The effect of water vapor on the oxidation behavior of Ni-Pt-Al coatings and alloys

Turbines fired with hydrogen or syngas from coal gasification will have significantly higher water vapor contents in the combustion gas than natural gas fired turbines. The effect of increased water vapor on alumina-forming coatings and model alloys was investigated at 1100 °C in furnace cyclic testing. Increasing the water vapor content from 10% to 50 vol.% increased the amount of scale spallation on undoped alumina-forming alloys. Compared to dry O₂, increased spallation was observed for β and γ/γ' phase coatings on the substrates of alloys 142 and N5. In all cases, the addition of water vapor appeared to reduce the formation of alumina whiskers and ridges at the scale-gas interface, but did not significantly change the alumina growth rate. The addition of water vapor may have a detrimental effect on the selective oxidation of Al in γ/γ' alloys and coatings.     

Influence of water vapor on the oxidation behavior of aluminized coatings under low oxygen partial pressure

FeCrNi alloy after aluminizing was oxidized at 1000 °C in dry and humid (2.23 vol.% water) H₂. Experimental results showed that H₂ promotes the formation of θ alumina and its transformation to α alumina. The morphology of surface alumina coating does not change significantly, but the oxidation rate of the aluminized layer accelerates by the addition of water vapor. As a result, more cracks are found beneath the alumina layer when water vapor is present. The addition of water vapor seems having a favorable effect on the selective oxidation of Al and concentration of oxygen vacancy in the aluminized alloys.     

Electrochemical behavior of Ti-Mo alloys applied as biomaterial

Electrochemical investigation on the as-cast Ti-Mo alloys (4-20 Mo wt.%) applied as biomaterials in Na₂SO₄ and Ringer physiological solutions is reported. Analyses of the open-circuit potential indicated that all alloys present spontaneous passivation. SEM and cyclic voltammograms obtained in the Ringer solution showed that the samples studied do not present pitting corrosion at potentials up to 8 V (SCE), indicating high corrosion resistance. Open-circuit potential profiles of the anodic oxides growth in both solutions show that the presence of chloride ions during the anodization does not influence the oxides’ chemical stability, and also clearly indicate that adding Mo to pure Ti improves the stability of the anodic oxides. All these results suggest Ti-Mo alloys promissory to be applied as biomaterials.     

Electrodeposition of Zn–Ni coatings as Cd replacement for corrosion protection of high strength steel

Electrodeposition of Zn–Ni coatings performed in acidic baths are not suitable for high strength steels due to their high susceptibility to hydrogen embrittlement.In this work, Zn–Ni coatings were deposited on a high strength steel (4340) upon stirring conditions from an alkaline bath. A complete characterisation of the coatings (corrosion, morphology and composition) has been accomplished, correlating the electrodeposition conditions with these features. The best protective properties of the grown coatings were achieved for the alloys with a single phase structure of γ-Ni₅Zn₂₁ and a denser morphology. Additionally, the hydrogen content incorporated is lower than even cadmium-coated 4340 steel which has undergone a postbaking dehydrogenation treatment.     

Effect of boron doping on the mechanical properties of ordered Ni–Mo alloys

HASTELLOY B-2 alloy was found to exhibit environmental embrittlement when tested in air and hydrogen at ambient temperature after atomic ordering introduced by a heat treatment at 700 °C for 24 h. Molybdenum in the HASTELLOY B-2 alloy was probably reactive enough to dissociate water vapor in air to generate atomic hydrogen, resulting in hydrogen embrittlement. The percentage of transgranular fracture increased as the test environment changed from hydrogen gas to moist air or water vapor, and vacuum or oxygen. With the addition of 100 wt ppm B, the environmental embrittlement was completely eliminated, with the tensile properties independent of both test environment and strain rate. The fracture mode remained the same, i.e. ductile dimpling, after B-doping when tested in different environments. The immunity of the B-doped B2 alloy to environmental sensitivity remained even after long-term heat treatment. Auger analysis does not detect any boron segregation at the grain boundaries. The mechanism of boron doping in eliminating the environmental embrittlement in the Ni–Mo alloy is apparently different from that in many L1₂-type alloys such as Ni₃Al and Ni₃Si.     

