Acoustic emission during pitting corrosion of 304 stainless steel

Acoustic emission (AE) during pitting corrosion of 304 stainless steel (304 SS) in H₂SO₄ solutions with different pH values and Cl⁻ concentrations was studied. Two types of AE signals are detected in all solutions. Each type of signals is characterized by AE parameters (rise time, counts number, duration and amplitude) and waveform carefully. It is believed that the hydrogen bubbles evolution inside the pits is the AE source.     

Corrosion behaviour of tin dioxide based electrode material at high temperature in molten soda-lime-silicate glass

Tin dioxide (SnO₂) based ceramics are used to form ceramic electrodes used in glass melting techniques. In this paper, MnO₂-doped SnO₂-based ceramics were prepared by pressureless sintering, and the evolution of the microstructure of SnO₂-based ceramics in molten glass liquid was studied by SEM, EPMA and EDX. The thin boundary phase has corrosion resistance to glass liquid at high temperatures. However, the thick intergranular layer is very easily attacked by glass liquid through the dissolution mechanism. Furthermore, the corrosion of SnO₂ grains should be controlled by the diffusion process. Based on our results, a schematic diagram illustrating the corrosion behaviour of SnO₂ based ceramics in molten soda-lime-silicate glass liquid is presented.     

Evolution of hydrogen at dissolving magnesium surfaces

The origin of hydrogen evolved at dissolving magnesium surfaces, including those subjected to anodic polarisation, remains a topical issue. In this work, a critical appraisal of existing theories is presented, along with data from simultaneous polarisation and hydrogen collection tests. The observations invalidate mechanisms invoking the presence of univalent Mg⁺. A combined experimental and thermodynamic analysis suggests that the ability of magnesium to support the cathodic reaction is enhanced during dissolution as the result of an increase in the exchange current density for the hydrogen evolution reaction, This mechanism can also explain high rates of hydrogen evolution in aluminium pits.     

Acoustic emission during the electrochemical corrosion of 304 stainless steel in H2SO4 solutions

Acoustic emission (AE) behaviour during the electrochemical corrosion of 304 stainless steel (304SS) in H₂SO₄ solutions was studied. AE signals which related to transpassive dissolution are detected in solutions with low pH, and are very slightly influenced by current density and pre-strain. During hydrogen bubble evolution, a weak correlation exists between the AE signal amplitude and the hydrogen bubble diameter. The concept of potential – pH – AE diagram is proposed and such a diagram is drawn based on AE activity and b-values. The main mechanisms of AE sources which are transpassive dissolution and hydrogen bubble evolution, are also discussed.     

Effects of pretreatment on the aluminium etch pit formation

The effect of chemical pretreatments on the electrochemical etching behavior of aluminium was investigated with the topographic studies of surface and the analysis of initial potential transients. Two-step pretreatments with H₃PO₄ and H₂SiF₆ result in a high density of pre-etch pits on aluminium surface by the incorporation of phosphate ion inside the oxide film and the removal of surface layer by aggressive fluorosilicic acid solution. It generates a high density of etch pits during electrochemical etching and results in the capacitance increase of etched Al electrode by expanding the surface area, up to 61.3 μF/cm² with the pretreatment solution of 0.5 M H₃PO₄ at 65 °C and 10 mM H₂SiF₆ at 45 °C.     

Facile synthesis of tin oxide nanocrystals and their photocatalytic activity

Tin oxide nanocrystals with diameters smaller than 10 nm were synthesized using Na₂SnO₃ and CO₂ as reactants and cetyltrimethylammonium bromide (CTAB) as stabilizer under mild conditions. As a mild acidic gas, CO₂ is favorable for the accurate adjustment of pH value of Na₂SnO₃ solution. Stannate salt is stable, cheap and easy in operation. The effects of Na₂SnO₃ concentration, CTAB concentration, aging temperature, and aging time on the nanocrystals were studied. It was found that, with the increasing Na₂SnO₃ concentration, aging temperature and aging time, SnO₂ nanocrystals size decreases. The formation of SnO₂ nanocrystals can be interpreted by electrostatic-interaction mechanism. SnO₂ nanocrystals show high photocatalytic activities in the degradation of Rhodamine B solution. The catalytic activity of small nanocrystals is higher than that of large ones.     

