Tailored AA1050 alloy surfaces by electrograining

The effects of anodic and cathodic polarisation during ac electrograining of aluminium AA1050 alloy in nitric acid on the resultant surface appearance have been examined. A strong dependency of the surface convolution on the amplitude of the anodic and cathodic polarisation is revealed. Cathodic polarisation promotes pit initiation and suppresses pit growth; consequently, a high population of fine hemispherical pits is observed after electrograining with cathodic bias. The passive region of anodic polarisation below the pitting potential is associated with dissolution of segregates and second phase material from the filmed macroscopic alloy surface, which influences the pit initiation stage. Anodic polarisation above the pitting potential encourages pit growth. Thus, by controlling the extents of anodic or cathodic polarisation and by consideration of the local interfacial solution conditions, the means for developing tailored and optimised pit morphologies is revealed.     

Metastable and stable pitting events on Al induced by chlorate and perchlorate anions—Polarization, XPS and SEM studies

The metastable and stable pitting events of Al were studied in 0.075 M deaerated acidic NaClO₃ and NaClO₄ solutions (pH 3) using potentiodynamic anodic polarization and potentiostatic measurements, complemented with SEM and XPS examinations of the electrode surface. Metastable pits (appeared here as oscillations in current in the nA range) form at potentials close to the pitting potential (E_pit). SEM examinations of the electrode surface showed that the current oscillations resulted in observable pits on sample surface. The repassivated metastable pitting sites are prone to become preferential sites for following metastable pits to nucleate, resulting in accumulated corrosion damages on the surface. Results showed that Cl⁻ ions produced in solution via the reduction of ClO₃⁻ and ClO₄⁻ anions at sufficiently negative cathodic potentials as well as their decomposition at high anodic potentials. Chloride production induced via the reduction of perchlorates is much slower than induced by chlorates. XPS examinations of the electrode surface showed that the amount of ClO₄⁻ and Cl⁻ anions detected on the electrode surface increases with both cathodic and anodic polarizations (even above E_pit). Experimental results revealed that addition of Cl⁻ ions to the ClO₄⁻ solution accelerates pitting corrosion, indicating that these anions cooperate together in passivity breakdown and initiation of pitting. The role of ClO₃⁻ and ClO₄⁻ ions, despite their large size, in pitting process is also discussed here. A point defect model (PDM) is employed to explain passivity breakdown induced by pitting corrosion as a result of the aggressive attack of Cl⁻ ions.     

碳钢—硝酸体系腐蚀与缓蚀行为研究

采用失重法、容量法和电化学方法研究了碳钢在50g/kg HNO_3中的腐蚀行为,并对Lan-5在碳钢—50g/kg NHO_3体系中的缓蚀行为进行了初步探索研究.实验结果表明:碳钢在50g/kg HNO_3中的阳极腐蚀产物是Fe~(3+),阴极反应主要不是析氢反应.Lan-5是碳钢—50g/kg HNO_3体系的高效缓蚀剂,其缓蚀效率可达到99.9%,缓蚀剂之间具有显著的协同作用.研究结果还发现,碳钢在硝酸—Lan-5介质中有孔蚀倾向,阳极极化加速孔蚀的形成.     

Role of intermetallic phases in localized corrosion of AA5083

The presence of intermetallic inclusions very often plays a crucial role for the susceptibility of different aluminium alloys to localized corrosion attack. The intimate details of localized corrosion of 5083 aluminium alloy have been studied in the present work. Local techniques such as scanning Kelvin probe force microscopy, in situ atomic force microscopy, scanning electron microscopy coupled with energy dispersive spectroscopy were used to investigate the mechanisms and the kinetics of localized corrosion attack. The importance of iron-rich and Mg₂Si intermetallic phases in the initiation of corrosion processes is demonstrated in the paper.The Mg₂Si phase has a potential lower relatively to the matrix. Moreover, the high reactivity of magnesium leads to the dissolution and consequently to the fast dealloying of these intermetallics. However, hydroxide (Mg(OH)₂ and SiO₂·nH₂O) deposits formed during corrosion act as an additional diffusion barrier hindering the deep propagation of pits.The iron containing intermetallics have the potential higher with respect to the aluminum matrix playing the role of effective cathodic centers for oxygen reduction causing anodic polarization and pitting in the surrounding alloy matrix. Dealloying of such intermetallics with subsequent iron enrichment was also revealed. Pitting initiation seems to be statistical and independent of the composition of Fe-rich intermetallics. However, the active growth of the pits prevents initiation of localized corrosion attack in nearby sites. A new pit can start to grow only when a neighbor one becomes passivated.     

