Corrosion behavior of two bipolar plate materials in simulated PEMFC environment by electrochemical noise technique

The corrosion behavior of 316L stainless steel (316L) and bulk amorphous Zr₇₅Ti₂₅ (AB) alloy was studied using electrochemical noise (EN) in simulated polymer electrolyte membrane fuel cell (PEMFC) conditions at 25 and 80 °C. At open circuit potential, under H₂ environment, AB exhibited a higher corrosion-resistance than 316L and the contrary was observed under O₂ environment. At imposed cathodic potential, under H₂ environment for PEMFC anodic simulation, the reduction mechanism of 316L at 80 °C gave the most important EN signals of all other conditions. At imposed anodic potential, under O₂ environment for cathodic simulation, the passive state was observed for 316L while severe localized corrosion for AB was obvious at 80 °C. It is concluded from the corrosion data of this work that in the anode environment of a PEMFC, the AB alloy could be a better candidate than 316L for bipolar plates. The contrary was observed in the simulated cathode environment.     

Electrochemical noise analysis of the corrosion of AZ91D magnesium alloy in alkaline chloride solution

The corrosion behavior of AZ91D magnesium alloy in alkaline chloride solution was investigated by electrochemical noise (EN). The wavelet transform, as well as noise resistance (R_n) and power spectral density (PSD), had been employed to analyze the EN data. It was revealed that there exist three different stages of corrosion for AZ91D magnesium alloy in alkaline chloride solution, including an anodic dissolution process companying with the growth, absorption and desorption of hydrogen bubbles, a development of pitting corrosion, an inhibition process by protective MgH₂ film. The results demonstrated that energy distribution plot (EDP) was a powerful tool to provide useful information about the dominant process for the different corrosion stages.     

Pitting mechanism on an austenite stainless steel nanocrystalline coating investigated by electrochemical noise and in-situ AFM analysis

The pitting corrosion behavior of an austenite stainless steel with polycrystalline (PC) and nanocrystalline (NC) microstructure in 3.5% NaCl solution was investigated by electrochemical noise (EN) and in-situ atomic force microscopy (AFM) under anodic potential. The power spectral density (PSD) and wavelet transform have been employed to analyze the EN data. The pitting corrosion resistance of the NC coating was much higher than that of the PC alloy, with different pitting mechanisms observed for both specimens; a slow metastable pit generation and healing for the PC alloy and a fast metastable pit initiation and death for the NC coating. There was good conformity between the EN and the in-situ AFM analysis. The pitting corrosion mechanism has been discussed according to the EN analysis and the in-situ AFM observations.     

Corrosion resistance of plasma-anodized AZ91D magnesium alloy by electrochemical methods

Anodic coatings formed on magnesium alloys by plasma anodization process are mainly used as protective coatings against corrosion. The effects of KOH concentration, anodization time and current density on properties of anodic layers formed on AZ91D magnesium alloy were investigated to obtain coatings with improved corrosion behaviour. The coatings were characterized by scanning electron microscopy (SEM), electron dispersion X-ray spectroscopy (EDX), X-ray diffraction (XRD) and micro-Raman spectroscopy. The film is porous and cracked, mainly composed of magnesium oxide (MgO), but contains all the elements present in the electrolyte and alloy. The corrosion behaviour of anodized Mg alloy was examined by using stationary and dynamic electrochemical techniques in corrosive water. The best corrosion resistance measured by electrochemical methods is obtained in the more concentrated electrolyte 3 M KOH + 0.5 M KF + 0.25 M Na₃PO₄·12 H₂O, with a long anodization time and a low current density. A double electrochemical effects of the anodized layer on the magnesium corrosion is observed: a large inhibition of the cathodic process and a stabilization of a large passivation plateau.     

Effect of impurities on the electrochemical behaviour of aluminium electrodes anodized in phosphoric acid

The electrochemical behaviour of anodic films formed on pure aluminium (99.999% and 1100) and 6061-Al alloy substrates in phosphoric acid has been investigated in an aluminium saturated phosphate solution of pH 6.23. The polarization resistance data obtained after one day immersion in the electrolyte showed a strong correlation with the pit number, assessed from an outdoor atmospheric exposure test after 4 months. The cathodic polarization measurements of anodized aluminium in the electrolyte were also carried out. Electrochemical behaviour, eg corrosion, hydrogen evolution reaction, was interpreted in terms of “active sites” within the anodic film. It is suggested that the active sites were mainly formed in the presence of impurities or alloy constituents in aluminium substrates.     

