Selective Dissolution Of Nickel Research Articles

Formation of self-supporting porous graphite structures by Spark Plasma Sintering of nickel–amorphous carbon mixtures

Graphitization of amorphous carbon in the presence of nickel has been reported for various configurations of the metal–carbon interface; however, no study has been performed to evaluate a possibility of forming self-supporting networks by sintering of the in situ formed graphite. In this work, we have shown that Spark Plasma Sintering (SPS) of nickel–amorphous carbon mixtures containing 50vol% of Ni at 1000°C results in the formation of networks formed by sintered graphite platelets 50–200nm thick and 0.3–2μm in diameter. Upon selective dissolution of nickel, a self-supporting porous 3D skeleton was revealed in 20mm-diameter compacts. Starting from the mechanically milled Ni–C mixture, porous graphite of uniform microstructure and containing submicron pores was obtained. A model study has been performed, in which a thin amorphous carbon film graphitized during annealing and formed a continuous graphite film with micron-sized grains covering an area of 2cm×2cm of the surface of a Ni foil. We discuss the role of the in situ formation of graphite by nickel-assisted graphitization in the formation of networks consisting of well sintered platelets during the SPS and the design possibilities of porous carbon materials produced by phase separation in nickel–graphite composites.

Corrosion Behavior of Ni-Based Alloys in Supercritical Water Containing High Concentrations of Salt and Oxygen

The Ni-based alloys Incoloy 800, Incoloy 825, Inconel 625, and Hastelloy C-276 exposed to subcritical water (350 °C, 25 MPa) and supercritical water (450 °C, 25 MPa) with high concentrations of chloride and oxygen were analyzed by using scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. There is a strong synergistic effect between salt and oxygen, leading to severe corrosion. The selective dissolution of nickel is the severest of all alloying elements, and a stable oxide in oxidizing condition is formed by chromium. Molybdenum improves the resistance to pitting corrosion when chromium is present. Without molybdenum, Incoloy 800 exhibits the severest pitting corrosion of the test alloys under subcritical condition. Inconel 625 and Hastelloy C-276 exhibit good corrosion resistance under the condition of oxygen and salt existing. NiO, NiCr2O4, and Cr2O3 are the three main components of oxide films on Ni-based alloys. The possible corrosion mechanisms of Ni-based alloys are...

Impedance of NiSi electrode in sulfuric acid in the active anodic dissolution range

Anodic behavior of NiSi electrode in 0.5 M H2SO4 in a potential range of active dissolution is studied. A conclusion about the selective dissolution of nickel from nickel silicide at a small anodic polarization (up to 0.15 V) is drawn based on the impedance data. The solid-phase diffusivity of nickel and the thickness of the diffusion zone are estimated.

Special features of fracture in materials used for soldering structures

1. Cracks that appear in soldering materials of different kind are a result of the simultaneous action of the appearing tensile stresses and the solder melt. 2. The appearance of tensile stresses in soldering is connected with the volume changes; in alloy EP202 they are caused by intensification of the decomposition of the solid solution accompanied by the segregation of particles of fine disperse phases and in steel VNS16 they are caused by the segregation of particles of carbide phases and the α→γ transformation. In addition, the appearance of stresses in a specific temperature range is promoted by technological factors, coarse grains, the deformation rate, the presence of hydrogen, etc. 3. The tensile stresses intensify the decomposition of the solid solution accompanied by segregation of hardening phases and their coarsening, especially over grain boundaries. 4. A crack appearing in materials during soldering has a wedge shape and a sharp notch in the frontal plane; its growth has an intercrystalline nature. The propagating cracks branch due to the pores and cracks appearing in the volume of the metal. 5. The growth of cracks in soldering occurs from the surface and in the bulk of the material. The growth of a crack from the surface is characterized by jumps due to the repeated processes of accumulation and consumption of elastic energy at the tip of the crack when it propagates and stops respectively. 6. Corrosion and diffusion processes develop in the zone of surface cracks. The corrosion process is characterized by selective dissolution of nickel, which forms the base of alloy EP202 and is an alloying element in steel VNS16, in the liquid solder. The diffusion process is characterized by predominant penetration of copper into the lattice of the materials. Corrosion and diffusion are not observed in fracture zones in the bulk of the specimens.

