Potentiostatic Tests Research Articles

Electrochemical Characterisitcs of Thin Carbon Coatings for Metal Bipolar Plates - Evaluation of the Relative Porosity Using EIS

The anticipation of a rapidly growing market demand for PEM fuel cell components, places the focus on the establishment of advanced, high-throughput processes to satisfy the material requests of the industry. Sandvik Materials Technology AB uses a roll-to-roll PVD process to produce carbon-coated steel for PEMFC bipolar plates that meet both technical requirements of corrosion resistance and electrical conductivity, as well as economic requirements. The main advantages of using pre-coated steel for bipolar plate production are increased productivity and decreased risks associated with parts handling. Furthermore, a consistently high material performance is achieved as the result of a very stable coating process combined with large batch sizes (several tons). For improved corrosion resistance, low porosity coatings are generally desired. A method for electrochemical impedance spectroscopy (EIS) has been developed in which the relative porosity has been evaluated for carbon coated steel. The carbon coating is deposited on top of an intermediate metallic layer, which serves to enhance adhesion and corrosion resistance. Using this method, we have been able to verify that increasing the coating thickness and the number of coating layers resulted in a decrease in the porosity index. More interestingly, however, we have been able to show that the relative porosity index can be related to interfacial contact resistance measured for coated samples subjected to potentiostatic testing (100 h at 0.7 V vs. AgCl). The interfacial contact resistance of studied samples varied between 3-13 mΩcm2 at a pressure of 140 N/cm2. Together these results indicate that the evaluation of relative porosity by means of EIS can be used as a relevant tool for assessing the corrosion resistance of coated metal intended for use as PEM bipolar plates.

Effects of Al incorporation on the interfacial conductivity and corrosion resistance of CrN film on SS316L as bipolar plates for proton exchange membrane fuel cells

Interfacial conductivity and corrosion resistance of bipolar plates are two significant parameters affecting the performance and durability of proton exchange membrane fuel cells. This study designs to investigate the effects of Al incorporation on the interfacial conductivity and corrosion resistance of CrN film coated on bipolar plates, ternary Cr–Al–N films with different Al content have been deposited on SS316L samples by closed unbalanced magnetron sputter ion plating (CFUBMSIP). Al content was adjusted by altering magnetron sputtering current of Al target. Scanning electron microscopy (SEM) results show that the deposited films are dense and continuous. The phase structures and bonding types before and after Al incorporation have also been investigated by X-ray diffractometry (XRD) and X-ray photoelectron spectroscopy (XPS). Interfacial contact resistance (ICR) between gas diffusion layer (GDL) and coated samples increases with the increase of Al content doped in CrN film, and the lowest ICR value is 5.1 mΩ cm2 at 1.4 MPa. The incorporation of Al has influence on the interfacial conductivity of CrN films by combining two competitive aspects actual contact area and the conductivity of the sample surface. Potentiodynamic polarization tests in the simulated corrosive circumstance of PEMFCs reveal that the corrosion potential of coated sample become more positive after Al incorporation and the corrosion current density obtained from Al doped CrN film after potentiostatic tests in cathode PEMFCs environment is 0.021 μA cm−2, which witnesses nearly one order of magnitude decrease compared with CrN without Al content. Electrochemical corrosion tests and inductively coupled plasma-mass spectrometry (ICP-MS) detection disclose that Al doped CrN film can improve the durability of bipolar plates by forming a dense passive film in real PEMFCs environments and reduce the metal ion contamination of membrane. Based on the results of ICR and electrochemical corrosion tests, it is demonstrated that low content of Al incorporation in CrN film can combine the two aspects, namely, better corrosion resistance and high interfacial conductivity which are beneficial for the commercial application of metallic bipolar plates.

Preparation of corrosion-resistant and conductive trivalent Cr–C coatings on 304 stainless steel for use as bipolar plates in proton exchange membrane fuel cells by electrodeposition

In this study, Cr–C-coated bipolar plates are produced by electroplating on the SS304 plates with a machined flow channel. The resulting plates were tested using potentiodynamic and potentiostatic measurements in simulated PEMFC environments, which show that the bipolar plate coated with Cr–C exhibited good anticorrosion performance. The corrosive current density of the Cr–C coating formed for a plating time of 10 min for 10 h exhibits a low stable value of 1.51 × 10−10 A/cm2 during the potentiostatic test in a 0.5 M H2SO4 + 2 ppm HF solution at 70 °C with an air purge, indicating that the Cr–C coating plated for 10 min is stable in a cathode environment. The interfacial contact resistance (ICR) of the bipolar plate with the Cr–C coating clearly improved, presenting an ICR of 19.52 mΩ cm2 at a pressure of 138 N/cm2. The results from scanning electron microscopy (SEM) and ICR before and after the corrosion tests indicate that the bipolar plate with the Cr–C coating is electrochemically stable. In this study, the maximum power density (212.41 mW/cm2) is obtained at a cell temperature of 80 °C and a gas flow rate of 300 standard cubic centimeters per minute (sccm) when Cr–C coated SS304 bipolar plates were used.