Hydrogen effects on the passive film formation and pitting susceptibility of nitrogen containing type 316L stainless steels

The effects of hydrogen on the passivity and pitting susceptibility of type 316L stainless steels have been investigated with alloys containing different nitrogen contents (0.015, 0.198 and 0.556 wt.% N). The study revealed that electrochemically pre-charged hydrogen significantly reduced the pitting resistance of alloys conatining 0.015 and 0.198 wt.% nitrogen contents. In alloy with highest nitrogen content (0.556 wt.% N), an increase in the passive film current density with hydrogen was observed without affecting breakdown potential. Auger electron spectroscopy (AES) analysis of the passive film indicated the presence of nitrogen in the passive film. On other hand, for hydrogen charged samples, nitrogen was found to be significantly less in the passive film. In Electrochemical impedance spectroscopy (EIS) measurement, the decrease in semi-circle radius of Nyquist plot, and the polarization resistance, R_P associated with the resistance of the passive film was observed with hydrogen, indicating that hydrogen decreased the stability of the passive film. The present investigation indicated that precharged hydrogen deteriorated the passive film stability and pitting corrosion resistance in these alloys, and the increase in nitrogen content of the alloy offsets the deleterious effect of precharged hydrogen.     

Effects of water vapour on isothermal oxidation of chromia-forming alloys in Ar/O2 and Ar/H2 atmospheres

Fe9Cr, Fe17Cr and Fe25Cr alloys were subjected to isothermal oxidation in Ar/O₂ and Ar/H₂ atmosphere at 700 °C as high temperature corrosion for 48 h. Oxidation weight change measurement showed increasing Cr content reduced the oxidation rate. The oxidized Cr alloys were analysed using SEM, TEM and XRD. The addition of water vapour accelerates the onset of breakaway oxidation kinetics for Fe9Cr. The presence of water vapour promotes internal oxidation of Cr within Fe9Cr. For Fe17Cr and Fe25Cr, the water vapour effect is not significant due to the large Cr reservoir due to continue growth of Cr₂O₃.     

Effects of electrochemical hydrogenation of Zr-based alloys with high glass-forming ability

Zr–(Ti)–Cu–Al–Ni metallic glasses exhibit a high thermal stability corresponding to a wide undercooled liquid region. Depending on their composition, the formation of metastable intermediate phases, e.g. a quasicrystalline phase is possible. The combination of early and late transition metals makes these alloys very interesting regarding their interaction with hydrogen. Amorphous Zr₅₅Cu₃₀Al₁₀Ni₅, Zr₆₅Cu_17.5Al_7.5Ni₁₀ and Zr₅₉Ti₃Cu₂₀Al₁₀Ni₈ ribbons were prepared by melt spinning and their microstructure and thermal behaviour was checked by X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. The cathodic reactivity of alloy samples at different microstructural states and after pre-etching in 1 vol.-% HF was investigated in 0.1 M NaOH by applying potentiodynamic polarisation techniques. Galvanostatically hydrogenated samples were characterised by XRD, DSC, TEM and thermal desorption analysis (TDA). For amorphous Zr₅₉Ti₃Cu₂₀Al₁₀Ni₈ samples an increase in electrochemical surface capacity by two orders of magnitude is observed after pre-etching. Compared to the quasicrystalline and crystalline alloy, the hydrogen reduction takes place at significantly lower overpotentials. Zr-based alloys cathodically absorb hydrogen up to H/M=1.65 while keeping the amorphous structure. Already small amounts of hydrogen cause a significant decrease of the thermal stability and changes in the crystallisation sequence. The hydrogen desorption is a two-stage process: (T<623 K) hydrogen desorption from high interstitial-site energy levels and (T>623 K) zirconium hydride formation and subsequent transformation under hydrogen effusion. Hydrogen suppresses the oxygen-triggered formation of metastable phases upon heating and supports primary copper segregation. At very high H/M ratios, severe zirconium hydride formation causes the crystallisation of new compounds.     

Optical reflection spectroscopy of thick corrosion layers on 304 stainless steel

Corrosion resistant structural materials of both iron and nickel based alloys are used in the electric power industry for the construction of the coolant loops of both conventional and nuclear power generating stations. These materials, in the presence of high temperature (e.g. 287 °C), high pH (e.g. 10.0 at 20 °C) water with dissolved hydrogen will oxidize and form corrosion films that are double metal oxides (or spinels) of the form AB₂O₄. This work describes optical reflectivity techniques that have been developed to study the growth of these films in situ. The optical technique uses a dual-beam specular reflection spectrometer to measure the spectrum of reflected light in small angle (i.e. <15°) scatter. The reflection spectra are then calibrated using a set of corrosion coupons with corrosion films that are well known. Results are compared with models based on multilayer reflection and Mie scattering from a particle size distribution. Surface roughness is found to be the dominant cause of reduced reflection as the films grow.     

Corrosion behaviour of magnesium/aluminium alloys in 3.5 wt.% NaCl

Corrosion behaviour of commercial magnesium/aluminium alloys (AZ31, AZ80 and AZ91D) was investigated by electrochemical and gravimetric tests in 3.5 wt.% NaCl at 25 °C. Corrosion products were analysed by scanning electron microscopy, energy dispersive X-ray analysis and low-angle X-ray diffraction. Corrosion damage was mainly caused by formation of a Mg(OH)₂ corrosion layer. AZ80 and AZ91D alloys revealed the highest corrosion resistance. The relatively fine β-phase (Mg₁₇Al₁₂) network and the aluminium enrichment produced on the corroded surface were the key factors limiting progression of the corrosion attack. Preferential attack was located at the matrix/β-phase and matrix/MnAl intermetallic compounds interfaces.     