Surface modification of Inconel-600 by growth of a hydrous oxide film

In this study, Inconel-600 (Ni–Cr–Fe alloy) was modified by repetitive potential cycling in 1M NaOH solution. This procedure induced the growth of a hydrous oxide film, following the same mechanism as previously reported for pure nickel in alkaline solution under similar experimental conditions. The electrode, modified by 30 repetitive potential cycles, exhibited about one order of magnitude lower current density in both the active and passive ranges of the anodic polarization curve. Selective dissolution of nickel and iron in acid solution was determined by rotating ring–disc electrode measurements. This process resulted in chromium enrichment as shown by use of X-ray electron spectroscopy. The proposed model for the enhanced stability of the modified electrode agrees with the percolation model of passivity of stainless steels and Fe–Cr alloys.

The selective corrosion of type 316 stainless steel in molten lead

Molten lead is a potential electrodynamic fluid for magneto-hydrodynamics (MHD) liquid metal (LM) generators in a project developed in the Center for MHD studies at Be n-Gurion University [1]. Obviously, the structural materials of the generator have to be corrosion-proof in liquid lead turbulent flow, and the material of the wall close to the electrode zone should not be ferromagnetic. Austenitic type 316 stainless steel (SS316) is a promising structural material which meets these requirements. To our knowledge, no data are yet available about the behaviour of SS316 in liquid lead turbulent flow. There is very little information about the behaviour of SS316 in contact with static or low speed flow molten lead in the literature [2]. The interaction of SS316 with Pb-17Li liquid alloy is described in a larger number of works, perhaps because this melt is a potential breeder in fusion reactors [3-5]. As a rule, selective nickel dissolution (leaching) from steel is mentioned. The selective process is related to higher nickel solubility in the liquid eutectic Pb-Li as compared with iron and chromium. In this letter the results of an investigation of the corrosion of SS316 samples in turbulent liquid lead flow are described. The experimental conditions determining either selective or uniform types of SS316 corrosion are discussed and the role of oxygen impurities is discussed in greater detail. An analysis of the equilibrium states in the P b P b O F e C r Ni-Mn system was also performed to estimate the characteristics of steel-liquid interaction under our experimental conditions. The samples under investigation were in the form of discs 40 mm in diameter and 8 m thick. The samples were polished and degreased with acetone. Tests were carried out by the rotating disc method described previously [6]. The test conditions were: temperature 500 °C, sample rotation rate 2400 rpm. The test container for the molten lead (99.97%) was made of carbon steel and the melt volume was approximately 1 I. Nitrogen (99.7%) was padded through the container above the melt surface at a flow rate of 4 1 min -1. The duration of the experiment was 800h. Every 200h the sample was removed from the container for X-ray diffraction (XRD) and scanning electron microscopy (SEM)/electron probe X-ray microanalysis (EPMA) [6]. According to EPMA, the steel composition was (wt %): Cr 17.9, Ni 10.5, Mo 2.5, Mn 1.8, Si 2.0, Fe remainder. The steel in the initial state proved to be two-phase: the main phase was 7 austenite (a = 0.3602 nm) and the second phase was oL-ferrite (a = 0.2870 nm). This corresponds to the equilibrium phase diagram of Fe -Ni -Cr [7]. It should be noted that the steel retained its phase composition during the entire test. The y phase cell parameter did not change during the test, but the parameter of the o~ phase increased to 0.2890 nm after 800 h. The concentrations of iron, nickel and chromium near the sample surface, according to EPMA data, did not change during the test period (Fig. 1). The presence of 7 and ol phases, complex oxide phase FeCr204 and small quantities of Fe2SiO4 (faylite) and o~-MnSiO3 was determined by XRD analysis of the sample surfaces. As follows from the above data, the composition of the subsurface zone remained unchanged for a long time. The dimensions of the samples also remained constant to within + 10 #m. An obvious question arises as to why there is no selective dissolution of nickel and, as a result, no build-up of o~-phase on the surface of the sample tested, despite the large solubility difference of Ni, on the one hand, and iron and chromium, on the other, in liquid lead (this difference amounts to four or five orders of magnitude at 800 K [8-10]). Firstly, the rate of nickel leaching from SS316 by liquid lead turbulent flow under the rotating disc regime was estimated. The rate of pure nickel dissolution, Jr~i, under our experimental conditions is expressed by [11]:

