High-speed anode dissolution of heat-resistant chrome-nickel alloys containing tungsten and rhenium: III. Chloride-nitrate solutions

The anodic dissolution rates of two heat-resistant tungsten (12 wt %) and tungsten and rhenium (8 wt % W : 6 wt % Re) alloys in chloride, nitrate, and chloride-nitrate (30 g/l NaCl : 120 g/l NaNO₃) electrolytes have been compared. An excess of the critical current density i _crit depending on the hydrodynamic conditions leads to (i) the decreasing dependence of the current efficiency on the current density and (ii) the independence of the dissolution rate on the solution content. The mass-transfer mechanism, which determines the dissolution rate and the structure of superficial layers at i > i _crit, is discussed. Data on the chemical content of the surface layers depending on the machining modes are presented. The modes of the electrochemical dimensional machining (ECDM) of the parts made of these alloys are offered to attain the best performance parameters for machining.     

High-speed anode dissolution of heat-resistant chrome-nickel alloys containing tungsten and rhenium: I. Chloride solutions

The results of a study on the anode dissolution of two heat-resistant chrome-nickel alloys containing tungsten (12 wt %) as well as tungsten and rhenium (8 wt % of W and 6 wt % of Re) are described. The experiments took place in 2 M NaCl at a current density of up to 40 μ/cm² using a rotating disk electrode. It is shown that the alloy with the greater tungsten content dissolves at a lower rate (due to the formation and accumulation of insoluble oxides layers on the surface) and at a current density lower than the maximum anode current density for the basic component (nickel, and, probably, cobalt) of the anodic dissolution. Transpassive dissolution takes place under conditions of thermokinetic instability of the electrode process. In this case, a decreasing dependence of the current efficiency on the current density is observed, and the dissolution rate is independent of the tungsten concentration in the alloy. In the region of the maximum anode currents and the transition from one area of dissolution to another, abnormal anode dissolution takes place due to chemical oxidation of intermediate products by oxidizers—anode-dissolution products or solution components. The results of varying the chemical composition on surfaces depending on the treatment mode are presented.     

The anodic dissolution of beryllium in non-aqueous solutions

The anodic dissolution of Be was studied in Cl^? and Cl0₄^? solutions of methanol, ethanol and N,N-dimethylformamide with water contents < 0.1 g/l at 25°C. The apparent valences of Be ions going into solution ranged from 0.37 to 1.8. The lower values were obtained in alcohol solutions with Cl^?. Considerable gas evolution was noted from the Be surface. The low and anomalous valences are attributed mainly to a chemical dissolution of the metal by reaction with the solution. This reaction is reported to be catalysed by Cl^?. Theeffect of the low watercontent is to hinder the reforming of an oxide or hydroxide film that protects the Be from chemical attack. The numerical value of the valence is mainly dependent on the rate that the Be will chemically react with the media once a current flows that disrupts the surface film.     

Scanning electrochemical diagnostics of initial stages of anodic dissolution of carbon steel in chloride containing solutions

An optical PC scanner is used in combination with electrochemical methods of controlling the pitting destruction by an example of the initial states of the anodic dissolution of carbon steel St3 in 0.1 M sodium chloride solution. It is shown that under anodic polarization at the active dissolution conditions, the carbon steel surface exhibits noticeable formation of local defects looking as pits. Under free corrosion conditions in chloride-containing media, this pitting includes at least two stages, namely, the pits initiation and their development, where the first stage is completed in 60 min of tests. Original Russian Text ? M.A. Petrunin, L.B. Maksaeva, D.N. Tyurin, V.A. Kotenev, 2008, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2008, Vol. 44, No. 5, pp. 564–568.     

Corrosion and anodic dissolution of magnesium alloys in the presence of inhibitor

The effect of NTPS inhibitor on the corrosion behavior of magnesium alloys with various contents of alloying components is studied. NTPS is found to be an effective corrosion inhibitor in the absence of energizing, but under the anodic polarization, it does not produce any noticeable effect on the self-dissolution process. Comparative analysis of the processes proceeding at the anodic dissolution of magnesium and electron-microscopic studies of the specimens showed that the dissolution is accelerated due to the active-surface development. A supposition that, under the effect of a constant anodic current, the development and formation of the magnesium surface relief takes place at the initial polarization stage is put forward.     

