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.     

Temperature dependence of oxide scale formation on high-Cr ferritic steels in Ar–H2–H2O

Four high chromium ferritic steels were oxidized in Ar/H₂/H₂O at temperatures between 500 and 900 °C. Polished specimens of all steels formed iron-rich oxides at temperatures below 600 °C, whereas increasing the temperature resulted in local formation of protective chromia scales. As the temperature was raised further, the specimens were totally covered with chromia scales. For the higher chromium steels this was also observed at 900 °C but not for the steel with a chromium content of 16%. The temperature dependence of the oxidation rates is governed by the competing diffusion processes in the alloy and the growing scales.     

Non-steady state carburisation of martensitic 9–12%Cr steels in CO2 rich gases at 550 °C

Two martensitic steels were reacted with Ar–50%CO₂ at 550 °C for exposure times up to 150 h. Microstructural analyses and glow discharge optical emission spectroscopy revealed that internal carburisation of both steels beneath external oxide scales occurred, in spite of the very low equilibrium carbon activity of the atmosphere. The carbon concentration at the metal–scale interface increased throughout the reaction and the carbon uptake varied approximately linear with time. A model is proposed to describe the non steady-state carburisation kinetics whereby the usual diffusion equation describing carbon movement into the alloy is modified to account for loss of carbon as precipitated carbides.     

Corrosion characteristics of LiBH4 film exposed to a CO2/H2O/O2/N2 mixture

LiBH₄ films were prepared by pulsed laser deposition using a LiB target in a background pressure of hydrogen. The corrosion characteristics of LiBH₄ films were measured by exposing them to a gas mixture of CO₂/H₂O/O₂/N₂ at ambient temperature for 1–24 h. Scanning electron microscopy images show some cracks on the surface of corrosion films, which could act as easy paths for H₂O and CO₂ to further react with Li⁺ and B³⁺. The X-ray photoelectron spectroscopy results and theoretical analysis show that LiBH₄ tends to react with H₂O and CO₂ to form Li₂B₄O₇, Li₂CO₃ and LiOH during the corrosion process.     

Oxidation of a chromia-forming nickel base alloy at high temperature in mixed diluted CO/H2O atmospheres

Corrosion of a chromia-forming nickel base alloy, Haynes 230^®, has been investigated under impure helium containing a few Pa of CO and H₂O at 900 °C. It has been found that this alloy reacts simultaneously with CO and H₂O. Oxidation by CO has been revealed to occur mainly in the first hours. CO diffuses through the scale via short-circuit pathways and oxidizes Al, Cr and Si at the oxide/metal interface. Kinetics of CO oxidation has been investigated and several rate limiting steps are proposed. In the long term, H₂O is the major oxidant of chromia-forming nickel base alloys in impure helium.     

Corrosion of austenitic and ferritic-martensitic steels exposed to supercritical carbon dioxide

Supercritical carbon dioxide (S-CO₂) is a potential coolant for advanced nuclear reactors. The corrosion behavior of austenitic steels (alloys 800H and AL-6XN) and ferritic-martensitic (FM) steels (F91 and HCM12A) exposed to S-CO₂ at 650 °C and 20.7 MPa is presented in this work. Oxidation was identified as the primary corrosion phenomenon. Alloy 800H had oxidation resistance superior to AL-6XN. The FM steels were less corrosion resistant than the austenitic steels, which developed thick oxide scales that tended to exfoliate. Detailed microstructure characterization suggests the effect of alloying elements such as Al, Mo, Cr, and Ni on the oxidation of the steels.     

