On the Influence of Non-Protective CuO on High-Temperature Oxidation of Cu-Rich Cu-Ni Based Alloys

A number of copper-rich Cu-Ni alloys wereoxidized from 750 to 1000^°C at differentoxygen pressures. The oxide scales formed consist of anouter copper oxide layer and an inner porous layer where internal oxide-derived NiO particles aredispersed in a copper oxide matrix. The copper oxide maybe both single-phase CuO and a two-phase(CuO+Cu₂O). At the lower part of thetemperature range, the oxidation kinetics and oxidemorphology depend strongly upon the formation of CuO.The CuO layer is nonprotective and further oxidation ofCu₂O, forming CuO, therefore changes the oxidation from being approximately parabolic tohaving a breakaway-like behavior. The relative thicknessof nonprotective CuO increases with increasing NiO andreflects that the solid-state flux of copper across the Cu₂O decreases due to abarrier effect of the NiO particles and porosity in theoxide and NiO particles in the alloy. The beneficialeffect of Ni in reducing the oxidation rate is lost due to the extensive formation ofnonprotective CuO.     

A study of the initial oxidation of copper in 0.1 MPa oxygen and the effect of purity by metallographic methods

To clarify the initial oxidation mechanism of copper, the oxidation was carried out at 400 °C in 0.1 MPa oxygen using 99.9999% (6 N) and 99.5% (2 N) pure specimens. Oxidation of 6 N copper after 60 s showed that the number density of the oxide nuclei varied with the face of copper crystals, while the nucleation occurred preferentially at the grain boundaries. A metallographic examination indicated that the products of initial oxidation consist of both CuO and Cu₂O. CuO is firstly formed as a thin uniform film on the copper surface, and then Cu₂O nucleates and grows beneath the CuO film. This result is different from the conclusion reached in the literature that CuO does not appear until the laterally growing Cu₂O nuclei have covered the whole surface using other methods. In contrast to 6 N copper, nucleation of Cu₂O was much delayed for 2 N copper, though a thin CuO film was similarly formed on 2 N copper surface. Impurities in 2 N copper should be responsible for slow nucleation of Cu₂O and slow growth of nuclei.     

Oxidation of copper and electronic transport in copper oxides

Oxidation of copper and electronic transport in thermally-grown large-grain polycrystals of nonstoichiometric copper oxides were studied at elevated temperatures. Thermogravimetric copper oxidation was studied in air and oxygen at temperatures between 350 and 1000°C. From the temperature dependence of the oxidation rates, three different processes can be identified for the oxidation of copper: bulk diffusion, grain-boundary diffusion, and surface control with whisker growth; these occur at high, intermediate, and low temperatures, respectively. Electrical-conductivity measurements as a function of temperature (350–1134°C) and oxygen partial pressure (10^–8–1.0 atm) indicate intrinsic electronic conduction in CuO over the entire range of conditions. Electronic behavior of nonstoichiometric Cu₂O indicates that the charge defects are doubly-ionized oxygen interstitials and holes. The calculated enthalpy of formation of oxygen ( ) and hole-conduction energy (E_H) at constant composition for nonstoichiometric Cu₂O are 2.0±0.2 eV and 0.82±0.02 eV, respectively.This work was supported by the U.S. Department of Energy, under Contract W-31-109-Eng-38.     

In situ oxidation studies on (001) copper-nickel alloy thin films

The (001)-oriented, single crystalline thin films of Cu-3% Ni, Cu-4.6% Ni, and Cu-50% Ni alloy were prepared by vapor deposition onto (001) NaCl substrates. The films were subsequently annealed at around 1100°K and oxidized at 725°K at an oxygen partial pressure of 5×10^–1 N · m^–2 (5× 10^–6 atm). High-resolution transmission electron microscopy was employed to observe the changes in situ. For all the alloy concentrations Cu₂O and NiO were observed to nucleate and grow independently; no mixed oxides were noted. The shape and growth rates of Cu₂O nuclei were similar to those found in previous work on pure copper films. For low-nickel alloy concentrations the NiO nuclei were larger and the number density of NiO was less when compared to the oxidation of pure nickel films. For the Cu-50% Ni films the shape and growth rates of NiO were identical to those for the oxidation of pure nickel films. Low nickel concentrations exhibited a reduced induction period for Cu₂O when compared to pure copper films. Cu-50%Ni films showed surface precipitation and growth of NiO first, followed by Cu₂O in a typical through -thickness growth after a prolonged induction period. The results are consistent with the previously established oxidation mechanisms of pure copper and pure nickel films.This work was performed at the Ames Research Center and funded by NASA Grants NCA2-OP390-403 and NSG-2025.     