Effect of chloride ions on the corrosion behavior of low-alloy steel containing copper and antimony in sulfuric acid solution

The influence of the addition of HCl on the corrosion behavior of low-alloy steel containing copper and antimony was investigated using electrochemical (potentiodynamic and potentiostatic polarization tests, and electrochemical impedance spectroscopy) and weight loss tests in a 1.6M H2SO4 solution with different concentrations of hydrochloric acid (0.00, 0.08, 0.15 and 0.20 M HCl) at 60 °C. The result showed that the corrosion rate decreased with increasing HCl by the formation of protective layers. SEM, EDS and XPS examinations of the corroded surfaces after the immersion test indicated that the corrosion production layer formed in the solution containing HCl was highly comprised of metallic Cu, Cu chloride and metallic (Fe, Cu, Sb) compounds. The corrosion resistance was improved by the Cu-enriched layer, in which chloride ions are an accelerator for cupric ion reduction during copper deposition. Furthermore, cuprous and antimonious chloride species are complex salts for cuprous ions adsorbed on the surface during copper deposition.

Effect of chloride and sulfate ions in simulated boiler water on pitting corrosion behavior of 13Cr steel

The pitting corrosion behavior of 13Cr steel was investigated in simulated boiler waters containing chloride ions (Cl−) and sulfate ions (SO42−) using potentiodynamic and potentiostatic polarization tests in addition to pit morphology analysis. The presence of 100ppm Cl− in the water caused pitting corrosion of the steel. Pit initiation was inhibited by the addition of 50ppm or 100ppm SO42− into the water containing 100ppm Cl−. Pit growth was also suppressed by the presence of 50ppm SO42− in the water with 100ppm Cl−; however, it was conversely promoted in the presence of 100ppm SO42−.

Electrochemical Studies on Inorganic Zero VOC Coated Steel in Atmospheric Exposure and REAP Test

The application of coatings for control of degradation of materials has been historically used since ancient civilizations, but usually the coating systems take a long time to show the physical evidence of corrosion on coated metals, so the evaluation of coatings in aggressive atmospheres and with a rapid way has been the dream of the experts and users through the time. Laboratory standard tests have been developed as a quality control for coatings, including salt fog, climate chamber or artificially damaged conditions.The paint systems utilized for the study were zero volatile organic solvents or paints water base with metallic zinc content. The electrochemical measurements were made before and after the potentiostatic test using the electrochemical techniques of Corrosion Potential (Ecorr), Linear Polarization Resistance (LPR), Electrochemical Impedance Spectroscopy (EIS) and the Polarization Curves.For the evaluation of the coatings exposed to the atmosphere, the samples were retired at different times to apply the electrochemical tests in the laboratory and then returned to the exposure rack. For the application of the Rapid Electrochemical Assessment of Paints (REAP), the galvanic current was registered during the potentiostatic conditions due that provides an alternative method for the qualification of paint systems, while the parameter of degradability for the paint systems can be calculated from the LPR results that are showed and compared with the results from both evaluation methods.

Plasma Arc Melting (PAM) and Corrosion Resistance of Pure NiTi Shape Memory Alloys

Plasma arc melting (PAM) as a suitable non-contaminating melting route for manufacturing high-quality NiTi alloy was successfully examined. The corrosion resistance of PAM Nitinol was evaluated by both potentiodynamic and potentiostatic tests and compared with lower purity NiTi produced by vacuum induction melting (VIM). For the electro-polished surfaces, excellent corrosion resistance of NiTi comparable with the Ti alloys was found with no pitting up to 800 mV versus saturated calomel electrode in simulated body fluid at 37 °C. Potentiostatic results of PAM Nitinol indicate slightly better corrosion resistance than the lower quality VIM alloy.

Effect of grain size on pitting corrosion of 304L austenitic stainless steel

The influence of grain size on pitting corrosion of 304L stainless steel in 3.5wt.% NaCl solution was investigated by electrochemical methods employing the statistical approach. The potentiodynamic results revealed that grain refinement has no effect on the pitting potential. To understand the reason of ineffectiveness of grain size on pitting potential of 304L stainless steel, potentiostatic tests were conducted. Evaluation of current transients observed in potentiostatic tests revealed that the frequency of metastable pit initiation decreases with grain refinement. However, grain size reduction increases the kinetics of metastable pits dissolution and the probability of transition from metastable pitting to stability.