Effects of hydrogen on the corrosion of pure magnesium

Electrochemical measurements and capacitance measurements were performed for better understanding of the effects of hydrogen on the corrosion of pure magnesium. Anodic polarization curves, activation energy (E_a), pitting initiation time and electrochemical noise (EN) were carried out, which showed that hydrogen had a strong influence on the corrosion of magnesium. There existed a highest corrosion resistance of magnesium, when a series of cathodic charging current density were applied to the specimens due to the optimum hydrogen concentration in the hydride coating (MgH₂) on the surface. Mott-Schottky results confirmed that there was a hydride coating on the charged magnesium. Hydrogen ionized as H⁻ and depleted donor/electron, which induced the inversion of semi-conductivity from N-type to P-type.     

Self Linear Polarization Resistance-Theory and Examples

Measuring the corrosion rate of a corroding metal is of interest in many situations, including monitoring industrial processes and undertaking fundamental research. The corrosion rate of a metal can be measured electrochemically by determining its polarization resistance, which is inversely proportional to the corrosion rate. What is described in this article is a technique for mathematically extracting from electrochemical noise (EN) data the polarization resistance as well as a measure of the frequency of anodic and cathodic transients. The theoretical framework for self-linear polarization resistance is based on a time-domain analysis of an electrical circuit model of an EN experiment. The analysis indicates that the polarization resistance for one electrode can be interpreted only if the second electrode alone is generating current transients during a given time record. One advantage of this approach, compared with other techniques for obtaining a polarization resistance from EN data, is that short time records, i.e. less than one minute, can be assessed. The self-consistency of the polarization resistance can be assessed with a correlation coefficient. Another advantage is that the nature of localized corrosion events can be attributed to either anodic or cathodic current transients from one of the electrodes.     

Electrochemical studies of the inhibition of the cathodic delamination of organically coated galvanised steel by thin conversion films

Scanning Kelvin Probe (SKP) measurements of thin amorphous conversion film coated galvanised steel in combination with current density-potential curves and electrochemical impedance spectroscopy (EIS) were performed with the aim to improve the understanding of electrode potentials at the coating/metal interface and their influence on corrosive de-adhesion. The thin hybrid conversion films contained Zn-phosphates, titanates and also complexing organic compounds and led to an inhibition of the cathodic oxygen reduction and anodic zinc dissolution. In the polymer coated area the conversion film leads to a cathodic shift of the potential as measured by means of the SKP. This cathodic potential shift is explained by the substitution of the n-semiconducting Zn-oxide with an insulating inorganic layer. When the SKP detects the potential of freely corroding iron at a defect, where no protective coating layer is, the interfacial potential for the conversion film coated zinc layer is more negative than the defect potential. This leads to a diminished driving force for an oxygen reduction induced delamination process which is of relevance for the understanding of cut-edge corrosion.     

In situ electrochemical Scanning Kelvin Probe Blister-Test studies of the de-adhesion kinetics at polymer/zinc oxide/zinc interfaces

Fundamental investigations of the polymer/zinc oxide/zinc interface corrosion stability were performed in situ by means of the electrochemical Height Regulated Scanning Kelvin Probe Blister-Test (HR-SKP-BT) under controlled atmospheric conditions. A hole under an adhesive layer film served as electrolyte reservoir to initiate cathodic de-adhesion processes. Then a combinatorial approach was undertaken to simultaneously study the influence of electrolyte pressure at constant defect polarisation and of relative atmospheric humidity on the de-adhesion rate. The time resolved blister growth and the propagation of the three phase boundary polymer/oxide covered zinc/interfacial electrolyte layer could be detected. It could be proven that the oxygen reduction induced electrochemical damage of the interface precedes the subsequent mechanical de-adhesion process. By variation of the relative atmospheric humidity the water concentration within the bulk adhesive and its interphase adjacent to the metal substrate could be adjusted. These processes were further analysed by peel-tests and in situ Attenuated-Total-Reflection Infrared Spectroscopy (ATR-IR) studies of water diffusion. A decrease of the interphasial water concentration led to a deceleration of the de-adhesion kinetics for constant defect conditions and to smaller interfacial ion transport rates. This could be assigned to an inhibition of the electron transfer reactions at the front of de-adhesion and an increased adhesion force between polymer film and oxide covered metal preventing the formation of an extended interfacial electrolyte layer.     