The influence of CO2, AlCl3 · 6H2O, MgCl2 · 6H2O, Na2SO4 and NaCl on the atmospheric corrosion of aluminum

The influence of salt deposits on the atmospheric corrosion of high purity Al (99.999%) was studied in the laboratory. Four chloride and sulfate-containing salts, NaCl, Na₂SO₄, AlCl₃ · 6H₂O and MgCl₂ · 6H₂O were investigated. The samples were exposed to purified humid air with careful control of the relative humidity (95%), temperature (22.0 °C), and air flow. The concentration of CO₂ was 350 ppm or <1 ppm and the exposure time was four weeks. Under the experimental conditions all four salts formed aqueous solutions on the metal surface. Mass gain and metal loss results are reported. The corroded surfaces were studied by ESEM, OM, AES and FEG/SEM equipped with EDX. The corrosion products were analyzed by gravimetry, IC and grazing incidence XRD. In the absence of CO₂, the corrosivity of the chloride salts studied increases in the order MgCl₂ · 6H₂O < AlCl₃ · 6H₂O < NaCl. Sodium chloride is very corrosive in this environment because the sodium ion supports the development of high pH in the cathodic areas, resulting in alkaline dissolution of the alumina passive film and rapid general corrosion. The low corrosivity of MgCl₂ · 6H₂O is explained by the inability of Mg²⁺ to support high pH values in the cathodic areas. In the presence of carbon dioxide, the corrosion induced by the salts studied exhibit similar rates. Carbon dioxide strongly inhibits aluminum corrosion in the presence of AlCl₃ · 6H₂O and especially, NaCl, while it is slightly corrosive in the presence of MgCl₂ · 6H₂O. The corrosion effects of CO₂ are explained in terms of its acidic properties and by the precipitation of carbonates. In the absence of CO₂, Na₂SO₄ is less corrosive than NaCl. This is explained by the lower solubility of aluminum hydroxy sulfates in comparison to the chlorides. The average corrosion rate in the presence of CO₂ is the same for both salts. The main difference is that sulfate is less efficient than chloride in causing pitting of aluminum in neutral or acidic media.     

Steam oxidation resistance of Ni-aluminide/Fe-aluminide duplex coatings formed on creep resistant ferritic steels by low temperature pack cementation process

Steam oxidation resistance and thermal stability were studied at 650 °C for a coating with an outer Ni₂Al₃ layer and an inner Fe₂Al₅ layer formed on P92 steel surface. The parabolic rate law of oxidation was obeyed only in less than 2000 h with positive deviations occurring at longer oxidation times. The outer layer of the coating was transformed to NiAl during oxidation, but it remained stable once it was formed. The mechanisms for the enhanced thermal stability were discussed and a simple approach to enhancing the lifetime of the coating was proposed.     

Corrosion of Ti3AlC2 at 800–1100 °C in Ar–0.2% SO2 gas atmosphere

Ti₃AlC₂ was corroded between 800 and 1100 °C in an Ar–0.2% SO₂ gas atmosphere according to the equation: Ti₃AlC₂ + O₂ → rutile-TiO₂ + α-Al₂O₃ + (CO or CO₂). The scales that formed on the Ti₃AlC₂ were thin and rich in α-Al₂O₃, whose growth rate was exceedingly slow. The TiO₂ was present either as the outermost surface scale or a mixture inside the α-Al₂O₃-rich scale. In the Ti₃AlC₂, the activity and diffusivity of Ti were low, whereas those of Al were high. This was the main reason for the superior corrosion resistance of Ti₃AlC₂ over TiAl.     

Simulation and experimental approach to CVD-FBR aluminide coatings on ferritic steels under steam oxidation

The ferritic steels used to produce structural components for steam turbines are susceptible to strong corrosion and creep damage due to the extreme working conditions pushed to increase the process efficiency and to reduce pollutants release. The response of aluminide coatings on the P-92 ferritic steel, deposited by CVD-FBR, during oxidation in a simulated steam environment was studied. The analyses were performed at 650 °C in order to simulate the working conditions of a steam turbine, and 800 °C in order to produce a critical accelerated oxidation test.The Thermo-Calc software was used to predict the different solid phases that could be generated during the oxidation process, in both, coated and uncoated samples. In order to validate the thermodynamic results, the oxides scales produced during steam tests were characterized by different techniques such as XRD, SEM and EDS. The preliminary results obtained are discussed in the present work.     

