Electrochemical behaviour of nickel anode in H2SO4 solutions and the effect of halide ions

The electrochemical behaviour of pure nickel in H₂SO₄ solutions has been potentiodynamically investigated. The effects of the following factors on the anodic dissolution and passivation of the metal are discussed: potential scan rate, successive cyclic voltammetry and progressive additions of Cl^?, Br^? and I^? ions. Increasing the potential scan rate increases the critical current density i_cc, denoting that the active dissolution of nickel in H₂SO₄ is a diffusion controlled process. Cyclic voltammetry shows that the reverse excursion does not restore the anode to its active state. On successive cycling, the height of i_cc decreases; this could be attributed to the decrease in the reduction efficiency of passivating oxide film during the cathodic half cycles. The presence of the halogen ions below a certain concentration specific to each anion inhibits the anodic dissolution both in the active and passive states. The inhibitive action of these additives decreases in the order I^?, Br^?, Cl^?. Beyond the specific concentrations, the halogen ions accelerate the anodic dissolution and shift the active passive transition to more positive values. The aggressiveness of these anions decreases in the sequence Cl^?, Br^?, I^?, Further increase in the halogen ion concentrations can lead to breakdown of the passive film and initiate pitting. The susceptibility of nickel to pitting attack enhances with increasing H₂SO₄ concentration.     

Hydrogen penetration rates of iron. Effect of halide ions,hydrogen sulfide and hexynol

The effect of the halides (Cl^?, Br^? and I^?) on the rates of the hydrogen evolution reaction (h. E. R.) and hydrogen penetration (h. P.) of iron in H₂SO₄ is determined. The H. E. R. and H. P. of Fe in these same electrolytes with the addition of H₂S are also studied. The inhibition of corrosion and h. p. in the H₂S environment by hexynol are compared. With halide addition her., i_corr and h. p. decreased in the following order, Cl^?> Br^?> I^?. H₂S catalyzed both the h. e. r. and h. p. The catalytic activity does not appear dependent on the H₂S concentration for the H. E. R. but does for H. P. The fraction of hydrogen generated which is absorbed by Fe is greater during corrosion when a new surface is formed continuously than during cathodic polarization. The addition of the acetylenic alcohol, hexynol, in H₂SO₄ containing H₂S and halides inhibited the h. e. r. corrosion and h. p.     

Der Einfluβ Der Spezifischen Adsorption Von Halogenidionen Auf das Elektrochemische Verhalten der Nickel-Elektrode

Small additions of the halide ions (F^?, Cl^?, Br^?, I^?) to 1N H₂SO₄ result in marked deviations of the current density/potential curves as well as the capacitance/potential curves from nickel, compared with those obtained in pure H₂SO₄. The different actions are explained by the varying tendency of the halide ions to specific adsorption at the nickel surface.The time-dependence of the capacitance demonstrates the existence of a two-stage process for the adsorption of iodide ions.In the presence of iodide ions the influence of their concentration was measured with respect to current density and capacitance.     

Mass spectrometric characterization of halogen doped polyacetylene

Mass spectrometry was used to investigate the volatile species given off when polyacetylene, which had been exposed to I₂, Br₂, Cl₂, or F₂, was heated in vacuo. A correlation of the conductivity of the samples with the amount of molecular halogen ions (X₂⁺) evolved was observed. These results are consistent with the present concept for the form of the dopant in the polymer. In the I₂ case, two sources of iodine in the (CH)_xwere found and these were related to the presence of I₃^? and I₅^? in the polymer films.     