Corrosion behaviour of multiwall carbon nanotube/magnesium composites in 3.5% NaCl

Corrosion behaviour of multiwall carbon nanotube (MWNT) modified pure magnesium (Mg) composites (MWNT/Mg) prepared by melt stirring technique was investigated by weight loss, H₂ gas collection and pH measurements, as well as electrochemical methods such as open circuit potential (OCP) monitoring, polarisation curves and electrochemical impedance spectroscopy (EIS). Results show that the corrosion rate of MWNT/Mg composites is significantly higher than the corrosion rate of pure Mg. The corrosion rate of the composites also depends on the degree of dispersion of MWNTs during melt stirring process. The role of the MWNTs in increasing the corrosion rate clearly can be attributed to their high cathodic activity. Characterization of the corrosion product layers indicates that dispersion of MWNTs in the matrix leads to a more homogeneous coverage of the surface by corrosion products, and the lowest thickness of the corrosion product layer. The corrosion product layers in all cases provide poor protection of the surface. The corrosion rate of pure Mg as well as Mg/MWNT composites significantly increases during 1-week immersion; this effect is the strongest for the composite with dispersed nanotubes and can be attributed to the increased cathodic kinetics with time.     

In situ electrochemical impedance and noise measurements of corroding stainless steel in high temperature water

An in situ corrosion study of austenitic stainless steel 08CH18N10T in high temperature water was performed. The material under study is used in the construction of steam generator of PWR (pressurized water reactor) nuclear power stations and is similar to AISI 321 stainless steel.In situ 300-h tests were performed under autoclave conditions at 280 °C and 8 MPa and consisted of impedance measurements, polarization measurements and electrochemical noise measurements. The experiments were performed in deionised water with the pH adjusted to 9.5, in the presence/absence of chlorides. An additional modification of corrosivity was achieved by changing oxygen concentration.A detailed analysis of the impedance data is presented identifying in the impedance spectra contributions of oxide, corrosion reaction, double layer and diffusion process. A good agreement was found between corrosion data from electrochemical impedance spectroscopy (EIS) and that from electrochemical noise (EN) measurements. It was confirmed that the oxide response cannot be attributed to the overall oxide layer but only to the part corresponding to the space charge layer, thus indicating the semi-conductive character of the oxide.     

Noise characterization of surface processes of the Li/organic electrolyte interface

The processes occurring in aprotic electrolyte on a lithium electrode in the steady state conditions and under polarization are studied using the method of electrochemical noise characterization. The evidence of the electro-chemical noise measurements on polarized lithium electrodes indicates that the discharge of lithium ions under cathodic polarization, as well as lithium anodic dissolution, is localized under the passive film rather than on its surface. An increase in the polarizing current results in local breakdown of the film; in this case, the electrochemical process emerges on the electrode surface affecting the character of potential fluctuations. The intensity of electrochemical noise significantly increases in the course of cathodic polarization with high currents. The reason is that lithium metal crystals, which are formed under the passive film, perforate the film, and dendrites grow on its surface. The method shows the dependence of electrochemical noise intensity on the nature of the electrolyte and establishes the correlation between the stability of the lithium electrode in the course of cycling and the intensity of fluctuations. This offers an opportunity of using the method of electrochemical noise for screening organic electrolytes for lithium batteries.     

