A study on zinc electrodeposition onto zinc cathode

Abstract

The electrodeposition of zinc onto a zinc cathode was studied through the comparison between theoretical and experimental polarisation curves. The former was constructed using the Butler–Volmer equation with electrochemical parameters, such as Tafel slope values, exchange current densities, equilibrium potential values, etc. The latter was obtained from data derived in a chloride based electrolyte of 0·6 mol dm^?3 Zn²⁺ ions with boric acid (25 g dm^?3), pH = 5. The electrochemical parameters such as exchange current density, limiting current density, equilibrium potential, Tafel slopes, can be obtained with good agreement with the values from the literature.     

Study of molybdate–phytic acid passivation on galvanised steel

Abstract

A combined molybdate–phytic acid passivation treatment on galvanised steel is studied in this paper. Both copper decoration drop tests and immersion mass loss tests indicated that the combined molybdate–phytic acid passivated samples showed better corrosion resistance than that of samples treated separately by molybdate or phytic acid. The electrochemical behaviours of the passivated samples were investigated using potentiodynamic polarisation and electrochemical impendence spectra in 0·5 mol L^?1 NaCl solution and showed corrosion current densities of 4·95×10^?5, 5·10×10^?5 and 1·19×10^?5 A cm^?2 respectively. Electrochemical impendence spectra also indicated that molybdate–phytic passivated samples exhibited better corrosion resistance. The mechanism of synergetic effect for molybdate–phytic acid passivation on galvanised steel is discussed in brief.     

Corrosion inhibition performance of polymer-alloy bilayer coatings: electropolymerised polyaniline and electrodeposited ZnNi with different layer orders

Polyaniline (PANI) conducting polymer coatings have been obtained galvanostatically with various current densities (from 0·1 to 0·4 mA cm^?2) and ZnNi alloy coatings have been obtained galvanostatically at 30 mA cm^?2 current density. Corrosion protection performances of monolayered PANI and ZnNi alloy coatings and multi-layered ZnNi/PANI and PANI/ZnNi coatings on st-37 low carbon mild steel (MS) have been investigated by an open circuit potential method, Tafel extrapolation method and electrochemical impedance method in 3·5 wt-% NaCl solution. In addition, the surface morphology of the coatings has been characterised by using scanning electron microscopy (SEM). Synthesising PANI films between two metal layers provided better corrosion protection to the steel. MS/PANI/ZnNi layer formation exhibited the biggest corrosion protection performance among all layer formations of the films and protected MS for up to 72 h.     

Anodic dissolution of nickel in acidic chloride solutions

The anodic dissolution of nickel was studied galvanostatically in hydrochloric acid solutions of various concentrations. The reaction orders of chloride ion and hydrogen ion concentrations were found to be 0.5 and 1.0, respectively. An anodic Tafel slope equal to 120 ± 10 mV · decade⁻¹ was obtained. The dissolution rate of nickel at constant acid concentration was increased with stirring of the solution and increasing temperature. The activation energy, ΔH, for the anodic dissolution process was found to be 12 kcal · mol⁻¹. The presence of oxygen in solutions assisted the passivation process. The effect of addition of aniline and some of its derivatives (o-, m-, and p-anisidine) as inhibitors on the dissolution kinetics of Ni in 1 M HCl was also investigated. These compounds inhibited the anodic dissolution of nickel without affecting the Tafel slope, indicating that the adsorption of such inhibitors could not interfere with the mechanism of metal dissolution.     

Nickel electrodeposition from novel lactate bath

Abstract

A new electroplating bath for nickel deposition has been developed. Lactic acid was used as a complexing agent in the bath replacing boric acid. The effect of bath composition, current density, pH and temperature on the cathodic polarisation, cathodic current efficiency, morphology and structure of the deposit was carried out. Optimum conditions for producing sound and satisfactory nickel deposits were: 0·30M NiSO₄.7H₂O, 0·50–1·0M lactic acid and 0·3M Na₂SO₄ at pH=10, c.d.=0·98 to 9·80 mA cm^?2 and 25°C. The surface morphology of the deposited nickel was investigated using SEM. The crystal structure was examined by X-ray diffraction analysis. The results showed that the nickel deposits have a face centred cubic structure.     