Comparison of the effect of non‐polluted and underwater‐volcano‐polluted seawater on the corrosion resistance of different stainless steels

This work compares the effect of non‐polluted and underwater‐volcano‐polluted seawater on the electrochemical behavior of two different alloys, notably an austenitic stainless steel (SS) and a duplex stainless steel. Polarization measurements, potentiostatic passivation tests, electrochemical impedance spectroscopy and capacitance measurements were performed. Results show that the composition of the polluted seawater negatively affects the passivation kinetics of both AISI 316 SS and Alloy 900, decreasing the corrosion resistance of both alloys. Additionally, when both steels are compared, it can be concluded that passive films formed on Alloy 900 presented better protective properties than those on AISI 316 SS.

RuxTi1−xO2 as the support for Pt nanoparticles: Electrocatalysis of methanol oxidation

Two binary Ru–Ti oxides, Ru0.1Ti0.9O2 and Ru0.7Ti0.3O2, were synthesized by the sol–gel method and used as an electrocatalyst support. The system was characterized by XRD, EDS, TEM and cyclic voltammetry. The Ru0.1Ti0.9O2 and Ru0.7Ti0.3O2 consist of two phases of anatase and rutile structure. An average size of the Pt nanoparticles supported on them is ∼3.5nm and they are deposited on both Ru and Ti-rich domains. The supports exhibited good conductivity and electrochemical stability. The onset potentials of COads oxidation on the synthesized catalysts and on commercial PtRu/C are similar to each other and lower than that on Pt/C. This suggests that in Pt/Ru0.1Ti0.9O2 and Pt/Ru0.7Ti0.3O2 the Pt nanoparticles are in close contact with Ru atoms from the support, which enables the bifunctional mechanism. The activity and stability of the catalysts for methanol oxidation were examined under potentiodynamic and potentiostatic conditions. While the activity of Pt/Ru0.1Ti0.9O2 is unsatisfactory, the performance of Pt/Ru0.7Ti0.3O2 is comparable to PtRu/C. For example, in the potentiostatic test at 0.5V the activities after 25min are 0.035mAcm−2 and 0.022mAcm−2 for Pt/Ru0.7Ti0.3O2 and PtRu/C, respectively. In potentiodynamic test the activities at 0.5V after 250 cycles are around 0.02mAcm−2 for both catalysts.

Influence of Ageing on the Corrosion Resistance of the ZnAl22Cu3 and ZnAl40Cu3 Alloys

Subject of examinations were ZnAl22Cu3 and ZnAl40Cu3 alloys. The samples after solutionizing 3850C/10h and ageing 1750C/10h and for comparative purposes, in the initial state (just after casting) have been studied. Corrosion tests were carried out in “acid rain” solution. The scope of the examinations included potentiodynamic and potentiostatic tests , examination of the surface and structure of the samples after corrosion. The research found that corrosion of ZnAl22Cu3 and ZnAl40Cu3 alloys in the "acid rain" environment leads to a depletion of zinc in the upper surface layer and its enrichment in aluminium and copper resulting in decreasing the corrosion rate. Depending on the parameters (temperature, time) solutioning and ageing of ZnAl22Cu3 and ZnAl40Cu3 alloys can lead to an increase or decrease of the corrosion resistance in the "acid rain" environment.

The Effect of Ti and REE Addition on the Corrosion Resistance of the AlZn12Mg3.5Cu2.5 Alloy in "Acid Rain" Environment

A significant problem associated with the use of high-strength aluminum-zinc alloys is their insufficient resistance to corrosion. A often used method of improving the resistance to corrosion is plating, resulting in a decrease of their strength. Improving the corrosion resistance can be achieved by application of micro - alloying elements. Titanium and rare earth elements are particularly interesting micro - alloying elements. Subject of examination were AlZn12Mg3.5Cu2.5 base alloy (without micro-alloying) and alloy with the addition of titanium alloy and alloy with the addition of rare earth elements. Corrosion examinations were carried out in a solution of "acid rain", pH = 3.5. The scope of the research included potentiodynamic and potentiostatic tests , examination of the surface and structure of the samples after corrosion.The research found that the addition of titanium does not cause a significant improvement in corrosion resistance of the AlZn12Mg3.5Cu2.5 alloy. A significant improvement was achieved with the addition of rare earth elements. It was found that corrosion tested alloys is local and leads to the formation of pits.

