Perchlorate pitting corrosion of tin in Na2CO3 solutions and effect of some inorganic inhibitors

The anodic behaviour of tin electrode in Na₂CO₃ solutions containing different concentrations of Na₂ClO₄ was studied by potentiodynamic technique and complemented by scanning electron microscope. In perchlorate free carbonate solutions; the polarization curves exhibit two anodic peaks assigned to the electroformation of Sn(II) and Sn(IV) species, respectively, prior to the permanent passivation region. The passivity is due to the presence of SnO and SnO₂ layers on the electrode surface. Addition of to the carbonate solution breaks down the passive layer and initiate pitting corrosion at a certain critical pitting potential. The pitting potential decreases with an increase in concentration but increases with increasing both Na₂CO₃ concentration and scan rate. Addition of increasing concentration of , or causes a shift of the pitting potential in the positive direction indicating the inhibition effect of added anions, while addition of anion accelerates the perchlorate pitting corrosion.     

Electrochemical behavior of tin in sodium borate solutions and the effect of halide ions and some inorganic inhibitors

The corrosion of tin electrode in sodium borate (Na₂B₄O₇) solutions was investigated using cyclic voltammetry and potentiostatic current transient techniques. In absence of halide ions, the E/j response exhibits active/passive transition. The active region involves one anodic peak corresponding to the formation of Sn(OH)₂ and/or SnO. Addition of Cl⁻, Br⁻ or I⁻ (C ? 0.01 M) ions inhibits the active dissolution of tin, but higher concentrations enhance the active dissolution and tend to breakdown the passive film and induce pitting attack. The effect of , , and as inorganic inhibitors on the pitting corrosion of tin in (0.1 M Na₂B₄O₇ + 0.1 M NaCl) solution has also been studied. The presence of these anions (except ) inhibits pitting corrosion. Chronoamperometry measurements showed that nucleation of pit takes place after an incubation time (t_i). The rate of pit nucleation () increases with increasing halide ions concentration and applied potentials, but decreases with increasing the concentration of the inorganic inhibitors (except ). The inhibition efficiency of these inhibitors decreases in the order:

     

Flexible polypyrrole/ClO4 films formed in aqueous solutions

The preparation of high quality polypyrrole film in aqueous solutions is described. The new polypyrrole films were produced in a phthalate buffer solution (pH 4) with perchlorate as the electrolyte. Very smooth and remarkably flexible free-standing films were grown electrochemically. The conductivity of the polypyrrole film exhibits an inverse dependency on the film thickness. The mechanical properties of these films, such as tensile strength and the modulus of elasticity, are better than the reported properties for polypyrrole/tosylate films prepared in organic solutions. In comparison with the polypyrrole films prepared in phosphate buffer solution, the stability of the polypyrrole films prepared in phthalate buffer are improved considerably. The morphology and composition of the conducting film is described and a mechanism for their formation is proposed.     

Galvanostatic study of the breakdown of Zn passivity by sulphate anions

The anodic behavior of Zn electrode in Na₂SO₄ solutions was studied by galvanostatic polarization technique. The polarization curves are characterized by a distinct arrest corresponding to ZnO formation, after which the potential rises linearly with time up to a well-defined value, the breakdown potential, at which the potential drops with time, down to more negative direction. This denotes the destruction of the passive film and initiation of pitting corrosion. It was found that, the breakdown potential, the time consumed till the breakdown potential and the rate of potential rise with time depend on the sulphate anions concentration, solution temperature and magnitude of the imposed current density. Addition of increasing concentration of phosphate, molybdate, tungstate or chromate anions causes a shift of the breakdown potential into the noble direction, indicating the inhibitive effect of these anions. The inhibitive effect of these inhibitors decreases in the order: CrO₄²⁻ > WO₄²⁻ > MoO₄²⁻ > HPO₄²⁻.     

Corrosion, passivation and breakdown of passivity of neodymium

The anodic polarisation behaviour of neodymium was investigated in various solutions (pH 1-12). While in sulphuric acid solution high dissolution rates were observed, spontaneous passivity occurred in phosphoric acid solutions, oxalic acid solutions and sodium hydroxide solutions. However, the stability of these protecting layers strongly depends on the kind and concentration of anions present, especially in acidic environments. A mechanism of the passive layer breakdown is proposed. During pitting corrosion erosive degradation of the electrode was observed. In all solutions, abnormal hydrogen evolution was observed when neodymium was in the active state during anodic polarisation.     