Oxide film growth on manganese in neutral aqueous solutions

Abstract

The open circuit potential of the Mn electrode is followed in aerated solutions of Cl^?, Br^?, I^?, SO?&SetFont Typeface="44";24, CrO?&SetFont Typeface="44";24, CO?&SetFont Typeface="44";23 and NO?2 with different concentrations, till steady state values are attained. In all solutions studied, the steady state potentials are approached from negative values indicating oxide film growth. The rate of oxide film thickening is determined from the linear relationship between the open circuit potential and the logarithm of immersion time, t, as evident from the relation: E?=?a+b logt, where a and b are constants. Oxide film growth is assumed to occur by ion conduction under a high field. The rate of oxide film thickening depends on anion type and concentration. The concentration of the inhibitive anions, CrO&SetFont Typeface="44";2?4, CO&SetFont Typeface="44";2?3 and NO?2 that can withstand a certain concentration of the aggressive ions, Cl^?, Br^?, and I^?, varies according to the relation: logC_inh.?=?A+n logC_agg., where A and n are constants.     

The inhibition of the dissolution of iron in sulfuric acid by halide ions

The dissolution rate of mild steel in 1 N H₂SO₄ without and with different additions of Cl^?, Br^? and I^? has been measured. Analysis of the results indicates that halide ions adsorbed on the metal surface inhibit the dissolution reaction. Adsorption occurs according to the Frumkin isotherm.     

Pitting Corrosion Currents on Steel in Relation to the Concentration of the Inhibitive and Corrosive Anions under Natural Corrosion Conditions

Abstract

The changes with time in the pitting corrosion current density on a steel electrode with the concentration of both the inhibitive anions (CrO₄^2?, HPO₄^2? and WO₄^2?) and aggressive anions (Cl^?, Br^? and I^?) was followed using a simple electrolytic cell. In chromate solutions the pitting corrosion currents started to flow after an induction period, τ, which varied, in one and the same inhibitor solution, with the concentration of the aggressive anion, according to the expression: log τ =a?b log C_agg.

The pitting corrosion currents finally reached steady-state values which depended on the type and concentration of both the inhibitive and corrosive anions. At a constant inhibitor concentration,the corrosion current varied with the concentration of the aggressive anion according to: log i_corr = a_l + b₁ log C_agg, and with constant aggressive ion concentration according to: log i_corr =a₂ - b₂ log C_inh Comparison was made between the experimentally obtained values of a_l (a₂ and b₁ (b₂), and the corresponding computed values. The a₂ values show that the corrosivity of the three aggressive anions decreases in the order: Cl^? > Br^? > I^?; on the other hand the inhibitive efficiency of the inhibiting anions decreases in the order: CrO₄^2? > HPO₄^2? > WO₄^2?.     

Effect of halide anions and temperature on initiation of pitting in Cr–Mn–N and Cr–Ni steels

Abstract

The pitting corrosion of Cr₁₈Mn₁₂N and Cr₁₈Ni₉ steels in halide solutions (F^?, Cl^?, Br^? and I^?) has been investigated. The study involved cyclic potentiodynamic polarisation tests with subsequent examination of the specimens by both optical and scanning electron microscopy. Values of the critical concentrations of halide ions, [X^?]_cr, beyond which pitting occurs, as well as breakdown potentials for pitting in chloride solution, have been established. In addition, the effect of the temperature over the range of 5–80°C on the critical chloride ion concentration [Cl^?]_cr has been investigated and it has been found that temperature has a negligible effect beyond 40°C.     

Effects of Halides on the Corrosion of Mild Steel in Molten NaNO3-KNO3 Eutectic

Abstract

The effect of alkali halides on the corrosion behaviour of mild steel in, (Na,K)NO₃ eutectic has been studied at temperatures ranging from 300°–450°c. Steady-state potentials of the steel electrode vary with the concentration of Cl^?, Br^?, and I^? ions according to: E = a + b log C. Gravimetric; measurements show that the corrosion process of steel in the melt without and with halide ions, at concentrations ≤0·l M, is essentially the same, the weight gain being parabolic with time. At higher halide concentrations the corrosion rate is linear with time. The corrosion rate also increases with increase in temperature. Both potential and weight gain measurements show that aggressiveness increases in the order: Cl^?<Br^?<I^?. It is suggested that in this system two opposing forces compete on the steel surface. The oxide ions help in building or repairing a passivating film of Fe₃O₄, whilst the aggressive halide ions attack weak areas in the film and assist the corrosion process. The process which prevails depends on the relative concentrations of the two species.

