Temperature dependence of the pitting potential of high purity aluminium in chloride containing solutions

Slow scan anodic polarization experiments were conducted on high purity (99.99%) aluminium exposed to chloride containing solutions at temperatures ranging from 1 to 70 °C. The total anodic charge passed prior to stable pitting shifts towards higher values with increasing chloride concentration and there is a transition point at 30 °C above which a significantly greater amount of charge is involved. For all chloride concentrations, the pitting potential decreases with temperature according to a linear relationship, and also shows a transition at approximately 30 °C, with a much steeper decrease observed for the higher temperatures. Possible explanations for these two effects are discussed.     

Influence of pitting and iron oxide formation during corrosion of carbon steel in unbuffered NaCl solutions

The corrosion evolution over time of a carbon steel rotating disk immersed in aerated NaCl solutions was analyzed using a superposition model. Using this approach, partial polarization curves for iron oxidation and oxygen reduction were synthesized from experimental current-potential data at different corrosion time in order to determine the kinetics parameters, corrosion potential and current density of the underlying anodic and cathodic subprocesses. The distinctive features of the polarization curves are well described in terms of the simplifying assumptions of the model. In particular, the time evolution of the corrosion current density was linked to the morphology of the corroding surface under different NaCl concentrations.     

Passivity breakdown of high purity iron and AISI 4340 steel in 0.5M NaCl solution

The anodic and cathodic electrochemical behaviour of high purity iron and of AISI 4340 high strength steel in unbuffered 0·5M NaCl solution has been studied. The results were correlated with the SCC failure mechanism of the steel. The pitting potentials of both high purity iron and AISI 4340 steel are coincident. Heat treatment affected the pitting potential of the steel very little. Failure of AISI 4340 steel under both anodic and cathodic polarization was found. SCC under anodic polarization was closely related to the pitting potential. Conditions at the metal-solution interface for pit maintenance are given.     

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.     

The localized corrosion of stainless steel in high purity sulphate solutions

The initiation of both pitting and crevice corrosion on Type 410 stainless steel has been observed in high purity acid sulphate solutions. Conditions of temperature, sulphate concentration and potential leading to the breakdown of passivity have been defined by free corrosion and potentiostatically controlled experiments.     

The pitting potential of aluminium in halide solutions

The pitting potential U_p is an important characteristic of every pitting system. It separates the region of passivity where the metal is practically immune from the region of heavy localized corrosion attack. On the basis of our own experimental results on pitting corrosion in the aluminum/halide-system, the present paper rediscusses fundamental questions concerning the pitting potential and the pitting process in general. At or above U_p, pitting of pure aluminum in chloride solutions is the result of the combined action of single, short-life tunnelling events, which themselves propagate discontinuously. The same type of attack, to a minor extent, also occurs down to potential values 300 mV below U_p. Therefore, the pitting potential is neither a lower limit for pit nucleation nor for pit growth, but appears to be the threshold value where the inactivation of tunnels is just balanced by tunnel nucleation. Taking the direct halogenation of the aluminum surface as the decisive step involved in tunnel nucleation, the pitting potential is given by the sum of the equilibrium potential of the aluminum/aluminum halide-electrode and the overpotential necessary for an appropriate nucleation rate to gurantee stable pitting. This pitting overvoltage has been evaluated, using the nucleation concept by Volmer, to calculate tunnel nucleation rate and experimental results on the life time of single tunnels. Values of the pitting potential calculated on this basis agree well with experimental results in chloride-, bromide-and iodide solutions. Only the case of fluoride pitting is different.     

Aluminization of high purity iron by powder liquid coating

A new powder liquid coating method is proposed for the aluminization of Fe. Mixed powder slurries of Al + Ti or Al + Al₂O₃ are pasted onto Fe specimens, and the specimens are then dried and heated in a vacuum. Unlike hot dipping or powder pack cementation, this technique can be used to aluminize specimens selectively without the need for special equipment or halides. The amount of Al adhering to the substrate is determined by the Al–Ti reaction or coalescence of molten Al in Al₂O₃ powder during heat treatment. The Al concentration profile of the modified layer can be controlled by adjusting the powder mixing ratio or heat treatment conditions. The properties of the modified layer are analyzed using a new formulation, where the diffusion equation is treated numerically with consideration of the concentration dependence of the interdiffusion coefficient. The calculated profiles are stable and in good agreement with the experimental data.     

Decreasing pitting susceptibility of passive films on X70 pipeline steel in NaCl solutions by illumination

The influence of UV illumination on passivity and pitting susceptibility on X70 pipoeline steel in a borate buffer （pH=8.4 ） solution containing NaCl is described. It is observed that illumination of the sample leads to a decrease in its pitting susceptibility as indicated by pitting potential and incubation time measurements in chloride containing electrolytes. This effect is strongly dependent on the applied potential during passivation. The electronic properties of the passive films on X70 steel were studied by Mott-Schottky analysis and photocurrent transient measurements. The results indicated that illumination during passivation led to modifications in the electronic properties of the passive films, mainly to a decrease of the bulk doping and an increase in the surface state density. The cause for the decrease in the pitting susceptibility is preliminary explained in terms of such modifications of the passive flm.     