On the reduction of ClO4 ions in the course of metal dissolution in HClO4 solutions

The problem of the application of perchloric acid and/or perchlorate supporting electrolyte in electrochemical and corrosion studies is discussed. It is demonstrated that in contrast to the general view in the literature the reduction of perchlorate ions could be an important step in the overall process occurring in the course of spontaneous dissolution of some metals in acid perchlorate medium. It is shown that especially in the case of the dissolution of iron the reduction of ClO₄⁻ leading to the formation of Cl⁻ plays a substantial role. Consequently, all interpretations concerning the dissolution of iron in the presence of ClO₄⁻ ions should be revisited taking into consideration the possibility of the reduction process.     

Effect of additive on corrosion resistance of NiFe2O4 ceramics as inert anodes

In order to improve the corrosion resistance of NiFe2O4 ceramics as inert anode, additive V2O5 was added to raw materials NiO and Fe2O3. The inert anodes of nickel-ferrite ceramics were prepared by powder metaUurgic method and the static corrosion rate in Na3AlF6-Al2O3 was determined by mass loss measurement. The effect of V2O5 on sintering property and corrosion resistance was studied. The results show that V2O5 can promote the grain to develop completely and improve sintering property. EDS results show the reaction product Ni2FeVO6 distributes along the grain boundary. The corrosion tests show that V2O5 is beneficial to improving corrosion resistance remarkably. The reasons that V2O5 can improve the corrosion resistance must be V2O5 promoting the gains to develop completely and Ni2FeVO6 distributes along the grain boundary. The stable structure can control the chemical dissolution of ceramics anode and the reinforced grain boundary can control the grain-boundary corrosion rate.     

Corrosion of an Al–Mg–Si alloy under MgCl2 solution droplets

The corrosion behavior of an Al–0.63Mg–0.28Si alloy under droplets of MgCl₂ solution in environments of 75% and 33% RH was studied using a Kelvin Probe. The equilibrium chloride concentrations in these two environments are 5.8 and 9.8 M chloride, respectively. In the 33% RH environment, metastable pitting was the main form of corrosion. In some cases at 75% RH, the potential baseline decreased slowly by hundreds of millivolts and remained at the lower value. These samples exhibited filiform-like corrosion inside micro-droplets that formed outside of the main MgCl₂ drop. A model for the filiform-like attack in a micro-droplet is presented.     

Passivity breakdown and stress corrosion cracking of α-brass in sodium nitrite solutions

The stress corrosion cracking (SCC) susceptibility of pure copper and four α-brasses of different zinc alloy concentration in NaNO₂ 1 M solution, at various pH values, was studied by means of potentiodynamic polarisation curves and constant potential slow strain rate experiments. The results confirmed that passivity rupture was a necessary condition for SCC of α-brass and copper in nitrite solutions. Susceptibility to SCC was observed only when the potential was equal to or higher than a certain critical value at which passivity breakdown was triggered by the slow dynamic straining of the metal. Cracks were found only in those metal areas where passivity rupture had taken place.     

Corrosion behavior of alumina ceramics in aqueous HCl and H2SO4 solutions

The corrosion behavior of cold isostatically pressed (CIP) high purity alumina ceramics in aqueous HCl and H₂SO₄ solutions with various concentrations has been studied simultaneously at room temperature (25 °C). Corrosion tests were also performed with 0.65 mol/l HCl and 0.37 mol/l H₂SO₄ solutions at 40, 55 and 70 °C for 48 h. Chemical stability was monitored by determining the amount of Al³⁺, Mg²⁺, Ca²⁺, Na⁺ Si⁴⁺ and Fe³⁺ ions eluted in different concentrations of HCl and H₂SO₄ solutions by means of atomic absorption spectrometry (AAS). By increasing the concentration from 0.37 to 6.5 mol/l, it was notified that the corrosion susceptibility in HCl and H₂SO₄ solutions for the CIP alumina specimens at room temperature decreases.     