Synergistic corrosion behavior of coated Ti60 alloys with NaCl deposit in moist air at elevated temperature

In the present paper, the corrosion behavior of Ti60 alloys with an aluminide, TiAlCr, and enamel coatings in moist air containing NaCl vapor at 700-800 °C were studied. The results showed that the TiAlCr and aluminide coatings failed to protect the substrate from corrosion due to the cyclic formation of volatile products during corrosion at 800 °C. However, an uneven continuous protective Al₂O₃ scale could form on the aluminide coating during corrosion at 700 °C. And the enamel coating could protect Ti60 from corrosion due to its high thermochemical stability and matched thermal expansion coefficient with substrates of Ti-base alloys during corrosion.     

Hydrogen sorption properties of Ti–Cr alloys synthesized by ball milling and cold rolling

Three Ti–Cr alloys with nominal compositions of TiCr_x (x = 2, 1.8 and 1.5) were synthesized by cold rolling and ball milling of as-cast ingots, and their microstructures and hydrogenation properties were studied. X-ray diffraction showed that TiCr_x transformed from a mixture of C14 and C15 Laves phases to a metastable BCC phase after 5 h of ball milling under argon. Cold rolling did not lead to the formation of a metastable BCC phase but only to the reduction of TiCr_x size particles under 20 nm. Surprisingly, the hydrogen absorption/desorption curves of cold rolled and ball milled samples at 323 K were quite similar. This result proves that hydrogen storage properties do not depend only on microstructure and that cold rolling could be an interesting method to synthesize hydrogen storage materials.     

Ni- and Cu-free Ti-based metallic glasses with potential biomedical application

In the present work Ti–Fe–Si and Ti–Fe–Si–X (X = Zr, Pd, Ge) glassy alloys are discussed as potential biomedical materials. Depending on composition and experimental conditions these alloys possess glassy, quasicrystalline or crystalline structure. The glassy state and crystallization behavior of the melt spun ribbons were studied by X-ray diffraction (XRD), transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and the Hank's solution was used as simulated body fluid for corrosion tests. Ternary Ti–Fe–Si alloys near the Ti₆₅Fe₃₀Si₅ eutectic point were prone to form quasicrystals if the cooling rate was not high enough to retain amorphous structure. The compositions on the steeper side of the eutectic point could be vitrified. The results indicate that small additions of Zr can have a positive effect on glass formation, while additions of Ge, Pd may have a detrimental effect by promoting crystallization. Ti–Fe–Si and Ti–Fe–Si–Zr alloys exhibited high corrosion properties, superior to that of pure Ti and most of Ti-based glassy alloys reported in the literature. Being free of Ni and Cu this group of alloys may be considered for possible biomedical application.     

Electrochemical characteristics of zirconium oxide treated Type 304 stainless steels of different surface oxide structures in high temperature water

Electrochemical polarization analyses were conducted to investigate the impact of different oxide structures on corrosion resistance of ZrO₂-treated Type 304 stainless steel specimens in high temperature water. All specimens were pre-oxidized in high temperature water containing either 300 ppb dissolved oxygen or 50 ppb dissolved hydrogen, followed by a hydrothermal deposition treatment with ZrO₂ nanoparticles. Experimental results revealed that the corrosion potentials and corrosion current densities on the ZrO₂-treated specimens were lower than those on the untreated ones, and the decreases in these parameters were more distinct on the ZrO₂-treated specimens with oxides developed under dissolved oxygen condition.     

Isothermal oxidation behaviour of Nb-W-Cr Alloys

A study of the effect of Cr content on the microstructure and isothermal oxidation behaviour of four alloys from the Nb-Cr-W system has been performed. Selection of specific alloy compositions has been based on the ternary isothermal sections. Oxidation experiments were conducted in air at 900 and 1300 °C for 24 h under isothermal conditions. Weight gain per unit area as function of the temperature has been used to evaluate the oxidation resistance. The phases present in the alloys and the oxide scales were characterized by XRD, SEM, and EDS. Microstructure consists of Nb solid solution and NbCr₂, Laves phase. The oxidation kinetics follows a parabolic behaviour at 1300 °C; the addition of 30% Cr resulted in the significant reduction of the parabolic oxidation rate. At 900 °C, alloys with higher Cr content exhibit higher oxidation rates in comparison to alloys with lower Cr content. The oxidation products are a mixture of CrNbO₄ and Nb₂O₅ and the amount of each oxide present in the mixture is related to the intermetallic phase content and the oxidation temperature. The characterization results delineate the effect of the Cr content on the oxidation mechanisms of these alloys that represent a promising base for high-temperature alloy development.     

