Dendritic Zinc Growth in Acid Electrolyte: Effect of the pH

In this paper, dendritic growth at the edges of electrogalvanized steel strip has been studied using a specially designed rotating washer electrode which simulates the fluid dynamic conditions and the current density distribution at the steel strip edge found in a production line. The effect of electrolyte pH and current density on dendritic growth in an acidic zinc plating bath (ZnSO₄ and H₂SO₄) was addressed. The temperature was kept constant at 60 °C. Solution pH was adjusted to 1, 2 or 3 using different amounts of H₂SO₄. In addition, the influence of temperature on the pH of the solution was determined. The current density was set at 40 or 60 A/dm², similar to that used in the industry. Deposits were characterized using SEM and XRD. The results showed that pH strongly affects dendrites shape, length and texture. Furthermore, the morphology of dendrites at the washer edge and of deposits on the flat portion of the washer changed considerably as solution pH was increased from 1 to 3. It was found that the morphology of dendrites at the washer edge stems from the morphology of the deposit on its flat portion, which in turn determines their shape.     

Evaluation of corrosion resistance of electrodeposited nanocrystalline Ni–Fe alloy coatings

Nickel–iron alloys with a compositional range of 24–80?wt-% iron were electrodeposited on a copper substrate from a sulphate-based bath and using a stirring rate of 100?rev?min^?1. The effect of applied current density and Ni²⁺/Fe²⁺ metal ion ratio of plating bath on the properties of alloy coatings was examined. Crystal structure and grain size of Ni–Fe alloy coatings were investigated using X-ray diffraction technique. Field emission scanning electron microscopy and energy dispersive X-ray spectroscopy were used to analyse the surface morphology and chemical composition of coatings. Microhardness test was applied to evaluate the hardness of the coatings. Finally, the electrochemical behaviour of the Ni–Fe alloy coatings was studied by a polarisation test in 10?wt-% H₂SO₄ solution. Results revealed that current density and plating bath composition had a strong effect on the characteristics of coatings. As the iron content of alloys produced increased, their corrosion resistance improved with the best corrosion resistivity being achieved at a metal ion ratio of 0.5 and applied current density of 2.5?A?dm^?2.     

Influence of Bath Compositions and Some Operating Conditions on the Electroplating of Cobalt from Aqueous Sulphate Baths

A systematic study has been made of the influence of aqueous sulphate bath constituents on the cathodic polarization curves during cobalt electrodeposition. The optimum bath composition has been established and it contains: 0.30 mol l^?1 CoSO₄.7 H₃0, 0.30 mol^?1 H₃BO₃. 0.05 mol l^?1 (NH₄)₂SO₄ and 0.10 mole l^?1 Na₂SO₄.10 H₂o. Cobalt electroplating from the optimized bath has been performed under a variety of conditions viz: pH, 1.5 to 5.3; current density, 0.2 to 2.0 A dm^?2 and temperature 25 to 53°C. Under the most suitable conditions, the optimum bath is characterized by a high cathodic efficiency (~ 99%), moderate throwing power (21.6%) and throwing index (1.47). The bath produced smooth, adherent and semibright plates characterized by a high microhardness (411.4 kgf mm^?2) and their morphologies, as revealed by scanning electron microscopy, showed a dependence on the plating variables.     

The Electrodeposition of Iron-Zinc Alloys

Methods are given for depositing iron-zinc alloys of 3 to 90% zinc content from sulphate baths and attention is drawn to the useful properties of these deposits. Under a given set of plating conditions the iron-zinc ratio in the deposit is directly proportional to that in the bath. Lowering either the current density or the pH raises the zinc content of the deposit. Some baths have a levelling action, since bright deposits can be prepared from them on an etched surface. Examples of such baths are: (i) FeSO₄-7H₂O 248, ZnSO₄-7H₂O 8·8, (NHJ₄)₂SO₄ 118, KCl 10, citric acid 0·5 g./l., operated at pH 1·7, 50°G, 200 amps./ft.² and giving a 6% zinc alloy of 560 D.P.N, hardness;

(ii) FeSO₄-7H₂O 174, ZnSO₄-7H₂O 88, (NH₄)₂ SO₄ 118, KCl 10, citric acid 0·5 g., Teepol 0·4 ml./l., operated at pH 1·7, 50°C, 180 amps./ft.² and giving a 60% zinc alloy of 350 D.P.N, hardness.

The throwing power of the baths is comparable with that of a bright nickel bath. Pitting can be overcome by using a wetting agent (Teepol or Lubrol W) and operating at high temperature (80° C.) and low pH (<1·8). Under these conditions the deposits are usually matt and light grey in colour.

