Recession behavior of Yb2Si2O7 phase under high speed steam jet at high temperatures

Recession behavior of Yb₂Si₂O₇ phase was examined under high speed steam jet environment between 1300 °C and 1500 °C. Yb₂SiO₅ phase was formed on the bulk surface by the decomposition of Yb₂Si₂O₇ phase and the elimination of silica component at elevated temperatures. The phase ratio of Yb₂SiO₅/Yb₂Si₂O₇ increased up to 1400 °C and then decreased above 1400 °C. The relative intensity of 2 2 0 peak for Yb₂Si₂O₇ phase increased with increasing the temperatures. Fine grains were generated on the bulk surface at 1300 °C. The phase decomposition caused on the grain interior. A porous structure was formed on the bulk surface during the test at 1400 °C. Surface cracks were generated for 1400 °C test sample. A smooth surface was generated on the surface of 1500 °C test sample. The triple points of the grains were bridged with a glassy phase.     

Reaction of modified SUS316 with tellurium under low oxygen potentials

Corrosion behavior of modified SUS316 has been studied under different oxygen potentials with tellurium vapor. Under the condition of oxygen potential ?−500 kJ/mol and tellurium vapor pressure of 8.9 × 10⁻⁴ atm, duplex corrosion layers formed on the surface of specimens, which mainly consisted of chromium telluride, iron telluride and chromium oxide. The thickness of corrosion layer was increased with decreasing the oxygen potential. Furthermore, the amount of oxygen uptake in sample corroded with tellurium vapor was much greater than that in sample oxidized without tellurium vapor.     

Surface spreading of intergranular corrosion on stainless steels

The spreading of intergranular corrosion was investigated using micrometer scale simulations and experimental verifications on sensitized stainless steel [UNS S30400]. The degree of sensitization, presence of a pit, and applied potential all affected spreading. The inputs used in the simulation were obtained from Fe-XCr(X = 10, 12, 14, 16 wt.%)-Mo-Ni alloys representing various grain boundary Cr depletion levels. Corroding grain boundaries and pits triggered corrosion of nearby sensitized boundaries due to Ohmic potential drop. Large connected clusters of corroding grain boundaries formed at high fractions of Cr-depleted grain boundaries. The metallurgical, electrochemical and geometric conditions for this behavior could be forecasted.     

In situ observation of formation and spreading of micro-droplets on magnesium and its alloy under cyclic wet-dry conditions

The formation and spreading of micro-droplets on pure magnesium and its alloy in an atmosphere of water vapor under cyclic wet-dry conditions was studied using an environmental scanning electron microscope (ESEM). The in situ observations indicated that the corrosion product stimulated the formation and spreading of micro-droplets, and the grain boundaries for pure magnesium and β phase for AZ91 magnesium alloy had no influences on this process. The oxygen concentration on the surface generally increased with the wetting time. The corrosion products decreased the corrosion rate.     

Characterization of microstructure and corrosion properties of cold worked Alloy 800

X-ray diffraction studies indicated that cold worked (∼50%) Alloy 800 was austenitic and transmission electron microscopy revealed the presence of a small volume fraction of hexagonal ε-martensite along with deformation bands, high dislocation density and primary TiN particle with a few dislocations within it. The passivity of cold worked alloy was very stable in H₂SO₄ solution but unstable in HCl solution at room temperature. The exposure of cold worked alloy in 673 K steam (initial pH of water was 10.1) for a period of 264 h showed almost nil corrosion rate. Scanning electron microscopy revealed a number of small oxide particles on the surface exposed in steam indicating initiation of oxide formation. Energy dispersive X-ray analyses of the surface containing small oxide particles indicated that the surface composition was similar to bulk composition of the alloy. X-ray photoelectron spectroscopy revealed that the alloy surface exposed in steam contained mixed oxides of iron and chromium as well as elemental form of iron, nickel and chromium.     

Microstructure and corrosion performance of a cold sprayed aluminium coating on AZ91D magnesium alloy

A pure Al coating was deposited on AZ91D magnesium alloy through cold spray (CS) technique. The microstructure of the coating was characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that the grain interfaces and subgrains formed close to the particle/particle boundaries. Electrochemical tests revealed that the cold sprayed pure Al coating had better pitting corrosion resistance than bulk pure Al with similar purity in neutral 3.5 wt.% NaCl solution. In addition, a mass-transfer step was found to be involved in the corrosion during 10 days immersion.     