In the case of fluoride, both the potential and weight gain of steel vary with time in an irregular manner, and no definite relation could be obtained. This has been attributed to the low solubility of fluoride in the melt and to the properties of the corrosion products, in that the film formed does not adhere to the metal surface.     

Corrosion behaviour of zirconium electrode in sodium azide electrolyte as compared to the alkali halide solutions

The kinetics of open circuit growth of an oxide film on zirconium electrode in NaN₃ solution of various concentrations was investigated using potential and capacitance measurements. The protective oxide film formed in azide solutions was found to thicken with time in two steps, the rate of oxide growth was found to decrease as the azide concentration increased. The corrosion behaviour of the electrode was characterized by ac impedance measurements to obtain detailed information about the effect of azide ion concentration on the electrical properties of the Zr electrode surface. The impedance response for the metal‐metal oxide‐electrolyte system was modeled with a transfer function. The potentiodynamic oxidation of Zr was also studied as a function of the azide ion concentration. The polarization curves showed the peak‐shaped active‐to‐passive transition and the corrosion rate was found to increase with increase of azide ion concentration. The activation energy of corrosion was calculated according to Arrhenius plot and found to be 14.5 kJ mol^?1. The polarization results in azide solutions were compared to those obtained in solutions of different halide ions, namely, F^?, Cl^?, Br^? and I^? ions. The rate of corrosion was found to decrease in the order Br^? > Cl^? > F^?, I^? > azide. Opposite to the behaviour in azide solutions, the halide ions do not show the active‐passive transition.     

Film Thickening on Nickel in Aqueous Solution in Relation to Anion Type and Concentration

Abstract

The variation of the open circutt potential of the Ni electrode in strongly aerated soluiion of different concentrations of Cl^?, Br^?, I^?, NO^?₃, SO^2?₄ and CrO^2?₄ is followdd till steady-stete values are established. In all solutions the steady potentials, E_sb, are approached from negative values, denoting film repair and thickening. Except in relatively concentrated Cl^? media, no Ni²⁺ is detected in solution. Plots of the Ni electrode potential, E, as a function of the logarithm of immersion time, t, are linear, satisfying the general relationship:

E = a_l + b₁log t ……(1)

where a₁ and b₁ are constants. Equation (1) is derived theoretically by taking into account the p-conducting properties of NiO, and assuming that the adsorption of the anions on the oxide film generates the electric field necessary to promoee the diffusion of Ni²⁺ through the oxide. The rates of oxide thickening in the presenee of different concentrations of the various anions are computed and discussed. Expressions are deduced relating E_st and the thicknsss of the oxide to the various parameters of the partial anodic and cathodcc reaction.The steady-stete potentials vary with the logarithm of the molar concentration of the various anions according to:E_st = a₂ ? b₂ log C ……(2)This relation is reached from a consideration of the structure of the double layer, and the effects produced thereon when the anions are specifically adsorbed on the surface of the electrod. Agreement between theory and experiment is established for Cl^?, I^? and SO^2?₄ soluiion. The adsorption of Br^? is assumdd to occur in the Br^2?₂ form. The results also indicaee that the CrO^2?₄ ion has a partial monovalent character. The adsorbability of the different anions tested decreases in the order: I^? (strong)> Cl^?> Br^?> SO^2?₄> NO^?₃ (weak)     