Oxidation, carburisation and metal dusting of 304 stainless steel in CO/CO2 and CO/H2/H2O gas mixtures

The oxidation and carburisation behaviour of 304 stainless steel was studied during thermal cycling in CO/CO₂ at 700 °C, and also in CO/H₂/H₂O at 680 °C. Thermal cycling caused repeated scale separation which accelerated chromium depletion from the alloy subsurface regions. The CO/CO₂ gas, with a_C=7 and , caused internal precipitation of oxides and carbides, some surface damage, but no dusting. In contrast, the CO/H₂/H₂O gas, with a_C = 19 and caused rapid graphite deposition and metal dusting. This was accompanied by internal oxidation and carburisation. The internal oxide was identified as spinel, which forms in the short term, but not at long reaction time. Its formation produced a significant volume expansion, which disrupted the material and resulted in surface damage in both gas atmospheres. In CO/H₂/H₂O, however, direct graphite deposition and metal disintegration into dust was the main reaction. The very different reaction morphologies produced by the two gas mixtures are discussed in terms of competing gas-alloy reaction steps.     

Platinum-induced oxidation of chromium in O2 at 800 °C

The effects of porous Pt on the oxidation of Cr at 800 °C have been studied with the ¹⁸O-SIMS technique, gas phase analysis and XPS. In oxide areas with Pt a pronounced inward oxygen transport takes place and a substantial oxide growth near the Cr substrate is observed. In oxide grown on areas without Pt the counts of CrO ions in SIMS and the binding energy of O (1s) in XPS depend on the distance from the area with Pt. The experimental observations are believed to be a consequence of a high dissociation efficiency of O₂ on areas with Pt in combination with a high diffusivity of O in external and internal oxide surfaces on areas both with and without Pt.     

Unusual oxidation behaviour of Zr50Cu50 alloy at high temperatures

The oxidation of Zr₅₀Cu₅₀ alloy at 500-700 °C is characterized by preferential oxidation of zirconium, while the excess of copper is accumulated at the alloy-oxide interface forming the Zr₁₄Cu₅₁ phase. The strong reaction at 800 and 850 °C resulted in the total corrosion of the specimens in 21 and 15 h, respectively. The oxidation at elevated temperatures showed an anomalous decrease of the oxygen consumption rate in the temperature range 930-1000 °C, corresponding to the preferentially oriented crystallites of ZrO₂ in the oxide scale at 900 and 1000 °C. The oxide layer consists of ZrO₂ and CuO in the whole temperature interval of the oxidation. The reaction kinetics obeys a parabolic rate law. An activation energy of 92.0 ± 0.3 kJ/mol has been estimated.     

The growth mechanism of CO2 corrosion product films

Carbon steel usually has high corrosion rate in water with dissolved CO₂ however the steel surface can be covered by protective corrosion product film. Therefore the carbon steel can be widely used in oil and gas industry. This paper discusses the structure, electrochemistry performances, initiation and growth of protective corrosion product films. A duplex layer structure of FeCO₃ films was found by electrochemical impedance spectrum (EIS). The physical and chemical situations of solution near carbon steel surface were investigated by ultraviolet spectrophotometry and chronopotentiometry. According to crystallography the growth mechanisms of protective CO₂ corrosion product films were discussed.     

Copper corrosion inhibition in O2-saturated H2SO4 solutions

Corrosion inhibition of copper in O₂-saturated 0.50 M H₂SO₄ solutions by four selected amino acids, namely glycine (Gly), alanine (Ala), valine (Val), or tyrosine (Tyr), was studied using Tafel polarization, linear polarization, impedance, and electrochemical frequency modulation (EFM) at 30 °C. Protection efficiencies of almost 98% and 91% were obtained with 50 mM Tyr and Gly, respectively. On the other hand, Ala and Val reached only about 75%. Corrosion rates determined by the Tafel extrapolation method were in good agreement with those obtained by EFM and an independent chemical (i.e., non-electrochemical) method. The chemical method of confirmation of the corrosion rates involved determination of the dissolved Cu²⁺, using ICP-AES (inductively coupled plasma atomic emission spectrometry) method of chemical analysis. Nyquist plots exhibited a high frequency depressed semicircle followed by a straight line portion (Warburg diffusion tail) in the low-frequency region. The impedance data were interpreted according to two suitable equivalent circuits. The kinetics of dissolved O₂ reduction and hydrogen evolution reactions on copper surface were also studied in O₂-saturated 0.50 M H₂SO₄ solutions using polarization measurements combined with the rotating disc electrode (RDE). The Koutecky-Levich plot indicated that the dissolved O₂ reduction at the copper electrode was an apparent 4-electron process.     