铝阳极氧化膜在氯化钠溶液中的孔蚀行为(Ⅱ)--膜层的极化行为与孔蚀形貌

采用动态恒电位极化曲线方法。结合金相分析对铝硫酸阳极氧化膜在ＮａＣｌ溶液中的孔蚀行为机理进行了研究初探,实验表明：铝经硫酸阳极氧化处理后具有较高耐孔蚀性能,这是由于它表面生成了一层由阻挡层与多孔层所构成的特殊双层结构所引起。     

The electrochemical study of pitting corrosion of aluminium in chloride solutions

The electrochemical behaviour of pure Al in different Cl^? media similar to those formed inside growing pits have been investigated. Artificial pit measurements confirm the previous assumption that saturated AlCl₃ must be formed at the pit bottom as a condition for the attack. Experimental results support the idea that a competitive process involving passivation and activation can occur depending on the potential, the pH and the Cl^? concentration. Rather than the potential for pitting initiation, the pitting potential of Al appears to be the point at which the passivation phenomena become very slow.     

Aluminization of high purity iron and stainless steel by powder liquid coating

Powder liquid coating is investigated metallographically as an aluminization technique for high-purity iron [Acta Mater., in press] and stainless steel. In this process, Fe₂Al₅ forms initially during heat treatment, with c axis preferentially aligned with the sample normal. In Fe–18mass%Cr alloy, Cr exhibits almost the same concentration profile as Fe except for the temporary formation of a Cr₅Al₈ network in the early stage of heat treatment. Fe–25Cr–18Ni alloy forms a thinner aluminized layer compared to the other substrates, and contains an Al–Ni-rich layer and spherical precipitates (ordered B2). The diffusion of Al and Ni in the system (B2/bcc/fcc) is simulated using a new formulation of the diffusion equation for the ternary Fe–Al–Ni system taking the concentration-dependent interdiffusion coefficient into account. The bcc layer is found to be predominantly in a steady state due to the large interdiffusion coefficients, and characteristic uphill diffusion of Al in the B2 layer is attributed to the existence of Ni.     

The nucleation and growth of metastable pitting on pure iron

The nucleation and growth of metastable pitting on pure iron surface in NaNO₂ + NaCl solution were investigated by potentiodynamic tests. The current fluctuations appear as cluster and overlapped. The active sites for nucleation depend mainly on the surface geometry. The growth behaviour of pure iron is different from stainless steels and carbon steels. On pure iron surface, many pits pile up together to form huge damage area. Pits are shallow, do not grow deeply and pits array one by one along the direction of abrasion grooves. The growth of stable pitting is different from metastable pitting.     

Chlorine inclusion mechanism in high purity copper electrorefining from nitric acid system

A practical approach was introduced to study the inclusion mechanism of chlorine in high purity copper electrorefining from nitric acid system via cyclic voltammetry (CV) combined with electrodeposition experiments. The CV curves display an obvious reduction peak of CuCl intermediate, which can provide an insight into the electrochemical behavior of this inclusion. Experimental results show that the increase of HNO₃ concentration is favorable to reducing the quantity of chlorine inclusion although there is a slight decline in cathodic current efficiency. The optimum conditions for copper electrorefining in nitric acid system are HNO₃ concentration in solution of 1−2 mol/L, moderate temperature of ~35 °C with current density not exceeding 25 mA/cm². Based on the theoretical studies, an optimized copper electrorefining experiment was designed to simulate the industrial electrolysis, by which high purity copper can be obtained with chlorine inclusion less than 10 μg/g and current efficiency higher than 90%.     

Anomalous dimension in pitting corrosion of SUS304 in a NaCl solution

The current-potential relations of SUS304 for a NaCl solution in water ranging from 0.01 to 2 mol/dm³ over a 298-333 K temperature range were investigated in order to exhibit the presence of anomalous dimensions that characterize the phase transition such as pitting corrosion. Using the anomalous dimension, all the experimental current points can be represented by a single curve in a reduced variable.     