Corrosion of a Type 304 stainless steel and a molybdenum-base TZM alloy in refluxing mercury with a small amount of potassium

Abstract Corrosion of a Type 304 stainless steel and a molybdenum-base TZM alloy (Mo-0.5Ti-0.08Zr-0.025C-0.025 oxygen) in refluxing mercury with and without about 0.004 mol fraction of potassium in mercury has been examined using small capsules made of the steel and the alloy at 600 and 700 ° C to recognize the corrosion mechanisms. The wall of the austenitic 304 stainless steel capsule was severely corroded due to selective dissolution of nickel and chromium, leading to a ferritic layer which is more stable than the austenitic phase. The corrosion products were deposited near the vapor-liquid interface of the mercury. In contrast, the TZM alloy showed no noticeable changes except for carburization of the surface after heating for 2000 h, probably due to the low solubility of molybdenum in mercury.

Sodium corrosion behavior of austenitic alloys and selective dissolution of chromium and nickel

The corrosion behavior of six austenitic alloys and reference Type 316 stainless steel (SS) has been examined in a flowing sodium environment at 700°C for up to about 4000 h. The alloys with a range of nickel content between ~ 15 and 43 wt% were designed and manufactured with an expectation of improved swelling resistance during fast neutron irradiation, compared to reference Type 316 SS. The corrosion loss of the alloys at zero downstream position and the concentrations of chromium, nickel and iron in the surface region were determined as a function of corrosion time. The selective dissolution of nickel and chromium played an important role in sodium corrosion of the alloys. During the initial period, accelerated corrosion took place and selective dissolution of chromium and nickel proceeded at a rapid rate. During the subsequent period, the overall corrosion rate and depletion of chromium and nickel decreased with increasing time until the corrosion rate and the surface concentrations of chromium, nickel and iron, which depended on composition of the alloys, reached the steady-state after about 2000 h. Also, the corrosion rate increased with increasing original nickel content of the alloys. Microstructural examination revealed surface attack of the alloys with higher nickel contents, in particular for the two precipitation strengthened Fe-Ni alloys. The alloys showed a trend of increasing carbon and nitrogen contents.

Studies of the influence of sulphur on the passivation of nickel by Auger electron spectroscopy and electron spectroscopy for chemical analysis

During the anodic polarization of sulphur-doped nickel, selective dissolution of nickel and surface segregation of sulphur are observed. These phenomena cause a complete inhibition of passive layer formation when the sulphur content is above a critical concentration. Auger electron spectroscopy and electron spectroscopy for chemical analysis were used to characterize the species formed on the surface during the sulphur enrichment. The spectra were interpreted by comparison with standard spectra from synthesized compounds: NiS, Ni 3S 2, Ni-adsorbed S. A comparison with previous results using radioactive sulphur ( 35S) allowed us to determine the mechanism of the sulphur enrichment and of the consequent inhibition of passivation by the formation of an adsorbed sulphur layer and subsequent growth of a nickel sulphide Ni 3S 2 which attains a stationary thickness of about 30 Å.