloys.     

Internal Nitridation of Nickel-Base Alloys. Part II. Behavior of Quaternary Ni-Cr-Al-Ti Alloys and Computer-Based Description

Whereas in Part I of this study the process ofinternal nitridation was described for binary andternary alloys within the Ni-Cr-Al-Ti system, this partfocuses on quaternary Ni-Cr-Al-Ti alloys, which are similar to commercial Ni-base alloys used inhigh-temperature applications regarding their chemicalcompositions. These alloys can simultaneously form twodifferent nitride-precipitation zones consisting of TiN and AlN. In order to quantify thenitridation process, thermogravimetric measurements inan oxygen-free nitrogen atmosphere in the temperaturerange 800-1100°C were carried out and supplemented by extensive microstructural studies. Whilesingle-nitride internal nitridation can easily bedescribed by Wagner's theory of internal oxidation,modeling of the more complex internal-precipitationreactions that involves more than one nitride requires anumerical treatment of both the diffusion and thethermochemical processes in the alloy. For this purpose,a computer simulation was developed in which the commercial thermodynamic software ChemApp iscombined with a finite-difference diffusion calculation.It was shown that this calculation technique can beapplied successfully to quantitatively describe the internal-nitridation process of theNi-Cr-Al-Ti model alloys used in this study.     

Internal Nitridation of Nickel-Base Alloys. Part I. Behavior of Binary and Ternary Alloys of the Ni-Cr-Al-Ti System

The internal-nitriding behavior of several modelalloys of the Ni-Cr-Al-Ti system in an oxygen-freenitrogen atmosphere at 800-1100°C was studied.Thermogravimetry as well as various metallographic techniques (SEM and TEM) were used. It wasshown that both the nitrogen solubility and the nitrogendiffusion coefficient are strongly affected by the Crcontent of the Ni alloy. Hence, in Ni-Cr-Ti alloys a higher chromium content leads to an increaseddepth of the internal precipitation of TiN. Nitridationof the alloying element Cr takes place only at highconcentrations of Cr. In general, the nitridation rate was found to obey Wagner's parabolic ratelaw of internal oxidation. Changes in the parabolic rateconstant with alloy composition can be understood bymeans of thermodynamic calculations in combination with microstructural observations.     

Numerical modeling of high-temperature corrosion processes

Numerical modeling of the diffusional transport associated with high-temperature corrosion processes is reviewed. These corrosion processes include external scale formation and internal subscale formation during oxidation, coating degradation by oxidation and substrate interdiffusion, carburization, sulfidation and nitridation. The studies that are reviewed cover such complexities as concentration-dependent diffusivities, cross-term effects in ternary alloys, and internal precipitation where several compounds of the same element may form (e.g., carbides of Cr) or several compounds exist simultaneously (e.g., carbides containing varying amounts of Ni, Cr, Fe or Mo). In addition, the studies involve a variety of boundary conditions that vary with time and temperature. Finite-difference (F-D) techniques have been applied almost exclusively to model either the solute or corrodant transport in each of these studies. Hence, the paper first reviews the use of F-D techniques to develop solutions to the diffusion equations with various boundary conditions appropriate to high-temperature corrosion processes. The bulk of the paper then reviews various F-D modeling studies of diffusional transport associated with high-temperature corrosion.     

Plasma nitriding of Fe-18Cr-9Ni in the range of 723–823 K

To clarify the mechanism of plasma nitriding, we examined the optical microstructure, the hardness, the precipitation, and the concentration of dissolved nitrogen in Fe-18Cr-9Ni nitrided using plasma in the range of 723–823 K. Compared with ammonia-gas nitriding, the features of plasma nitriding are the formation of small chromium-nitride precipitates (CrN), the absence of an externally nitrided layer, the high concentration of dissolved nitrogen, and the high hardness (HV=1200). The diffusion coefficient of nitrogen in the present alloy was determined using the growth rate of the internally nitrided layer, based on calculations used in internal oxidation. Plasma- and gas-nitriding were also compared with respect to the growth rate of the nitrided layer.     

