Electrochemical impedance spectroscopy study on the corrosion of the weld zone of 3Cr steel welded joints in CO_2 environments

The welded joints of 3Cr pipeline steel were fabricated with commercial welding wire using the gas tungsten arc welding （GTAW） technique. Potentiodynamic polarization curves, linear polarization resistance （LPR）, electrochemical impedance spectroscopy （EIS）, scan- ning electron microscopy （SEM）, and energy-dispersive spectrometry （EDS） were used to investigate the corrosion resistance and the growth of a corrosion film on the weld zone （WZ）. The changes in electrochemical characteristics of the film were obtained through fitting of the EIS data. The results showed that the average corrosion rate of the WZ in CO2 environments first increased, then fluctuated, and finally de- creased gradually. The formation of the film on the WZ was divided into three stages： dynamic adsorption, incomplete-coverage layer forma- tion, and integral layer formation.

ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY STUDY OF CORROSION INHIBITION OF MODIFIED LIGNOSULPHONATE FOR CARBON STEEL

The corrosion inhibition for carbon steel in circulating cooling water by modified lignosulphonate has been investigated using electrochemical impedance spectroscopy technique. Results show that the inhibition efficiency of modified lignosulphonate GCL2 is a great improvement on that of lignosulphonate. The maximum inhibition efficiency of GCL2 reaches 99.21% at the concentration of 400mg·L^-1 at 303K. The corrosion inhibition of GCL2 is attributed to forming adsorption film on the metal surface for the electrochemical impedance spectroscopy in GCL2 solution shows more than one time-constant.Moreover,results also indicate that it is more efficient in stirring solution than in still solution for GCL2 because the constant of adsorption in stirring solution is much larger than that in still solution. The adsorption of inhibitor GCL2 follows Langmuir＇s adsorption isotherm.

Exfoliation corrosion susceptibility of 8090 Al-Li alloy examined by electrochemical impedance spectroscopy

The exfoliation corrosion susceptibility and electrochemical impedance spectrosc opy(EIS) of rolled and peak-aged 8090 Al-Li alloys in EXCO solution were studi ed,and the EIS after exfoliation was simulated. Once exfoliation occurs,two ca pacitive arcs appear in the EIS at high-mediate frequency and mediate-low freq uency respectively. The exfoliation-attacked alloy surface consists of two part s,an original flat alloy surface and a new inter-face exposed to EXCO solution due to the exfoliation. The capacitance corresponding to the new exfoliation in ter-face increases approximately linearly with time at early exfoliation stage,due to the enlargement of the new inter-face. Then it maintains stable,due to the corrosion product covering on the new inter-face. The exfoliation suscepti bility can be judged through the average slope of the capacitance vs time curve of the early exfoliation stage. This average slope of the rolled 8090 alloy is m uch higher than that of the peak-aged 8090 alloy,accordingly the rolled 8090 a lloy is more susceptible to exfoliation than the peak-aged 8090 alloy.

Study on pitting process of 316L stainless steel by means of staircase potential electrochemical impedance spectroscopy

Pitting corrosion of 316L stainless steel in NaCl solution was investigated by means of staircase potential electrochemical impedance spectroscopy（SPEIS）.The investigation focused on the transition of stainless steel from the passive state to pitting corrosion.Based on the evolution of electrical parameters of the equivalent electrical circuit,it is suggested that the most probable mechanism of pit creation is the film breaking model.The result demonstrates that staircase potential electrochemical impedance spectroscopy is an effective method for the investigation of pitting corrosion.

Evaluation of electrochemical impedance and biocorrosion characteristics of as-cast and T4 heat treated AZ91 Mg-alloys in Ringer’s solution

