DC Electrochemical Techniques Research Articles

Anodization of medical grade stainless steel for improved corrosion resistance and nanostructure formation targeting biomedical applications

Stainless steel is extensively used in various biomedical engineering and hospital applications, including surgical equipment and furniture. Strong adhesion of bacteria and viruses on metal surfaces can restrict long-term utilization for biomedical applications. This study aims to develop an improved electrochemical etching protocol for the anodization of 316 L grade stainless steel (10 × 15 mm) to fabricate nanostructures for biomedical and hospital applications. Anodizing conditions were optimized using two different electrolyte solutions; HNO3: H2SO4 (1:1) and HNO3, by varying applied potential, electrolyte concentration and anodizing time. Morphology and topography of the anodized surfaces were characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM) and scanning Kelvin probe force microscopy (SKPFM). AC and DC electrochemical techniques were used to further characterize the corrosion behaviour of the nanostructured surfaces. Electrochemical optimization produced two different nanostructured surfaces with the anodizing conditions of (1) 50% HNO3 at 0.465 A/cm2 for 1 min (surface 1), and (2) 0.5 M HNO3 + H2SO4 (1:1) at 0.366 A/cm2 for 5 min (surface 2). Both processes produced nanoscale surface roughness with varying corrosion susceptibility. Surfaces anodized using 50% HNO3 comprised of ‘hierarchical roughness’ with dense spikes (10 – 20 nm in diameter), covering rock candy-like protrusion (10 - 15 µm diameter). Whereas the second set of conditions produced single scale roughness with a terrace-like topography with nanoscale ridges of 34.8 ± 1.2 nm in width atop microscale hills. Surface 2 possessed improved corrosion resistance through the formation an oxide film, while the surface 1 was more susceptible to corrosion. Overall, this study demonstrates the importance of the careful optimization of electrochemical surface treatment for medical grade stainless steel in terms of roughness of nanostructures and corrosion susceptibility.

Corrosion protection by poly(3-hexylthiophene)/poly(methyl-methacrylate) coating of cracked 410 stainless steel

PurposeThis paper aims to study the corrosion protection of 410 stainless steel (410SS) cracked by fatigue tests. The purpose of this study is to show that using polymeric coatings, it is possible to reduce the corrosion rate in metallic structures in operation.Design/methodology/approachPoly(3-hexylthiophene) (P3HT)/poly(methyl-methacrylate) (PMMA) composite was used as a coating to protect the cracked 410SS in the corrosive environment 0.5 M NaCl at 25°C and 80°C. Physicochemical characterization was carried out by adhesion tests, thermogravimetric analysis, nuclear magnetic resonance and size exclusion chromatography. Surface morphology was studied before and after the electrochemical tests by scanning electron microscopy. Uncoated and coated cracked 410SS were characterized by DC electrochemical techniques: linear polarization resistance and potentiodynamic polarization curves.FindingsP3HT/PMMA coating reduced corrosion rate and crack propagation of 410SS in the corrosive medium NaCl 0.5M. The P3HT/PMMA coating increased the polarization resistance by one order of magnitude and decreased the corrosion current density by one order of magnitude, relative to the values obtained with uncoated cracked 410SS. The coating helped to have a less damaged surface and less crack propagation on the cracked 410SS. The feasibility of increasing the useful life of cracked metal structures in a saline environment was demonstrated through polymeric composite coatings.Originality/valueIn the literature, no works were detected that report the use of organic coatings to protect cracked metals against corrosion. This is the first reported work on the corrosion protection of 410SS in a saline medium using coatings based on P3HT/PMMA.