Corrosion product formation during NaCl induced atmospheric corrosion of magnesium alloy AZ91D

Magnesium alloy AZ91D was exposed in humid air at 95% relative humidity (RH) with a deposition of 70 μg/cm⁻² NaCl. The corrosion products formed and the surface electrolyte were analysed after different exposure times using ex situ and in situ FTIR spectroscopy, X-ray diffraction and Ion Chromatography. The results show that magnesium carbonates are the main solid corrosion products formed under these conditions. The corrosion products identified were the magnesium carbonates hydromagnesite (Mg₅ (CO₃)₄ (OH)₂4H₂O) and nesquehonite (MgCO₃ 3H₂O). The corrosion attack starts with the formation of magnesite at locations with higher NaCl contents. At 95% RH, a sequence of reactions was observed with the initial formation of magnesite, which transformed into nesquehonite after 2-3 days. Long exposures result in the formation of pits containing brucite (Mg(OH₂)) covered with hydromagnesite crusts. The hydromagnesite crusts restrict the transport of CO₂ and O₂ to the magnesium surface and thereby favour the formation of brucite. Analysis of the surface electrolyte showed that the NaCl applied on the surface at the beginning was essentially preserved during the initial corrosion process. Since the applied salt was not bound in sparingly soluble corrosion products a layer of NaCl electrolyte was present on the surface during the whole exposure. Thus, Na⁺ and Cl⁻ ions can participate in the corrosion process during the whole time and the availability of these species will not restrict the atmospheric corrosion of AZ91D under these conditions. It is suggested that the corrosion behaviour of AZ91D is rather controlled by factors related to the microstructure of the alloy and formation of solid carbonate containing corrosion products blocking active corrosion sites on the surface.     

Corrosion of ultra-high-purity Mg in 3.5% NaCl solution saturated with Mg(OH)2

Corrosion was evaluated for ultra-high-purity magnesium (Mg) immersed in 3.5% NaCl solution saturated with Mg(OH)₂. The intrinsic corrosion rate measured with weight loss, P_W = 0.25 ± 0.07 mm y⁻¹, was slightly smaller than that for high-purity Mg. Some specimens had somewhat higher corrosion rates attributed to localised corrosion. The average corrosion rate measured from hydrogen evolution, P_AH, was lower than that measured with weight loss, P_W, attributed to dissolution of some hydrogen in the Mg specimen. The amount of dissolution under electrochemical control was a small amount of the total dissolution. A new hydride dissolution mechanism is suggested.     

Effects of solution pH and Cl on electrochemical behaviour of an Aermet100 ultra-high strength steel in acidic environments

In this work, the effects of solution pH and Cl⁻ on the electrochemical behaviour of an Aermet100 ultra-high strength steel in 0.5 M (Na₂SO₄ + H₂SO₄) solution were studied by polarization curve and electrochemical impedance spectroscopy (EIS) measurements, combined with scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) characterization. The results show that, when solution pH is below 4, the steel is in the active dissolution state, and corrosion current decreases with the increase of pH. There exists a critical pH value, above which the steel is passivated. Moreover, the oxides and hydroxides of Fe, Co, Ni and Cr are the primary components of the passive film. With addition of Cl⁻, pits are initiated on the steel electrode.     

The effect of H2S concentration on the corrosion behavior of carbon steel at 90 °C

The electrochemical behavior of SAE-1020 carbon steel in 0.25 M Na₂SO₄ solution containing different concentrations of H₂S at 90 °C was investigated using the methods of weight loss, electrochemical measurements, scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results showed that the corrosion rate of carbon steel increased significantly with the increase of H₂S concentration. H₂S accelerated the corrosion rate of SAE-1020 carbon steel by a promoted hydrogen evolution reaction. Severe corrosion cavities were observed on the carbon steel surface in the solutions containing H₂S due to cementites stripped off from the grain boundary. The loose corrosion products formed on the steel surfaces were composed of mackinawite.     