MIC of stainless steel in freshwater and the cathodic behaviour of biomineralized Mn-oxides

Microbially influenced corrosion by manganese oxidizing microorganisms (MIC by MOMOs) is a corrosion phenomenon occurring in freshwater systems affecting stainless steels. Typically, chloride induced corrosion is observed at chloride concentrations normally not considered as critical. The initiation of corrosion is attributed to an anodic shift of the open circuit potential of the passive stainless steel caused by manganese dioxide biomineralized by the specific organisms. However, not only the potential of the passive steel seems of interest. For the stabilization of localized corrosion, the ability of the cathodic reaction to drive enough current for promoting the switch from metastable pitting to stable pit growth is considered important. Consequently, experiments have been carried out in order to compare the dynamic cathodic properties of the usual oxygen reduction reaction with that of biofilm incorporating biomineralized MnO₂. The results indicate a significantly better cathodic performance of such biofilm which might explain the fast and extensive damages observed in certain cases of MIC by MOMOs.     

Mechanistic investigation of hydrogen-enhanced anodic dissolution of X-70 pipe steel and its implication on near-neutral pH SCC of pipelines

The effects of hydrogen-charging on anodic dissolution of pipe steel under near-neutral pH condition were studied by electrochemical techniques. Hydrogen-charging enhances the anodic dissolution rate of the steel. The hydrogen-enhanced dissolution increases with increasing charging current density. The hydrogen effect is attributed to the alteration of chemical potential and exchange current density of steel. Hydrogen-charging affects the corrosion process of the steel. In particular, at a high charging current density, a layer of corrosion product forms on the electrode surface to change corrosion potential and interfacial double-charge layer capacitance as well as charge-transfer resistance. The hydrogen effect factor for enhanced anodic dissolution of steel at an anodic potential of −0.4 V (SCE) is 1.53 only. Hydrogen-enhanced anodic dissolution of steel by itself may not be the major factor contributing to the high rate of crack growth in pipe steel in near-neutral pH electrolyte. A further investigation of the synergistic effect of hydrogen and stress on dissolution at the crack-tip is essential to determine the mechanism of near-neutral pH stress corrosion cracking of pipelines.     

Steady-state polarization response of chloride-induced macrocell corrosion systems in steel reinforced concrete — numerical and experimental investigations

Chloride-induced macrocell corrosion of steel is recognized as a frequent and harmful problem affecting reinforced concrete structures. For this specific corrosion mechanism, anodic and cathodic areas at steel-concrete interface are spatially separated. Thereby, at equilibrium, anodic and cathodic potentials are different due to electrical resistivity of concrete, which means there is a potential gradient and a macrocell corrosion current flowing in the concrete volume. Despite some available works on electrochemical modelling of free macrocell corrosion systems in reinforced concrete, the literature is very scarce regarding the response of such systems under polarization. Due to this lack of knowledge, the assessment of steel corrosion rate in reinforced concrete is still commonly based on the uniform corrosion assumption. In this paper, the detailed polarization response of the different components of macrocell systems is studied in steady-state condition on the basis of numerical simulations and the qualitative conclusions are validated by original experiments. Some important results are achieved, with potentially major consequences regarding the non-destructive evaluation of steel corrosion in reinforced concrete. In particular, the distribution of the polarizing current in the macrocell system is found to be dependent on the polarization nature (anodic or cathodic) and magnitude. Moreover, in case of chloride-induced macrocell systems, the apparent linear polarization range is actually much higher than usually assumed and significant differences are observed between anodic and cathodic responses.     