The present study aims at understanding the electrochemical impedance and biocorrosion characteristics of AZ91 Mg-alloy in Ringer’s solution.As-cast AZ91 Mg-alloy was subjected to T4 heat treatment in a way to homogenize its microstructure by dissolving most of theβ-Mg 17 Al 12 phase at the vicinity of grain boundaries.The electrochemical impedance and biocorrosion performances of these two different microstructures(as-cast and T4 heat treated AZ91 Mg-alloys)in Ringer solution were evaluated by electrochemical impendence spectroscopy,potentiodynamic polarization and weight loss method.EIS spectra showed that both microstructures exhibit similar dynamic response as a function of the immersion time;however,the value of impedance and maximum phase angle are about 50%higher in as-cast AZ91 Mg-alloy as compared to that of homogenized AZ91 Mg-alloy.Weight loss measurement indicated that corrosion resistance of as-cast AZ91 was significantly better than that of homogenized AZ91.Microstructural and XRD analysis revealed that as-cast AZ91 contains a passive film of MgCO_(3)and CaCO_(3)precipitates with near spherical morphologies,whereas homogenized AZ91 comprised mainly unstable Mg(OH)_(2)film featured by irregular plate-like morphologies.

Effect of sulphate-reducing bacteria on the electrochemical impedance spectroscopy characteristics of 1Cr18Ni9Ti

The electrochemical characteristics of 1Cr18Ni9Ti in sulphate-reducing bacteria （SRB） solutions and the biofilm of SRB on the surface of the 1Cr18Ni9Ti electrode were studied by electrochemical, microbiological, and surface analysis methods. Electrochemical impedance spectroscopy （EIS） of 1Cr18Ni9Ti was measured in the solutions with and without SRB at the culture time of 2, 4, 8 d, respectively. The measurement used two test methods, the nonimmersion electrode method and the immersion electrode method. It was found that the polarization resistance （Rp） of 1Cr18Ni9Ti in the solutions without SRB is the greatest for each test method. When using the nonimmersion electrode method, Rp shifts negatively at first and then positively, and the time constant is only one. Although using the immersion electrode method, the Rp shifts positively at first and then negatively, and the time constant also changes when the biofilm forms. The biofilm observed through SEM is with pores. It was demonstrated that SRB has accelerated corrosion action on 1Cr18Ni9Ti. The protection effect of the biofilm on the electrode depends on the compact degree of the film.

Electrochemical Behavior of Alumix 321 PM and AA6061 Alloys in 3.5 wt% NaCl Solution by Electrochemical Impedance Spectroscopy Measurements

Powder metallurgy (PM) is a very interesting metal manufacturing technique in the production of automotive components of a net or near net shape. In this research, the electrochemical corrosion behavior of a commercially available aluminum powder alloy known as Alumix 321 was investigated and compared to wrought alloy AA6061 in 3.5 wt% NaCl solution using Electrochemical Impedance Spectroscopy Measurements (EIS). Alumix 321 alloy samples were prepared by pressing the powder at pressures ranging from 50 to 500 MPa and subsequently sintering them for 30 min at 630°C. It was found that the presence of pores and their morphology strongly affect the corrosion behavior the PM samples.

Electrochemical corrosion behavior of arc sprayed Al-Zn-Si-RE coatings on mild steel in 3.5% NaCl solution

Al-Zn-Si-RE coating with high Al content was deposited on mild steel by arc spraying. The electrochemical behavior of Al-Zn-Si-RE coating in 3.5%NaCl solution was systematically studied by potentiodynamic polarization, corrosion potential (φcor ) and electrochemical impedance spectroscopy techniques (EIS). The impedance data were fitted to appropriate equivalent circuits to explain the different electrochemical processes occurring at the electrode-electrolyte interface. The results indicate that Al-Zn-Si-RE coating reveals the similar polarization behavior as Zn-15Al coating. The coating has no passive region in the anodic polarization, but far lower corrosion current and much higher corrosion potential. Al-Zn-Si-RE coating provides effective sacrificial protection for steel substrate and the sacrificial anodic protection plays dominant role during the immersion process. In addition, theφcor evolution and EIS plots indicate that the corrosion process can be divided into five stages: pitting-dissolution-redeposition, activation corrosion, cathodic protection, physical barriers and the coating failure.