Effects of hydrogen on the anodic behavior of Alloy 690 at 60 °C

Polarization and electrochemical impedance spectroscopy (EIS) measurements, Mott-Schottky (M-S) analysis and X-ray photoelectron spectroscopy (XPS) were used to investigate the effects of hydrogen on the anodic behavior of a one-dimensionally (1D) 25% cold worked (CW) Alloy 690 thermal treated (TT) in a boric acid and sodium sulphate solution at 60 °C. The pre-hydrogen-charged specimen exhibited a higher anodic current than that of the uncharged specimen below the transpassive potential. The charged hydrogen can be trapped in the metal. Electrochemical impedance spectroscopy (EIS) showed that the resistance capacitance loop of the hydrogen-charged specimen was significantly smaller than that of the uncharged specimen. Mott-Schottky analyses indicated that the passive film formed on Alloy 690 at −0.2 V_SCE was an n-type semiconductor, with a p-n hetero-junction at 0.2 V_SCE. Charged hydrogen increased the carrier density and the thickness of the passive film both at −0.2 V_SCE and 0.2 V_SCE. The Ni/Cr ratio in the surface film decreased after hydrogen charging, indicating that charged hydrogen could enhance the oxide film growth by increasing the OH⁻ (O²⁻) concentrations through its reaction with vacancies.     

An exploratory study of the corrosion of Mg alloys during interrupted salt spray testing

A first systematic investigation was carried out to understand the corrosion of common Mg alloys (Pure Mg, AZ31, AZ91, AM30, AM60, ZE41) exposed to interrupted salt spray. The corrosion rates were also evaluated for these alloys immersed in 3 wt.% NaCl by measuring hydrogen evolution and an attempt was made to estimate the corrosion rate using Tafel extrapolation of the cathodic branch of the polarisation curve. The corrosion of these alloys immersed in the 3 wt.% NaCl solution was controlled by the following factors: (i) the composition of the alpha-Mg matrix, (ii) the volume fraction of second phase and (iii) the electrochemical properties of the second phase. The Mg(OH)₂ surface film on Mg alloys is probably formed by a precipitation reaction when the Mg²⁺ ion concentration at the corroding surface exceeds the solubility limit. Improvements are suggested to the interrupted salt spray testing; the ideal test cycle would be a salt spray of duration X min followed by a drying period of (120-X) min. Appropriate apparatus changes are suggested to achieve 20% RH rapidly within several minutes after the end of the salt spray and to maintain the RH at this level during the non-spray part of the cycle. The electrochemical measurements of the corrosion rate, based on the “corrosion current” at the free corrosion potential, did not agree with direct measurements evaluated from the evolved hydrogen, in agreement with other observations for Mg.     

The Oxidation of Ferritic Stainless Steels in Simulated Solid-Oxide Fuel-Cell Atmospheres

The cyclic oxidation of a variety of chromia-forming ferritic stainless steels has been studied in the temperature range 700–900°C in atmospheres relevant to solid-oxide fuel-cell operation. The most detrimental environment at 800°C and 900°C was found to be air with 10% water vapor. This resulted in excessive oxide spallation or rapid scale growth. Impurities in the alloys, particularly Al and Si, were found to have a significant effect on the oxidation behavior. Oxide growth was slow at 700°C but the higher-Cr-content alloys were observed to form sigma-phase at this temperature. The sigma phase formation was accelerated by higher silicon contents, and remarkably, by the presence of water vapor in the exposure environment. Alloys containing Mn were observed to form an outer layer of MnCr₂O₄ over the chromia scale. The potential for this overlayer to suppress reactive evaporation of the chromia scales has been analyzed.     

Electrochemical response of AlNiLa amorphous and devitrified alloys

The electrochemical response of Al_94−xNi₆La_x alloys (x = 4, 5, 6, 7) after different stages of devitrification was studied in 0.05 M Na₂SO₄ as well as in different concentrations [0.001 M, 0.01 M and 0.1 M] NaCl solutions. Complementary crystallization studies were carried out to elucidate the composition dependent phase evolution in these alloys. It was observed that the primary crystallization did not cause any deterioration in the corrosion resistance of the alloys as compared to the amorphous alloys. In the case of Al₈₇Ni₆La₇, there was actually an improvement in the passivating ability in benign media. The various primary crystalline phases in the different alloys investigated did not cause different electrochemical responses. However, the onset of secondary crystallization caused a reduction in the corrosion resistance in the NaCl media through a loss in passivating ability of all the alloys. This is due to increased galvanic activity as well as the loss of the amorphous phase.