Mechanisms of Hydrogen Evolution in Weak Acid Solution Investigated By Using EIS

EIS was used in this mechanistic study of the electrochemical reaction mechanisms associated with corrosion of mild steel in the presence of weak acids, such as carbonic acid found in CO2 saturated aqueous solutions. Over the past decade, comprehensive mechanistic studies using DC electrochemical techniques have indicated that direct reduction of carbonic acid is not a significant reaction in CO2 aqueous solutions, contrary to what the conventional wisdom was for a long time. The new results suggest that the only cathodic reaction is hydrogen reduction, and that the main role of carbonic acid is to increase the limiting current by the so-called buffering effect. However, the problem is that the same limiting current is obtained theoretically whether one assumes that the direct reduction of carbonic acid or the buffering effect is true. Therefore, by using DC electrochemical techniques it is impossible to distinguish these two mechanisms in the limiting current region which is always seen in the polarization sweeps. An attempt has been made here to use the EIS technique to identify the role of carbonic acid in accelerating the cathodic reaction. Results indicate that the buffering effect is dominant, as recently proposed.

Influence of Surface Finishing on Corrosion Behaviour of 3D Printed TiAlV Alloy

Additive manufacturing is currently one of the promising methods for the fabrication of products of complex shapes. It is also used in medical applications, thanks to technological progress, which also enables the printing of metallic materials. However, the final products often have to undergo a final surface treatment. In this work, the influence of surface finishing on the corrosion behavior of the medical alloy Ti-6Al-4V prepared by the selective laser melting technique is studied. The samples were subjected to mechanical, chemical and electrochemical treatments. Corrosion behavior was investigated using DC and AC electrochemical techniques such as potentiodynamic and potentiostatic curves and electrochemical impedance spectroscopy. Furthermore, the influence of surface treatments on the possibility of localized corrosion attack was evaluated. The results showed that the surface treatments have a positive effect on the corrosion resistance and reduce the risk of crevice corrosion.

Electrochemical techniques implementation for corrosion rate measurement in function of humidity level in grounding systems (copper and stainless steel) in soil samples from Tunja (Colombia)

In this work, DC electrochemical techniques were used to determine the corrosion rate of copper and stainless-steel electrodes used in grounding, varying the level of humidity, in sandy loam and clay loam soils. The maximum corrosion potentials were: for copper -211 and -236mV and for stainless steel of -252 and -281mV, in sandy loam and clay loam respectively, showing that in sandy loam the values are higher, about 30mV. The mechanism by which steel controls corrosion is by diffusion, whereas in copper it is carried out by transfer of mass and charge, which affects the rate of corrosion, which in copper reached a maximum value of 5mm/yr and in Steel 0.8mm/yr, determined by Tafel approximations. The behaviour of the corrosion rate was mathematically adjusted to an asymptotic model that faithfully explains the C.R. as a function of humidity, however, it is necessary to define the relation between the factor □ established in the model and the precise characteristics of the soil, such as the permeability or quantity of ions present.

Synthesis and Characterization of Mg-Based Coatings for Sacrificial Cathodic Protection of Mg Alloy AZ31B