Corrosion and electrochemical behaviors of 7A09 Al-Zn-Mg-Cu alloy in chloride aqueous solution

The corrosion and electrochemical behaviors of 7A09 Al？Zn？Mg？Cu alloy were investigated in 3.5% NaCl （mass fraction） solution using complementary techniques such as scanning electron microscopy （SEM）, metallographic microscopy and electrochemical measurements. The results show that both pitting corrosion from or around the intermetallic particles and intergranular corrosion are observed after the immersion test due to the inhomogeneous nature of the microstructure of the 7A09 alloy. The preferential dissolution of the anodic Cu-depleted zone along grain boundaries is believed to be the possible cause of intergranular corrosion. The passivation and depassivation of this alloy show significant dependence of immersion time, owing to the formation and dissolution of various passive films on the sample surfaces. Furthermore, the corrosion process and corrosion mechanism were also analyzed.

Effects of Hg and Ga on microstructures and electrochemical corrosion behaviors of Mg anode alloys

The effects of Hg and Ga on the electrochemical corrosion behaviors of the Mg-2%Hg, Mg-2%Ga and Mg-2%Hg-2%Ga (mass fraction) alloys were investigated by measurements of polarization curves, galvanostatic tests and measurements of electrochemical impedance spectroscopy. Scanning electron microscopy, X-ray diffractometry and energy dispersive spectrometry were employed to characterize the microstructures and the corroded surface of the above alloys. The results demonstrate that the microstructure of the Mg-2%Ga alloy is solid solution and the Mg-2%Hg and Mg-2%Hg-2%Ga alloys have white second-phases at the grain boundaries. The Mg-2%Ga alloy has the worst electrochemical activity and the best corrosion resistance, showing a mean potential of -1.48 V and a corrosion current density of 0.15 mA/cm2. The Mg-2%Hg-2%Ga alloy has the best electrochemical activity and the worst corrosion resistance, showing a mean potential of -1.848 V and a corrosion current density of 2.136 mA/cm2. The activation mechanism of the Mg-Hg-Ga alloy is dissolution-deposition of the Hg and Ga atoms.

Electrochemical and corrosion behaviors of pure Mg in neutral 1.0% NaCl solution

The corrosion behaviors and corresponding electrochemical impedance spectroscopy（EIS） and polarization curves of pure Mg in neutral 1.0% NaCl solution were investigated.The fractal dimension of EIS at different time was studied.The corrosion process and EIS evolution are divided into three stages.In the initial stage,EIS is composed of two overlapped capacitive arcs,the polarization resistance and charge transfer resistance increase rapidly with immersion time,and the corrosion rate decreases.Then,two well-defined capacitive arcs appear,and the charge transfer resistance and corrosion rate remain stable.After a long immersion time,inductive component appears in a low frequency range,the charge transfer resistance decreases and the corrosion rate increases with the immersion time.The fractal dimension obtained from the time records of EIS seems to be a promising tool for the analysis of corrosion morphology because of its direct relationship with the metal surface.

ELECTROCHEMICAL NOISE FEATURES OF PURE ALUMINUM DURING PITTING CORROSION IN NEUTRAL NaCl SOLUTION

Spontaneous electrochemical noise (EN) can be a rich source of information concerning the processes simultaneously occurring at a corroding interface. Potential noise fluctuations during the free corrosion of pure aluminum in different concentration of neutral sodium chloride solution are investigated, and the breakdown and restoration of passive metal's film are studied using potentiodynamic scanning (PDS) measurements and electrochemical impedance spectroscopy (EIS) technique. Two capacitance loops are observed in the Nyquist plots in two kinds of concentration, and the corrosion process is under activation control at first, then become diffusion control within the oxide film and corrosion products of (Al(OH)p-mCl-m) accumulated on the surface of the corroding electrode. It is suggested that the pitting corrosion is much easier to occur for pure aluminum in 7.0wt% than in 2.0wt% NaCl solution, and the high concentration of chloride ion in solution inhibits the repassivation of a metastable pit. The corrosion rate deterministic step does not involve Cl-.