Alloys with zinc contents >ca. 30% have electrode potentials in N/10 KCl nearly equal to that of pure zinc. In the C.R.L. beaker test, the alloys with zinc contents between 30 and 90% are, in general, more corrosion resistant than pure zinc. Various applications of these alloys are proposed, including their use as an undercoat for paints and chromium plating and for decorative finishes indoors.

Deposition of iron-zinc alloys from chloride baths is dealt with briefly. A matt, corrosion-resistant alloy of 60% zinc content can be obtained, at pH 1·8, 50° C., and 50 amps./ft.², from a vigorously stirred bath of the following composition:—FeCl₂·4H₂O 177, ZnCl₂ 42, NH₄Cl 100, KCl 15, citric acid 0·5 g./l.

A colorimetrie method for the analysis of zinc in the presence of iron is described.     

Das anodische und kathodische Verhalten des Eisens in sulfathaltigen Elektrolyten. Chaire d'Electrochimie,Faculté des Sciences,Strasbourg, France

The anodic and cathodic behaviour of iron in sulphate containing electrolytes The formation of Fe₂(SO₄)₃ on passive iron at p_H = 1 appears probable from a thermodynamical point of view. At high SO₄^2? concentrations the equilibrium system contains but low concentrations of Fe³⁺, and no Fe²⁺ ions, a fact showing the relatively elevated stability of the Fe₂(SO₄)₃ layer on passive iron. In slightly acid solution (p_H = 4) the passivity of the iron is determined by iron oxide layers. The formation of FeSO₄ from metallic iron and sulphate ions is restricted to the transpassive zone (p_H 4 to 7), in alkaline solutions even to the active zone. In the p_H region 2 to 14 the passive layer on iron has about the same composition in the systems Fe|H₂O + SO₄^2? and Fe|H₂O.     

Role of Thiamine Hydrochloride and Gelatin on the Electrodeposition of Zinc

A search for a non-cyanide zinc plating bath resulted on the development of a zinc acetate bath. To obtain bright zinc deposits, thiamine hydrochloride and gelatin were added. Hull cell studies revealed that in presence of 3 g dm^?3 thiamine hydrochloride a bright deposit was obtained at a current density above 2.5 A dm^?2. Addition of 3 g dm^?3 gelatin produced a bright yellow tinge deposit above 2.5 A dm^?2. Voltammetric studies carried out at a glassy carbon electrode in the plating solution revealed that in the pH range of 4.5-5.5, zinc acetate complexes underwent successive reduction to zinc. Acetate ions and pH affected both the dissolution and deposition of zinc. The dissolution of zinc took place with the participation of OH^? ions. Gelatin molecules adsorbed on the electrode surface. They favoured both zinc dissolution and deposition and prevented hydrogen evolution in the potential range of interest. Thiamine hydrochloride caused a reduction in zinc dissolution and deposition rates but favoured hydrogen evolution.     

Effect of KSCN and its concentration on the reactivation behavior of sensitized alloy 600 in sulfuric acid solution

The effects of KSCN addition and its concentration on the reactivation behavior of sensitized Alloy 600 in an H₂SO₄ solution were investigated. Single-loop electrochemical potentiokinetic reactivation (SL-EPR), potentiostatic and potential decay tests were conducted in H₂SO₄+KSCN solutions, including intermittent adjustment of the electrolyte composition and concentration. The SL-EPR results showed that the maximum current densities of the reactivation peaks increased with an increase in KSCN concentration from 0 to 0.05 M in the 0.5 M H₂SO₄ solutions. A decrease in the peak current density was observed, however, if the KSCN concentration was increased to 0.5 M. The potentiostatic test results at +400 mV_SCE showed that sensitized Alloy 600 could be passivated in plain H₂SO₄ and KSCN solutions as well as in mixed H₂SO₄+KSCN solutions. Potentiostatic tests at +30 mV_SCE revealed that the presence of low KSCN concentrations made the passive film less stable in H₂SO₄ solutions, but it enhanced passivation at 0.5 M. The adsorption and desorption of SCN⁻ and its subsequent redox reaction at different concentrations was discussed in relation to the reactivation of passive film formed on sensitized Alloy 600.     

