Corrosion of Ni-Ti alloys in the molten (Li,K)2CO3 eutectic mixture

The corrosion of pure Ni and of binary Ni-Ti alloys containing 5, 10, and 15 wt.% Ti respectively in molten (0.62Li,0.38K)₂CO₃ at 650°C under air has been studied. The corrosion of the single-phase Ni-5Ti alloy was slower than that of pure Ni, forming an external scale composed of NiO and TiO₂. The two-phase Ni-10Ti and Ni-15Ti alloys underwent much faster corrosion than pure Ni, producing an external scale containing NiO and TiO₂, and a thick internal oxidation zone of titanium mainly involving the intermetallic compound TiNi₃ in the original alloys. The rates of growth of the external scales for the Ni-Ti alloys were reduced with the increase of their titanium content, while the internal oxidation was significantly enhanced. The corrosion mechanism of the alloys is also discussed.     

Oxidation behavior of an Fe-38Ni-13Co-4.7Nb-1.5Ti-0.4Si superalloy at high temperature in Ar-H2O atmospheres

The oxidation of an Fe-38Ni-13Co-4.7Nb-1.5Ti-0.4Si superalloy (Incoloy 909 type alloy), was investigated at temperatures between 1000 K and 1400 K in Ar-(1, 10%)H₂0 atmosphere using metallographic, electron probe microanalysis, and X-ray diffraction techniques. The oxide scales consist of an external scale and an internal scale which has an intergranular scale (above 1200 K) and an intergranular scale. The oxide phases in each scale are identified as-Fe₂,O₃ (below 1200 K) or FeO (above 1300 K) and CoO · Fe₂O₃ and FeO · Nb₂O₅, respectively. The morphologies, the oxide phases and the oxidation rates do not depend on the partial pressure of H₂O in the range between one and ten percent in Ar gas. The rate constants for the intergranular-scale formation in this alloy are about one-tenth as large as those in Fe-36%Ni alloy reported previously. At all the temperatures the scales grow according to a parabolic rate law and the apparent activation energies for the processes are estimated.     

Transitions Between Different Oxidation Modes of Binary Cu–Zn Alloys in 0.1 MPa O2 at 1,073 K

Oxidation of five Cu–Zn alloys at 1,073 K in 0.1 MPa O₂ showed the existence of three possible oxidation modes, including the internal oxidation of zinc beneath external CuO _x scales, the growth of mixtures of the two oxides and the exclusive growth of external ZnO scales. The critical Zn contents required for the transitions between these oxidation modes have been calculated and compared with the experimental results.     

High-Temperature Scale Formation of Fe–36% Ni Bicrystals in Air

Fe–36% Ni bicrystals were oxidized at temperatures from 1100 to 1250 K in air, both under and without tensile stress. The scales were divided into three categories: external scales, intragranular subscales, and intergranular subscales. A linear relationship exists beween the thickness of scales and subscales and the square root of oxidation time. Tensile stress significantly accelerates intergranular oxidation, but has essentially no influence on either the formation of intragranular subscales or that of external scales. The external scales are composed of -Fe₂O₃ and Fe₃O₄ after a longer oxidation time. The intergranular and intragranular subscales consist of Fe₃O₄ and FeO particles and an Ni-enriched matrix. Cellular or dendritic oxide nodules sometimes exist in the intragranular subscales, causing their frontier interfaces to significantly undulate.     

The nature of the third-element effect in the oxidation of Fe-xCr-3 at.% Al alloys in 1 atm O2 at 1000 °C