Effect of grain size on corrosion of nanocrystalline copper in NaOH solution

Effect of grain size on corrosion of bulk nanocrystalline copper was investigated using potentiodynamic polarization measurements in 0.1 M NaOH solution. Bulk nanocrystalline copper was prepared by inert gas condensation and in situ warm compress (IGCWC) method. The grain sizes of all bulk nanocrystalline samples were determined to be 48, 68 and 92 nm using X-ray diffraction (XRD). Results showed that bulk coppers displayed an active-passive-transpassive behaviour with duplex passive films. From polycrystalline to nanocrystalline, grain size variation showed little effect on the overall corrosion resistance of copper samples.     

Increase of the concentration of dissolved copper in drinking water systems due to flow-induced nanoparticle release from surface corrosion by-products

Standard measurements of dissolved copper are made by filtering water samples through 0.45 μm pore-size membranes. However, the surface of corroding metallic surfaces may be covered by topographic features < 0.2 μm and structures that can be detached into the bulk water as nano-sized particles. A SEM, EDX, and AFM characterization of a corroding pipe after flow events revealed surface cavities, detached particles and attached particles with sizes between 0.05 and 0.2 μm. Our findings show that the release of colloidal and nanoparticles of corrosion by-products into the water can result in an increase of the dissolved copper measurements.     

The effect of H2S concentration on the corrosion behavior of carbon steel at 90 °C

The electrochemical behavior of SAE-1020 carbon steel in 0.25 M Na₂SO₄ solution containing different concentrations of H₂S at 90 °C was investigated using the methods of weight loss, electrochemical measurements, scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results showed that the corrosion rate of carbon steel increased significantly with the increase of H₂S concentration. H₂S accelerated the corrosion rate of SAE-1020 carbon steel by a promoted hydrogen evolution reaction. Severe corrosion cavities were observed on the carbon steel surface in the solutions containing H₂S due to cementites stripped off from the grain boundary. The loose corrosion products formed on the steel surfaces were composed of mackinawite.     

Transgranular SCC mechanism of thermally treated alloy 600 in alkaline water containing lead

This work aims to understand a SCC failure mode of thermally treated steam generator tubing materials in high temperature water containing lead. The effect of lead contents on the anodic polarization curves of alloy 600 (UNS NO6600) and alloy 690 (UNS NO6900) has been studied in a solution of pH 10 at 200 °C and 315 °C. Lead increased the active peaks of alloy 600 and alloy 690 in mild alkaline water at high temperatures. A reduction of PbO to a metallic lead in alloy 690 is easier than that of alloy 600. When lead was added into the solution, a relative ratio of Cr from among the main metallic elements (Cr, Fe, and Ni) of alloy 600 and alloy 690 decreased in the outer corrosion film. Alloy 690 TT showed a transgranular stress corrosion cracking (TGSCC) in a 10 M NaOH solution with 5000 ppm of lead. Intergranular stress corrosion racking (IGSCC) was observed in the 100 ppm lead condition, and some TGSCC was detected on the fracture surface of the alloy 600 MA cracked in the 10,000 ppm lead solution. IGSCC seemed to be retarded by a crack blunting around the grain boundaries, and the TG cracking mode of the thermally treated alloy 600 and 690 seemed to be related to a crack blunting at the grain boundary carbide and a film dissolution by lead in an alkaline solution.     

The relation between microstructure and corrosion behavior of AZ80 Mg alloy following different extrusion temperatures

Cast AZ80 alloy was subjected to conventional extrusion pressing at 250 °C, 300 °C, and 350 °C. In order to characterize the changes in their microstructure a thorough study was done through various microscopy analyses including Optical Microscope, SEM, and TEM.Corrosion performance of each condition was investigated in 3.5% NaCl solution saturated with Mg(OH)₂ (pH ≈ 10.5) using immersion and AC and DC polarization tests. The local potential difference on the surface resulting from different compositions of second phase particles to the matrix was investigated using scanning Kelvin probe force microscopy (SKPFM) technique.The results show grain refinement by a factor of about 15-20 and obvious evidence of dynamic recrystallization were identified leading to the formation of nano-sized grains after the extrusion process.The corrosion resistance of cast AZ80 alloy drastically decreases after the thermo-mechanical processes and the main factor is high dependence on different phase rearrangements before and after the extrusion process, especially β phase. For the extrusion conditions, different corrosion resistances are attributable especially to dislocation rearrangement results by grain growth after dynamic recrystallization.     

