The effects of peak temperature and cooling rate on the susceptibility to intergranular corrosion of alloy 690 by laser beam and gas tungsten arc welding

This study investigates the effects of welding method, peak temperature, and cooling rate on the susceptibility to intergranular corrosion of alloy 690 weldments. The experimental results reveal that the laser beam welding process with cooling rate of around 212.6 °C/s can be produced with much less mass loss and a lower value of maximum reactivation current density/maximum anodic current density than with the gas tungsten arc welding process, where cooling rate is at around 17-20.6 °C/s. This is because the very rapid cooling rate during welding leads to an insufficient exposure time of around 2.1 s within the chromium (Cr)-carbide precipitation temperature range, suppressing Cr-carbide precipitation and Cr-depletion along grain boundaries in the weld decay region of the heat affected zone.     

An investigation on oxidation wear mechanisms of Ti-46Al-7Nb-0.7Cr-0.1Si-0.2Ni intermetallic-based alloys

The results of investigation on oxidation wear mechanism of Ti-46Al-7Nb-0.7Cr-0.1Si-0.2Ni-based intermetallic alloy are presented. Oxidation was carried out in air at temperatures: 900 °C, 925 °C and 975 °C taking into account the micro-geometry of surfaces being heated and oxidized. It was determined that the rise of surface roughness to Ra = 5.8 μm definitely reduces scale of chipping in higher temperature (975 °C). Investigation of the material structure of the specimen and chemical composition of oxidation products was performed. Possibilities of an increase in the heat resistance of the tested alloy by means of application of high roughness surfaces were shown.     

Kinetics inversion in isothermal oxidation of uncoated WC-based carbides between 450 and 800 °C

Oxidation of uncoated WC-based carbides (hard metals) has been investigated between 450 and 800 °C. An anomalous change in the oxidation behaviour has been found in the temperature range between 528 and 630 °C. Instead of the normal increase of the oxidation rates when the temperature increases, a kinetics inversion is produced: an anomalous decrease of the oxidation rates has been observed in this temperature range. Furthermore, this anomalous behaviour produces an important change in the activation energy. At temperatures below the anomaly (below 528 °C), an activation energy of 119 ± 8 kJ/mol has been found, whereas above the anomaly (above 630 °C), an activation energy of 208 ± 8 kJ/mol has been found. This anomalous temperature dependence of the oxidation behaviour can be related to a higher amount of complex oxide CoWO₄ formed on the oxide scale, and to the effect that this oxide produces in the oxidation kinetics of the other oxides.     

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.     

Study of the corrosion and microstructure with annealing conditions of a β-quenched HANA-4 alloy

The variation of microstructure and corrosion characteristics with the applied annealing conditions of a HANA-4 (Zr-1.5Nb-0.4Sn-0.2Fe-0.1Cr) alloy were studied by utilizing transmission electron microscopy and a corrosion test at 360 °C in a water environment. The samples were annealed at temperature ranges from 540 to 660 °C up to 16 h after β quenching at 1050 °C. The corrosion behaviour with the annealing conditions was divided into two groups following the second phase characteristics. The suitable annealing temperature to obtain good corrosion resistance in the HANA-4 alloy ranged from 570 to 600 °C.     

Impact of temperature excursion on stress corrosion cracking of stainless steels in chloride solution

The impact of a temperature excursion on the subsequent stress corrosion crack growth at the normal operating temperature has been investigated for 321 stainless steel (UNS32100) and 316L stainless steel (UNS31603) using precracked compact tension specimens. Although the data are preliminary the indication is that once crack growth has initiated in 321 SS at the elevated temperature, 130 °C in this study, the crack growth may be sustained at the lower temperature (40 °C), at least over the exposure time of about 700 h. However, the growth rate of 316L SS at the lower temperature was significantly lower than for 321 SS and tended to zero after 2000 h. For the 316 SS a temperature transient should not impact on structural integrity, provided it is short in duration.     

On the influence of water vapour on the oxidation behaviour of pure Ti

The influence of water vapour on the oxidation behaviour of pure titanium in air at 700 °C and 900 °C was evaluated through thermogravimetric tests. The influence of water vapour during oxidation at 700 °C was small, resulting in slightly higher mass gains in the moist atmosphere. At 900 °C, a multilayered rutil scale was developed from the earliest stages of oxidation. In this case, presence of water resulted in a considerable decrease of the oxidation kinetics. It is proposed that water promotes densification of the innermost part of the rutile layer by increasing rutile creep rate.     