The oxidation of an Fe-Al alloy containing 3 at.% Al and of four ternary Fe-Cr-Al alloys with the same Al content plus 2, 3, 5 or 10 at.% Cr has been studied in 1 atm O₂ at 1000 °C. Both Fe-3Al and Fe-2Cr-3Al formed external iron-rich scales associated with an internal oxidation of Al or of Cr+Al. The addition of 3 at.% Cr to Fe-3Al was able to stop the internal oxidation of Al only on a fraction of the alloy surface covered by scales containing mixtures of the oxides of the three alloy components, but not beneath the iron-rich oxide nodules which covered the remaining alloy surface. Fe-5Cr-3Al formed very irregular external scales where areas covered by a thin protective oxide layer alternated with others covered by thick scales containing mixtures of the oxides of the three alloy components, undergrown by a thin layer rich in Cr and Al, while internal oxidation was completely absent. Conversely, Fe-10Cr-3Al formed very thin, slowly-growing external Al₂O₃scales, providing an example of third-element effect (TEE). However, the TEE due to the Cr addition to Fe-3Al was not directly associated with a prevention of the internal oxidation of Al, but rather with the inhibition of the growth of external scales containing iron oxides. This behavior has been interpreted on the basis of a qualitative oxidation map for ternary Fe-Cr-Al alloys taking into account the existence of a complete solid solubility between Cr₂O₃ and Al₂O₃.     

High-Temperature Oxidation Kinetics and Behavior of Ti–6Al–4V Alloy

Owing to the high-temperature reactivity of titanium, the oxidation and alloying of titanium during hot working processes is an important variable. The oxidation behavior of Ti–6Al–4V alloy in air was investigated at various temperatures between 850 and 1100 °C for different times. The oxidation kinetics were determined by isothermal oxidation weight gain experiments. The results showed that the oxidation kinetics approximately obeyed a parabolic law. The activation energy of oxidation was estimated to be 199 and 281 kJ mol^?1 when temperature was above and below the beta transformation temperature (T _β), respectively. A model to predict oxidation extent was established based on experimental observations. The oxide scales mainly consisted of TiO₂ with a small amount of Al₂O₃ and TiVO₄. The alpha case was defined as solid solution formed because of oxygen diffusion into the substrate. The difference in the morphology and the formation mechanism of the alpha case at different temperature ranges was mainly owing to the participation of the grain boundary and grain orientation of the nucleation site.     

Air Oxidation of Ni–Ti Alloys at 650–850°C

The oxidation of pure Ni and three Ni–Ti alloys containing 5, 10, and 15 wt.% Ti over the temperature range 650–850°C in air was studied to examine the effect of titanium on the oxidation resistance of pure nickel. Ni–5Ti is a single-phase solid solution, while the other two alloys consisted of nickel solid solution (-Ni) and TiNi₃. The oxidation of Ni–Ti alloys at 650°C follows an approximately parabolic rate law and produces a decrease in the oxidation rate of pure Ni by forming an almost pure TiO₂ scale. At higher temperatures, Ni–Ti alloys also follow an approximately parabolic oxidation, and their oxidation rates are close to or faster than those of pure Ni. Duplex scales containing NiO, NiTiO₃ and TiO₂ formed. Some internal oxides of titanium formed, especially at 850°C. In addition, the two-phase structure of Ni–10Ti and Ni–15Ti was transformed into a single-phase structure beneath the scales.     

Einfluß von HCl und Cl2 auf die Hochtemperaturkorrosion des 2 1/4 Cr 1 Mo-Stahls in Atmosphären mit hohen Sauerstoffdrücken

Influence of HCl and Cl₂ on high temperature corrosion of 2 1/4Cr 1 Mo Steel in atmospheres with high oxygen pressures The oxidation of the 2 1/4 Cr 1 Mo steel was investigated at 773 K in oxidizing He-O₂-HCl atmospheres. The addition of HCl to He-O₂ atmospheres leads to accelerated oxidation rates. Below porous and cracked oxide scales condensed chlorides are formed. At low HCl pressures 0–1000 vppm the “active oxidation” is determining the corrosion process; i.e. oxidation of evaporating chlorides within the oxide scale. For higher HCl contents 1000–3000 vppm the corrosion behaviour changes to paralinear; i.e. simultaneous parabolic oxide growth and linear mass loss by chloride evaporation.     

