The anodic dissolution of Mg in NaCl and Na2SO4 electrolytes by atomic emission spectroelectrochemistry

The dissolution of Mg has been investigated with atomic emission spectroelectrochemistry at open circuit and during potentiostatic control in chloride (NaCl) and sulfate (Na₂SO₄) electrolytes. A dissolution stoichiometry of approximately n = 2 is observed under all the conditions of these experiments with no evidence for the commonly invoked Mg⁺ intermediate. Nevertheless, in chloride electrolyte it is shown that anodic polarization results in a cathodic activation of the surface with increased hydrogen production during the polarization and an increased corrosion rate following the pulse. The formation of insoluble Mg oxides/hydroxides was also investigated in the sulfate electrolyte.     

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.     

A comparative H2S corrosion study of 304L and 316L stainless steels in acidic media

H₂S corrosion of 304L and 316L in oxygen-free Na₂SO₄ + Na₂S solution at pH 3 and temperature of 60 °C were investigated by EIS, potentiodynamic polarisation, multi-component Pourbaix diagrams and microstructure characterization. At similar conditions, lower corrosion rate was observed on 316L, attributed to its denser (1.5 times) and smoother (6%) surface layer and confirmed by SEM micrograph. During polarisation, H₂S increases significantly the critical current density on 304L and passivation current density, i_p, on 316L. Higher i_p on 316L was associated to simultaneous FeS₂–MoS₂ preservation, confirmed by XRD examination. H₂S could have an inhibiting effect on 304L in passivity region.     

Observations of anomalies in the anodic behaviour of microcrystalline aluminium

The anodic behaviour of sputtered microcrystalline Al (mc-Al) was investigated in neutral Na₂SO₄ electrolyte under varied conditions. Our results revealed that Cl⁻ addition led to a reduction in the anodic current density, which we considered unusual. Mott-Schottky analysis showed that Cl⁻ introduction altered the semiconducting property of the passive film from n-type to p-type, implying that the p-type film can possessed a relative higher stability. Immersion of mc-Al in other electrolytes yielded films with n-type, p-type and positive p-n junction structure. The results also indicate that the p-type film was most stable and the positive p-n junction film least stable.     

Development of anodic coatings on aluminium under sparking conditions in silicate electrolyte

Spark anodizing of aluminium at 5 A dm⁻² in sodium metasilicate/potassium hydroxide electrolytes is studied, with particular emphasis on the mechanism of coating growth, using transmission electron microscopy and surface analytical techniques, with coatings typically 10 μm, or more, thick. Two-layered coatings develop by deposition of an outer layer based on amorphous silica, associated with low levels of alkali-metal species, at the coating surface and growth of an inner, mainly alumina-based, layer, with an amorphous region next to the metal/coating interface. Formation of crystalline phases in the inner layer, mainly γ-Al₂O₃, with some α-Al₂O₃ and occasional δ-Al₂O, is assisted by local heating, and possibly also by ionic migration processes, arising from the rapid coating growth at sites of breakdown. Due to local access of electrolyte species in channels created by breakdown events, the silicon content in the inner coating regions varies widely, ranging from negligible levels to about 10 at.%. Silica deposition at the coating surface and formation of Al₂SiO₅ and Al₆Si₂O₁₃ phases is promoted by increased time of anodizing and concentration of metasilicate in the electrolyte. However, at sufficiently high concentration of metasilicate and pH, when more extreme voltage fluctuations accompany breakdown, the two-layered nature of coatings is replaced by a mixture of aluminium-rich and silicon-rich regions throughout the coating thickness.     

Effect of sulphate ion on the electrochemical polymerization of pyrrole and N-methylpyrrole

Pyrrole and N-methylpyrrole were electrochemically polymerized in an identical manner in aqueous electrolytes containing (Et₄N)BF₄, LiClO₄ or Na₂SO₄ to give films of the corresponding p-doped polymers. The polymer obtained from the aqueous Na₂SO₄ electrolyte, [(poly N-methylpyrrole)^+y (SO₄)_y/2^2-]_x, differed from the other polymers in that it exhibited a very low conductivity (≈ 10^?7 S/cm), contained a high concentration of > C=O groups and had an EPR linewidth, ΔH_pp, more than an order of magnitude greater than that found in the corresponding polymers obtained from the (Et₄N)BF₄ and LiClO₄ electrolytes. A sample of [(polyN-methypyrrole)^+y(BF₄)_y^?]_x electrochemically synthesized in CH₃CN and subsequently converted electrochemically to the corresponding sulphate in the Na₂SO₄ aqueous electrolyte exhibited normal behaviour. A mechanism is proposed for the introduction of > C=O groups into the poly(N-methylpyrrole) polymer during its electrochemical polymerization in the Na₂SO₄ aqueous electrolyte.     