Magnesium (Mg) alloys are becoming increasingly popular for weight reduction in the automotive industry due to their low density and nominal specific strength [1]. However, the deleterious intrinsic corrosion rates of Mg alloys have hindered their widespread application. To date, protection strategies for Mg alloys has consisted primarily of barrier coatings such as Rockhard™ and Tagnite™ which have not demonstrated the ability to protect bare Mg at scratches, cracks, and other defects [2] while conversion coatings have not been shown to survive harsh working conditions and do not provide sacrificial cathodic protection or inhibitor release [3]. The drawbacks of these coating strategies provide motivation for additional research. In addition, sacrificial cathodic protection of Mg alloys has not been widely studied and presents opportunities of high interest. Recent studies have shown that sacrificial cathodic protection of Mg and its alloys is possible and shows some promise albeit complicated by cathodic corrosion of Al rich phases and large local cathodic reaction rates [4]. However, an optimal sacrificial coating has not been produced to date. As such, the overall goal of this research is to design a tunable coating which acts as (1) a corrosion barrier, (2) provides sacrificial cathodic protection, and (3) functions as a reservoir for ionic inhibitor release. Such a coating has been designed recently for aluminum alloy AA2024 using an amorphous Al-Co-Ce alloy [5]. The present work aims to study sacrificial Mg-based coatings produced by pulsed laser deposition (PLD) tuned for the Mg alloy AZ31B-H24 (Mg-3Al-1Zn). Binary and ternary coatings using rare earth elements known to decrease the OCP of Mg [6] and exhibit some ability for inhibitor release (RE = La, Gd, Y) as well as elements such as Ge and Sn [7] which poison cathodic kinetics, will be explored. Preliminary results of a Mg88Gd9Y3 based coating on AZ31B revealed a two-phase microstructure consisting of an extended solid solution Mg-rich matrix with a grain size on the order of 100 nm as well as a small fraction of Gd-Y rich particles < 1 μm in diameter. Immersion testing of PLD coated AZ31B in 0.6 M NaCl revealed an open circuit potential of -1.58 VSCE which is mildly cathodic to uncoated AZ31B (-1.56 VSCE) after a 24-hour exposure. In addition, the coating was observed to dissolve preferentially on an AZ31B surface demonstrating ability as a sacrificial anodic coating. Future testing will systematically explore the effects of alloying element concentration on the corrosion behavior of PLD coated AZ31B utilizing both AC and DC electrochemical techniques. Such an understanding will provide a framework to optimize a sacrificial cathodic coating for AZ31 based on critical factors such as open circuit potential, corrosion rate, galvanic couple potential, and galvanic couple current. Acknowledgments: This project is funded by the US Air Force Academy under agreement number FA7000-13-2-0020 as part of Corrosion University Pilot Program under the direction of Mr. Daniel Dunmire and by the National Science Foundation under NSF DMR#1309999 as well as the Army Research Office under agreement number W911NF-14-2-0005 with project manager Joe Labukas. The U.S. Government is authorized to reproduce and distribute reprints for government purposes notwithstanding any copyright notation thereon. The views and conclusions contained are those of the authors and should not be interpreted as necessarily representing the official policies or endorsement of the US Air Force Academy or the US Government.

Connection between Atomic Scale Characterization and Electrochemical Behavior during Passivation of Polycrystalline and Single Crystal Ni-Cr and Ni-Cr-Mo Alloys