Initiation and propagation of pitting and crevice corrosion of hydrogen-containing passive films on X70 micro-alloyed steel

Hydrogen promoted initiation and propagation of pitting and crevice corrosion of X70 micro-alloyed steel were characterized by potential dynamic measurements, the scanning reference electrode technique (SRET) and electrochemical impedance spectra (EIS). At open circuit potential, in situ SRET results show that hydrogen accelerates the nucleation and propagation of pitting of X70 steel. The pitting potential E_p of X70 steel gradually decreases with an increase of chloride ion concentration in NaHCO₃ solution. Pre-charged hydrogen does not have a significant effect on the pitting potential E_p and open circuit potential E_corr of the steel in 0.5 M NaHCO₃ solution. However, a synergistic effect of hydrogen and Cl⁻ on the anodic dissolution and pitting potential of X70 steel is observed in 0.5 M NaHCO₃ solution containing chloride ions. When crevices are present in X70 steel, hydrogen accelerates the initiation and progress of crevice corrosion. The mechanisms by which hydrogen promotes the initiation and propagation of pitting and crevice corrosion are proposed and discussed.     

Mechanism of steel corrosion in concentrated NaOH solutions

A study has been made of the mechanism of corrosion of carbon steel in 1–19 N NaOH at 25–80°C. The polarization curves were obtained with a rotating steel disk electrode, and the rotating ring-hemisphere electrode technique was used to identify soluble corrosion products. It was found that at the active anodic dissolution potentials, steel dissolves to form HFeO⁻₂ ions. Both the anodic and cathodic polarization curves exhibited a well defined Tafel regime, and the electrokinetic parameters were obtained for the corrosion of steel. The results were interpreted with a corrosion mechanism based on the decomposition of molecular water to hydrogen at the cathodic sites, and the active dissolution of iron to HFeO⁻₂ ions via two adsorbed intermediates, FeOH_ads and Fe(OH)_2,ads, at the anodic sites. The theoretically derived anodic Tafel slope, anodic reaction order, corrosion potential, and corrosion current density agreed quantitatively with the experimental values. At the potentials above the anodic Tafel regime and in the neighborhood of the active—passive transition potential, steel dissolved to form both the bi-valent and tri-valent iron species, HFeO⁻₂ and FeO⁻₂ ions. The thermodynamic considerations revealed that FeO⁻₂ ion was formed from the oxidation of Fe(OH)_2,ads intermediate, rather than from the oxidation of HFeO⁻₂ ion on steel surface.     

牺牲阳极技术在埋地管道中的应用

深圳机场埋地管道经过20年地下杂散电流,各种土壤微生物和电解介质的腐蚀,容易发生电化学及穿孔。但在活泼阳极优先腐蚀溶解和释放出的电子与被保护体表面发生阴极还原反应,抑阻了阳极溶解进程。2006年进行检测维护,增加部分阳极,保证正常的电流电位。2011年在开挖埋地管道回收中,发现埋地管道及设施免受其电化学腐蚀。     

Electrochemical behaviour of AISI 321 stainless steel in methanol-H2SO4 mixtures

The electrochemical corrosion behaviour (passivity, pitting and repassivation) of austenitic stainless steel AISI 321 containing titanium has been investigated in methanol containing different concentrations of sulphuric acid (0.001–1 M). The cathodic reaction is observed to be hydrogen evolution. Anodic polarization of the stainless steel reveals a characteristic cathodic loop in methanol with higher concentrations of H₂SO₄ (0.1 M and 1 M). A clear and stable passivity is also observed in each solution mixture which breaks down at higher potentials due to pitting. The passivation current, passivity range, pitting intensity and pitting potential are found to vary with variation of acid concentration in the methanol. The kinetics of pitting and the morphology of the pits have been studied. A variation in concentration of acid and scan rate has been found to influence the protection potential of the stainless steel.     

Pitting on the crevice wall prior to crevice corrosion: Iron in sulfate/chromate solution

Experimental results are presented for the induction period that precedes the onset of crevice corrosion. In situ visual examination and ex situ microscopy revealed that the first corrosive attack on the (passive) crevice wall occurs as a pit near the bottom of the crevice. Additional pits and corrosion product form higher and higher on the crevice wall during the induction period. The measured current is initially low in the passive range (10 μA), increasing gradually, but the measured current does not record the large increases in anodic current that produce the pits. The latter result and the observation that gas bubbles form within the pits indicate that the pitting initially is a local cell process of metal dissolution and cathodic reactions on the sample, including H₂ formation which occurs inside the pits in this iron/sulfate-chromate solution (pH 8.8, room temperature). Then, a transition from merging pits to corrosive attack across the width of the crevice wall occurs accompanied by a much steeper increasing current. This latter morphology (horizontal boundary that moves up the wall towards the crevice opening, heaviest attack just below this boundary and passive crevice wall above this boundary) is representative of stable crevice corrosion of the IR type reported in the literature.     