Nitridation in NH3‐H2O‐mixtures

Exposures were conducted of iron, nickel, ferritic 1‐18%Cr steels, austenitic 18%Cr‐9%Ni‐ and 20%Cr‐31%Ni‐steels and a 16%Cr‐Ni‐base alloy at 500°C in He‐30%H₂O and 70%H₂O‐30%NH₃, to compare the corrosion behaviour of these materials in water vapor as in conventional power plants with their behaviour in a NH₃‐H₂O mixture, i.e. under conditions of the “Kalina‐cycle”. After 50 h in He‐H₂O generally a dense oxide scale had grown on iron and on the steels, whereas the scale grown in NH₃‐H₂O was porous, due to initial formation of the γ′‐ and ε‐nitrides, which are converted to Fe₃O₄ later. The porous scale allows internal nitridation of the Cr‐steels, nitrogen is transferred into the metal phase and reacts to finely dispersed CrN‐precipitates. This process causes stresses in the material and formation of cracks. The higher the Cr‐content of the material, the worse is the damage of the materials surface. Least corrosion damage occurs for iron and the 1%CrMo‐steel, however, the inward penetration of nitridation is greatest, and after 5 years on the low Cr‐steel a layer of about 15 mm would be embrittled by internal nitridation, formation of γ′ and ε‐nitride layers and external oxidation. Nickel is strongly damaged by intermediate formation of instable Ni₃N, which causes internal stresses and cracking, but also pore formation by its decomposition. The surface region of the 15%Cr‐Ni‐base alloy is also destroyed by internal nitridation and extrusion of Ni‐particles, while for this material the inward penetration of nitridation is relatively slow due to the low solubility and diffusivity of N in Ni and Ni‐alloys.     

The internal nitriding behavior of Co-3Cr,Co-3Al,and Co-3Ti (a/o) over the range of 700–1100°C

Internal nitridation of Co containing 3 a/o of either Cr, Al, or Ti was studied over the range of 700–1100° C in NH₃/H₂ (694 ratio). The kinetics of thickening of the reaction zone followed the parabolic rate law, suggesting that solidstate diffusion was rate controlling. Activation energies obtained were 51.2 Kcal/molfor Co-3Cr, 46.6 Kcal/mol for Co-3Ti, and 27.9 Kcal/mol for Co-3Al. XRD showed only CrN, AlN, and TiN. Deep etching revealed that AlN formed hexagonal plates near the surface when formed at high temperature, the precipitates becoming more massive (blocky morphology) near the reaction front. TiN formed elongated dendritic precipitates, whereas CrN tended to form spheroids. The precipitate size varied with temperature, decreasing with decreasing temperature. In some cases, nitriding formed a case but no visible precipitates even at very high magnifications in the SEM. The solubility of nitrogen in cobalt was determined by long-time equilibration and subsequent chemical analyses. The diffusivity of nitrogen in cobalt was determined from measured permeabilities and the experimentally determined solubilities. Mechanisms are discussed, and the behavior of internal nitridation is compared with internal carburization and oxidation in cobalt alloys.     

Untersuchungen zum Verhalten von Hochtemperaturwerkstoffen gegenüber Aufstickung

Investigations of nitridation behaviour of high temperature materials Furnace rolls and inner tube walls of industrial bright annealing furnaces are often subject to attack by nitrogen atmospheres. In order to select appropriate materials for application in nitriding atmospheres, three commercial stainless steels (AISI 314, alloy DS, alloy 800H) and four nickel base alloys (alloy 45-TM, alloy 600H, alloy 601H and alloy 602CA) with different concentrations of nickel, chromium, silicon and aluminium were exposed to both N₂/H₂ gas atmospheres at temperatures of 1000°C, 1100°C and 1200°C. The impact strength, the mass change due to nitrogen pick-up and the depth of internal nitridation were determined after exposure. At 1000°C nitrogen pick-up and loss of impact strength, was low for all alloys investigated. At 1100°C and 1200°C, however, all alloys suffered internal nitridation. Both internal nitridation and loss of ductility were more severe in the iron base alloys than in the nickel-base alloys. The corrosion attack by nitridation decreased with increasing nickel content. The highest resistance to nitridation was found in the nickel base alloys 600 H and 602 CA.     