Corrosion behaviour of Y2O3-ZrO2 coatings on IN713LC in a LiCl-Li2O molten salt

Hot corrosion studies were performed on superalloy specimens. The IN713LC superalloy were sprayed with an aluminized NiCrAlY-bond coat and then with an yttria-zirconia top coat. The bare superalloy reveals an obvious weight loss due to spalling of the scales by the rapid scale growth and thermal stress. The top coatings showed a much better hot corrosion resistance in the presence of LiCl-3 wt.% Li₂O molten salt when compared with those of the bare superalloy and the aluminized bond coatings. These coatings have been found to be beneficial for increasing hot corrosion resistance of structural materials for lithium molten salts.     

The role of microstructure in the high temperature oxidation mechanism of Cr3C2-NiCr composite coatings

Composites of Cr₃C₂-NiCr provide superior oxidation resistance to WC-Co composites, which has seen them applied extensively to components subjected to combined high temperature erosion and oxidation. This work characterises the variation in oxidation mechanism of thermally sprayed Cr₃C₂-NiCr composites at 700 °C and 850 °C as a function of heat treatment. Carbide dissolution during spraying increased the Ni alloy Cr concentration, minimising the formation of Ni oxides during oxidation. Compressive growth stresses resulted in ballooning of the oxide over the carbide grains. Carbide nucleation with heat treatment reduced the Ni alloy Cr concentration. The oxidation mechanism of the composite coating changed from being Cr based to that observed for NiCr alloys.     

Anomalous temperature dependence of oxidation kinetics during steam oxidation of ferritic steels in the temperature range 550-650 °C

The oxidation behavior of a number of selected ferritic steels in a simulated steam environment at temperatures between 550 and 650 °C was studied. In the prevailing test gas, some of the studied 9-12% Cr steels tended to exhibit an anomalous temperature dependence of the oxidation behavior. This means, that the oxidation rates do not steadily increase with increasing temperature. At higher temperatures, some of the studied steels tend to form a very thin and protective oxide scale whereas at lower temperature rapidly growing, less-protective oxides are being developed. The anomalous temperature dependence is related to differences in chromium distribution in the inner part of the oxide scale. The effect is observed for steels with intermediate-Cr contents (∼10-12%) whereas steels with either lower or higher Cr contents exhibit an increasing oxidation rate with increasing temperature.     

Numerical simulation of a single corrosion pit in CO2 and acetic acid environments

The present paper bears on a numerical model of steady-state behaviour of a single corrosion pit in CO₂-containing media in the presence of acetic acid. The kinetics of electrochemical reactions occurring on the electrode surface is described by Tafel equations fitted to experimental data obtained on API 5L X65 pipeline steel. Simulation results provide an interesting insight into both the chemistry and electrochemistry inside the active pit. Particular attention is paid to the effect of HAc on the coupling behaviour between the attack base and the outer surrounding surface and to the comparison with experimental data obtained in similar conditions.     

Localized corrosion of magnetite on ferritic–martensitic steels exposed to supercritical water

Magnetite usually forms on ferritic–martensitic (F–M) stainless steels as a protective barrier when the steels are exposed to supercritical water (SCW). However, a novel localized corrosion was observed on magnetite induced by adsorbed Cu₂S. The morphology and chemistry of the localized corrosion were studied by means of scanning electron microscopy, focused-ion beam, energy dispersive X-ray spectroscopy, and Auger electron spectroscopy. The mechanism of the development of the localized corrosion is elucidated in this paper. The presence of Cu₂S or related species should be eliminated for the applications of F–M stainless steels in SCW environment.