EVOLUTION OF THE ELECTROCHEMICAL CHARACTERISTICS DURING PITTING CORROSION OF PURE ALUMINUM IN SODIUM CHLORIDE SOLUTION

The corrosion behavior of pure aluminum in neutral 3.0% （mass fraction） sodium chloride （Na-Cl） solution has been studied using electrochemical impedance spectroscopy （EIS） measurement in conjunction with the scanning electron microscopy （SEM） technique. EIS information on the evolution of pitting corrosion over a period of 15 days has been obtained and analyzed with equivalent circuit technique. The results shown that, during the ensemble constant immersion time, two time constants involved, the high frequency one originated from the protective layer on the corroding surface while the low frequenO, one from the diffusion process or the corrosion reaction and so on. And there existed a period for oxide film to growth and thickening prior to the of the attacking of chloride ions to the substrate. Meanwhile, good relationship between EIS and the material corrosion type/severity has been obtained, which has been interpreted according to the characteristics of corrosion process such as auto acceleration of pitting corrosion and the protection of local anodic reaction to the area around them.

ELECTROCHEMICAL FEATURES DURING PITTING CORROSION OF LY12 ALUMINUM ALLOY IN DIFFERENT NEUTRAL SOLUTIONS

The electrochemical features of commercial airfrarne material, Al alloy LY12, in 0.349mol/L neutral sodium chloride (NaCl) and sodium sulfate (NazSO^) solutions were investigated by electrochemical impedance spectroscopy (EIS) and potentiody-namic polarization techniques. The microstructure of the as-tested samples was studied by scanning electron microscopy. The results show that the Nyquist plots of LY12 at different immersion time displayed different features, indicating that the Cl- ions elevate the corrosion rate and inhibit the repassivation of a metastable pit. It also shows that the corrosion product of LY12 formed in SO42- solution isn't easy to dissolve, and it will cover the surface of working electrode in the electrolyte. SEM images indicate that the corrosion apparent area and pit number of LY12 in NaCl solution are greater than that in Na2SO4 solution.

THE ELECTROCHEMICAL BEHAVIOR OF OCEANIC MICROBIOLOGICAL INFLUENCED CORROSION ON CARBON STEEL

The corrosion behavior of carbon steel in the medium of marine microorganisms was investigated by electrochemical impedance spectra, polarization curves, and so on. Experimental results showed that the corrosion potential of carbon steel moved in a negative direction in the unpurified marine microorganism solution, and the polarization style of the cathodic process did not change. The electrochemical impedance spectra showed that the impedance value of the electrode decreased in the medium with bacteria, which indicated that the existence of microorganism could accelerate the corrosion progress of carbon steel.

Effect of Ca(OH)_2, NaCl, and Na_2SO_4 on the corrosion and electrochemical behavior of rebar

The corrosion of rebar in reinforced concrete in marine environments causes significant damage to structures built in ocean environments. Studies on the process and mechanism of corrosion of rebar in the presence of multiple ions may help to control damage and predict the service life of reinforced concrete structures in such environments. The effect of interactions between sulfate and chloride ions and calcium hydroxide on the electrochemical behavior ofrebar are also important for evaluation of structure durability. In this work, electrochemical impedance spectroscopy （EIS） plots of rebar in Ca（OH）2 solution and cement grout, including NaC1 and Na2SO4 as aggressive salts, were measured for different immersion times. The results show that corrosion of rebar was controlled by the rate of charge transfer as the rebar was exposed to chloride solution. In the presence of high concentrations of sulfate ions in the electrolyte, generation and dissolution of the passive film proceeded simultaneously and corrosion was mainly controlled by the diffusion rate. When Na2SO4 and NaC1 were added to Ca（OH）2 solution, the instantaneous corrosion rate decreased by a factor of 10 to 20 as a result of the higher pH of the corroding solution.

Electro-Chemical Impedance Spectral (EIS) Study of Patinated Bronze Corrosion in Sulfate Media: Experimental Design Approach