Corrosion resistance of the Ti–50Zr at.% alloy after anodization in different acidic electrolytes

The stability of oxide films potentiodynamically (50 mV s⁻¹) grown up to 8.0 V(SCE) on Ti–50Zr at.% in H₂SO₄, HNO₃, CH₃SO₃H, and H₃PO₄ (pH ≈ 1) was assessed in the growth electrolyte itself or in a Ringer solution. For all anodizing electrolytes, oxide films become stabler as their thickness increases. In the Ringer solution, the oxide film stability is affected by the anodizing electrolyte; for oxides grown up to 8.0 V(SCE), films obtained in H₃PO₄ are slightly less stable than the ones obtained in the other acids, whereas films obtained in H₂SO₄ are clearly the stablest ones.     

Electrodeposition,characterisation and mathematical model of nickel–iron film in a parallel plate flow system

Abstract

Nickel–Iron alloy films were electrodeposited in a parallel plate flow system. The volumetric flow rate of electrolyte was fixed at 12 dm³ min^?1 through the 1 cm thick and 9 cm wide slit parallel plate. Fluid velocity was ca 0.22 m s^?1 under fully turbulent convective flow. Alloy films with iron content varying from 7·5 to 40 wt-% were deposited as a function of solution pH, temperature, bath ingredient concentration and applied current density. It is shown that the magnetic property is strongly correlated to the alloy content: the saturation moment, B _s, increases with the iron composition, while the coercivity, H _c, increases with nickel content. Current efficiency increases with pH and applied current density. The nickel deposition rate is inhibited in the presence of ferrous ion in the plating bath. The microhardness of the deposit is increased as the iron content is increased over the range studied. A mathematical model that considers the convective mass transfer of Fe(II) and Ni(II) species in the diffusion layer, the competition of adsorbed metal species on surface active sites, and Tafel electrochemical kinetics describe the alloy plating system well.     

Leaching of gallium from corundum flue dust using mixed acid solution

In order to extract gallium from a high-silica-content flue dust generated in corundum production, a mixed acid solution of H₂SO₄ and HF was used for leaching, and test parameters of the leaching process were optimized. Experimental results show that the leaching rate of gallium was only 38% when H₂SO₄ was used as leaching agent. Composition analysis results of micro areas in this corundum flue dust indicate that the content of gallium in silica-enriched phases was high; this portion of gallium was insoluble in H₂SO₄ solution. The leaching rate of gallium increased significantly with addition of HF due to corrosion of silica. Effects of reaction time, temperature, and concentrations of HF and H₂SO₄ on leaching rates of gallium were investigated. The leaching rate of gallium reached 91% when this corundum flue dust was leached in a mixed acid solution of H₂SO₄ and HF for 4 h, at a temperature of 80 °C, with a liquid-to-solid ratio of 5:1 (mL/g). The optimal concentrations of H₂SO₄ and HF in the mixed acid solution were 1.5 and 6.4 mol/L, respectively.     

Effects of temperature and pH level on the corrosion behavior of amorphous Co69Fe4.5Nb1.5Si10B15 alloy

The corrosion behavior of the amorphous Co₆₉Fe_4.5Nb_1.5Si₁₀B₁₅ (at.%) alloy ribbon in H₂SO₄ solutions (0.001 M or 0.07 M), NaCl solution (0.07 M), and HCl + NaOH solution were examined as functions of solution temperature and pH. The corrosion potential decreased when either the temperature or pH of the solutions increased. The corrosion resistance of the Co₆₉Fe_4.5Nb_1.5Si₁₀B₁₅ alloy in the 0.07 M NaCl solution was higher than the 0.001 M or 0.07 M H₂SO₄ solutions for a given temperature. The corrosion rate increased exponentially with an increase in temperature and was inversely proportional to the pH in the range of 10^?6 A/cm²～10^?4 A/cm².     

Peculiarities of electric conductivity changes of solutions of acids and alkalis under the effect of a glow discharge

The influence of atmospheric pressure glow discharges on the electric conductivity of H₂SO₄, Na₂SO₄, and NaOH solutions as electrolyte cathodes has been investigated. The electric conductivity has been calculated taking into account changes in the solution pH. These values are compared with experimental ones. The qualitative differences of the experimental and calculated dependencies are explained by the modification of structural features of the solution under the action of the glow discharge.     