Studies on magnesium and its alloys in battery electrolytes

Abstract

Studies on the anode efficiency of magnesium and its alloys in battery electrolytes such as magnesium perchlorate, magnesium bromide, and magnesium chloride solutions were made by galvanostatic polarisation and corrosion rate measurements. The open circuit potential and corrosion rate of magnesium increase when lead is present in the alloy. The corrosion rates were observed to follow the order Mg> AP65 (Mg?(6·0?7·0) Al?(4·4?S·0)Pb) > AZ61(Mg?(5·8?7·2)Al?(0·5?1·5)Zn) > AZ31(Mg?(2·5?3·5)Al?(0·6?1·4)Zn) in all three electrolytes. Anode efficiency increased with increasing current density up to 40 mA cm^?2, but decreased above this level. Galvanostatic polarisation results indicated that corrosion of magnesium and its alloys in these electrolytes occurs under cathodic control. Of the tested alloys, AZ31 and AZ61 were found to be most suitable in respect of corrosion rate and anode efficiency in 2M Mg(ClO₄)₂ solution.     

Electrochemical study on corrosion behaviour of zircaloy-2 in aqueous medium containing permanganate ion

Abstract

The corrosion compatibility of zircaloy-2 was studied in strong oxidising permanganate based acidic (HMnO₄) medium using impedance spectroscopy and potentiodynamic anodic polarisation methods at room temperature (28°C) and elevated temperatures (65 and 90°C). The average corrosion rate of zircaloy-2 in HMnO₄ medium (0·417–8·33 mM) was found to be 1·32×10^?4 μm h^?1 at 28°C, 2·59×10^?4 μm h^?1 at 65°C and 3·53×10^?4 μm h^?1 at 90°C. At 28°C, the polarisation resistance R_p values obtained from impedance spectrum were higher (in kΩ) when compared to R_p values at 65 and 90°C, indicating a lower corrosion rate at lower temperature. Comparative studies with 2·5 mM H₂SO₄ and platinum (as a working electrode) showed the effects of oxidising nature and participation of additional redox reactions in HMnO₄ medium. Cyclic polarisation studies showed the absence of pitting attack on zircaloy-2.     

Studies on copper-semiconducting layer-electrolyte systems—I. Electrode potentials of copper and electrodeposited copper sulphide in S2− and Cu2+ media

Copper sulphide layers electrodeposited from a Na₂S bath consist chiefly of cuprous sulphide. Calculated values of the specific conductance of the deposited sulphide average around 3·8 × 10^?5 ohm^?1.cm^?1.Electrode potentials, E_H, of the Cu/Cu_2?δS in unbuffered Na₂S solutions and in solutions buffered at pH 9, correspond to the behaviour of a Cu/Cu_2?δS, overlayered with CuS/electrolyte. Steady reproducible potentials are obtained with the above electrodes in 0·5–10^?3M CuSO₄ solutions, closely comparable with those of the reversible Cu/Cu²⁺ couple.     

Redox behavior of indium in molten chlorides

An electrochemical study on the redox behavior of indium in the eutectic LiCl-KCl system at 450 °C was carried out with the transient techniques of cyclic voltammetry and chronopotentiometry on an inert molybdenum electrode. The reduction of In(III) was found to be a two-step process involving In(III)/In(I) and In(I)/In couples at the potentials of about ?0.4 and ?0.8 V versus Ag/AgCl, respectively. The redox mechanism was further confirmed by the theoretical evaluation of the number of transferred electrons based on cyclic voltammetry and characterizations of the precipitates generated by the potentiostatic electrolysis. The diffusion coefficients of indium ions in the eutectic LiCl-KCl melt at 450 °C were estimated by cyclic voltammetry and chronopotentiometry. The results obtained through the two methods are in fair agreement, delivering an average diffusion coefficient of approximately 1.8×10^?5 cm²/s for In(III), and 1.4×10^?4 cm²/s for In(I).     