Pitting corrosion of intermetallic compound Ni3(Si,Ti) in sodium chloride solutions

The pitting corrosion of intermetallic compound Ni₃(Si,Ti) was investigated as functions of test temperature and chloride concentration in sodium chloride solutions by using a potential step method. In addition, the pitting corrosion of solution-annealed austenitic stainless steel type 304 and pure nickel was also studied under the same experimental condition for comparison. The pitting potential obtained for the intermetallic compound decreased with increasing chloride concentration and test temperature. A critical chloride concentration below which no pitting corrosion took place was found to exist and to decrease with increasing test temperature. The specific pitting potential at the critical chloride concentration also decreased with increasing test temperature. In addition, the pitting potential at various constant chloride concentrations above the critical chloride concentration decreased with increasing test temperature. The pitting potential of Ni₃(Si,Ti) was higher than pure nickel, but lower than that of type 304.     

Copper corrosion inhibition in O2-saturated H2SO4 solutions

Corrosion inhibition of copper in O₂-saturated 0.50 M H₂SO₄ solutions by four selected amino acids, namely glycine (Gly), alanine (Ala), valine (Val), or tyrosine (Tyr), was studied using Tafel polarization, linear polarization, impedance, and electrochemical frequency modulation (EFM) at 30 °C. Protection efficiencies of almost 98% and 91% were obtained with 50 mM Tyr and Gly, respectively. On the other hand, Ala and Val reached only about 75%. Corrosion rates determined by the Tafel extrapolation method were in good agreement with those obtained by EFM and an independent chemical (i.e., non-electrochemical) method. The chemical method of confirmation of the corrosion rates involved determination of the dissolved Cu²⁺, using ICP-AES (inductively coupled plasma atomic emission spectrometry) method of chemical analysis. Nyquist plots exhibited a high frequency depressed semicircle followed by a straight line portion (Warburg diffusion tail) in the low-frequency region. The impedance data were interpreted according to two suitable equivalent circuits. The kinetics of dissolved O₂ reduction and hydrogen evolution reactions on copper surface were also studied in O₂-saturated 0.50 M H₂SO₄ solutions using polarization measurements combined with the rotating disc electrode (RDE). The Koutecky-Levich plot indicated that the dissolved O₂ reduction at the copper electrode was an apparent 4-electron process.     

Reaction mechanism of roasting Zn2SiO4 using NaOH

The reaction kinetics of roasting zinc silicate using NaOH was investigated. The orthogonal test was employed to optimize the reaction conditions and the optimized reaction conditions were as follows: molar ratio of NaOH to Zn₂SiO₄ of 16:1, reaction temperature of 550 °C, and reaction time of 2.5 h. In order to ascertain the phases transformation and reaction processes of zinc oxide and silica, the XRD phase analysis was used to analyze the phases of these specimens roasted at different temperatures. The final phases of the specimen roasted at 600 °C were Na₂ZnO₂, Na₄SiO₄, Na₂ZnSiO₄ and NaOH. The reaction kinetic equation of roasting was determined by the shrinking unreacted core model. Aiming to investigate the reaction mechanism, two control models of reaction rate were applied: chemical reaction at the particle surface and diffusion through the product layer. The results indicated that the diffusion through the product layer model described the reaction process well. The apparent activation energy of the roasting was 19.77 kJ/mol.     

Susceptibility of stainless steel alloys to crevice corrosion in ClO2 bleach plants

Stainless steels, including duplex stainless steels, are extensively used for equipment in pulp bleaching plants. One serious corrosion problem in chlorine dioxide bleach plants is crevice corrosion of stainless steels, which is frequently the factor that limits their use in bleach plants. Crevice corrosion susceptibility of alloys depends on various environmental factors including temperature, chemical composition of environment and resulting oxidation potential of system. Upsets in the bleaching process can dramatically change the corrosivity of the bleaching solutions leading to temperatures and chemical concentrations higher than those normally observed in the bleach process. When the environmental limits are exceeded the process equipment made of stainless steel can be severely affected. Environmental limits for crevice corrosion susceptibility of eight stainless steel alloys with PRE numbers ranging from 27 to 55 were determined in chlorine dioxide environments. Alloys used in this study included austenitic, ferritic-austenitic (duplex), and superaustenitic stainless steels. The performance of the different stainless steel alloys mostly followed the PRE numbers for the respective alloys. The 654SMO alloy with the highest PRE number of 55 showed the highest resistance to crevice corrosion in this environment. Under the most aggressive chlorine dioxide bleach plant conditions tested, even alloys Nicr3127 and 654SMO with PRE numbers 51 and 55 respectively were susceptible to crevice corrosion attack. The two factors that seem to contribute the most to crevice corrosion and pitting in the investigated environments are temperature and potential.     