Ni-Cr and Ni-Cr-Mo alloys owe their superior corrosion resistance to the surface enrichment of passivating Cr-rich oxides or hydroxides and the synergistic effect of Mo in the film1–3. However, the specific role of alloying elements such as Mo are not well-understood, especially with respect to their position relative to the oxide/metal interface and atomistic influence on film growth, localized breakdown, and repassivation processes. The composition, structure, and thickness of the passivating oxide films are additionally challenging to characterize in-situ due to their nanoscale dimensions and the high electric field imposed by aqueous growth. The key processes which take place in oxide films and regulate their behavior in aqueous environments are atomic, ionic, and electronic in nature and currently poorly understood, often requiring study at the resolution and detection limits of characterization methods4. The main goal of this work is to integrate bulk polycrystalline and single crystal electrochemical measurements with nanoscale characterization to advance the fundamental understanding of the structure, molecular, and electronic properties of oxides formed during aqueous passivation and uncover the role of Mo on surface stability in corrosive environments. The alloys under study are Ni-11Cr, Ni-11Cr-6Mo, Ni-22Cr, and Ni-22Cr-6Mo, wt%, polycrystalline solid solutions with additional focus on individual grain orientations in isolation from grain or twin boundaries. DC and AC electrochemical techniques were utilized for exploring the controlled growth of passive oxide films on these materials in aqueous chloride and chloride-free solutions of various acidity. The repassivation kinetics of polycrystalline and single grain crystal surfaces were determined potentiostatically and galvanostatically after previously formed air oxides were reduced. A major challenge to analysis of the oxidation rate from electrochemical data is that film thickening rate must be assessed independently from the total oxidation rate. Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) results indicate that a significant portion of the applied anodic charge during oxidation is associated with direct cation ejection and/or dissolution rather than film thickening. Single frequency electrochemical impedance spectroscopy (SF-EIS) and ICP-MS were thus applied as novel characterization methods to model the electrochemically grown oxides and measure in-situ changes in oxide thickness and elementary dissolution reaction rates during passivation and film dissolution and/or cation emission. The oxide thicknesses calculated using these methods were related to the charge that goes toward oxide film growth and the related oxidation current density. Atomic force microscopy (AFM) topography images and profiles across single grain surfaces at intermediate stages of potentiostatically and galvanostatically controlled oxide growth were obtained to give insight on the nucleation and morphology of oxides formed. The surface roughness was derived from these images in order to quantify the influence of the minor alloying elements, and chloride presence on the morphology and degree of film breakdown during the stages of passivation. 3D atom probe tomography and X-ray photoelectron spectroscopy analysis of the oxides following potentiostatic and galvanostatic growth provided additional information regarding the location and chemical identity of elements across the films. This pairing of operando mesoscale electrochemical techniques with ex-situ microscopy describes the competitive oxidation and dissolution processes in aqueous environments and achieves an atomic-scale understanding of the oxide surface morphology along with the influence of minor alloying elements. The fate of all elements during fast repassivation was tracked operando for analysis of the significant impact of dissolution and cation ejection during passivation using ICP-MS, SF-EIS, and AFM measurements. The role of kinetics on passivation was found to play a crucial role over thermodynamics, resulting in non-conformal and unexpected oxide growth dictated by the minimization of interfacial energy and stresses. Finally, Mo was found to serve a number of roles during passivation and collects at the film/solution interface as well as throughout oxides. ACKNOWLEDGEMENTS This work was funded by the U.S. Office of Naval Research Multi-University Research Initiative under a subcontract from Northwestern University SP0028970-PROJ0007990 with Dr. David A. Shifler. REFERENCES 1. F. Bocher, R. Huang, and J. R. Scully, Corrosion, 66, 1–15 (2010). 2. A. C. Lloyd, J. J. Noël, S. McIntyre, and D. W. Shoesmith, Electrochim. Acta, 49, 3015–3027 (2004). 3. A. C. Lloyd, J. J. Noël, N. S. McIntyre, and D. W. Shoesmith, JOM, 57, 31–35 (2005). 4. P. Jakupi, D. Zagidulin, J. J. Noël, and D. W. Shoesmith, Electrochim. Acta, 56, 6251–6259 (2011).

Directionally solidified dilute Zn-Mg alloys: Correlation between microstructure and corrosion properties

Zinc–Magnesium (Zn–Mg) alloys have been addressed as potential biodegradable biomaterials. These alloys enable to overcome the main drawbacks of Mg, its high corrosion rate and hydrogen evolution. Moreover, homogeneous corrosion degradation is also an interesting issue to ensure the success of load-bearing biodegradable implants. In this work, two directionally solidified dilute alloys, Zn-0.3wt-%Mg and Zn-0.5wt-%Mg were studied. Both alloys having different microstructural morphologies (cellular and dendritic arrays) depending of the distance to the cooling bottom were evaluated to correlate their microstructural features with corrosion properties. Kinetics and corrosion mechanism have been evaluated by means of DC and AC electrochemical techniques in low chloride containing solution, 0.06 M NaCl. Coarser microstructures are shown to promote higher corrosion rate. Like Zn the corrosion mechanism occurs throughout a multistep dissolution process involving the formation of intermediate monovalent ZnI, and soluble zinc hydroxychloride compounds resulting of the reaction of zincate ions with chloride ions.