Corrosion of dissimilar alloys: Electrochemical noise

A new approach for interpreting the electrochemical noise generated by freely corroding electrodes is presented. For a single electrode, with well-defined anodic and cathodic regions, it is proposed that the instantaneous values of the corrosion current and open circuit potential are functions of the time-invariant potential difference between the anodic and cathodic reactions and the time-dependent impedances associated with the anodic and cathodic reactions. The fluctuations in the values of anodic and cathodic impedances are determined by the interaction between the aggressive environment and the oxide, hydroxide or adsorbed surface layer covering the anodic and cathodic regions; the instantaneous values of impedances modulate the corrosion current. The mathematical implications of these assumptions are discussed and the results are compared with the traditional approach, based on time-dependent current sources acting on constant impedances. Subsequently, the findings for a single electrode are transferred to two dissimilar electrodes and an experimental technique that enables the estimation of the time evolution of anodic and cathodic resistances for each of the dissimilar electrodes is presented.     

Model for corrosion of metals covered with thin electrolyte layers: Pseudo-steady state diffusion of oxygen

A one-dimensional mathematical model is presented for the free corrosion of a bare metal surface (devoid of any oxide film) under a thin electrolyte layer using mixed potential theory where anodic metal dissolution is controlled by oxygen diffusion through the electrolyte layer and by the oxygen reduction at the metal surface. A pseudo-steady state is considered wherein the oxygen diffusion is at steady state while the metal and hydroxyl ions keep accumulating in the thin electrolyte layer due to a decoupling arising from the assumed Tafel laws for corrosion kinetics. Under free corrosion the oxygen diffusion is shown to depend on a non-linear boundary condition with a non-integer power on oxygen concentration at the metal surface which makes the model non-trivial. Analytical and numerical results for the oxygen concentration at the metal surface, corrosion potential, and corrosion current density are reported which depend on several kinetic, thermodynamic and transport parameters in the system. The model is applied to iron and zinc systems with input data taken from the literature. The experimental utility of the model for gathering thin-film corrosion parameters from a study of the corrosion current and potential as a function of the thickness of the electrolyte layer is discussed. Precipitation and passivity, though not the main object of study in this work, are briefly discussed.     

原油储罐底板外侧阴极保护技术

以固体电解质涂层为导电层,石墨导电涂料为阳极涂层,石墨为参比电极,进行原油储罐底板外侧的气相阴极保护。通过实验确定了固体电解质、导电涂料的配方,测定了Q235钢在涂层中的自腐蚀电位、最小保护电位和最小保护电流密度等电化学参数。     

Mechanistic aspects of electrochemical corrosion of aluminum alloy in ethylene glycol–water solution

The electrochemical corrosion behavior of 3003 aluminum (Al) alloy in ethylene glycol–water solutions was studied by electrochemical measurements and surface analysis techniques through an impingement jet system. Al corrosion in aerated, stationary solution is a mixed-controlled process, i.e., both activation and mass-transfer steps control the corrosion reaction. Upon flowing of solution, no matter if sand particles are contained, Al corrosion is an activation-controlled process. In addition to Al oxide film, a layer of Al-alcohol film will also be formed on the electrode surface in ethylene glycol–water solution, contributing to inhibition of anodic dissolution of Al. In the absence of oxygen, a stable passivity could also be achieved on Al alloy. The main cathodic reaction is either reduction of water or reduction of ethylene glycol. However, the reduction of water and ethylene glycol would not be as significant as that of oxygen. As a consequence, the resultant film is not as stable as that formed in the presence of oxygen, as shown in polarization measurements. Al corrosion reaction in mixed-controlled by activation step and mass-transfer step through the anodic surface film. The coverage of the adsorbed intermediate product on Al electrode surface is expected to be very small under the impingement of fluid and sand particles, which results in the generation of an inductive loop in Nyquist diagrams measured on Al electrode in ethylene glycol–water solution.     

Effect of process parameters of plasma electrolytic oxidation on microstructure and corrosion properties of magnesium alloys

In this work, a plasma electrolytic oxidation process was applied to AZ91 and AM50 magnesium alloys and commercially pure magnesium to produce a protective surface layer. The plasma electrolytic oxidation process was carried out in an alkaline phosphate solution with a DC power supply, using relatively high current densities and short treatment times. The influence of some important process parameters such as current density, treatment time and voltage was studied. The layers were characterised by scansion electron microscopy, X-ray diffraction and X-ray photoelectron spectrometry, in order to investigate the effect of the process parameters on the microstructure and chemical composition. The corrosion resistance properties of the obtained layers were investigated by potentiodynamic anodic polarization and electrochemical impedance spectroscopy tests. The current density, applied during the treatment, influenced the morphology and the thickness of the coatings, and, consequently, the corrosion resistance. The corrosion tests evidenced that the layers obtained with plasma electrolytic process provided a good corrosion protection to the magnesium and magnesium alloys.     