Experimental analysis and computer‐based simulation of nitridation of Ni‐base alloys—the effect of a pre‐oxidation treatment

Nickel‐base superalloys are commonly used for high‐temperature applications in power‐generation industry, e.g., gas‐turbine blades or heat exchangers. They are designed to resist high creep loading and severe corrosion attack during operation. Nitridation is one of these corrosion processes, in particular when the alloys need to be exposed to a N₂ atmosphere. Based on past assumptions, a dense oxide layer should be an efficient barrier against N₂ ingress. But is this really the case? This work is focused on the nitridation behavior of commercial Ni‐base alloys and the influence of a pre‐oxidation treatment. To model the growth of the internal‐nitridation zone, the diffusion processes were solved using the numerical implicit finite‐difference method in combination with the subroutine ChemApp for thermodynamic calculations.     

Internal nitridation of nickel-chromium alloys

The nitriding behavior of nickel-chromium alloys was studied in ammonia-hydrogen mixtures over the range of 700–900°C. Nitridation rates decreased with increasing chromium content, but the critical amount of chromium for transition from internal nitridation to continuous-nitride film formation was found to be much greater than the critical value to form a continuous-Cr ₂O₃ film during oxidation. In general, internal-nitridation rates were found to obey the parabolic rate law. Parabolic rate constants and activation energies for the diffusion of nitrogen were measured. Very fine precipitates formed at the lowest temperature, increasing in size with increasing temperature. The precipitate number density was found to vary within the internally nitrided zone, decreasing with distance from the gas/metal surface. The precipitate morphology changed also with temperature and distance, becoming Widmanstätten at higher temperatures and/or increasing distance within the zone. CrN formed for all exposure conditions. No Cr ₂ N was detected under any conditions studied.     

Einfluß der Legierungszusammensetzung auf das Oxidations- und innere Nitrierungsverhalten von Nickelbasis-Superlegierungen

Influence of the alloy composition on the oxidation and internal-nitridation behaviour of nickel-base superalloys Internal nitridation of nickel-base superalloys takes place as a consequence of the failure of protecting oxide scales (Al₂O₃ and Cr₂O₃, respectively) and leads to a deterioration of the material properties due to near-surface embrittlement caused by the nitrides precipitated (TiN and AlN, respectively) and due to near-surface dissolution of the γ′ phase. By using thermogravimetric methods in a temperature range between 800 °C and 1100 °C supplemented by microstructural examinations, the failure potential due to internal nitridation could be documented. A quantification was carried out by extending the experimental program to thermogravimetric studies in a nearly oxygen-free nitrogen atmosphere which was also applied to various model alloys of the system Ni-Cr-Al-Ti. It could be shown that the nitrogen diffusivity and solubility in nickel-base alloys is influenced particularly by the chromium concentration. An increasing chromium content leads to an enhanced internal-nitridation attack.     

Effect of magnesium on nitridation and infiltration of aluminum powder

We investigated the effect of Mg on the nitridation behavior of Al particles during thermal treatment for a range of Mg contents and temperatures. The addition of Mg stimulates the initiation of nitridation because it diffuses and forms an Al-Mg alloy with a lower melting point than that of pure Al. In the initial stages of the nitridation, nitrides without a perfect crystallographic structure are formed on the surface of Al particles and then AlN with a superlattice structure is formed within the reaction product. Since the Al-Mg powder bed is rapidly densified during heat treatment compared to its pure Al counterpart, the pathways in it for nitrogen diffusion are limited in comparison and further nitridation is much suppressed, resulting in much lower total nitridation compared to a pure Al powder bed. This behavior is observed regardless of the Mg content or the nitridation temperature (up to 900 °C), demonstrated in experiment for Al powder beds containing <5 wt% Mg.     

The Effect of Supersaturation on the Internal Oxidation of Binary Alloys

According to the theory of Bohm and Kahlweit ofthe internal oxidation of binary A-B alloys, theparabolic rate constant for the formation of reasonablystable internal BO oxides as well as theconcentrations of O and B at the oxidation front arecontrolled only by the degree of supersaturationnecessary for the nucleation of new oxide particles. Theeffects of this factor on the previous parameters arecalculated for various values of the solubility product ofthe oxide and of the diffusion coefficients of O and B.Moreover, an alternative procedure for the calculationof the critical degree of supersaturation behind the precipitation front required for oxideprecipitation, which is a function of the concentrationof the reactants at the internal oxidation front, isproposed. A simple modification of Wagner's theory of internal oxidation is also presented, andits results are compared with those of the treatment byBohm and Kahlweit. Finally, the limitations of the twomethods are examined.     