Anodic polarization of tungsten in neutral and alkaline solutions under conditions of anode film growth

Anodic polarization of W has been studied in neutral (Na₂SO₄, NaCl) and alkaline solutions (ammonium borate, Na₂CO₃, Na₃PO₄, and NaOH) by the galvanostatic method under conditions which lead to anode film growth. The rate of oxide growth has been calculated as a function of current density, nature and concentration of electrolyte. Film growth to potentials above 100 V is achieved in alkaline solutions, provided the solution is dilute and the current density is high. The applicability of the high field ionic conduction has been verified from the dependence of both reciprocal capacity and potential on the logarithm of current density. Data are recorded for the coulometric increase of oxide thickness. The constants of the exponential law, and the electric field.     

Electropolishing behavior of pure titanium in sulfuric acid–ethanol electrolytes with an addition of water

The electropolishing behavior of commercially pure Ti was studied in sulfuric acid–ethanol electrolytes with a small amount of H₂O. The experimental results showed that commercially pure Ti could be electropolished well in the 3 M H₂SO₄–ethanol electrolyte, while inadequately electropolished in 0.5 and 1 M H₂SO₄–ethanol electrolytes. However, the brightening effect on the Ti surface could be clearly observed by the addition of a few volume percent of H₂O to the 0.5 and 1 M H₂SO₄–ethanol electrolytes. The effect of a small amount of H₂O in the H₂SO₄–ethanol electrolytes on the electropolishing behavior of pure Ti was discussed.     

Effect of applied voltage and fluoride ion content on the formation of zirconia nanotube arrays by anodic oxidation of zirconium

Anodised zirconium (Zr) foil at 20 V for 1 h in Na₂SO₄ with 0.5 wt.% NH₄F resulted in 6 μm thick anodic oxide film with nanotubular structure. At 50 V the anodic layer is consisted of an irregular inner layer with the nanotubular structure only at the top of the layer. Nonetheless, the anodic oxides formed are crystalline and photocatalytic ability of the anodic oxide was assessed by photodegradation of methyl orange.     

Corrosion protection of magnesium alloys by cerium, zirconium and niobium-based conversion coatings

A new Ce, Zr and Nb-based conversion coating was designed for AZ91 and AM50 magnesium alloys. The corrosion protection provided by this coating was evaluated by electrochemical measurements (polarization curves, electrochemical impedance spectroscopy) in Na₂SO₄ electrolyte, and accelerated atmospheric corrosion tests (humid, SO₂ polluted air, and salt spray). Its chemical composition was characterized by X-ray photoelectron spectroscopy (XPS). Electrochemical measurements showed that Mg alloys treated during 24 h in the Ce-Zr-Nb conversion bath exhibit: (i) increased corrosion potential, (ii) decreased corrosion and anodic dissolution current densities, and (iii) increased polarization and charge transfer resistances. The accelerated corrosion tests revealed excellent atmospheric corrosion resistance for all Ce-Zr-Nb-treated samples, with or without an additional layer of epoxy-polyamide resin lacquer or paint. XPS analysis showed that the coating includes CeO₂, Ce₂O₃, ZrO₂, Nb₂O₅, MgO, and MgF₂ as main components. No significant modification of the chemical composition was observed after cathodic and anodic polarization in Na₂SO₄. This new coating provides improved corrosion resistance, and excellent paint adhesion. It offers an alternative to the chromate conversion coating for magnesium alloys.     

Properties of ‘Sulpho-Chromispel-I’ electrolyte of standard concentration for the electrodeposition of chromium

The most important properties of non-standard highly efficient “Sulpho-Chromispel- I” type electrolytes for chromium plating have been investigated. They contain as side anions catalysing the cathodic process, both SO^2-₄ and I^?. The effects of cathodic current density and concentration of hydroiodic acid on the total current distribution are evaluated. The deposition rate is determined. The effect of operating parameters on the throwing power is established. The properties of the electrolytes studied are compared with those of standard sulphate electrolytes and “Chromispel-I” compositions. The most efficient electrolyte comprises CrO₃:I- = 20:1 and 20 cm³ dm^?3 sulphuric acid. The deposition rate can reach a value of 250 μm h^?1 at a current density of 200 A dm^?2. The throwing power of this electrolyte is close to that of standard sulphate electrolytes. As a whole “Sulpho-Chromispel-I” electrolytes exhibit lower throwing power compared to standard sulphate electrolytes or pure “Chromispel-I” electrolytes. They are recommended for chromium plating details of simple profile.     