Scaling behaviour of a Cu 27.6% Sn alloy at 550–725°C under 1 atm oxygen

The corrosion in oxygen of a bronze containing 27.6 wt% tin has been studied in the range 550–725°C. The oxidation rate of this alloy is considerably smaller than that of pure copper and compares favourably with that of the more dilute 13 wt% tin alloy. In fact at 650°C and below the reaction stops after a relatively short time of quasi-parabolic kinetic (about 5 hr). At higher temperatures instead the initial parabolic stage is followed by a second stage of linear kinetics. The results are interpreted by assuming the formation of a continuous protective layer of SnO₂ at the base of the scale at 650°C and below and of a dispersion of SnO₂ particles in copper oxide at higher temperatures. The importance of the practical absence of Cu₂O from these scales in relationship of the approximate values of the parabolic values of the parabolic rate constant for this alloy as compared to those on pure copper is also examined.     

Composition of Oxide formed on Copper and Copper–Manganese Alloys (5–40% Mn) in Oxygen at 150°–600°c

Abstract

The proportions of CuO and Cu₂O in films formed on copper and copper-manganese alloys in oxygen at temperatures of 150–400°c (600°c for higher Mn alloys) were estimated by coulometric reduction. Only Cu₂O was observed at 150°c. The proportion of CuO increased as the temperature was raised, reaching 82% for copper at 300°c and somewhat lower values for the alloys containing Mn. At temperatures of 350–400°c, the proportion of CuO declined. The oxide remained adherent above 400°c only for alloys with 20% and 40% Mn. From 400° to 600°c the proportion of CuO increased for the 20% alloy but neither CuO nor Cu₂O was found on the 40% alloy.     

Comparison of the Oxidation Behavior of Nanocrystalline and Coarse-Grain Copper

The Oxidation of an electro-deposited nanocrystalline Cu (nc Cu) and a conventional coarse-grain Cu (cg Cu) was investigated at 30–800 ° C under 1 atm of oxygen. Both Cu samples formed external scales of copper oxide (Cu₂O+CuO). At the lower temperature (30–300 °C) the very slow oxidation rates of both the nc and cg Cu might be attributed to the formation of a protective Cu₂O surface layer. However at the higher temperature (300–700 °C), oxidation rates of the nc Cu were obviously faster than those of the cg Cu, which was attributed to faster diffusion of various species along grain boundaries both in the metal and in the scale. In particular, the scale grew faster on the nc Cu by means of not only rapid external oxidation as a result of outward diffusion of Cu-ions but also a significant contribution from inward diffusion of oxygen along the grain boundaries in the scales. Compared with the cg Cu, dissolved O₂ in the nc Cu may have a certain effect on the faster oxidation of the nc Cu. Above 700°C, the difference seemed to disappear as a result of the ineffectiveness of grain-boundary diffusion.     

Oxidation Mechanism of Cu2O to CuO at 600–1050°C

To clarify the oxidation mechanism of Cu₂O to CuO, Cu₂O oxidation was studied at 600–1050 °C under 1atm O ₂. The Cu₂O specimens were prepared through completely oxidizing 99.99999 and 99.5 pure copper at 1000°C in an Ar + 1 O ₂ atmosphere. The oxidation kinetics of Cu₂O specimens prepared from both purity levels followed the logarithmic law, not the parabolic law or the cubic law as reported in the literature. The activation energy for Cu₂O oxidation is relatively high in the lower-temperature range, but becomes very small or even negative at higher temperatures. The logarithmic oxidation rate law can be explained by Davies et al.s model related to grain-boundary diffusion in the oxide layers. The very small or negative activation energies in the higher-temperature range can be attributed to the very small thermodynamic driving force and the fast lateral growth of CuO grains related to a sintering effect. The influence of small amount of impurities is also discussed.     