Laboratory studies of atmospheric corrosion—I. Weight loss and electrochemical measurements

Electrochemical studies have been performed with the atmospheric corrosion monitor (ACM) under thin layers of electrolyte which were drying out at R.H. < 100%. Galvanic couples (Cu/steel, Cu/zinc) and one-metal (steel, zinc) ACMs were used. Measurements were carried out as a function of R.H. and Na₂SO₄ concentration. In addition, weight loss data were collected under identical conditions in thin layer experiments for steel and zinc in 0.01N solutions of NaCl, Na₂SO₄, HCl, H₂SO₄ and distilled H₂O in air, air + 1 ppm SO₂, argon and argon + 1 ppm SO₂. The data obtained in air and air + SO₂ were compared to weight loss results in bulk solutions.The electrochemical technique makes it possible to follow the changes of corrosion rates with time. As observed in outdoor exposure, a large increase of corrosion rates occurs when the electrolyte layers become very thin, shortly before the surface dries out. These findings explain the results of the weight loss data which show for most environments a much larger corrosion rate than in the bulk electrolyte. An accelerating effect of SO₂ was observed for steel at higher R.H. values, while for zinc, no effect occurred in NaCl, Na₂SO₄ and H₂SO₄, but an inhibiting effect was measured in HCl and in distilled H₂O.Since weight loss and electrochemical data were recorded under identical conditions, it is possible to determine how accurately the ACM data reflect the true corrosion rate. It was found for Cu/steel ACMs that the electrochemical data follow the same trends as the weight loss data, but account for only about 20% of the corrosion rate. Due to larger scatter in the weight loss data, a similar efficiency factor could not be determined for Cu/zinc. For steel and zinc ACMs, the true Tafel slopes are not known, which makes a calculation of corrosion rates doubtful. The low cell efficiency is considered to be due to local corrosion of single cell plates and to i.r.-drop effects.Despite the fact that exact corrosion rates cannot, at present, be obtained from ACM data, the technique appears very valuable for following the changes of atmospheric corrosion behaviour and for time-of-wetness measurements.     

Effect of ferric ions in AISI 316L stainless steel pickling using an environmentally‐friendly H2SO4‐HF‐H2O2 mixture

A mixture of hydrogen peroxide, sulphuric and hydrofluoric acids has been used as pickling solution at pH 2.0 for AISI 316L austenitic stainless steel (SS). The stability of the H₂SO₄‐HF‐H₂O₂ mixture is assessed varying the ferric ions content from 0 to 40 g/L, the temperature from 25 to 60°C, and with and without stirring of the pickling solution. The AISI 316L SS pickling rate at 50°C was 2.6 and 0.2 mg/dm² day (mdd) in the absence and presence of 40 g/L ferric ions, respectively. p‐toluene sulphonic acid (PTSA) has been used as stabiliser of hydrogen peroxide.     

Enhanced lithium leaching from lepidolite in continuous tubular reactor using H2SO4+H2SiF6 as lixiviant

An acidic mixture of sulfuric and fluosilicic acid (H₂SO₄+H₂SiF₆) was employed as lixiviant to enhance leaching of lithium from lepidolite. The H₂SiF₆ was obtained as a byproduct of anhydrous hydrofluoric acid production, aiming to provide HF molecules. It was found that the HF molecules were the main reaction component and played a key role in strengthening the dissolution of lepidolite. Different factors, including mass ratio of ore/H₂SO₄/H₂SiF₆, concentrations of H₂SO₄ and H₂SiF₆, leaching temperatures (40−80 °C) and time (15−75 min), were investigated. Moreover, an efficient tubular reactor was employed to improve this acid leaching system. Under the optimal conditions (ore/H₂SO₄/H₂SiF₆ mass ratio of 1:0.8:1.6, 80 wt.% H₂SO₄, 15 wt.% H₂SiF₆, 80 °C, 15 min), 97.9% of Li, 96.4% of K, 97.6% of Rb, 96.7% of Cs and 81.4% of Al (mass fraction) were leached. Additionally, a two-step thermal process was proposed to remove fluorine of leaching slurry. This acid treatment using an acidic mixture of H₂SO₄ and H₂SiF₆ in a continuous tubular reactor shows potential as an alternative process to extract lithium from lepidolite.     

Methanol oxidation on aluminium-copper-silicon alloys coated with polyprrole

Methanol oxidation on the aluminum-copper-silicon alloys which are coated with polyprrole were investigated in 1 N H₂SO₄ by using electrochemical method. For this purpose, first the current densitypotential curve of alloys were obtained in 1 N H₂SO₄ + x M pyrrole solutions, determined passive zones are coated with polyprrole alloys in H₂SO₄ solutions. Then the current-potential curves were obtained in 1 N H₂SO₄ + x M methanol with aluminum alloys and these are expected at +1.5 V against to the standard calomel electrode (SCE) at the different times in 1 N H₂SO₄ + 10⁻³ M pyrrole that were obtained with different scan rates. The extreme methanol oxidation was seen on the E110 and E140 alloys which are coated with polyprrole. Embedded pH electrode solutions which were coated with polypyrrole in 1 N H₂SO₄ and 1 N H₂SO₄ + 0.5 M methanol solutions were measured after 30 minutes at +2.1 V.     