Anodisches Verhalten und Korrosion von Titan in methanolischen Lösungen

Anodic behaviour and corrosion of titanium in methanolic solutions The anodic behaviour of titanium in methanolic solutions containing halides or water in various concentrations has been investigated and the result were compared with the result obtained by corrosion tests performed on ?U”? bend specimens in similar solutions. Polarization curves can predict the susceptibility to titanium to stress corrosion cracking as in the case the metal surface is not passivable. These conditions can be obtained in the presence of low water content (i.e. 0.1% H₂O) and of activating ions as Cl^? and Br^? even in very small concentration (10^5?, 10^4? M). Presence of higher water contents and/or of F^? or I^? ions can to some extent produce passivation of the metal surface and cracking does not occur. A lower water amount in the solution (i.e. 130 ppm) eliminates any possibility of passivation of the metal surface, so that cracking can occur even in the absence of chlorides or bromides. High Cl^? concentration in the solution. (i.e. 0.1 M NaCl + 0.1% H₂O) can produce intergranular attack even in the absence of applied stress.     

The Effects of KCl,NaCl and K<Subscript>2</Subscript>CO<Subscript>3</Subscript> on the High-Temperature Oxidation Onset of Sanicro 28 Steel

The present study investigates the early stages in the oxidation process of Sanicro 28 (Fe31Cr27Ni) stainless steel when exposed to an alkali salt (KCl, NaCl or K₂CO₃) for 2 h at 450 and 535 °C. After the exposure, the oxidized samples were analyzed with a combinatory method (CA, XPS and SEM–EDX). It was found that all three salts were corrosive, and the overall oxidation reaction rate was much higher at 535 °C than at 450 °C. There were clear differences in terms of the impact of cations (Na⁺, K⁺) and anions (Cl^?, CO₃ ^2?) on the initial corrosion process at both temperatures. When focusing on the cations, the presence of potassium ions resulted in a higher rate of chromate formation than in the presence of sodium ions. When studying the effect of anions, the oxidation of iron and chromium occurred at higher rates in the presence of both chloride salts than in the presence of the carbonate salt, and chloride salts seemed to possess higher diffusion rate in the gas phase and along the surface than carbonate salts. Moreover, at the higher temperature of 535 °C, the formed chromate reacted further to chromium oxide, and an ongoing oxidation process of iron and chromium was identified with a significantly higher reaction rate than at 450 °C.     

Microstructure,microhardness and corrosion resistance of laser cladding Ni–WC coating on AlSi5Cu1Mg alloy

The microstructure, microhardness, and corrosion resistance of laser cladding Ni–WC coating on the surface of AlSi5Cu1Mg alloy were investigated by scanning electron microscopy, X-ray diffraction, microhardness testing, immersion corrosion testing, and electrochemical measurement. The results show that a smooth coating containing NiAl, Ni₃Al, M₇C₃, M₂₃C₆ phases (M=Ni, Al, Cr, W, Fe) and WC particles is prepared by laser cladding. Under a laser scanning speed of 120 mm/min, the microhardness of the cladding coating is 9–11 times that of AlSi5Cu1Mg, due to the synergistic effect of excellent metallurgical bond and newly formed carbides. The Ni–WC coating shows higher corrosion potential (−318.09 mV) and lower corrosion current density (12.33 μA/cm²) compared with the matrix. The crack-free, dense cladding coating obviously inhibits the penetration of Cl⁻ and H⁺, leading to the remarkedly improved corrosion resistance of cladding coating.     

Initiation and Inhibition of Pitting Corrosion in Alkali Solutions under potentiostatic polarization conditions

The variation of the critical pitting potential of a zincalloy was studied in aerated NaOH solutions as a function of the concentration of the aggressive ions, Cl^?, Br^? and I^?. Curves with segmented nature were obtained when E_pitting was plotted versus logarithm of the halogen ion concentrations. Initiation of pitting corrosion was discussed on the basis of formation of complex halo-compounds with the oxides/hydroxides that constitute the passive surface film. Addition of chromate, phosphate and carbonate ions to the halogen-containing solutions causes the shift of the critical pitting potential in the noble direction, accounting for increased resistance to pitting attack. Nitrite-ion additions contribute with the halogen ions in the destruction of the passive film. Sufficient concentrations of the carbonate ions cause complete inhibition of pitting corrosion.     