Relation between individual properties of alloy components and anodic behavior of multicomponent alloys in acidic environments: I. Active dissolution and passivation of Fe-Cr alloys in sulfuric acid solutions

The active dissolution and passivation of Fe-Cr alloys are analyzed with the use of concepts of the appearance, operation, and blockade of active dissolution sites on the actual metal surface. General regularities of the effect of chromium on the rate of the anodic process at the change in the alloy composition and electrode potential are considered. The less noble, but easily passivable chromium component provides a dual effect on the anodic behavior of the binary alloys; it stimulates the anodic reaction at the low content in the alloy and suppresses the process at the high content. The principally different electrochemical properties of the alloy components result in the step-by-step passivation of the alloy with an increase in the potential, which is reflected by regular breaks in perfect anodic curves.     

Chronovoltammetry of the anodic dissolution of Ag-Au alloys in nitrate environments

A theory of quasi-reversible anodic chronovoltammetry of the selective dissolution of a homogeneous A-B alloy controlled by the solid-phase diffusion is proposed. The effects of the equilibrium solid-phase adsorption of the components and the roughness of the electrode surface with either homogeneously or statistically normally distributed microscopic irregularities of diverse kind on the shape of chronovoltammogram (the I _A current dependence on the η(t) overpotential) are successively taken into account. An expression for the I _{A, m} current at the voltammogram maximum, which is a generalization of the Randles-Sevik formula, is derived. A technique of estimating the recording-time boundaries of voltammograms, within which the surface roughness effect may be taken into account by simply multiplying the current by the roughness factor f _r , is founded. Outside the time interval, the I _A,m − ν^1/2 dependence (ν is the potential scanning rate), which is criterion for the diffusion kinetics, is shown to be substantially nonlinear. The diffusion-zone parameters (δ thickness and D _Ag heterodiffusion coefficient) obtained by processing the experimental data for Ag-Au alloys with a gold content of 15 or 20 at.% with the use of three models discussed are compared. Original Russian Text ? O.A. Kozaderov, V.V. Lozovskii, A.V. Vvedenskii, 2008, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2008, Vol. 44, No. 4, pp. 359–368.     

On the electrochemical behaviour of tungsten: the formation and dissolution of tungsten oxide in sulphuric acid solutions

The formation and dissolution of the anodic oxide film of tungsten were investigated in sulphuric acid solution of different concentrations. The dissolution process was found to follow a first order mechanism under different conditions. It seemed that, in highly diluted solutions, the hydrate formation is favoured, whereas in concentrated solutions, the process takes place via the formation of WO₂²⁺. The rate of dissolution was found to increase by increasing the initial current density of formation.     

An XPS study of anodic behavior of amorphous nickel-phosphorus alloys containing chromium,molybdenum or tungsten in 1 m CHl

The surfaces of amorphous Ni-18P, Ni-IOCr-20P, Ni-9Mo-19P and Ni-5W-18P alloys immersed or anodically polarized in 1 M HCl solution were analyzed in connection with their corrosion and anodic behavior. All alloys were more corrosion-resistant than crystalline nickel metal because of formation of phosphate-containing surface films on the Ni-18P, Ni-9Mo-19P and Ni-5W-18P alloys and because of spontaneous passivation due to formation of passive hydrated chromium oxyhydroxide film on the Ni-10Cr-20P alloy. The latter alloy was stable up to the transpassive region of chromium although intrusion of phosphate in the film was responsible for the higher passive current density in comparison to the amorphous Fe-Cr-13P-7C alloy of the same chromium content. The formation of thick porous phosphate films containing nickel, and molybdenum or tungsten by anodic polarization was not effective in passivating the Ni-18P, Ni-9Mo-19P and Ni-5W-18P alloys, and they suffered pitting corrosion by anodic polarization.     

Effect of vegetal tannin on anodic copper dissolution in chloride solutions

The corrosion inhibition of copper in a 0.1 M NaCl solution in the presence of vegetal tannin using potentiodynamic techniques was studied. The tannin was extracted from Takaout galls (Tamarix articulata). The gallic acid has been used as a representative of the tannin species. The tannin are anodic inhibitors. The inhibition efficiency of the tannin evaluated from anodic polarization curves and impedance measurements is about 93.2% for a tannin concentration of 2 g/l. We noted that the presence of tannin changes the mechanism of copper dissolution in a 0.1 M NaCl solution. Microscopic scanning studies of the surface state revealed the formation of corrosion layer products. The impedance investigation allows the explanation of the inhibition process.     