钢样在磷酸钠溶液中氯离子存在的极化曲线与腐蚀曲线

<正> 分析极化曲线的特微成功地探测金属在溶液中进行的腐蚀过程.把阳极极化曲线的直线部分延长到对应静电位的电流密度以代表自发腐蚀的阳极电流密度,同样得到阴级电流密度.认为阴极极化曲线和阳极极化曲线的直线部分的延是线交点,在近似情况,可代表自发腐蚀的溶解初速度.     

Pitting corrosion behavior of stainless steel in ultrasonic cell

Under potentiostatic conditions on stainless steel electrodes in chloride solution, a novel electrochemical cell equipped with an ultrasonic transducer has been designed to evaluate the metastable and stable pitting corrosion behaviors. The advantage of the cell is that the rupture of pit cover can be controlled during the metastable or stable pitting growth periods, and in which the effect of the cover on pitting stability is affirmed. Some shallower stable pits, which were formed at higher anodic potential, were found to repassivate after ultrasonic cover rupture due to their shorter diffusional length. In terms of the relative susceptibility to localized corrosion, repassivation potential (E_r) determined by the cyclic potentiodynamic polarization curve was found to describe more properties of the occluded pit cover than the others.     

Pitting corrosion of Al and Al–Cu alloys by ClO4 ions in neutral sulphate solutions

The influence of various concentrations of NaClO₄, as a pitting corrosion agent, on the corrosion behaviour of pure Al, and two Al–Cu alloys, namely (Al + 2.5 wt% Cu) and (Al + 7 wt% Cu) alloys in 1.0 M Na₂SO₄ solution was investigated by potentiodynamic polarization and potentiostatic techniques at 25 °C. Measurements were conducted under the influence of various experimental conditions, complemented by ex situ energy dispersive X-ray (EDX) and scanning electron microscopy (SEM) examinations of the electrode surface. In free perchlorate sulphate solutions, for the three Al samples, the anodic polarization exhibits an active/passive transition. The active dissolution region involves an anodic peak (peak A) which is assigned to the formation of Al₂O₃ passive film on the electrode surface. The passive region extends up to 1500 mV with almost constant current density (j_pass) without exhibiting a critical breakdown potential or showing any evidence of pitting attack. For the three Al samples, addition of ClO₄⁻ ions to the sulphate solution stimulates their active anodic dissolution and tends to induce pitting corrosion within the oxide passive region. Pitting corrosion was confirmed by SEM examination of the electrode surface. The pitting potential decreases with increasing ClO₄⁻ ion concentration indicating a decrease in pitting corrosion resistance. The susceptibility of the three Al samples towards pitting corrosion decreases in the order: Al > (Al + 2.5 wt% Cu) alloy > (Al + 7 wt% Cu) alloy. Potentiostatic measurements showed that the rate of pitting initiation increases with increasing ClO₄⁻ ion concentration and applied step anodic potential, while it decreases with increasing %Cu in the Al samples. The inhibitive effect of SO₄²⁻ ions was also discussed.     

Time-dependent polarization behaviour of pipeline grade steel in low ionic strength environments

A bench-top experimental approach is described for estimation of the polarization behaviour of pipeline steel as a function of the time-dependent formation of scale and calcareous deposits in simulated soil leachates. A three-time constant process model provided a common set of parameters for a given soil environment and level of aeration. The parameters estimated could be applied to a broad range of current values and were independent of time, potential and applied current. The experimental approach, model and regression procedure are general and could be used to determine the physical parameters associated with the seasonal variations (wet–dry cycles) in the soils surrounding pipelines or with other factors that influence general corrosion. The polarization model could provide a boundary condition for mathematical models for cathodic protection of pipelines or other buried structures. The separation of current contributions implicit in the model can be used to assess the reduction of corrosion current associated with specific CP criteria.     