The aim of the present investigation was to model the experimental conditions of tin bronze patination using full factorial experimental design. In this sense, a full factorial design approach was developed to model the corrosion behavior of patinated tin bronze alloy in sulfate electrolyte. Three experimental factors (the immersion time in the chloride electrolyte, the potential limit for the anodic sweep Elim, and the potential scan rate) were chosen to identify the significant factor on the patina growth process at the bronze substrate. The experimental responses were the kinetic parameters extracted from the electro-chemical spectra (EIS) for eight different experiments. An equivalent electrical circuit containing an electrolyte resistance (Re), a double layer capacitance (CPEdl), a charge transfer resistance (Rt) and Gerischer element (G), was developed to model the patinated bronze corrosion process. The electro-chemical spectra (EIS) show that the corrosion process of the patinated bronze alloy occurred from a chemical reaction is followed by an electrochemical one. Analysis of the experimental responses showed that while the scan rate is the most influent factor for the corrosion potential (Ecorr), the electrolyte resistance (Re), and the double layer capacitance CPEdl variation, the potential limit is the significant factor for charge transfer resistance Rt, reciprocal of the admittance parameter Y0 and the effective transfer rate of the chemical reaction k variation.

Effect of NiO-NiCr2O4 nano-oxides on the microstructural,mechanical and corrosion properties of Ni-coated carbon steel

Pure Ni and its composites with different percentages of Ni-Cr nano-oxides were coated over carbon steel to assess the coating features and mechanical and corrosion behavior.A nano-oxide composite of Ni-Cr was first synthesized through chemical coprecipitation with uniform distribution constituents.Electrodeposition was employed to coat pure Ni and Ni-(Ni-Cr)oxides(10,20,30,40,and 50 g/L)on the steel sheets.Transmission electron microscope and field emission scanning electron microscope were adopted to examine the microstructure of powders and coatings,and X-ray diffraction analysis was employed to study the chemical composition.The microhardness,thickness,and wear resistance of the coatings were assessed,polarization and electrochemical impedance spectroscopy(EIS)tests were conducted to analyze the corrosion behavior,and the corresponding equivalent circuit was developed.Results showed flawless and crack-free coatings for all samples and uniform distribution of nano-oxides in the Ni matrix for the samples of 10-30 g/L.Agglomerated oxides were detected at high concentrations.Maximum microhardness(HV 661),thickness(116μm),and wear resistance of coatings were found at 30 g/L.A three-loop equivalent circuit corresponded satisfactorily to all EIS data.The corrosion resistance increased with the nano-oxide concentration of up to 30 g/L but decreased at 40 g/L.The sample of 50 g/L showed the best corrosion resistance.

Microstructure characterization and electrochemical corrosion behavior of Zn and Zn/Mg alloys in H_2SO_4 solution

The three systems of pure Zn, Zn-0.10% Mg(mass fraction), and Zn-0.15% Mg(mass fraction) were cast under controlled atmosphere and their microstructures were characterized by SEM/EDS analysis. The electrochemical corrosion behavior of these three samples was examined in the very aggressive solution of 50% H2SO4(mass fraction) using electrochemical impedance spectroscopy(EIS) and potentiodynamic polarization measurements. The results show that magnesium improves in some extent the corrosion resistance of pure Zn in 50% H2SO4(mass fraction) confirmed by EIS test. Results of polarization measurment also demonstrate that small amount of Mg significantly improves the passivation of Zn in the test solution. Results of surface morphology of the samples and EDS analysis reveal that Mg reduced the corrosion attacks to pure Zn.

Electrochemical techniques for monitoring stress corrosion cracking of Type 40Cr steel in acidified chloride solution

Electrochemical impedance spectroscopy(EIS) and electrochemical noise(EN) techniques were used to detect stress corrosion cracking(SCC) on 40Cr steel specimens exposed to the acidified chloride solution at ambient. To test these two techniques,slow strain rate tensile(SSRT) tests were performed with 40Cr specimen in the identical corrosive solution at room temperature. In impedance measurements,phase shifts in frequency range from 1 to 1 000 Hz show a clear difference between the stressed and non-stressed specimens,suggesting that stress corrosion cracks are detected by the impedance measurements. EN signals in the process of SCC were recorded and then analyzed by standard deviation(STD). On the other hand,the mechanical properties,such as maximum tensile strength(MTS) and fracture strain(FS) measured by the SSRT,decrease significantly when the specimens are exposed to the corrosive solution relative to that in an inert medium. The SSRT results are consistent with fractography of the tested specimens by scanning electron microscopy(SEM). Analysis of the fracture surface clearly shows intergranular attack,suggesting that stress corrosion cracks are formed.