Cl-污染大气环境下T91钢孔蚀萌生的电化学噪声检测

采用电化学噪声(EN)技术研究了T91马氏体耐热钢在Cl^-污染大气环境下的电化学行为,并针对EN数据进行统计分析,得到表征腐蚀萌生和发展的特征参数。运用Thevenin等效电路模型得到统计参数与腐蚀速度之间的关系并用实验数据进行验证。实验结果表明,EN技术可以有效监测大气环境下T91钢的腐蚀过程,电流噪声幅值大小一定程度上反映了局部腐蚀萌生和发展过程。理论分析结果表明,溶液电阻可影响电位噪声和电流噪声的测量以及噪声电阻的计算结果,但只要溶液电阻值远小于工作电极的阻抗,其影响可以忽略。谱噪声电阻不等于研究电极的阻抗模值,二者之间存在一定的关系。实验数据的进一步分析证实了理论预测的正确性。     

Evaluation of the corrosion protection performance of epoxy coatings containing Mg nanoparticle on carbon steel in 0.1 M NaCl solution by SECM and EIS techniques

The effect of corrosion protection performance of epoxy coatings containing magnesium (Mg) nanoparticles on carbon steel was analyzed using scanning electrochemical microscopy (SECM) and electrochemical impedance spectroscopy (EIS). Localized measurements such as oxygen consumption and iron dissolution were observed using SECM in 0.1 M NaCl in the epoxy-coated sample. Line profile and topographic image analysis were measured by applying ?0.70 and +0.60 V vs the Ag/AgCl/saturated KCl reference electrode as the tip potential for the cathodic and anodic reactions, respectively. The tip current at ?0.70 V for the epoxy-coated sample with Mg nanoparticles decreased rapidly, which is due to cathodic reduction in dissolved oxygen. The EIS measurements were conducted in 0.1 M NaCl after wet and dry cyclic corrosion test. The increase in the film resistance (R _f) and charge transfer resistance (R _ct) values was confirmed by the addition of Mg nanoparticles in the epoxy coating. Scanning electron microscope/energy-dispersive X-ray spectroscope analysis showed that Mg was enriched in corrosion products at a scratched area of the coated steel after corrosion testing. Focused ion beam–transmission electron microscope analysis confirmed the presence of the nanoscale oxide layer of Mg in the rust of the steel, which had a beneficial effect on the corrosion resistance of coated steel by forming protective corrosion products in the wet/dry cyclic test.     

Study of the copper anode passivation by electrochemical noise analysis using spectral and wavelet transforms

The passivation of copper anode in sulphuric acid industrial electrolyte at 65 °C was investigated by electrochemical noise (EN) technique using three imposed anodic currents 153, 180 and 210 mA cm⁻² (galvanostatic or chronopotentiometric technique). The wavelet transforms and power spectral density (PSD) analyses have been employed to analyse the EN data. It has been found that during active dissolution the electrode surface is dominated by long time scale process and the change of the position of the maximum relative energy from D7 to D8 could be an indication of future passivation. This is associated in fast Fourier transform analyses by the disappearance of the low frequency plateau (white noise). During anodic dissolution the electrode surface was mainly undertaking general corrosion and the relative energy distribution was concentrated on large timescale crystals (D7-D8) with range of scale between 25.6 and 12.6 s. During passivation the electrode surface was dominated by short time scale process (D3), and during trans-passivation by medium (D4) followed by short time scale process (D2) with range of scale between 0.8 and 0.4 s, 1.6 and 0.8 s and 0.4 and 0.2 s, respectively. Crystals D2 and D3 could be related to metastable pitting and crystal D4 to localized corrosion.