The Effect of Antisite Disorder and Particle Size on Li Intercalation Kinetics in Monoclinic LiMnBO3

In materials containing 1D lithium diffusion channels, cation disorder can strongly affect lithium intercalation processes. This work presents a model to explain the unusual transport properties of monoclinic LiMnBO3, a material determined by scanning electron microscopy and synchrotron X‐ray diffraction to contain a wide particle size distribution and Mn/Li antisite disorder. First‐principles calculations indicate that Mn occupying Li sites obstruct the 1D lithium diffusion channel along the [001] direction. While channel blockage by the antisites significantly lowers Li mobility in large particles, Li kinetics in small particles and particle surfaces are found to be less sensitive to the presence of antisite disorder. Thus, in an electrode containing a large particle size distribution, smaller particles have higher Li mobility, and the measured Li diffusivity as determined by potentiostatic intermittent titration test varies as a function of particle size. The Li capacity in monoclinic LiMnBO3 is kinetically controlled by the fraction of large particles with antisite disorder, but is not intrinsically limited. These results strongly suggest that particle nanosizing will significantly enhance the electrochemical performance of LiMnBO3.

Effects of Boron on Microstructure and Metastable Pitting Corrosion Behavior of Super304H Austenitic Stainless Steel

The effects of boron on the microstructure and the pitting corrosion behavior, especially the metastable pitting, were investigated in Super304H austenitic stainless steels with 0–130 ppm boron solution treated at 1100°C. The microstructures were analyzed using scanning electron microscopy, transmission electron microscopy with energy dispersive spectroscopy and secondary ion mass spectroscopy. Meanwhile, the pitting corrosion behavior was evaluated by potentiodynamic and potentiostatic polarization tests in 3.5% NaCl solution at 25°C. Microstructural characterizations showed that boron addition favored the precipitation of Cr-rich boride (M2B), which resulted in an asymmetric Cr-depleted zone around M2B and reduced the chromium content in the austenite matrix. The Cr-depleted zones could act as extra sites for pit initiation, along with the interfaces between the austenite matrix and non-metallic inclusions. The potentiodynamic polarization test results indicated that boron addition decreased the pitting potential. The potentiostatic test results revealed that boron addition promoted the initiation frequency and growth rate of metastable pits, and increased the probability of stable pitting.

Synthesis of mesoporous nanocomposites for their application in solid oxide electrolysers cells: microstructural and electrochemical characterization.

Fabrication routes for new SOEC mesoporous nanocomposite materials as electrodes are presented in this paper. NiO-CGO and SDC-SSC are described as possible new materials and their synthesis and the cell fabrication are discussed. The obtained materials are microstructurally characterised by SEM, TEM and XRD, where the mesoporous structures are observed and analysed. The obtained samples are electrochemically analysed by I-V polarisation curves and EIS analysis, showing a maximum current density of 330 mA cm(-2) at 1.7 V. A degradation of the cell performance is evidenced after a potentiostatic test at 1.4 V after more than 40 hours. Oxygen electrode delaminating is detected, which is most probably the main cause of ageing. Even taking into account the lack of durability of the fabricated cells, the mesoporous electrodes do not seem to be altered, opening the possibility for further studies devoted to this high stability material for SOEC electrodes.

Effect of Organic Inhibitors on Chloride Corrosion of Steel Rebars in Alkaline Pore Solution

The inhibition properties of aspartic and lactic acid salts are compared with nitrite ions with regard to their effect on critical chloride concentration. The tests were carried out on carbon steel specimens in simulated pore solutions with initial pH in the range of 12.6 to 13.8. The critical chloride concentrations were estimated through multiple specimen potentiostatic tests at potentials in the usual range for passive rebar in noncarbonated concrete structures. During tests, chloride ions were progressively added until all specimens showed localized attack, obtaining cumulative distribution curves reporting the fraction of corroded specimens as a function of chloride concentration. The presence of the organic inhibitors on the passivity film was detected by IR spectra. The results confirm that 0.1 M aspartate exhibits an inhibiting effect comparable with nitrite ions of the same concentration. Calcium lactate does not increase critical chloride concentration; however it appears to promote the formation of a massive scale, reducing the corrosion propagation.