High temperature corrosion properties of ionic liquids

The corrosion behaviour of several metals and metal alloys (copper, nickel, AISI 1018 steel, brass, Inconel 600) exposed to a typical ionic liquid, the 1-butyl-3-methyl-imidazolium bis-(trifluoromethanesulfonyl) imide, ([C₄mim][Tf₂N]), has been investigated by electrochemical and weight-loss methods. Corrosion current densities have been determined by extrapolation from Tafel plots and by polarization resistance measurements and 48 h immersion tests were performed at 150, 250, 275 and 325 °C. Room temperature results show low corrosion current densities (0.1-1.2 μA/cm²) for all the metals and alloys investigated. At 70 °C, the corrosion current for copper dramatically increases showing a strongly dependence on temperature. At 150 °C copper shows significant weight-loss while nickel, AISI 1018, brass and Inconel do not. At higher temperatures (?275 °C), the copper sample crumbles and localized corrosion occurs for the other metals and alloys.     

The structure and high temperature corrosion performance of medium-thickness aluminide coatings on nickel-based superalloy GTD-111

Simple and Si-modified aluminide coatings having medium-thickness (40-60 μm) have been applied on the superalloy GTD-111 by a slurry technique. Hot corrosion and cyclic oxidation performance of the uncoated and the coated superalloy were investigated by exposing samples to a molten film of Na₂SO₄-40 %wt NaVO₃-10%wt NaCl at 780 °C and 1 h cyclic oxidation at 1100 °C in air, respectively. The presence of silicon in the aluminide structure increased the oxidation resistance by a factor of 1.7 times. In addition, a SiO₂-containing scale, which formed on the Si-containing coating surface, was stable during of the hot corrosion testing.     

Temperature and water vapour effects on the cyclic oxidation behaviour of Fe-Cr alloys

Binary Fe-Cr alloys were subjected to cyclic oxidation at 600, 700 and 950 °C in flowing gases of Ar-20O₂ and Ar-20O₂-5H₂O (vol.%). The minimum chromium concentration required to achieve protective scale growth decreased as temperature increased from 600 to 700 °C. This change is attributed to faster chromium diffusion at higher temperature. Conversely, this minimum chromium level increased when the temperature was raised from 700 to 950 °C. This is attributed to faster scale growth, leading to its rapid mechanical failure, along with formation of slow-diffusing austenite. Water vapour accelerated scaling, leading to a need for higher chromium concentrations to resist breakaway oxidation.     

The influence of relative humidity and temperature on the acetic acid vapour-induced atmospheric corrosion of lead

The acetic-acid induced atmospheric corrosion of lead was studied at 22.0 °C and 30-95% RH and at 4 °C and 95% RH. The samples were exposed to synthetic air with careful control of relative humidity, temperature, acetic acid concentration (170 ppb) and flow conditions. Reference exposures were carried out in clean humid air. Samples were analysed by gravimetry, ion chromatography, quantitative carbonate analysis, ESEM and XRD. Traces of acetic acid vapour strongly accelerate the atmospheric corrosion of lead. The corrosion rate is only weakly dependent on relative humidity in the range 95-50% RH. The accumulated amount of acetate is independent of RH in the range 95-40%. Lead corrosion in humid air in the presence of acetic acid vapour exhibits a negative correlation with temperature. The crystalline corrosion products formed on lead in the presence of acetic acid vapour were lead acetate oxide hydrate (Pb(CH₃COO)₂ · 2PbO · H₂O) and massicot (β-PbO) together with plumbonacrite (Pb₁₀O(CO₃)₆(OH)₆) or hydrocerussite (Pb₃(CO₃)₂(OH)₂). The transformation of lead acetate oxide hydrate into hydrocerussite and vice versa was also studied. The mechanism of corrosion is addressed, and the implications of this study for combating the corrosion of lead organ pipes in historical organs are discussed.     