Corrosion processes of Mg–Y–Nd–Zr alloys in Na2SO4 electrolyte

The corrosion behavior of Mg–Y–Nd–Zr (WE43 commercial alloy) was investigated in Na₂SO₄ electrolyte using potentiodynamic polarization curves, X-Ray Photoelectron Spectroscopy (XPS), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) depth profiles, Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectrometry (EDS) analyzes. SEM and EDS data show that Nd-rich precipitates are mainly located at the grains boundaries. Zr/Y-rich zones are distributed inside the most of the grains. XPS study indicates a depletion of Mg on surface that could be attributed to Mg dissolution and an enrichment of the addition element oxides. XPS and ToF-SIMS analyzes demonstrate that the corrosion films are made up of a magnesium hydroxide (Mg(OH)₂) outer layer and an inner layer containing magnesium oxide (MgO), yttrium oxide (Y₂O₃) and hydroxide (Y(OH)₃), mixed with a small amount of MgH₂, zirconium oxide (ZrO₂) and neodymium oxide (Nd₂O₃). The Y₂O₃ and Y(OH)₃ signals increase slightly in the inner layer towards the corrosion film/alloy interface. Unlike these compounds, ZrO₂ and Nd₂O₃ compound signals are constant inside the inner layer. It is concluded that: (i) neodymium, zirconium and yttrium play a key role in the slightly improved corrosion resistance of the alloy and (ii) the cathodic reaction is slower on WE43 than on pure Mg and AZ91.     

Mechanochemical preparation and investigation of properties of magnesium,calcium and lithium–magnesium alanates

Ball milling of MgCl₂ and CaCl₂ with NaAlH₄ or LiAlH₄ can be used for the preparation of magnesium, calcium and lithium–magnesium alanates in mixture with NaCl or LiCl. Using wet chemical separation methods, it was possible to obtain these alanates in nearly pure state. The alanates were characterized by X-ray diffractometry, solid-state ²⁷Al NMR and IR spectroscopy and thermovolumetric (TV) and differential scanning calorimetry (DSC) measurements. Mg(AlH₄)₂ dissociates thermally in one step to MgH₂, Al and hydrogen; at a higher temperature, MgH₂ and Al transform to Mg–Al alloy and hydrogen. Thermal dissociations of Ca(AlH₄)₂ and of LiMg(AlH₄)₃ (in mixture with NaCl or LiCl) proceeds in several steps, of which the first two can be assigned to the formation of CaH₂ and of a MgH₂/LiH mixture, respectively, in addition to Al and H₂. Possible intermediates of these two steps are CaAlH₅ and LiMgAlH₆. Higher temperature dissociations include formation of MgH₂ (LiH) and Ca–Al alloys from CaH₂, CaH₂ and Al, respectively. Upon ball milling of MgCl₂ or CaCl₂ with NaAlH₄ or LiAlH₄ in the presence of Ti catalysts, only the thermal dissociation products of the expected alanates are obtained. This indicates that dehydrogenation discharge of earth alkali metal alanates can be catalyzed by Ti. According to DSC measurements, the thermodynamic stability of Mg(AlH₄)₂ (ΔH = 1.7 kJ/mol) is too low for the purpose of reversible hydrogen storage. Determination of ΔH values for the second, endothermal step of calcium and lithium–magnesium alanate dissociations gave values of around 31.6 and 13.1 kJ/mol, respectively.     