Oxidation resistance and corrosion behavior of hot-dip aluminized coatings on commercial-purity titanium

Aluminizing is an effective method to protect alloys from oxidation and corrosion. In this article, the microstructure, morphology, phase composition of the aluminized layers and the oxide films were investigated by SEM, EDS and X-ray diffraction. The high temperature oxidation resistance and electrochemical behavior of hot dip aluminizing coatings on commercial-purity titanium had been studied by cyclic oxidation test and potentiodynamic polarization technique. The results show that the reaction between the titanium and the molten aluminum leads to form an aluminum coating which almost has the composition of the aluminum bath. After diffusion annealing at 950 °C for 6 h, the aluminum coating transformed into a composite layer, which was composed of an inner layer and an outer layer. The inner layer was identified as Ti₃Al or Ti₂Al phase, and the outer layer was TiAl₃ and Al₂O₃ phase. The cyclic oxidation treatment at 1000 °C for 51 h shows that the oxidation resistance of the diffused titanium is 13 times more than the bare titanium. And the formation of TiAl₃, θ-Al₂O₃ and compact α-Al₂O₃ at the outer layer was thought to account for the improvement of the oxidation resistance at high temperature. However, the corrosion resistance of the aluminized titanium and the diffused titanium were reduced in 3.5 wt.% NaCl solution. The corrosion resistance of the aluminized titanium was only one third of bare titanium. Moreover, the corrosion resistance of the diffused titanium was far less than bare titanium.     

Oxidation behavior of molten magnesium in atmospheres containing SO2

The microchemistry and morphology of the oxide layer formed on molten magnesium in atmospheres containing SO₂ were examined. Based on the results and the thermodynamic and kinetic calculations of oxide-growth process, a schematic oxidation mechanism is presented. The results showed that the oxide scales with network structure were generally composed of MgO, MgS, and MgSO₄ with different layers, depending on the SO₂ content, the time and the temperature. The formation of MgSO₄ was important for the formation of the protective oxide scales. The growth of the oxide scales followed the parabolic law at 973 K and was controlled by diffusion.     

Effect of nanocrystallization on the corrosion resistance of K38G superalloy in CO + CO2 atmospheres

The corrosion resistance of the cast superalloy K38G and a sputtered nanocrystalline coating of the same material was investigated in pure CO in the temperature range, 850–1000°C and in CO-20 vol.% CO₂ at 900°C. The cast K38G alloy formed Cr₂O₃ and TiO₂ scales, and a zone of internal Al₂O₃ precipitation. Weight-gain kinetics followed the parabolic rate law under all conditions investigated. The sputtered K38G nanocrystalline coating, however, formed a single-phase Al₂O₃ scale and no internal-oxidation zone. The parabolic rate constants for nanocrystalline coating oxidation were about one order of magnitude smaller than those of the cast alloy. The changes in reaction morphology and rate are attributed to the more rapid grain-boundary diffusion of aluminum in the nanocrystalline material.     

Comparison of the Isothermal Oxidation Behavior of As-Cast Cu–17%Cr and Cu–17%Cr–5%Al. Part II: Scale Microstructures

The isothermal oxidation kinetics of as-cast Cu–17%Cr and Cu–17%Cr–5%Al in air were studied between 773 and 1,173 K under atmospheric pressure. Details of the oxidation kinetics of these alloys were discussed in Part I. This paper analyzes the microstructures of the scale and its composition in an attempt to elucidate the oxidation mechanisms in these alloys. The scales formed on Cu–17%Cr specimens oxidized between 773 and 973 K consisted of external CuO and subsurface Cu₂O layers. The total thickness of these scales varied from about 10 μm at 773 K to about 450 μm at 973 K. In contrast, thin scales formed on Cu–17%Cr–5%Al alloys oxidized between 773 and 1,173 K. The exact nature of these scales could not be determined by X-ray diffraction but energy dispersive spectroscopy analyses were used to construct a scale composition map. Phenomenological oxidation mechanisms are proposed for the two alloys.     

The corrosion behavior of Ni-Nb alloys in a mixed gas of H2-H2O-H2S

The corrosion behavior of Ni-Nb alloys containing up to 40 wt.% Nb was studied over the temperature range of 550–800°C in a mixed H₂/H₂O/H₂S gas. The scales formed on all alloys were multilayered. The outer scale was single-phase Ni₃S₂, while the structure and constitution of the inner scale depended on alloy composition and reaction conditions. Internal oxidation has been found in Ni-20Nb and Ni-30Nb, external oxidation has been observed on Ni-34Nb. Platinum markers were located at the interface between the outer scale and inner scale. The decrease in corrosion rate with increasing Nb content may be attributed to the presence of increasing amounts of Ni-Nb double sulfides as well as to the presence of Nb₂O₅ in the inner region of the scale.     