In-situ measurements by impedance spectroscopy of highly resistive α-alumina

In-situ impedance spectroscopy has been used for characterisation of oxides at elevated temperatures. However, for highly resistive oxides, the influence of electrode contact and leakage currents due to gas phase and surface conduction needs to be taken into account. In this study, IS measurements of pure and dense α-alumina (α-Al₂O₃) samples were performed in the temperature range 400-1000 °C with different types of electrode contact, in air and in nitrogen. The results show that above 700 °C the influence is negligible, whereas at lower temperatures the surface leakage current was substantial, and a so-called guard electrode recommendable.     

Corrosion behavior of iron-based alloys in the LiBr + ethylene glycol + H2O mixture

The corrosion resistance of 1018 carbon steel, 304 and 316 type stainless steels in the LiBr (55 wt.%) + ethylene glycol + H₂O mixture at 25, 50 and 80 °C has been studied using electrochemical techniques which included potentiodynamic polarization curves, electrochemical noise and electrochemical impedance spectroscopy techniques. Results showed that, at all tested temperature, the three steels exhibited an active-passive behavior. Carbon steel showed the highest corrosion rate, since both the passive and corrosion current density values were between two and four orders of magnitude higher than those found for both stainless steels. Similarly, the most active pitting potential values was for 1018 carbon steel. For 1018 carbon steel, the corrosion process was under a mixed diffusion and charge transfer at 25 °C, whereas at 50 and 80 °C a pure diffusion controlled process could be observed. For 316 type stainless steel, at 25 and 50 °C a species adsorption controlled process was observed, whereas at 80 °C a diffusion controlled mechanism was present. Additionally, at 25 °C, the three steels were more susceptible to uniform type of corrosion, whereas at 50 and 80 °C they were very susceptible to localized type of corrosion.     

Temperature effect on mild steel corrosion in acid media in presence of azoles

The inhibiting properties of four azoles (indole, benzimidazole, benzotriazole, benzothiazole) were investigated in case of mild steel corrosion in 1 M HCl. The effect of temperature was followed. Impedance spectroscopy, polarization resistance, gravimetric and polarization curves methods were applied. An adequate structural model of the interface in inhibitors presence is offered and the values of the corresponding parameters are calculated. The apparent activation energy of the process taking place in an inhibitor presence was determined on the ground of five temperature values in the range from 20 °C to 60 °C using the data obtained by two independent methods. Comparative investigations were carried out in 1 M H₂SO₄ aiming to elucidate the effect of the acid’s anion. The generalization of the data obtained provided to make conclusions concerning the mechanism of the inhibitors’ adsorption.     

Superheater fireside corrosion mechanisms in MSWI plants: Lab‐scale study and on‐site results

Combustion of municipal waste generates highly corrosive gases (HCl, SO₂, NaCl, KCl, and heavy metals chlorides) and ashes containing alkaline chlorides and sulfates. Currently, corrosion phenomena are particularly observed on superheater's tubes. Corrosion rates depend mainly on installation design, operating conditions i.e., gas and steam temperature and velocity of the flue gas containing ashes. This paper presents the results obtained using an innovative laboratory‐scale corrosion unit, which simulates MSWI boilers conditions characterized by a temperature gradient at the metal tube in the presence of corrosive gases and ashes. The presented corrosion tests were realized on carbon steel at fixed metal temperature (400 °C). The influence of the flue gas temperature, synthetic ashes composition, and flue gas flow pattern were investigated. After corrosion test, cross sections of tube samples were characterized to evaluate thickness loss and estimate corrosion rate while the elements present in corrosion layers were analyzed. Corrosion tests were carried out twice in order to validate the accuracy and reproducibility of results. First results highlight the key role of molten phase related to the ash composition and flue gas temperature as well as the deposit morphology, related to the flue gas flow pattern, on the mechanisms and corrosion rates.     

Corrosion loss of mild steel in high temperature hard freshwater

This paper presents an interpretation of corrosion loss data for mild steel coupons exposed for up to 2.6 years to freshwater under a range of dissolved oxygen levels and temperatures 50-60 °C. The water total alkalinity was 0.5-0.6 mmol/l and pH 8.7-9.2. It is shown herein that the data is consistent with the early stages of the corrosion loss model proposed earlier for steel exposed to seawater, brackish water or freshwaters in the usual environmental range of 0-30 °C. The new data was found to be consistent with the effect of dissolved oxygen and the effect of water hardness on the model.     