Pitting and pitting inhibition of iron in sodium sulphate solutions

The anodic behaviour of high purity iron in 0.5 M sodium sulphate solutions was studied. Experiments were made in both acid and alkaline solutions (pH 2.7, pH 9.0, pH 10.0 without buffers; and pH 9.2 with borate buffer). Anodic polarization curves, and surface scratching experiments, showed pitting potentials in 0.5 M Na₂SO₄ pH 9.0 and pH 10.0 solutions. Their values were very close to the corrosion potential obtained in a 0.5 M Na₂SO₄, pH 2.7, pit-like solution. The pitting potential in a borate buffered 0.5 M Na₂SO₄ solution was 50 mV higher than that in the unbuffered solutions. The pitting inhibition potential measured in a 0.5 M Na₂SO₄ solution, pH 10.0, was very close to the passivation potential found in the pit-like solution. All these facts can be explained by the localized acidification mechanism for pitting. The pitting potential is the minimum potential at which an acidified solution can be produced and maintained in contact with the dissolving metal. Similarly the pitting inhibition potential is the electrode potential at which the metal becomes passive in the pit-like solution.     

Separation and Precipitation of Nickel from Acidic Sulfate Leaching Solution of Molybdenum-Nickel Black Shale by Potassium Nickel Sulfate Hexahydrate Crystallization

Nickel was separated and precipitated with potassium nickel sulfate hexahydrate [K₂Ni(SO₄)₂·6H₂O] from acidic sulfate solution, a leach solution from molybdenum-nickel black shale. The effects of the potassium sulfate (K₂SO₄) concentration, crystallization temperature, solution pH, and crystallization time on nickel(II) recovery and iron(III) precipitation were investigated, revealing that nickel and iron were separated effectively. The optimum parameters were K₂SO₄ concentration of 200 g/L, crystallization temperature of 10°C, solution pH of 0.5, and crystallization time of 24 h. Under these conditions, 97.6% nickel(II) was recovered as K₂Ni(SO₄)₂·6H₂O crystals while only 2.0% of the total iron(III) was precipitated. After recrystallization, 98.4% pure K₂Ni(SO₄)₂·6H₂O crystals were obtained in the solids. The mother liquor was purified by hydrolysis-precipitation followed by cooling, and more than 99.0% K₂SO₄ could be crystallized. A process flowsheet was developed to separate iron(III) and nickel(II) from acidic-sulfate solution.     

Effect of Some Operating Variables on the Characteristics of Electrodeposited Cu-α-Al2O3 and Cu-TiO2 Composites

Copper- χ-Al₂O₃, and Copper-TiO₂ composites were electrodeposited from acidic baths containing 150 CuSO₄.5H₂O₂ 50 Na₂SO_4.10H₂O, 25 H₃BO₃ and either 1–20 gl^?1 α-Al₂O₃ or TiO₂ particles suspended in the bath. The effects were studied of particle size, particle concentration and plating conditions on the cathodic polarization, current efficiency, inert particle content in the composite deposit and on the surface morphology and microhardness of the as-received plate. A mechanism of coelectrodeposition of the inert particles with copper could be suggested from measurements of the cathodic polarization. The efficiency of copper electrodepostion from the selected bath was relatively high (98.5%) and was increased further by increasing the current density and by raising the pH of the bath. The inert particle content reached 1.7% for α-Al₂O₃ and 2.3% for TiO₂. At a current density of 0.66A dm^?2, the microhardness of the copper plate approached 100 kgf mm and the inclusion of the inert particles in copper composites led to an increase of about 25% in the microhardness. A further increase in the microhardness of copper-α-Al₂O₃, was achieved by raising of the deposition current density, where it reached 165 kgf mm^?2 at 1.66 A dm^?2. Scanning electron microscopy examination showed a direct correlation between both the inert particles content, their morphological shape and the microhardness of the copper composites.     

Initial reactions of SO2 after adsorption on to metals

An electrophoretic method for separation of labelled corrosion products from metals exposed to atmospheres containing natural concentrations of ³⁵SO₂ has been developed. Using this method, it was found that on iron and zinc such products are exclusively sulphur (IV) species and sulphate. The conversion of S(IV) → S(VI) on both iron and zinc was found to have a half-life of approx 24 h.The implications of these results are discussed with respect to a different chemistry of surface electrolyte layers from that of bulk solutions.