Effect of surface finishing of a Zr-based bulk metallic glass on its corrosion behaviour

Zr-based metallic glasses passivate spontaneously, but exhibit also a certain pitting susceptibility. On the example of the Zr₅₉Ti₃Cu₂₀Al₁₀Ni₈ alloy studied in 0.01 M Na₂SO₄ + x M NaCl (x = 0-0.1) electrolytes it is demonstrated that the surface finishing state and the pre-exposure conditions can significantly influence the free corrosion and anodic polarisation behaviour. Mechanical fine-polishing procedures can lead to extremely smooth topographies but also to Cu enrichment at the surface. This yields a pronounced Cu dissolution at low anodic polarisation prior to stable passivity and increases the pitting initiation susceptibility as compared to mechanically ground surface states.     

Formation, structure and composition of anodic films on AM60 magnesium alloy obtained by DC plasma anodising

Anodising of AM60 magnesium alloy (6% Al + 0.27% Mn) was studied in a solution containing 1.5 M KOH + 0.5 M KF + 0.25 M Na₂HPO₄ · 12H₂O with addition of various NaAlO₂ concentrations. The experiments were carried out in DC current galvanostatic mode. Observations of phenomena occurring at the sample surface plus voltage monitoring revealed three stages: traditional anodising, followed by microarc anodising and finally arcing. The film was porous and cracked, with poor bonding to the substrate. It was composed of magnesium and aluminium oxide, and contained all the elements present in the electrolyte. The aluminium concentration in the film was dependent on the concentration of aluminate ions in the electrolyte. The transition from microarc to arcing stage took place when the alloy surface was completely covered by the anodic film.     

Micro arc oxidation of S-containing TiO2 films by sulfur bearing electrolytes

Porous S-containing titania (TiO₂) films were fabricated by micro arc oxidation (MAO) in sulfur bearing electrolytes. Two kinds of electrolytes, Na₂SO₃ solution and Na₂S₂O₃ solution, were used in the experiment. The influence of electrolyte and applied voltage on the structure and surface morphology of prepared TiO₂ films has been evaluated. The critical voltage for film generation is 150 V in Na₂SO₃ solution and 100 V in Na₂S₂O₃ solution. Under applied voltage of 250 V, the thickness of the TiO₂ film is 15 μm for Na₂SO₃ electrolyte and 25 μm for Na₂S₂O₃ electrolyte, respectively. The films were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) later. Two main phases, anatase and rutile, are contained in the porous film. Detailed reaction mechanism of MAO process was discussed later, and it is concluded that TiO₂ films are more easily prepared in Na₂S₂O₃ solution due to sulfur deposition.     

Hot corrosion behaviour of Nbss/Nb5Si3 in situ composites in the mixture of Na2SO4 and NaCl melts

Hot corrosion behaviour of Nb–16Si–24Ti–6Cr–6Al–2Hf (at.%) in the mixture of Na₂SO₄ and NaCl melts at 900 °C was studied. The results show that the corrosion kinetics of the alloy fit parabolic law. The oxides consist of a loose and porous outer layer and an internal oxidation zone. Outer oxides are mainly composed of TiO₂, TiNb₂O₇, Nb₂O₅, CrNbO₄ and SiO₂ while the internal oxidation zone is composed of TiO₂. Hot corrosion mechanism of the alloy in the presence of Na₂SO₄ and NaCl deposits is discussed.     

Corrosion of low-temperature nitrided molybdenum-bearing stainless steels

Austenitic stainless steels with up to 6.1 wt.% Mo were nitrided at 425 °C and examined in 0.1 M Na₂SO₄ without and with chlorides at pH 3.0 and 6.5. Nitrided steels exhibited an increased resistance to pitting, but at pH 3.0 they had a decreased resistance to general corrosion. After corrosion at pH 3.0 surface films contained chromium nitrides and oxides of Mo, Cr and Fe. It is proposed that the improved pitting resistance of nitrided steels is associated with the initially accelerated dissolution which leads to the accumulation of corrosion resistant CrN and of oxidised steel components.     