Potentiodynamic studies on anodic dissolution and passivation of tin, indium and tin–indium alloys in some fruit acids solutions

The anodic dissolution and passivation of tin, indium and tin–indium alloys were studied in 0.5 M solutions of both malic and citric acids, using potentiodynamic technique and characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The E/I curves showed that the anodic behavior of all investigated electrodes exhibits active/passive transition. The active region of tin involves two anodic peaks (I and II) prior to the passive region in both the investigated acids, while indium exhibits two peaks (I and II) in malic and one peak in citric acid. These two peaks (I and II) correspond to the formation of InOOH and In(OH)₃/In₂O₃ system, respectively, but that observed peak in citric acid is to InOOH. The active region for tin–indium alloys (I, II and III) in citric acid showed one peak (I) and shoulders (II). This shoulder predominates with increasing temperature due to little In₂O₃ formation and its dissolution at higher temperatures. The disappearance of this shoulder for the alloys (IV and V) with high indium percent may be due to the formation of large amounts of In₂O₃ with tin oxides on the surface.     

The kinetics of the anodic dissolution of copper in acid chloride solutions

The anodic dissolution of copper has been studied in different acid chloride solutions under potentiostatic conditions. A rotating disk electrode technique was employed.The results showed that the slow cuprous chloride complexes transferred towards the bulk is the r.d.s. all over the polarization range. Anodic limiting currents have been also observed and explained.     

High rate anodic dissolution in chloride solutions of steel after electrothermochemical treatment

High rate (up to 15 A/cm²) anodic dissolution of St3 steel in chloride solutions (1–6 M NaCl) was investigated after its electrothermochemical al treatment (nitriding and thermal treatment). Various potential ranges (or current density ranges) can be emphasized: the region of anodic anion activation (sublimiting currents), the diffusion limitation area (caused by salt passivation), and the transpassive dissolution region. It is shown that the heat treatment influence is the most substantial in the region of salt passivation (or the limiting current region). The double role of the hydrodynamics is revealed during dissolution under these conditions: the influence of the ionic mass transfer rate in the solution and of the removal of the hard carbonaceous products produced as a result of the selective dissolution of the components of the nitrided steel. It is shown that, in sublimiting currents and transpassive dissolution ranges, the specific dissolution rate depends on the nitrogen content of the studied steel.     

Electrochemical behavior of nanocrystalline Ni–P alloys containing tin and tungsten

Autocatalytic ternary Ni-Sn-P, Ni-W-P and quaternary Ni-W-Sn-P films were prepared using an alkaline bath. Plain Ni-P films were also prepared for comparison. Corrosion resistance of the films was evaluated in 3.5% sodium chloride solution in non-deaerated condition by potentiodynamic polarization and electrochemical impedance spectroscopy methods. Deposits were also immersed in 3.5% sodium chloride solution for 7 days. All the coatings attained stable equilibrium potential within 30 minutes in NaCl medium. Lower corrosion current density values were obtained for ternary Ni-Sn-P coatings compared to the plain Ni-P coatings. Ternary Ni-W-P and quaternary Ni-W-Sn-P alloys did not show improved corrosion resistance compared to the ternary Ni-Sn-P coatings. Similar behavior of these coatings was further confirmed by the electrochemical impedance studies. After the potentiodynamic polarization test deposits were examined by scanning electron microscope. It was found that more corrosion occurred for the quaternary deposit compared to other deposits. Energy dispersive analysis of X-ray results indicated that more amount of Fe present on NiWP and NiWSnP coated samples. Similar behavior was confirmed from the optical images of the surfaces obtained for the deposits after the immersion test. The article is published in the original.     

Localised dissolution of iron in buffered and non-buffered chloride containing solutions

Pitting corrosion of pure iron was studied by using conventional samples, and artificial pit electrodes. Experiments were conducted in solutions of 0.01, 0.1 and 1 mol dm⁻³ NaCl at pH values of 7, 10, 11 and 12; and in borate buffer solutions with the same chloride concentrations and pH 8.7 and 9.2. Four times higher concentration of borate salt was required to reach inhibiting capacity of the hydroxyl anions, as determined via pitting potentials. From measurements of solution resistance, the increase in local conductivity due to dissolved corrosion products exuded from the pit was calculated for each bulk solutions. For the artificial pit electrodes, contribution of the borate species to the internal pit solution conductivity in low chloride solution was associated with the difference between transition potential, E_T, values for buffered and non-buffered solutions, and this contribution was also used to explain the non-linear dependence of E_T on [Cl⁻]. Further analysis was conducted using the concept of the critical i.x parameter for stable pit propagation.     

Anodic dissolution of mild steel in HCl solutions containing thio-ureas

The effect of thio-urea and its derivatives on the anodic dissolution of mild steel in HCl solutions has been studied. Thio-urea and N-substituted derivatives show maximum anodic polarisation at low concentrations. The anodic Tafel slope shows an abrupt change in the intermediate concentration range, i.e. 4–40 mM. This has been explained in terms of protonated species taking part in metal dissolution. S-substituted derivatives exert increasing anodic polarisation with increasing concentration.