Numerical simulation of a single corrosion pit in CO2 and acetic acid environments

The present paper bears on a numerical model of steady-state behaviour of a single corrosion pit in CO₂-containing media in the presence of acetic acid. The kinetics of electrochemical reactions occurring on the electrode surface is described by Tafel equations fitted to experimental data obtained on API 5L X65 pipeline steel. Simulation results provide an interesting insight into both the chemistry and electrochemistry inside the active pit. Particular attention is paid to the effect of HAc on the coupling behaviour between the attack base and the outer surrounding surface and to the comparison with experimental data obtained in similar conditions.     

Effect of Ni content on corrosion behavior of Ni/（10NiO-90NiFe2O4） cermet inert anode

Ni/（10NiO-90NiFe2O4） cermet inert anodes with metal Ni content of 0, 5, 10, 15 and 20 （mass fraction, %） were prepared and their corrosion behavior in Na3AlF6-Al2O3 melts was investigated in laboratory electrolysis tests. The results indicate that the content of metal Ni in anodes has little effect on the steady-state concentration of impurities Ni and Fe in electrolyte and the values range in （114-173）×10^-6 and （287-385）×10^-6, but the content of impurities in the metal aluminum manifolds. There is preferential corrosion for metal Ni in NiO-NiFe2O4 based cermet anodes. Considering the corrosion resistance and electrical conductivity, the cermet containing 5%Ni （mass fraction） behaves best among NiO-NiFe2O4 based cermet anodes studied, and should be further studied.     

Al-In-Mg系铝合金阳极在NaOH溶液中的电化学行为

以Al-In-Mg合金为基础，选择添加Pb、Mn、sn制备了4类铝合金阳极材料。通过自腐蚀、开路电位、阳极极化方式研究了该系列合金阳极在c(NaOH)=4mol／L溶液中的电化学腐蚀行为，初步分析了合金成分、介质温度及溶液缓蚀剂对合金阳极性能的影响。结果发现：制备的3#(Al-In-Mg-Sn)合金、4#(Al-In-Mg-Sn-Pb)合金阳极在c(NaOH)=4mol／L c(Na2SnO3)=0．04mol／L溶液中表现出更好的电化学性能，具有比纯铝(99．95％)更负的开路电位，更小的自腐蚀速率和更低的阳极极化。     

Investigation of the specific adsorption of HSO4(SO4) and Cl ions on Co and Fe by radiotracer technique in the course of corrosion of the metals in perchlorate media

The adsorption of sulphate and chloride ions on Co and Fe was studied in 0.5 mol dm⁻³ NaClO₄ supporting electrolyte at various pH values by radiotracer technique in the course of the corrosion (dissolution) of the metals. The effect of the reduction of ClO₄⁻ ions (leading to the formation of Cl⁻ ions) on the radiotracer measurements was investigated. It was found that the adsorption of chloride and sulphate ions occurs in different pH ranges. This phenomenon was explained by the assumption that the adsorption strength of the two species is very different on oxide covered and “pure” metal surfaces.     

Localised corrosion of carbon steel in NaHCO3/NaCl electrolytes: role of Fe(II)-containing compounds

Pitting corrosion processes of carbon steels in 0.1 M NaHCO₃ + NaCl aqueous solutions induce rapidly the precipitation of Fe(II)-containing phases. The mechanisms leading to those transient compounds were studied by coupling classical electrochemical methods (cyclic voltammetry, chronoamperometry) with electrochemical microprobes and analyses by micro-Raman spectroscopy. Anodic polarisation above the breakdown potential induces the formation of a white corrosion product, identified as iron carbonate by Raman spectroscopy. The activity of a given pit, followed by Scanning Vibrating Electrode Technique, reaches a maximum before to decrease, in agreement with the evolution of the overall current density that stabilises whereas the number of pits increases. Iron carbonate is oxidised by dissolved O₂ into the hydroxycarbonate Green Rust. The study of the oxidation of aqueous suspensions of Fe(II) compounds precipitated from FeCl₂ · 4H₂O, NaOH and NaHCO₃ solutions allowed to clarify the mechanisms, that are governed by the concentration ratios and . The results were confronted to a thermodynamic approach.