Isothermal oxidation behavior of Ni3Al-0.1B base alloys containing Ti,Zr, or Hf additions

The isothermal oxidation behavior of Ni₃Al-0.1B and of this alloy containing additions of approximately 2% Ti, Zr, or Hf was studied in purified oxygen at atmospheric pressure over the temperature range 1300 to 1500 K and for periods up to 400 ks. Ni₃Al was also studied similarly for comparison. The oxidation of Ni₃Al and Ni₃Al-0.1B resulted in an extensive formation of geometric voids on the substrate surface leading to poorly adherent scales. The addition of B to Ni₃Al did not change the oxidation behavior much, however, both the activation energy of oxidation and the relative thickness of the Al₂O₃ layer in the scale were increased. The addition of Ti led to the formation of adherent scales at 1300 K, but the oxidation was accelerated after an initial period at 1300 and 1400 K. At higher temperatures the scale was protective, and the parabolic rate constant, k_p, decreased, however, the scale adherence was impaired by the formation of interfacial voids. The addition of Zr resulted in very adherent scales at all temperatures, but at the same time it increased k_p considerably. The addition of Hf also resulted in very adherent scales at all temperatures and decreased k_p except at 1300 K. The improved scale adherence can be accounted for by the keying effect, since the additives resulted in roughened scale/alloy interfaces. A complex of oxide particle and an associated void was found on the exposed surface of the Hf-containing alloy. This supports the vacancy-sink mechanism.     

The influence of yttrium additions on the oxidation resistance of a directionally solidified Ni-Al-Cr3C2 eutectic alloy

Isothermal oxidation of a directionally solidified Ni-Al-Cr₃C₂ eutectic alloy results in development of an external -Al₃O₃-rich scale. However, this scale breaks down after relatively short times at temperature and a less protective Cr₂O₃-rich scale is formed, together with substantial internal oxide in the alloy. In an attempt to maintain the external -Al₂O₃-rich scale and prevent damaging subscale oxidation, modified yttrium-containing directionally solidified alloys have been developed. The oxidation resistance of these alloys at 1000 and 1100°C in flowing air has been investigated and found to be considerably better than that of the corresponding yttrium-free alloy. At both temperatures an external -Al₂O₃-rich scale is produced and is retained for much longer periods than on the yttrium-free alloys during isothermal and thermal cycling oxidation. Some scale breakdown does occur during thermal cycling at 1100°C, but -Al₂O₃ is able to re-form as the surface oxide. However, although external -Al₂O₃-rich scales are retained for long periods on these alloys, some oxide penetration into the alloy beneath these scales does occur where coarse carbide fibers intersect the alloy surface. This is associated with relatively poor scale integrity at these intersections.     

Effect of scale constitution on the carburization of heat resistant steels

Several austenitic heat-resistant steels were exposed to alternating periods of carburization at 1273 K [a _c= 1,po₂<10^–28 atm] and oxidation at 973°K [a _c O,po₂ = 0.2 atm]. In all cases the depth of internal carbide precipitation increased with cumulative carburization time. It was found that the carburization rates of high nickel content alloys were unaffected by intermittent oxidation cycles, whereas the low nickel, high iron content alloys experienced a reduction in carburization rate subsequent to oxidation treatment. The latter group of alloys formed external scales of chromium-rich M₇C₃ which were shown by sulfur tracing experiments to be gas permeable. It was concluded, therefore, that oxidation of these materials led to blockage of cracks and holes in the scales, thereby decreasing the surface carbon activity and hence the carburization rate. High nickel, low iron alloys formed external scales of chromium-rich M₇C₃ covered by Cr₃C₂. These scales were shown to have very low gas permeabilities. It was concluded that the carbon activity at the surface of these alloys was controlled by scale-alloy equilibration, and was therefore not affected by brief periods of oxidation. The pattern of carbide scale formation is qualitatively consistent with the thermodynamics of the Fe-Cr-C system.     