Analysis of the Passivation and Local Dissolution of Ni-Cr-Based Alloys Using the Cabrera-Mott and Point Defect Models

ABSTRACT In Fe-Cr based steels, the effects of low concentrations of alloying elements such as N, Mo, and W are somewhat understood with respect to corrosion and passivation. Such alloying often achieves significant improvements in corrosion resistance [1]. However, the roles of alloying elements in a Ni-Cr system are much less well understood. Upon adding small to intermediate concentrations of Mo to Ni-Cr, the corrosion resistance improves dramatically [2]. The Pitting Resistance Equivalent Number can forecast corrosion resistance based on major and minor alloying element concentrations but is empirical and does not provide predictive capabilities nor scientific insights [4]. The goal of this study is to identify the mechanism(s) by which alloying elements improve aqueous passivation in the Ni-Cr-Mo system at the atomic scale and to investigate the oxide properties of the protective passive film. The alloys under study are Ni-11Cr, Ni-11Cr-6Mo, Ni-22Cr, Ni-22Cr-6Mo, and Ni-22Cr-6Mo-3W, wt%, polycrystalline solid solutions. Previous studies have postulated the influence of different elements on passivity in acidic and high [Cl-] environments [3,5]. Cr has been shown to provide passivity during repassivation and enhance resistance to corrosion in oxidizing acids while Mo enables repassivation during oxide film breakdown and increases the resistance to reducing acids [3]. Electrochemical impedance spectroscopy (EIS) studies have been used to analyze the passive film for a Ni-22Cr-13Mo-3W-3Fe-2.5Co, wt%, alloy in varying environments [4]. This presentation explores DC and AC electrochemical techniques for exploring controlled growth of passive oxide films on Ni-Cr alloys. Potentiostatic and galvanostatic holds were used in aqueous chloride and chloride-free solutions of increasing acidity. The data was analyzed using the Cabrera-Mott (C-M) and Point Defect (PDM) models in order to investigate their applicability to the case of Ni-Cr oxide growth with varying concentrations of minor alloying elements [6,7]. A major challenge to the application of either C-M or PDM is that the film thickening rate must be assessed independently from the total oxidation rate. Inductively Coupled Plasma-Optical Emission Spectroscopy indicates that a significant portion of the applied anodic charge during oxidation is associated with direct cation ejection and/or dissolution of the oxide into solution rather than thickening of the film. Therefore, the oxide growth rate and charge must be extracted from the total current for C-M and PDM analysis. EIS techniques were used to characterize and model the electrochemically grown oxides, represented as a constant phase element, to enable the calculation of oxide thickness. The impedance data following potentiostatic oxide growth was compared to the data produced from single frequency EIS where the impedance was continuously recorded in order to extract the variation of oxide charge and thickness over time. The oxide thicknesses calculated using these methods was related to the charge that goes toward oxide film growth. By comparing these values to the potentiostatic or galvanostatic total charge, the portion involved in dissolution was removed for C-M and PDM analysis. Subsequently, diagnostics were used to test the applicability of C-M and PDM models. Results are reported herein. Ex situ Raman and X-ray Photoelectron Spectroscopy were utilized to characterize the chemical and molecular identities of the oxide film, accurately calculate the thickness, and test the accuracy of the EIS-dependent oxide calculations. These ex situ methods corroborated the determination of the potentiostatic and galvanostatic charges associated with the growth of the oxide. ACKNOWLEDGEMENTS This work was funded by the U.S. Office of Naval Research Multi-University Research Initiative under a subcontract from Northwestern University SP0028970-PROJ0007990 with Dr. David A. Shifler. REFERENCES [1] Sedriks, A.J. (1996). Corrosion of stainless steels (2nd ed.). New York: Wiley [2] Bocher, F., Huang, R., & Scully, J. R. (2010). Prediction of critical crevice potentials for Ni-Cr-Mo alloys in simulated crevice solutions as a function of molybdenum content. Corrosion, 66(5). [3] Lloyd, A. C.; Noel, J. J., McIntyre, S.; & Shoesmith, D. W. (2004). Cr, Mo, and W alloying additions in Ni and their effect on passivity. Electrochimica Acta, 49, 3015-3027. [4] Combrade, P. (2001). Corrosion mechanisms in theory and practice (2nd ed.). CRC Press: Marcus. 349-397. [5] Jakupi, P.; Zagidulin, D.; Noel, J.J.; & Shoesmith, D.W. (2011). The impedance properties of the oxide film on the Ni-Cr-Mo Alloy-22 in neutral concentrated sodium chloride solution. Electrochimica Acta, 56, 6251-6259. [6] Davenport, A.J. & Lee, B. K. (2012). Passive film growth kinetics for iron and stainless steel, Electrochemical Society Proceedings, 13, 187-197. [7] Marshall, P. I. & Burstein, G. T. (1984). Effects of alloyed molybdenum on the kinetics of repassivation on austenitic stainless steels, Corrosion Science, 24(5), 463-478.