The influence of an intermediate annealing treatment on the oxidation of copper and nickel

A study has been made of the effects of an intermediate, isothermal annealing treatment in argon on the oxidation kinetics of copper and nickel in 1 atm oxygen at 800 and 1100°C, respectively, using a semiautomatic microbalance. Changes in scale morphology and composition have been investigated using various physical techniques. The outer CuO layer formed on copper during oxidation dissociates very rapidly on annealing to give CU₂O and oxygen since the partial pressure of oxygen in the gas is below the dissociation pressure of CuO but above that of Cu₂O at 800°C. The CuO layer is quickly re-formed on reoxidation in oxygen. There are relatively few other changes in the oxide morphologies of either metal during annealing, although the small grains present in the scale adjacent to the metal after oxidation are able to grow. During reoxidation both metals show a reduction in oxidation rate constant because of the decrease in total cation vacancy concentration in the scale and the reduced cation vacancy gradient across the scale brought about by the reduction in oxygen partial pressure at the oxide-gas interface during annealing. The reoxidation rate constants following annealing approach those recorded prior to annealing as the equilibrium cation vacancy levels in the scales are reestablished in the oxidizing environment. Rosenberg's method for analysis of the kinetics of reoxidation has enabled the equilibrium concentrations and diffusion coefficients of cation vacancies in CU₂O and NiO during oxidation in 1 atm oxygen at the appropriate temperatures to be estimated approximately. These show reasonable agreement with literature values.     

Oxidation Mechanisms of Copper and Nickel Coated Carbon Fibers

Differential-Thermal Analysis (DTA) and X-ray diffraction analysis were applied to determine the mechanisms of high-temperature oxidation of copper- and nickel-coated carbon fibers. Both kinds of coatings were deposited by electroless plating onto the fiber surface. The as-deposited copper film was crystalline, whereas the nickel coating consisted of an amorphous Ni–P alloy. Coated fibers were heated from room temperature to 900 °C in air at 10 °C min^?1. For the copper coating, the main oxidation product formed at low temperatures was Cu₂O, while at higher temperatures was CuO. The crystallization of Ni–P took place at 280–360 °C with the formation of Ni and Ni₃P. The final compounds were NiO, Ni₂P and Ni₃(PO₄)₂. After complete oxidation of the carbon fibers, copper and nickel-oxidized microtubes were obtained. Besides, while copper reduced the temperature of the fiber oxidation, nickel coatings increased the minimum temperature needed for this reaction.     

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.     

Effects of blending on surface characteristics of copper corrosion products in drinking water distribution systems

Copper scales formed over 6-months during exposure to ground, surface and saline waters were characterized by EDS, XRD and XPS. Scale color and hardness were light red-brown-black/hard for high alkalinity and blue-green/soft for high SO₄ or Cl waters. Cl was present in surface or saline copper scales. The Cu/Cu₂O ratio decreased with time indicating an e transfer copper corrosion mechanism. Cu₂O, CuO, and Cu(OH)₂ dominated the top 0.5-1 A° scale indicating continuous corrosion. Cu₂O oxidation to CuO increased with alkalinity, and depended on time and pH. Total copper release was predicted using a Cu(OH)₂ model.     