A new understanding of intergranular stress corrosion cracking resistance of pipeline steel through grain boundary character and crystallographic texture studies

The roles of grain boundary character and crystallographic texture on the intergranular stress corrosion cracking (IGSCC) of API X-65 pipeline steel has been studied using scanning electron microscope (SEM) based electron backscattered diffraction (EBSD) and X-ray texture measurements. It has been found that low angle and special coincident site lattice (CSL) boundaries, mainly Σ11 and Σ13b and, possibly Σ5, are crack-resistant while the CSL boundaries beyond Σ13b and the random high angle boundaries are prone to cracking. However, several cracks were found to have been arrested even when the random high angle grain boundaries were available for them, both at the crack-tips and areas immediately ahead of them, to continue propagating. Texture studies in the vicinities of these crack-arrest regions, as well as in the cracked areas, provided a new understanding of crystallographic orientation-dependent IGSCC resistance: the boundaries of {1 1 0}‖rolling plane (RP) and {1 1 1}‖RP textured grains, mainly associated with 〈1 1 0〉 and 〈1 1 1〉 rotation axes, respectively, were crack-resistant due to their low energy configurations, while the cracked boundaries were mainly linked to the {1 0 0}‖RP textured grains.     

Preparation and hot corrosion behaviour of an Al-gradient NiCoCrAlYSiB coating on a Ni-base superalloy

A gradient NiCoCrAlYSiB coating was prepared on a Ni-base superalloy using arc ion plating (AIP) and subsequent gaseous phase aluminisation techniques. Hot corrosion of normal NiCoCrAlYSiB and the gradient coating in pure Na₂SO₄ and Na₂SO₄/NaCl (75:25, wt./wt.) salts was performed at 900 °C in static air. The corrosion results indicated an enhanced corrosion resistance to both salts for the gradient NiCoCrAlYSiB coating, which the improved performance of it should be attributed to the β aluminide ‘‘pool” at the surface layer. By partially sacrificing Al₂O₃ (i.e. Al), the gradient NiCoCrAlYSiB coating specimen behaved excellently in the two kinds of salts. The grain growth during the gaseous phase aluminisation and the corrosion mechanism, including the role NaCl played in the mixture salt corrosion, are discussed.     

Pitting behavior of a bulk Ni-18 wt.% Fe nanocrystalline alloy

Three electrodeposited Ni-18 wt.% Fe samples were annealed at 400 °C for 3 h (hrs), 8 h, and 24 h to study the effects of grain size on the electrochemical properties of bulk Ni-18 wt.% Fe in 3.5 wt.% NaCl. The electrochemical results from the annealed samples are compared with those measured for the as-received Ni-18 wt.% Fe material consisting of an average grain size of 23 nanometers (nm). Of the four materials studied, the as-received nanocrystalline alloy less sensitive to localized corrosion.     

Electropolishing effects on corrosion behavior of 304 stainless steel in high temperature, hydrogenated water

The corrosion rate of electropolished 304 stainless steel surfaces (UNS S30400) is found to be lower by more than a factor of three relative to that determined previously for machined surfaces in mildly alkaline, hydrogenated water at 260 °C. This favorable result is attributed to significant changes in nanocrystallinity of the corrosion oxide layer caused by the removal of surface microstrain, which had been imparted during the machining process. In the absence of microstrain, a low-porosity, protective, corrosion layer forms that is composed of extremely small and uniformly-sized spinel oxide crystals. Application of scanning electron microscopy (FEG-SEM), X-ray diffraction and X-ray photoelectron spectroscopy (XPS) in conjunction with ion milling and target factor analyses, found the corrosion layer to consist of micrometer-size crystals of a ferrite-based spinel oxide (non-protective) over-laying nanometer-size crystals of a chromite-based spinel oxide (protective). Composition of both phases is unchanged from that previously observed on corroded, machined surfaces and is representative of solvus phases in the immiscible Fe(Fe_1−nCr_n)₂O₄ spinel binary. The smaller size (10 vs. 26 nm) and greater surface density (∼10,000 vs. 835 μm⁻²) of the chromite-based crystals relative to those formed on machined (i.e., cold-worked) surfaces, however, is consistent with the absence of preferred high energy nucleation sites on strain-free surfaces. Therefore, electropolishing, which removes surface microstrain induced by cold-working, represents a preferred reference surface condition.     