Investigations in the field of synthetic metals. VI. Synthesis and properties of dibenzotetrathiafulvalene halides

Direct oxidation of dibenzotetrathiafulvalene (DBTTF) by halogens in acetonitrile and nitrobenzene yielded DBTTF₂I₃, DBTTF·I₃, DBTTF·Cl_x (x = 0.7 ? 1), DBTTF·Br_x (x = 0.8 ? 1), and DBTTF·Br_1.2 complexes. Ir-, u.v.-, and e.p.r.-spectra of these compounds have been studied. X-ray data for the complex DBTTF·I₃ are discussed. Conductivities of the above mentioned complexes lie in the range 10^?2 – 10^?4 ohm^?1 cm^?1.     

Dissolution Current and Pitting Potential of Zinc in KOH Solutions in Relation to the Concentration of Aggressive Ions

Abstract

Potentiodynamic anodic polarisation curves for zinc were obtained in dilute (10^?3 M) solutions of KOH that favoured metal dissolution with only a slight tendency to passivate the zinc, and in more concentrated (10^?2 M) solutions that favoured passivation of the zinc. Addition of aggressive halide ions (Cl^?, Br^?, I^?) to the 10^?3 M solution caused a shift of the dissolution potential to more negative values and a marked increase in corrosion rate, suggesting adsorption of the anions on to the bare metal surface. The difference between the dissolution current density, i_o, in 10^?3 M KOH alone, and in the presence of halide ions, i_agg, was given by: log (i_agg?i_o) = a + b log C_agg, where a and b are constants.

In 10^?2 M KOH, small additions (2 × 10^?4 M) of aggressive ions did not affect the dissolution kinetics. At higher concentrations the c.d. increased sharply at a well defined potential, or pitting potential, E_p, given by the relation: E_p = α ? β log C_agg, α and β being constants. It is assumed that in 10^?2 M KOH the aggressive anion acts by penetrating the oxide, or by becoming incorporated in it as an impurity, altering its physical and electrical properties.     

Electrochemical Corrosion Investigations on Anaerobic Treated Distillery Effluent

Present study is focused on the corrosivity of anaerobic treated distillery effluent and corrosion performance of mild steel and stainless steels. Accordingly, electrochemical polarization tests were performed in both treated distillery and synthetic effluents. Polarization tests were also performed in synthetic solutions and it was observed that Cl^? and K⁺ increase whereas SO₄ ^?, PO₄ ^?, NO₃ ^?, and NO₂ ^? decrease the corrosivity of effluent at alkaline pH. Further, comparison in corrosivity of distillery and synthetic effluents shows the former to be less corrosive and this is assigned due to the presence of amino acids and melanoidins. Mild steel experienced to have the highest corrosion rate followed by stainless steels—304L and 316L and lowest in case of SAF 2205. Relative corrosion resistance of stainless steels is observed to depend upon Cr, Mo, and N content.     

The influence of alkali-halide additions on the stress corrosion cracking of an austenitic stainless steel in MgCl2 solution

The influence of NaCl, NaBr, NaI additions on the stress corrosion cracking (s.c.c.) of Type 304 stainless steel in 38 wt% aqueous MgCl₂ solution at 135°C has been investigated. Slow strainrate and U-Bend tests, potentiodynamic measurements and fractographic observation were employed. The results indicate clearly that the addition of 1N NaI or NaBr to the boiling MgCl₂ solution prevents stress corrosion cracking of the steel while NaCl additions accelerates cracking. It is suggested that the addition of NaI inhibits mainly the cathodic process. The I^? ion is oxidized by the dissolved oxygen through the following reaction: 41^? + 4H⁺ + O₂ → 2I₂ + 2H₂O.Due to this reaction O₂ and H⁺ are not reacted cathodically with the metal. This process enhanced the cathodic inhibition effect and results in preventing s.c.c. of the AISI 304 stainless steel in 38 wt% MgCl₂ boiling solution.