The annealing and re-oxidation of oxidized CuNi alloys

A study has been made of the effects of an intermediate, isothermal annealing treatment in argon on the oxidation kinetics in dry oxygen of Cu10%Ni and Cu24%Ni at 800°C and Cu80%Ni at 1000°C using a semi-automatic microbalance. Changes in scale morphology and composition have been investigated using various techniques.Oxidation of Cu10%Ni and Cu24%Ni produces external scales consisting of a thick, inner Cu₂O layer and a thin, outer CuO layer, together with nickel-rich internal oxide in the adjacent alloy. Oxidation of Cu80%Ni yields an external scale consisting of a thick, inner NiO layer and a thin, outer CuO layer, together with a few nickel-rich internal oxide particles in the adjacent alloy. During annealing, the outer CuO layers on the three alloys dissociate to give Cu₂O and oxygen. With Cu80%Ni only, further dissociation to give copper metal takes place after long annealing times. In all cases, the annealing treatment leads to a reduction in the cation vacancy gradient across the scale and a reduction in the total vacancy level in the scale due to the reduction in oxygen activity at the oxide-gas interface.In Cu10%Ni and Cu24%Ni, internal oxide formation during annealing stimulates dissociation of the Cu₂O scale near the oxide-alloy interface to give copper metal and oxygen. Similarly, internal oxide formation stimulates dissociation of the NiO layer on Cu-80%Ni, although to a lesser extent than for the copper-rich alloys.During re-oxidation, the kinetics are partly determined by the extent of scale thinning during the prior annealing treatment, giving more rapid weight gains than expected from those recorded during the initial oxidation period. Nevertheless, particularly for Cu-80%Ni, where the scale thinning is relatively small, the reduction in the cation vacancy gradient across the scale and the reduction in the total vacancy level in the scale do result in a reduced oxidation rate compared with the rate recorded during the initial oxidation period.     

The oxidation of cupronickel alloys—I. XPS study of inter-diffusion

The outer surface of thick oxides formed on the high copper-cupronickel alloys is invariably copper oxide, yet at room temperature the oxide film is entirely nickel oxide. The ESCA (or XPS) technique has been used to determine the temperature at which overgrowth of the initial nickel oxide by cuprous oxide takes place on a alloy. By means of interrupted oxidation runs on a heated probe in the ESCA spectrometer, the physical and chemical displacement of Cu₂O by NiO has been followed. It is concluded that the diffusion step in this displacement reatcion is probably rate controlling during oxidation of the alloy at temperatures up to 650°C.The XPS spectra have been interpreted quantitatively and excellent agreement is found between the calculated stoichiometry and the chemical shifts of the Auger spectra.     

Influence of Small Amounts of Impurities on Copper Oxidation at 600–1050°C

In order to study the influence of small amount of impurities on the copper oxidation kinetics, the oxidation was examined at 600–1050°C in 0.1 MPa oxygen atmosphere using 99.5% (2N) and 99.9999% (6N) pure copper specimens. The influence of impurities has been discussed considering the roles of the nonprotective CuO layer, the impurity layer at the Cu₂O–Cu interface, and the diffusion of copper in the Cu₂O layer. The nonprotective CuO layer for 2N copper can greatly enhance copper oxidation. However, the impurities concentrated at the region near the Cu₂O–Cu interface for 2N copper can slow oxidation. Contrary to the presence of metallic impurities, such as Ni, Sb, and Pb, the nonmetallic elements As and Se dissolved in Cu₂O have a deleterious influence on the outward diffusion of copper. Grain-boundary diffusion in Cu₂O can somewhat contribute to 2N copper oxidation at 850–1050°C, but its effect in enhancing oxidation at 600–800°C is weaker than the effect of the impurity layer at the Cu₂O–Cu interface in impeding oxidation.     

Anomalous high-temperature oxidation in the zirconium-copper system

The high-temperature oxidation of the Zr-3 mass% Cu alloy and Zr₂Cu in oxygen is characterized by selective oxidation of zirconium while the excess of copper is accumulated at the alloy-oxide interface forming the Zr₈Cu₅ phase. The oxidation of Zr₂Cu at elevated temperatures shows an anomalous decrease of the oxygen consumption rate in the temperature range 890-975 °C. The oxide layer consists of monoclinic ZrO₂ mainly, with preferentially oriented crystallites in depth region at 900 °C and tetragonal ZrO₂ on the surface below 600 °C, and small amounts of CuO and Cu₂O. The reaction kinetics obeys a parabolic rate law. The activation energy of 117.5 and 54.4 kJ/mol has been estimated for the oxidation of the Zr-3 mass% Cu alloy and Zr₂Cu, respectively.     