The Oxidation of TiB2 Ceramics Containing Cr and Fe

The oxidation behavior of TiB₂, TiB₂–0.5 wt.% Cr–0.5 wt.% Fe and TiB₂–1 wt.% Cr–1 wt.% Fe was studied at 800, 900, and 1000°C in static air. These ceramics oxidized rather rapidly and formed thick oxide scales. The oxidation rates of TiB₂-base ceramics were comparable to TiO₂ formation on pure titanium. The scale formed on TiB₂ consisted of TiO₂ and B₂O₃. For TiB–Cr–Fe ceramics, a small amount of Cr- and Fe-oxides was additionally formed. B₂O₃ formed during oxidation tended to evaporate because of its high vapor pressure, making oxide scales porous and fragile. The oxidation of the TiB₂-base ceramics appeared to be governed by the inward transport of oxygen via the highly porous oxide scale. The oxidation resistance of TiB₂–Cr–Fe ceramics was similar to or better than that of TiB₂.     

In Situ observation of the effect of temperature on Cu2O scale morphology

The effect of temperature on copper oxide scale morphology was studied during in situ oxidation of OFHC copper in a hot-stage environmental scanning electron microscope (HSESEM). Cuprous oxide scales grown at low temperature [0.2T_m(Cu₂O)] and intermediate temperatures 0.6–0.7T_m(Cu₂O)] were found to be crystallographically oriented. At intermediate temperatures, scales exhibited nonplanar features such as ridges and growth pyramids. At high temperatures [T> 0.8T_m(Cu₂O)], scales had planar morphologies and a few dislocation growth pits. Downquenching and upquenching of the Cu₂O scales from steady-state oxidation temperatures induced morphological changes such as cavity formation and surface reconstruction.     

Cyclic Oxidation Behavior of IN 718 Superalloy in Air at High Temperatures

Ni-base superalloy IN 718 was cyclically oxidized in laboratory air at temperatures ranging from 750 to 950 °C for up to 12 cycles (14 h/cycle). The kinetic behaviour as well as the surface morphology, and the oxide phases of the scales were characterized by means of weight gain measurements, cyclic oxidation kinetics, scanning electron microscopy equipped with energy dispersive spectroscopy (SEM-EDS), and X-ray diffraction (XRD) analysis techniques. The results showed that as the oxidation temperature increased, the oxidation rate, the external scale thickness, and internal oxidation zone increased. It was suggested that the oxidation rate was controlled by the diffusion of substrate elements in the alloy and the inward diffusion of oxygen through the oxide scale. The oxidation kinetics followed a sub-parabolic rate law and, the activation energy of oxidation was 249 ± 20 kJ mol^?1. The scaling process was controlled mainly by the diffusion of chromium, titanium, manganese, and oxygen ions through the chromia scale. IN 718 showed low weight gain and very slow reaction rates of substrate elements at 750 °C. At 850 °C, a continuous and very thin oxide scale was formed. At 950 °C, XRD and EDS-elemental mapping analysis revealed that a complex oxide scale had formed. It consisted of an outermost layer of TiO₂?CMnCr₂O₄ spinels, inner layer of Cr₂O₃, and the inner most layer composed of Ni₃Nb enriched with Nb, Ti and Al oxides underneath the chromia layer. The oxide scale at this temperature seemed to be thicker layer, significant spallation and volatilization had apparently occurred, and greater internal corrosion was identified. The doping effect of titanium was observed, where it was found to be diffused through the chromia scale to form TiO₂ at the oxide-gas interface as well as internally and at the oxide alloy interface. The amount of rutile (TiO₂) at the oxide surface increased with temperature. In view of Mn contents in the alloy, the manganese?Cchromium spinel oxide was inferred to have played an important role in cyclic oxidation behaviour of IN 718, where the change in oxidation kinetic was noted. The Al contents would cause internal Al-rich oxide formation at grain boundaries.     

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.