Selective recovery of Sn from copper alloy dross and its heat-treatment for synthesis of SnO2

Preliminary study on concentration and separation of tin (Sn) from copper alloy dross by selective dissolution method was conducted. The tin in the copper alloy dross did not dissolve in an aqueous nitric acid solution which could allow separation of tin from the copper alloy dross. The tin as H₂SnO₃ (metastannic acid) phase was precipitated in the solution with centrifuging process and transformed to tin dioxide (SnO₂) after drying process. The dried sample was heat-treated at low temperature and its phase characteristics, surface morphology and chemical composition were investigated.     

Corrosion properties ofAZ31 magnesium alloy and protective effects of chemical conversion layers and anodized-coatings

The corrosion properties of AZ31 magnesium alloys were studied by potentiodynamic polarization curves and electrochemical impedance spectroscopy（E1S） techniques, meanwhile, the protective properties of two environmentally protective types of chemical conversion layers and anodized coatings of AZ31 magnesium alloys were also discussed. The component of chemical conversion bath is NaH2PO4·12H2O 20 g/L, H3PO4 7.4 mL/L, NaNO2 3 g/L, Zn（NO3）2·6H2O 5 g/L and NaF 1 g/L, and components of the anodization bath is Na2SiO3 60 g/L, C6H5Na3O7·2H2O 50 g/L, KOH 100 g/L and Na2B4O7·2H2O 20 g/L. The results show that the corrosion resistance of AZ31 magnesium increases with the increase of pH value of the corrosive medium. For the chemical conversion layer acquired at 80 ℃, 10 min is the best processing time and the charge transfer resistance of the chemical conversion layer is enhanced nearly by 10 times. The optimum processing time for the anodization of AZ31 is 60 min, the charge transfer resistance value of the anodized sample at the early immersion stage is nearly 26 times of that of the blank sample and the corrosion type of the anodized samples is pitting.     

Self-assembled 3-D flower-shaped SnO2 nanostructures with improved electrochemical performance for lithium storage

Flower-shaped SnO₂ nanoplates were successfully synthesized via a simple hydrothermal treatment of a mixture of tin(II) dichloride dihydrate (SnCl₂·2H₂O) and sodium citrate (Na₃C₆H₅O₇·2H₂O) in alkali solution. The obtained SnO₂ nanoplates were less than 5 nm thick and self-assembled into flower-shaped nanostructures. The introduction of citrate was essential for the preparation of the SnO₂ nanoplates. The nanoscale shape and self-assembled architecture of SnO₂ nanoparticles were mainly controlled by the alkalinity of the solution. When the self-assembled SnO₂ nanostructures were used as anode materials in Li-ion batteries, they exhibit a reversible capacity of 670 mA h g^?1 after 30 cycles and an average capacity fading of 0.95% per cycle after the second cycle. The good electrochemical performance of the SnO₂ sample prepared via the hydrothermal synthesis indicates the possibility of fabricating specific self-assembled three-dimensional nanostructures for Li-ion batteries.     

Effects of the addition of Na2SnO3 to NaCl electrolytes on Mg-14Li-3Al-1Gd electrode electrochemical behavior

We report on the electrochemical performance of Mg-14Li-3Al-1Gd electrodes prepared by the accumulation roll bonding technique in a 0.7 M NaCl solution. To explore the effects of adding different concentrations of Na₂SnO₃ to 0.7 M NaCl solutions, potentiodynamic polarization, potentiostatic oxidation, electrochemical impedance spectroscopy, and scanning electron microscopy were utilized. The results show that the addition of Na₂SnO₃ to a 0.7 M NaCl solution increases the corrosion potential of the Mg-14Li-3Al-1Gd electrodes. Samples with 0.1 mM Na₂SnO₃ retained the highest discharging current density and lowest polarization resistance of all the specimens. Electrodes in an electrolyte solution mixed with 0.1 mM Na₂SnO₃ presented a larger active reaction area, deeper channels, and higher discharging currents than those with other additive concentrations. In conclusion, to improve the electrochemical behavior of Mg-14Li-3Al-1Gd electrodes in a 0.7 M NaCl solution, the optimal concentration of Na₂SnO₃ is 0.1 mM.     