Surface characterization of passive film formed on nitrogen ion implanted Ti-6Al-4V and Ti-6Al-7Nb alloys using SIMS

The aim of this paper is to study the effect of N⁺ ion implantation on corrosion and phase formation on the implanted surfaces of Ti-6Al-4V and Ti-6Al-7Nb alloys. Nitrogen ion was implanted on Ti-6Al-4V and Ti-6Al-7Nb alloys at an energy of 70 and 100 keV, respectively using a 150 keV accelerator at different doses ranging from 5 × 10¹⁵ to 2.5 × 10¹⁷ ions/cm². Electrochemical studies have been carried out in Ringer’s solution in order to determine the optimum dose that can give good corrosion resistance in a simulated body fluid condition. The implanted surfaces of such modified doses were electrochemically passivated at 1.0 V for an hour. Secondary ion mass spectroscopy was used to study and characterize titanium oxide and titanium nitride layers produced on implanted surface and to correlate them with the corrosion resistance. The nature of the passive film of the implanted-passivated specimen was compared with the unimplanted-passivated as well as as-implanted specimens.     

Intergranular corrosion resistance of nickel-based alloy 690 weldments

This study focused on the temperature distributions and thermal histories of alloy 690 weldments, as well as the residual stress, grain boundary character distribution, degree of sensitization, and carbide precipitation, to evaluate the combined effects of these factors on the intergranular corrosion resistance of the weldments. Both laser beam welding and gas tungsten arc welding were performed for comparison. The results show that laser beam welding, with its narrower temperature distribution and rapid heating/cooling, resulted in more low energy Σ (1 ≦ Σ ≦ 9) boundaries, lower residual stress, and considerable suppression of Cr₂₃C₆ carbide formation. Therefore, laser weldment had a low degree of sensitization, and thus the intergranular corrosion resistance was significantly improved.     

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.     

Long-term trends in the corrosion state and surface properties of the stainless steel tubes of steam generators decontaminated chemically in VVER-type nuclear reactors

At some VVER-type pressurized water nuclear reactors (Russian-type PWR) different versions of the so-called AP-CITROX method have been widely used for the chemical decontamination of the heat exchanger tubes of steam generators. In the period of 2000–2007, within the frame of a joint-project dealing with the comprehensive investigation of the corrosion state of the steam generators of the Paks Nuclear Power Plant, Hungary, effects of the AP-CITROX chemical decontamination procedure on the corrosion and surface characteristics of the heat exchanger tubes have been studied. These studies provide evidences that some adverse features (formation of a “hybrid” layer with accelerated corrosion rate and great mobility) can be detected after 1–3 years of applying the AP-CITROX procedure. The present work is a continuation of the above program and focused on the long-term trends in the corrosion state and structure of protective oxide-layer grown on the decontaminated surfaces. The results of electrochemical (voltammetric), surface analytical (SEM–EDX, CEMS) and mobility (ICP–OES) studies have revealed that (1) some beneficial changes in the corrosion characteristics, mobility and chemical composition of the inner surfaces of decontaminated heat exchanger tubes can be observed in the long run, and (2) the passivity of the oxide-layers formed on decontaminated surfaces of steel tubes exhibits favorable tendency after 4–7 years under normal operation conditions.     

Enhanced oxygen exchange and incorporation at surface grain boundaries on an oxide ion conductor

This study reports spectrometric and spectroscopic evidence indicating a pronounced role of surface grain boundaries in enhancing oxygen incorporation on oxide ion conducting ceramic yttria stabilized zirconia (YSZ). Oxygen isotope exchange measurements were carried out using high spatial resolution (50 nm) secondary ion mass spectrometry (SIMS) under cathodically biased conditions as well as on bare YSZ surface. Surface mapping of the ¹⁸O distribution by SIMS clearly shows high activity at surface grain boundaries, suggesting that these boundary regions provide preferential pathways for oxygen incorporation into YSZ in both cases, albeit the effect is less pronounced without bias. The results are supported by a.c. impedance spectroscopy measurements conducted on polycrystalline YSZ membranes with surface grains engineered for different sizes, which indicate that smaller grains (i.e. higher grain boundary densities) exhibit lower electrode impedances. These results open up the possibility of engineering nanostructured YSZ surfaces containing a high density of grain boundaries to achieve enhanced performance of electrochemical devices, particularly for solid oxide fuel cells operating at low temperatures.