Erosion of oxide scales formed on Cr3C2-NiCr thermal spray coatings

Cr₃C₂-NiCr thermal spray coatings are extensively used to mitigate high temperature erosive wear in fluidised bed combustors and power generation/transport turbines. The aim of this work was to characterise the variation in oxide erosion response as a function of the Cr₃C₂-NiCr coating microstructure. Erosion was carried out at 700 °C and 800 °C with erodent impact velocities of 225-235 m/s. The erosion behaviour of the oxide scales formed on these coatings, was influenced by the coating microstructure and erosion temperature. Development of the carbide microstructure with extended heat treatment lead to variations in the erosion-corrosion response of the Cr₃C₂-NiCr coatings.     

Hot corrosion behaviour of Yb2Zr2O7 ceramic coated with V2O5 at temperatures of 600-800 °C in air

To better understand the hot corrosion behaviour of Yb₂Zr₂O₇ ceramic in molten V₂O₅, hot corrosion experiments were performed in a temperature range of 600-800 °C in air. Different reaction products of ZrV₂O₇, YbVO₄ and m-ZrO₂ were identified depending upon the hot corrosion conditions, for example, ZrV₂O₇ and YbVO₄ at 600 °C for 2 h and 8 h; ZrV₂O₇, m-ZrO₂ and YbVO₄ at 700 °C for 2 h; m-ZrO₂ and YbVO₄ either at 800 °C for 2 h or at 700-800 °C for 8 h. The hot corrosion reaction mechanisms were further discussed based on the thermal instability of ZrV₂O₇ at elevated temperatures.     

Critical pitting temperature for Type 254 SMO stainless steel in chloride solutions

The variation with time of the open circuit potential of high molybdenum containing stainless steel (Type 254 SMO) was measured in 4% sodium chloride solution in the temperatures range 30-100 °C. The plot of steady state potentials as function of temperature showed an inflection at 50 °C, attributed to the decrease of oxygen solubility in test solution above 50 °C. Potentiodynamic cycling anodic polarization technique was used to determine the critical pitting potential (E_pit) and the critical protection potential (E_prot) of the steel in 4-30% NaCl solutions at temperatures between 30 and 100 °C. By plotting the two values versus solution temperature, the corresponding critical pitting (CPT) and the critical protection (CPrT) temperatures were determined. Both parameters decreased with increasing chloride content. Above the CPT, E_pit and E_prot decreased linearly with log[Cl⁻]. The addition of bromide ions to the solution shifted both E_pit and E_prot towards positive values. In 4% NaCl, E_pit increased linearly with pH in the range 1-10. The combined effect of chloride ion concentration and pH on the morphology of the pits was examined by scanning electron microscopy (SEM) following potentiodynamic cycling anodic polarization.     

Oxidation kinetics of high strength low alloy steels at elevated temperatures

High strength low alloy (HSLA) steels are candidate Rockbolt materials for use as underground roof supports at Yucca Mountain nuclear waste repository. Oxidation kinetics of International Rollforms Split Set Friction Rock Stabilizers (SS46), and Swellex Mn24 steels have been determined by temperature modulated thermogravimetry at temperatures ranging between 600 and 900 °C in pure oxygen atmosphere for 100 hr. The imposed sinusoidal temperature modulations (±5 °C for a period of 1 cycle per 200 s) on the isothermal temperature did not have any noticeable effect on the weight gain characteristics during oxidation. Weight gain data on the steels indicate two distinct regions with different oxidation profiles, where a definite change in rates of oxidation is observed: a first oxidation regime where the steels followed a rate law y = kt^0.40-0.63 (changing index of rate law depending upon steel and temperature) and a second stage oxidation regime that follows the parabolic law. The results of characterization of the oxide films using SEM/EDAX, X-ray diffraction and Synchrotron white beam X-ray microdiffraction are presented. The oxidation data of the steels presented here is expected to be useful for characterizing those steels for use in underground rock bolt system and as roof support for the DOE proposed Yucca Mountain Nuclear Waste Repository. To the best of our knowledge this is the first time thermogravimetric studies of this kind have been done on these steels.