The mechanism of caustic cracking of carbon steels—I. Influence of electrode potential and film formation

The anodic behaviour of a 0·1% carbon steel wire in 10M sodium hydroxide solution at 121°C has been studied at different electrode potentials under static conditions, and while the wire yields at strain rates between 1·5 and 436%/min.The static material forms a fairly coherent and adherent film of magnetite in the potential range ca. ?0·60 to ?0·85 V(she), while the anodic current density falls to ca. 0·2 mA/cm² (at ?0·70 V) in ca. 40 min. When such a specimen is made to yield at 436%/min, the overall current density continuously rises to over 8 mA/cm² at the bared metal area produced by the cracking of the oxide film. This current density is sufficient to account for the rate of crack propagation measured microscopically on specimens strained at 1·5%/min. Equivalent results are reported for potentials over the whole range ?0·60 to ?0·85 V(she).On either side of this potential range little or no adherent film is formed, the anodic current density does not fall below ca. 1 mA/cm² and on straining rises to no more than ca. 5 mA/cm²; and no cracks are produced.The results strongly support the theory that crack propagation occurs because bared metal at the yielding advancing edge of a stress-raising crack can dissolve several hundred times as fast as that at the static crack sides, which are continuously protected by the growth of film. The crack thus maintains its acuity, and the raised stress at its edge maintains ductile yielding of the metal so that the electrochemical crack advancement proceeds without the need for any mechanical cracking.A comparison with previously reported work on the same steel in hot concentrated nitrate solutions shows that the range of anodic current densities found on bared metal, and the corresponding range of crack propagation rates, are about an order of magnitude less in the present hot concentrated hydroxide solution.     

Zur Korrosion des Hafnium – Oxide und Hydride

Corrosion of hafnium – oxides and hydrides Investigations concerning the growth and behaviour of hafnium as well as electrode kinetics have been performed on hafnium (Goodfellow and Material Research Company) in various electrolytes (acidic, alkaline and Cl^?-containing solutions). Conventional electrochemical methods and impedance spectroscopy were applied. Corrosion rates derived from electrochemical measurements were found to be significantly higher than the published data obtained from the loss weight observed usually over long of time. – The DK was evaluated to be 11.5. – SIMS profiles indicate that the oxide layer in not uniform, due to the nonstoichiometry of the Hf oxide at 22°C. The layer shows a very low n-conductivity – quasi insulator behaviour; the Mott-Schottky-plot leads to the flat band potential U = ? 10 mV. The initial oxide layer was determined by capacity measurements to 2.7 nm. Cl₂-evolution begins at U = 0.5 V in 0.5 M NaCl solution. In the cathodic potential region hydrogen is generated at Hf oxide at U = ? 1.69 ? 0.058 V. With respect to the hydrogen induced cracking of Hf the behaviour of hydrogen in Hf was investigated using the nuclear physics ¹⁵N-method. After cathodic polarisation hydrogen is absorbed by hafnium and the hydride HfH_1.6 is formed. The hydrogen diffusion coefficient is determined to D_20° = 3.8 × 10^?11 cm²s^?1. At 385°C hydrogen is released from HfH_1.6. The results are discussed and compared to those found for titanium and zirconium.     

Corrosion of Metals by Applied Alternating Currents

Abstract

The corrosion of mild steel in 0·05 M NaCl at 30°c under the influence of an applied square wave a.c. signal was studied. A mechanism for a.c. corrosion is presented, stressing the importance of reactions during a preceding cathodic half-cycle to the reactions during an anodic half-cycle and vice versa.

The corrosion rates of copper, lead and zinc under applied a.c. were also determined for comparative purposes.