The Effect of Impurities on the Formation of the Inner Porous Layer in the Cu2O Scale During Copper Oxidation

The effect of impurities on the formation of the inner porous layer in the Cu₂O scale during copper oxidation was examined using 99.99 (4N), 99.9999% (6N) and floating-zone-refined (FZR, >99.9999) copper specimens at 800° C. Oxidation for 240 min shows that an inner porous layer was formed in the Cu₂O scale near the Cu₂O/Cu interface for 4N copper, but not for 6N and FZR copper. The results support that the inner porous layer in the oxide scale is related to impurities from the metal base.     

Photoacoustic study on cathodic reduction of anodic oxide films formed on copper in borate solution

An in-situ photoacoustic (PAS) technique, using a piezoelectric detector with high sensitivity was applied to the study on duplex oxide films anodically formed on copper in pH 8.4 borate solution. The PAS signals from the copper electrode were produced by an irradiation of light beam with a wavelength of 514.5 nm. The PAS amplitude during cathodic reduction of the outer oxide layer to Cu₂O changed in the opposite direction, depending on the anodic potential of film formation and oxidation time. Assuming that the change in PAS amplitude is proportional to both optical absorption coefficient and film thickness, it was deduced from comparison of the estimated absorption coefficients for Cu (OH)₂, CuO and CuO_0.67 films that dehydration of the outer layer having an average composition of CuO_x (OH)_2?2x proceeded with increasing anodic potential of film formation and oxidation time during growth of the duplex oxide film. Moreover, it was found that the change in PAS amplitude during cathodic reduction of the total Cu₂O film involving the inner layer to metallic copper was proportional to the electric charge required for cathodic reduction, i.e., the film thickness, irrespective of anodic potential of film formation and oxidation time, which proved the validity of the above assumption.     

Differences between corrosion products formed on copper exposed in Tokyo in summer and winter

We analyzed the copper corrosion products that formed during a month in summer and a month in winter at three sites in Tokyo using several analytical techniques. The X-ray diffraction patterns revealed that cuprite Cu₂O and posnjakite Cu₄SO₄(OH)₆·H₂O formed on copper exposed in summer. By contrast, only cuprite was found in winter exposed copper. The X-ray fluorescence results indicated that the amounts of sulfur and chlorine on the copper plates exposed in summer were much greater than those in winter. This could be explained by the change in particulate sulfate and sea salt concentrations. Depth profiling analysis by Auger electron spectroscopy revealed that the oxide layer formed in summer was thicker than that in winter. This difference in oxide layer thickness could have been due to the differences in temperature, relative humidity, and the amount of sulfur and chlorine on the copper plate.     

Enhanced oxidation of the 9%Cr steel P91 in water vapour containing environments

The short term (∼100 h) oxidation behaviour of the 9%Cr steel P91 was studied at 650 °C in N₂-O₂-H₂O gas mixtures containing a relatively low oxygen level of 1%. The oxidation kinetics were measured thermogravimetrically and the oxide scale growth mechanisms were studied using H₂¹⁸O-tracer with subsequent analyses of oxide scale composition and tracer distribution by MCs⁺-SIMS depth profiling. The corrosion products were additionally characterised by light optical microscopy, SEM-EDX and XRD. It was found that the transition from protective, Cr-rich oxide formation into non-protective mixed oxide scales is governed by the ratio H₂O^(g)/O₂ ratio rather than the absolute level of H₂O^(g). The results of the tracer studies in combination with the data obtained from experiments involving in situ gas changes clearly illustrated that under the prevailing conditions the penetration of water vapour molecules triggers the enhanced oxidation and sustains the high growth rates of the poorly protective Fe-rich oxide scale formed in atmospheres with high H₂O^(g)/O₂ ratios. The experimental observations can be explained if one assumes the scale growth to be governed by a competitive adsorption of oxygen and water vapour molecules on external and internal surfaces of the oxide scales in combination with the formation of a volatile Fe-hydroxide during transient oxidation. The formation of the non-protective Fe-rich oxide scales is suppressed in atmospheres with low H₂O^(g)/O₂ -ratios, and the healing of any such scale is promoted.