Improvement of Oxidation Behavior of a Ti–48Al–2Cr Alloy by a Nitridation Treatment

The effect of a nitridation treatment for 10 hr at 800°C on the oxidation resistance of a Ti–48Al–2Cr (at.%) alloy in air at 800°C was evaluated. Results prove that nitridation decreases by about 40% the total mass gain of nonnitrided material, although the oxidation mechanism is the same for both materials. The oxidation can be divided into two stages. The formation of a nonprotective mixed alumina–rutile scale during the transient stage results in a high oxidation rate. A further decrease in the oxidation rate arises from the establishment of an external alumina-rich layer during the steady stage. The main difference between the scale developed on both materials is the continuous nature of the nitride layer present in the nitrided material during the entire exposure. The thin continuous nitride layer formed during the nitridation treatment acts beneficially as a diffusion barrier, preventing oxygen dissolution in the ₂-Ti₃Al phase during the transient stage. Furthermore, the oxygen gradient through the oxide scale is kept low, because no oxygen is removed at the scale–alloy interface.     

NiCrAlY涂层对单晶镍基合金高温氧化行为的影响

通过对有无NiCrAlY涂层镍基单晶合金进行不同温度的恒温氧化动力学曲线测定及组织结构观察,研究了纳米晶NiCrAlY涂层对高Cr单晶镍基合金高温氧化行为的影响。结果表明：在高温氧化期间,无涂层试样发生明显的氧化、内氧化和内氮化,在表层为Al2O3、Cr2O3的混合氧化物,在次表层氧化物中富含元素Ta,而元素Al贫化,并在近基体区域存在内氧化物;随氧化温度升高,元素Al的贫化区尺寸增大,其中,富Ta相可抑制基体中元素Al向外扩散,延缓合金的氧化速率。合金在氧化初期增重迅速,而恒温氧化增重动力学曲线呈现起伏波动的原因,归结于表面氧化膜的形成与剥落。高Cr单晶合金经溅射NiCrAlY纳米晶涂层,可有效改善合金的抗氧化性能;有涂层试样在不同温度的恒温氧化动力学曲线仅在氧化初期有轻微增重而后趋于平稳,其形成的Al2O3氧化膜不发生明显的剥落,仅在基体近涂层/基体界面区域存在少量AlN内氮化物。     

The Oxidation Properties of an Iron-1.5 wt.% Silicon Alloy in Carbon Dioxide-Carbon Monoxide Atmospheres at 1000°C

An investigation is reported on the growth and structure of the scale formed on a ferritic Fe-1.5 wt.% Si alloy exposed to carbon-dioxide-carbon monooxide atmospheres at 1000°C. The amorphous silica film on the metallographically polished specimens crystallized to β-cristobalite. Wustite and fayalite developed within nodules which grew laterally to cover the alloy surface. Oxygen diffusion into the underlying alloy led to precipitation of silica as α-tridymite. This internal oxidation zone was sufficiently depleted of silicon for its transformation to an austenitic phase. A fully developed scale was composed of an external wustite layer and an inner wustite-fayalite conglomerate layer, interspersed with discontinuous fayalite bands. A model and a reaction sequence are advanced to account for the form of the oxidation curves and the reaction rate constants in terms of surface reaction steps similar to those for wustite formation on pure iron in these atmospheres.     

Internal oxidation phenomenon in pure copper

This paper presents two special kinds of internal oxidation phenomenon that can take place in pure metals containing a high concentration of non-equilibrium defects. These processes are Internal Oxidation (IO) and Internal Carbonisation (IC). Both processes start with the dissolution of oxidant (O or C) into the pure metal at the free surfaces, and continue with the diffusion of oxidant atoms into the metal matrix volume, where they are trapped at numerous defects within the crystal lattice. Increasing oxidant activity at these places causes local oxidation of the matrix and, consequently, precipitation of fine oxide or graphite particles. The IO and IC processes were tested on the rapidly solidified pure copper which was produced by the Chill-Block Melt Spinning Technique. Analysis of the IO process showed the formation of Cu-Cu₂O, and the formation of Cu-C composite from the IC process.