Dodigen 213-N as corrosion inhibitor for ASTM 1010 mild steel in 10% HCL

This article describes a study of the behavior of a mixture of amines and amides, commercially known as Dodigen 213-N (D-213 N), as a corrosion inhibitor for ASTM 1010 mild steel in 10% w/w HCl solution. The concentration range used was 1 × 10⁻⁵ M to 8 × 10⁻⁴ M. The weight loss and electrochemical techniques used were corrosion potential measurement, anodic and cathodic polarization curves, and electrochemical impedance spectroscopy (EIS). The solution temperature was 50 ± 1 °C and it was naturally aerated. The corrosion potential values shifted to slightly more positive values, thus indicating mixed inhibitor behavior. The anodic and cathodic polarization curves showed that D-213 N is an effective corrosion inhibitor, since both the anodic and the cathodic reactions were polarized in comparison with those obtained without inhibitor. For all concentrations the cathodic polarization curves were more polarized than the anodic ones. The inhibition efficiency was in the range 75–98%, calculated from values of weight loss and corrosion current density, i _corr, obtained by extrapolation of Tafel cathodic linear region.     

Passivity and passivity breakdown of a zinc electrode in aerated neutral sodium nitrate solutions

The passivation and pitting corrosion behaviour of a zinc electrode in aerated neutral sodium nitrate solutions was investigated by cyclic voltammetry and chronopotentiometry techniques, complemented by ex situ scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive X-ray (EDX) examinations of the electrode surface. Measurements were conducted under different experimental conditions. The potentiodynamic anodic polarization curves do not exhibit active dissolution region due to spontaneous passivation. The passivity is due to the presence of thin film of ZnO on the anode surface. The passive region is followed by pitting corrosion as a result of breakdown of the passive film. SEM images confirmed the existence of pits on the electrode surface. The breakdown potential decreases with an increase in NO₃⁻ concentration and temperature, but increases with increasing potential scan rate. Addition of SO₄²⁻ ions to the nitrate solution accelerates pitting corrosion, while addition of WO₄²⁻ and MoO₄²⁻ ions inhibits pitting corrosion. The chronopotentiometry measurements show that the incubation time for pitting initiation decreases with increasing NO₃⁻ concentration, temperature and applied anodic current density. Addition of SO₄²⁻ ions decreases the rate of passive film growth and the incubation time, while the reverse changes produced by addition of either WO₄²⁻ or MoO₄²⁻ ions.     

Electrochemical behaviour of copper in potassium thiocyanate solution—II. Analysis of potentiostatic current transients

The kinetics of passivity and pitting corrosion of copper in borate-boric acid buffer containing potassium thiocyanate is studied at 25°C by means of potentiostatic transients recorded at different constant potential values. Three different potential regions are distinguished. Region I, as the applied potential is increased, implies the successive formation of cuprous thiocyanate, cuprous and cupric oxide. This corresponds to the formation of a composite anodic layer. Region II extends from the potential of the third anodic voltammetric peak up to the breakdown potential. Region III involves the breakdown of the passive layer caused by thiocyanate ions, and the formation of different copper salts in the pits. The kinetics of the various stages is discussed in terms of an extension of the nucleation and growth mechanism following an approach which was earlier applied for explaining the initiation and growth of pitting corrosion of metals.     

Corrosion of the stressed pipe steel in carbonate-bicarbonate solution studied by scanning localized electrochemical impedance spectroscopy

Corrosion of X-70 pipe steel as a function of stress distribution was investigated by scanning localized electrochemical impedance spectroscopy (LEIS) technique. The deformation-induced stress, if not sufficiently high, has an inhibitive effect on corrosion reaction, pitting occurrence and crack initiation in pipe steel under high-pH condition. Such an inhibitive effect is due to the enhanced generation of carbonate product and the resultant surface block effect at the stressed zones. The tensile and compressive stresses have identical effect on inhibition of dissolution and pitting of the steel. However, tensile stress enhances the steel dissolution more significantly than compressive stress, and thus, generates more carbonate product, resulting in higher localized impedance. Pits are easy to occur around the neutral axis of the U-bend specimen, where the steel deformation and the resultant stress are ignorable. For pipelines encountering non-uniform stress distribution, the role of stress in crack initiation is critically different. Scanning LEIS technique provides a promising method to map and characterize corrosion of the steel as a function of the stress distribution.