Influence of anions on the formation and structure of artificial zinc rusts

To simulate the atmospheric corrosion of steels galvanized with Zn under different conditions, artificial zinc rusts of basic zinc salt (BZS) were prepared by hydrolyzing ZnO particles in aqueous solutions including ZnCl₂, ZnSO₄ and Zn(NO₃)₂. In ZnCl₂–ZnSO₄, ZnSO₄–Zn(NO₃)₂ and ZnCl₂–Zn(NO₃)₂–ZnSO₄ systems, zinc hydroxysulfate (Zn₄(OH)₆(SO₄)·nH₂O) was formed while zinc hydroxychloride (Zn₅(OH)₈Cl₂·H₂O) was generated in ZnCl₂–Zn(NO₃)₂ system. Zinc hydroxynitrate (Zn₅(OH)₈(NO₃)₂·2H₂O) was yielded in only Zn(NO₃)₂ system. All the formed artificial zinc rusts were hexagonal plate particles. These results suggest that SO_x is the most effective corrosive gas on the formation of BZS rusts on galvanized steel.     

Corrosion behaviour of magnesium in ethylene glycol

Corrosion of magnesium engine components by coolant is an important issue in the automotive industry where magnesium alloys may be used. It is of significance to understand the corrosion behaviour of pure magnesium in ethylene glycol solutions, as this can provide a basis for developing new coolants for magnesium alloy engine blocks. In this paper, through corrosion and electrochemical tests, it was found that the corrosion rate of magnesium decreased with increasing concentration of ethylene glycol. Individual contaminants, such as NaCl, NaHCO₃, Na₂SO₄ and NaCl can make aqueous ethylene glycol solution more corrosive to magnesium. However, in NaCl contaminated ethylene glycol, NaHCO₃ and Na₂SO₄ showed some inhibition effect. The solution resistivity played an important role in the corrosion of magnesium in ethylene glycol solutions, and the competitive adsorption of ethylene glycol and the contaminants on the magnesium surface was also responsible for the observed corrosion behaviours. The corrosion of magnesium in ethylene glycol can be effectively inhibited by addition of fluorides that react with magnesium and form a protective film on the surface.     

Preparation of Sn4+-Doped Titanium Dioxide Photocatalytic Films on 304 Stainless Steel by Duplex Treatment

Sn⁴⁺-doped titanium dioxide photocatalytic films were synthesized on 304 stainless steel (SS) by a duplex treatment. The SS substrates were alloyed with titanium (Ti) through cathodic-arc ion plating followed by a microarc oxidation (MAO) treatment in different electrolytes. Field-emission scanning electron microscopy, x-ray diffraction, and energy dispersive spectroscopy were used to characterize the films surface morphology, crystalline phase, and composition, respectively. Photocatalytic activity was measured using an UV-Vis spectrophotometer. It was found that the films with a porous structure are mainly composed of TiO₂, which exists in an anatase and rutile state. Furthermore, small quantities of SnO₂ have been found in the Sn⁴⁺-doped titanium dioxide films. The fraction of anatase varies with the MAO time and electrolytes, whereas the pore size remains the similar with the same MAO current intensity and density and the surface roughness increases slightly with increasing MAO time. It was also found that the photocatalytic activity of the Sn⁴⁺-doped porous film improved, and the film synthesized with a shorter MAO time in a lower Na₂SnO₃-containing electrolyte is superior to the films with longer MAO times and higher Na₂SnO₃ concentrations.     

Hot corrosion behaviour of a cobalt-base super-alloy K40S with and without NiCrAlYSi coating

A NiCrAlYSi coating was deposited by arc ion plating on a cobalt-base super-alloy K40S to improve its hot corrosion resistance at 1173 K in air. The K40S suffered from accelerated corrosion and formed non-protective scale with poor adherence when its surface was beneath Na₂SO₄ and Na₂SO₄ containing 25 wt.% NaCl salt deposits. After the K40S was coated with NiCrAlYSi coating, a protective α-Al₂O₃ scale was formed on the coating. Although the NiCrAlYSi coating changed into NiCoCrAlYSi during corrosion processes, it still possessed good corrosion resistance. In addition, the corrosion mechanisms were discussed on a basis of basic fluxing model.