Pitting corrosion of super martensitic stainless steel 00Cr13Ni5Mo2

Purpose – The purpose of this study was to investigate the pitting resistance and assess the critical pitting temperature (CPT) of a super martensitic stainless steel, 00Cr13Ni5Mo2, made in China, considering especially the difference in the pitting corrosion resistance between the domestic super martensitic stainless steel and an imported one. Design/methodology/approach – Potentiodynamic sweep tests were applied to investigate the effects of four NaCl concentrations (weight per cent) of 1, 3.5, 9 and 17, and four testing temperatures of 30, 50, 75 and 90°C on the pitting resistance of the domestic super martensitic stainless steel in the presence of CO2. Potentiostatic sweep tests were utilized to determine the CPT. Furthermore, chemical immersion exposures, implemented according to the appropriate standard were used to evaluate the difference in the pitting corrosion resistance between the domestic super martensitic stainless steel and an imported one. In addition, the morphology of pits was analyzed using a scanning electron microscope. Finding – The pitting potential of the domestic super martensitic stainless steel decreased with an increase in NaCl concentration and temperature in the presence of CO2. The CPT of the domestic super martensitic stainless steel measured by potentiostatic polarization was 41.16°C. Two types of typical corrosion pits, closed pits formed at 35°C and open pits formed at 50°C, were observed. Furthermore, compared to the super martensitic stainless steel made in Japan, the domestic one was better in terms of pitting potential, corrosion rate and the density of the pits, but worse in terms of the depth of the pits, which may result in a risk of corrosion perforation of tubing and casings. Originality/value – The paper highlights that chloride ions, temperature and the presence of CO2 play an important role on the pitting resistance of super martensitic stainless steel.

Corrosion and tribocorrosion behavior of Ti/TiN PVD coating on 316L stainless steel substrate in Ringer's solution

Using physical vapor deposition technique, Ti/TiN coating was deposited on 316L stainless steel with 2 μm thickness. The analysis showed the formation of Face Centered Cubic crystalline structure of TiN with (220) preferred orientation and a hardness of ∼2600 VHN. The results also revealed that Ti/TiN coating not only improved the corrosion resistance of stainless steel in Ringer's solution at 37 °C, but also increased the breakdown potential significantly. Tribocorrosion tests at open circuit potential demonstrated that both stainless steel and Ti/TiN coating samples become repassive after the sliding is stopped. This indicated their ability to repair of passive film in wear scars after sliding. On the other hand, for stainless steel, a significant rise in current was noticed at the beginning of the sliding in anodic potentiostatic tribocorrosion test, indicating a high degree of anodic dissolution. However, the lost volumes of Ti/TiN coated sample in open circuit potential, anodic and cathodic conditions were lower than those for stainless steel, due to its higher hardness and corrosion resistance. For Ti/TiN coated sample, the effect of synergy between wear and corrosion was more important than mechanical wear and/or anodic dissolution.

Corrosion behavior of polyphenylene sulfide–carbon black–graphite composites for bipolar plates of polymer electrolyte membrane fuel cells

The aim of this work was to study the corrosion behavior of polyphenylene sulfide (PPS) – carbon black – graphite composites regarding their application as bipolar plates of polymer electrolyte membrane (PEM) fuel cells. Electrochemical impedance spectroscopy (EIS), potentiostatic and potentiodynamic polarization tests were used to characterize the electrochemical response of the composites in a simulated PEM fuel cell environment. Cross-sectional views of fractured specimens were observed by scanning electron microscopy (SEM). The results showed that the corrosion behavior depends on the carbon black content incorporated into the composite formulation. There was a trend of decreasing the corrosion resistance for higher carbon black contents. This behavior could be explained based on the porosity and electrical conductivity of the composites.

Electrochemical Studies on Zero VOC Coated Steel through Atmospheric Exposure and REAP Test in the Laboratory

The application of coatings for control the degradation of materials has been historically used since ancient civilizations, but usually the coating systems take a long time to show the physical evidence of corrosion on coated metals, so the evaluation of coatings in aggressive atmospheres and with a rapid way has been the dream of the experts and users through the time. Cabinete standard tests have been developed as a quality control for coatings, including salt fog, climate chamber or artificially damage conditions.The paint systems utilized for the study were zero volatile organic or water base paints with Zinc phosphate content. The electrochemical measurements were made before and after the potentiostatic test using the electrochemical techniques of Corrosion Potential (Ecorr), Linear Polarization Resistance (LPR), Electrochemical Impedance Spectroscopy (EIS) and the Polarization Curves. For the evaluation of the atmospheric exposed coatings they were retired at different times and applied the electrochemical tests in the laboratory and returned to the exposure rack. For the application of the Rapid Electrochemical Assessment of Paints (REAP), the galvanic current was registered during the potentiostatic conditions providing an excellent parameter for the qualification of paint systems, while a degradability parameter for the paint systems can be calculated from the LPR results that can be compared with results from atmospheric exposure.