Formation of poorly crystalline iron monosulfides: Surface redox reactions on high purity iron, spectroelectrochemical studies

In the use of iron for reductive dehalogenation of chlorinated solvents in ground water, due to presence of sulfate-reducing bacteria the formation of hydrogen sulfide is expected. To simulate those processes the interface between 99.99% pure iron and 0.1 M NaHCO₃ deoxygenated solution with 3.1 × 10⁻⁵-7.8 × 10⁻³ M Na₂S · 9H₂O added was studied. The surface processes were characterised by the in situ normal Raman spectroscopy (NRS) and ex situ techniques; X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy dispersive X-ray (EDX). The open circuit potential (OCP) was monitored during in situ NRS measurements, and potentiodynamic anodic polarization measurements were carried out to reveal electrochemical behaviour of iron electrode.Open circuit potential-time transients indicated that the native oxide is unstable in deaerated bicarbonate solution and undergoes reductive dissolution (i.e. autoreduction) leaving the metallic Fe covered by Fe(OH)₂, adsorbed OH⁻, and patches of ‘magnetite-like’ oxide. Immediately upon injection of the Na₂S-solution the iron interface undergoes complex redox surface processes and a poorly crystalline FeS film forms. Potentiodynamic anodic polarization measurements indicated a mechanical breakdown of the FeS film. The origin and initiation of this breakdown process is not clear but is probably a result of internal stress developed during film growth. Based on surface studies supported by electrochemical measurements, a conceptual model for the complex redox processes occurring at the iron interface is proposed. This model describes the structural development of a poorly crystalline FeS, which breaks down, allowing further dissolution of the Fe and formation of FeOOH at the interface. Simultaneously and despite the existence of thick layer of FeS the entrance of hydrogen was evident as the typical hydrogen cracks in bulk of the iron material. This work shed the light on the complexity of the iron/sulfide solution interface, this knowledge is important to understand the kinetic of reduction of organic groundwater contaminants.     

Inhibition mechanism of sodium laurate to underdeposit corrosion of carbon steels in NaCl solutions

Abstract

In a self-designed occluded corrosion cavity (OCC) simulated cell, the inhibition mechanism of sodium laurate (C₁₂H₂₃O₂Na) to the underdeposit corrosion of the N80 steel in a neutral 0·2 mol L^–1 NaCl solution was studied by electrochemical methods, including measurements of polarisation curves, coupling currents, polarisation resistances, electrochemical impedance spectra (EIS) and ion selectivity of the scale film. The results show that C₁₂H₂₃O₂^– could form a monomolecular adsorption film on the N80 steel surface in the studied solution, whose adsorption behaviour would follow Flory–Huggins isotherm model, and inhibit both anodic and cathodic processes. C₁₂H₂₃O₂^– displays good inhibition performance to the underdeposit corrosion of the N80 steel in the studied system, which could be mainly attributed to the inhibition to the electrode processes of the bulk cathode and occluded anode, the increase in the resistance of the corrosion deposit layer and the prevention to the autocatalysing acidification effect in OCC. The last two factors might be related to the conversion of the deposit layer from anion selectivity to cation selectivity and the formation of a hydrophobic layer on the wall of the micropores in the deposit layer by adsorption of C₁₂H₂₃O₂^–.     

The mechanism of formation of iron oxide and oxyhydroxides in aqueous solutions at room temperature

For a better understanding of the atmospheric rusting of iron and steels, the present work is aimed to explore the mechanism of formation of green rusts, Fe₃O₄, α-FeOOH, β-FeOOH, γ-FeOOH, δ-FeOOH and amorphous ferric oxyhydroxide in aqueous solution at room temperature. The formation processes on which end products are determined are strongly affected by the oxidation rate, pH and the structure and composition of initial and intermediate species of iron. The systematic diagram of formation processes of iron oxide and oxyhydroxides has been presented, in which both dissolved and solid species of iron are included.     

On the pitting corrosion of tin in aqueous solutions

Four independent techniques were employed to prove that Sn undergoes pitting corrosion. The first was based on the measurement of the variation of the open circuit potential of the Sn electrode in aerated Cl^? solutions of various concentrations. Steady-state potentials were attained slowly and erratically from negative values, and were more positive the higher the dilution. Sn electrodes prepassivated in CrO₄^2? solutions responded readily to additions of Cl^? ion. The potentials developed were more noble than those measured in presence of the passivator alone, and changed to positive values with the increase of the concentration of the pitting corrosion agent. Attack was under cathodic control.Galvanostatic polarization of the Sri electrode was carried out in 0.005 to 0.1M NaOH, in the presence of various additions of Cl^?. Above a certain Cl^? content, contingent upon the alkali concentration, the aggressive anion prevented the evolution of Oa on the electrode, and oscillations in the E?t curves were recorded. Competitive adsorption of Cl^? and OH^? is assumed to occur, which affected both the quantity of electricity, Q_p, consumed along the oxide formation steps, and the rate of potential increase, dE/dt, following oxidation. Plots of the two variables as a function of the Cl^? ion concentration exhibited a definite break at the value characteristic for the initiation of pitting attack. Potentiodynamic polarization showed that the pitting corrosion potential progressively shifts towards negative values as the concentration of the aggressive agent in solution was increased. The pitting corrosion currents resulting from the addition of Cl^? to Sn electrodes prepassivated in CrO₄^2? solutions were measured. The dependence of the maximum currents on solution composition is explained on the basis of competitive adsorption.