Corrosion relationships as a function of time and surface roughness on a structural AE44 magnesium alloy

Pit initiation, growth, and coalescence corrosion mechanisms of an AE44 magnesium alloy subjected to a salt-water environment were quantified. Stereological quantities were evaluated using optical microscopy, scanning electron microscopy, and laser beam profilometry. Three corrosion mechanisms clearly arose: pitting, intergranular, and general. Pitting began as the result of localized galvanic dissolution between the intermetallics and magnesium matrix. Intergranular corrosion arose as pits coalesced. General corrosion arose by dissolution and regeneration of a Mg(OH)₂ film at a continuous rate. Stereological quantification demonstrated that the corrosion pit number density and pit radius size distribution initially increased before decreasing due to pit coalescence.     

The inhibition of mild steel corrosion in acidic medium by 1-methyl-3-pyridin-2-yl-thiourea

The effect of 1-methyl-3-pyridin-2-yl-thiourea on the corrosion resistance of mild steel in H₂SO₄ solution was investigated by different techniques. The results show that the inhibition efficiency increases with the increase of inhibitor concentration. This compound affects both the anodic dissolution of steel and the hydrogen evolution reaction in 0.5 M H₂SO₄. The adsorption of this inhibitor is also found to obey the Langmuir adsorption isotherm. From the adsorption isotherm, value of the ΔG_ads for the adsorption process was calculated. From the corrosion rate obtained at 25-45 ± 1 °C E_a, ΔH_a and possible mechanism have been proposed.     

Efficient recycling of WC-Co hardmetal sludge by oxidation followed by alkali and sulfuric acid treatments

We present a process to recycle strategic metals, viz. tungsten and cobalt, from a WC-Co hardmetal sludge (WCHS) via oxidation followed by a two-step hydrometallurgical treatment with alkali and acid solutions. The oxidation of WCHS was investigated in the temperature range of 500 to 1000 °C and optimized at 600 °C to transform the maximum WC into an alkali-soluble WO₃. The conditions for the selective dissolution of WO3 in stage-I were optimized as follows: 4.0 M NaOH, pulp density of 175 g/L, and temperature of 100 °C for 1 h, yielding maximum efficacy. Subsequently, in the second step, the optimal conditions for cobalt leaching from the alkali-treated residue were established as follows: 2.0 M H₂SO₄, 25 g/L pulp density, and 75 °C temperature for 30 min. Downstream processing of the obtained metal ions in solutions was also easier, as the only impurity of dicobaltite ions with the Na₂WO₄ solution was precipitated as Co(OH)₃ under atmospheric O₂; meanwhile, the CoSO₄ solution obtained through the second step of processing can be treated via electrolysis to recover the metallic cobalt. The present process is simpler in operation, and the efficient use of eco-friendly lixiviants eliminates the previously reported disadvantage.     

The effect of synthetic zinc corrosion products on corrosion of electrogalvanized steel. II. Zinc reactivity and galvanic coupling zinc/steel in presence of zinc corrosion products

The reactivity of zinc under synthetic zinc patinas and the galvanic coupling in steel/patina/Zn are studied. Zn₅(OH)₆(CO₃)₂ and Na₂Zn₃(CO₃)₄⋅3H₂O inhibit zinc anodic dissolution in NaCl, while Zn₅(OH)₈Cl₂ H₂O and Zn₄(OH)₆SO₄ nH₂O do not. The galvanic current in steel/patina/NaCl/Zn is smaller as compared to steel/NaCl/Zn. The inhibiting effect decreases with time for Na₂Zn₃(CO₃)₄⋅3H₂O or Zn₄(OH)₆SO₄ nH₂O due to the transformation into Zn(OH)₂. In NaHCO₃, the polarity between zinc and steel can reverse. The effect of confinement on the cathodic current is stronger than the initial effect of patina which is explained by the instability of the patinas under rapid pH-increase.