It has been established that for 50 Hz sine wave a.c. applied to mild steel a peak current density of 471 mA.cm^?2 must be exceeded before serious corrosion sets in. This has been verified by a comparison of the corrosion rates as encountered in two electrode boilers operating at current densities above and below this critical value.     

Measurements of hydrogen activity in iron during cathodic protection using a potentiometric concentration cell

A solid - state potentiometric sensor, based on the well - tried principle of the thermodynamic concentration cell, has been constructed. The sensor uses a conventional bi-electrode design with an entry side and an exit side at which the hydrogen is detected. The sensor has been used for the quantitative determination of the hydrogen activity (equivalent H₂ pressure) generated during cathodic protection. Thus, the background hydrogen pressure in uncharged steel is 10^?17.7 (2 × 10^?18 atm.) with a variation of about 20 times. During cathodic protection this value rises by large amounts. Thus, using a zinc anode, the equivalent hydrogen pressure rose by 10^10.3 (to 4.4 × 10^?8 atm.) in 3.5% NaCl and by 10^16.9 (to 0.16 atm.) in artificial sea water, indicating the much larger amounts of hydrogen present in the latter case. This compares, for example, with literature data of 0.11 μA cm^?2 and 0.60 μA cm^?2 for hydrogen permeation at ?1000 mV (SCE) in 3.5% NaCl and artificial sea water respectively. Hydrogen entry and exit was also considerably slowed in sea water. These differences are caused by local surface pH buffering and deposit formation.     

Microstructure and corrosion resistance of tantalum after nitrogen ion implantation

This paper investigates the effect of nitrogen ion implantation on surface structure as well as resistance against tantalum corrosion. Bulk Ta surface was implanted with 30?keV nitrogen ions at a temperature of 100°C with doses between 1?×?10¹⁷ and 1?×?10¹⁸?ions/cm². The implanted samples were characterised by atomic force microscopy, X-ray diffraction analyses and the corrosion test to identify structural, compositional and electrochemical changes at various doses. The experimental results indicate the formation of hexagonal tantalum nitride (TaN_0.43), in addition to the fact that by increasing the ion dose, nitrogen atoms occupy more interstitial spaces in the target crystal, a case which can significantly improve corrosion resistance. The maximum extent in the improvement of the micro hardness was 75% and the reduction in the corrosion current was 83%. According to scanning electronic microscopy and corrosion results, in the dose of 1?×?10¹⁸?ions/cm² the highest corrosion resistance was received against the H₂SO₄ corroding media.     

Low-energy ion beam treatment of α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>(0001) and improvement of photoluminescence of ZnO thin films

A low energy N₂ ^? ion beam impinged on a α-Al₂O₃(0001) single crystal surface in the range of fluence 5×10¹⁵/cm²?1×10¹⁸/cm² at room temperature. After ion bombardment, chemical bonding on the modified sapphire surface was investigated by x-ray photoelectron spectroscopy. Below a fluence of 1×10¹⁵/cm², only a non-bonded N1s peak at the binding energy 398.7 eV was found, but further irradiation up to 2×10¹⁷/cm² induced Al?O?N bonding at around 403 eV. The occurrence of Al?N bonding was identified at ion fluence higher than 5×10¹⁷/cm² at 396.6 eV. II–VI ZnO thin films were grown on an untreated/ion-beam-induced sapphire surface by pulsed laser deposition (PLD) for the investigation of the modified-substrate effect on photoluminescence. The ZnO films grown on modified sapphire containing Al?O?N bonding only, and both Al?O?N and Al?N bonding showed a significant reduction of the peak related to deep-level defects in photoluminescence. These results are explained in terms of the formation of Al?N?O and Al?O?N layers and relaxation of the interfacial strain between Al₂O₃ and ZnO.     

Effect of phosphate ions on electrochemical behaviour of copper in sulphate solutions

Abstract

The effect of phosphate ions on the corrosion behaviour of copper in sulphate solutions was investigated at different temperatures (293, 313, 333, and 353 K) and at different pH values (pH = 2·1, 7·2, and 12·3). A three electrode electrochemical technique was used for the measurements, using a saturated calomel electrode (SCE) for reference purposes. Both anodic and cathodic polarisation curves were established, from which the anodic and cathodic Tafel constants were determined. Corrosion potentials were determined by extra trapolation of the anodic and cathodic Tafel lines to the point of zero current density. The results showed that for a pH value of 2·1, the corrosion rate of copper showed a marked tendency to increase with increasing temperature in solutions containing either sulphate alone or sulphate plus phosphate. However, at pH values of 7·2 and 12·3, this tendency became less pronounced in solutions containing sulphate alone and in solutions containing sulphate plus phosphate together, max imum corrosion rates were greatly reduced and showed less tendency to increase with increasing temperature. Anodic and cathodic reaction mechanisms are proposed to explain the observations for the corrosion of copper.     

In-situ synthesis of silicide coatings on molybdenum substrates by electrodeposition in chloride-fluoride molten salts

Silicide coatings including MoSi₂ and Si-MoSi₂ were prepared on molybdenum substrates by electrodeposition in NaCl-KCl-NaF-K₂SiF₆ molten salt. The experimental was conducted under different cathodic current densities at the temperature of 1073 K. The effect of the current density on the microstructure, phase composition, cross-section morphologies and elemental distribution of the as-prepared coatings were investigated by means of SEM, XRD and EDX. The results revealed that the type of the silicide coatings was strongly dependent on the current density and the relevant deposition mechanism was discussed. Besides, the electrochemical behavior of silicon ion in the chloride-fluoride molten salts was also studied using cyclic voltammetry and chronopotentiometry to reveal the electroreduction mechanism. The reduction of Si(IV) to Si was proved to be a quasi-reversible or irreversible diffusion-controlled single-step reaction and the diffusion coefficient of Si(IV) calculated from chronopotentiograms was (1.28 ± 0.25) × 10⁻⁵ cm²·s⁻¹.     

Hydrogen evolution reaction on spheroidal graphite cast iron with different pearlite areas in sulphuric acid solutions

The rate equation of hydrogen evolution reaction of spheroidal graphite cast iron with different pearlite area has been studied in sulphuric acid solutions at 298 K. The cathodic Tafel slope of ?0·130 V/decade and the reaction order with respect to the activity of hydrogen ion of 1 are obtained by linear potential sweep technique. The rate equation of hydrogen evolution reaction does not depend on the area of pearlite. There is no difference in hydrogen evolution reaction mechanisms between pure iron and spheroidal graphite cast iron.     

Behaviour of Aluminium in Alkaline and Neutral Tartrate and Citrate Solutions

Abstract

The anodic polarisation of aluminium (2S) in a solution containing 1 M-sodium hydroxide, 0·3/ sodium chloride and 1–10% sodium tartrate and saturated with calcium hydroxide has been studied. The anode utilisation efficiency has been determined at various current densities starting from 2 mA cm^?2. It has been found that the corrosion of aluminium in a solution containing 10% tartrate, alkali and chloride is reduced by addition of calcium hydroxide. It has, however, been observed that under similar conditions, tartrate causes a greater degree of polarisation than citrate, indicating that tartrate may not be as effectiveas citrate in complexingaluminium and may even favour anodic oxidation.     

Mechanism of Action of Acidified Sodium Phosphate Solution as a Corrosion Inhibitor of Iron in Tap Water

Abstract

The influence of phosphate as acid Na₃PO₄ solution in tap water was investigated using gravimetric and potentiostatic techniques. It was found that the phosphates are adsorbed in the form of positively charged colloidal particles on the cathodic areas of the metal. The degree of coverage, θ, vs log concentration of PO₄^3? shows good agreement with Frumkin's isotherm in the form:

θ/1?θ exp (?2.55θ) = 2530.CpO^3?₄

The calculated standard free energy of adsorption was found to be ? G_ads° = ?28·8 kJ.mol^?1 (or ?6·9 kcal.mol^?1).