Dynamic Characterization of Dendrite Deposition and Growth in Li-Surface by Electrochemical Impedance Spectroscopy

The evolution of dendrite formation is characterized by DC and AC electrochemical techniques. Interfacial mechanisms for lithium deposition are described and quantified by electrochemical impedance spectroscopy (EIS) between a lithium electrode and a graphite electrode. The initiation and growth of dendrites in the lithium surface due to the cathodic polarization conditions following anodic dissolution emulate long term cycling process occurring in the lithium electrodes. The dendrite initiation at the lithium/organic electrolyte interface is proposed to be performed through a combination of layering and interfacial reactions during different cathodic conditions. The growth is proposed to be performed by surface geometrical deposition. In this work, we use EIS in galvanostatic mode to assess the initiation and growth stages of dendrites by the accumulation of precipitates formed under different current conditions. The lithium/organic solvent experimental system using frequency domain techniques is validated by the theoretical approach using a deterministic model that accounts for the faradaic processes at the interface assuming a coverage fraction of the electrodic surface affected by the dendritic growth.

Laser alloyed Al-W coatings on aluminum for enhanced corrosion resistance

A tungsten precursor deposit was spray coated on aluminum 1100 substrate and was subsequently surface alloyed using a continuous wave diode-pumped ytterbium laser at varying laser energy densities. For the laser energy input of 21–32J/mm2 the melt depth ranged between 135 and 150μm. Scanning electron microscopy observations indicated the formation of uniform and continuously dense laser alloyed coatings with sound interface between the modified surface and substrate along with an equi-axed grain structure with second phase precipitates in the intergranular region. X-ray diffraction analysis confirmed that laser processing has resulted in the formation of Al4W, as the major phase with retention of W in Al within the alloyed region. The corrosion resistance of laser alloyed coatings was evaluated in near natural chloride solution using ac and dc electrochemical techniques. After laser processing potential–time measurements has indicated the relatively stable and high potential values over the longer exposure times. Cyclic polarization results showed the reduction in the corrosion current density by a factor of 8, compared to untreated Al 1100. Besides, the electrochemical impedance spectroscopy confirmed the increase in the total resistance (47–70kΩcm2) with the increase in the laser energy density.

CORROSION BEHAVIOR OF ZIRCONIUM TREATED MILD STEEL WITH AND WITHOUT ORGANIC COATING: A COMPARATIVE STUDY

In this study, the anti-corrosion performance of phosphated and zirconium treated mild steel (ZTMS) with and without organic coating was evaluated using AC and DC electrochemical techniques. The topography and morphology of the zirconium treated samples were studied using atomic force microscopy (AFM) and field emission scanning electron microscope (FE-SEM) respectively. The results revealed that the anti-corrosion performance of the phosphate layer was superior to the zirconium conversion layer without an organic coating due to very low thickness and porous nature of the ZTMS. Additionally, the corrosion behavior of the organic coated substrates was substantially different. It was found that the corrosion protection performance of the phosphate steel and ZTMS with an organic coating is in the same order.

Corrosion evaluation of multi-pass welded nickel–aluminum bronze alloy in 3.5% sodium chloride solution: A restorative application of gas tungsten arc welding process

In this research, the corrosion behavior of a gas tungsten arc welded nickel–aluminum bronze (NAB) alloy is investigated by DC and AC electrochemical techniques in 3.5% sodium chloride solution. Regarding the electrochemical impedance spectroscopy and potentiodynamic results, uniform corrosion resistance of instantly immersed weld and base samples are almost analogous and increased (more in weld region) during the immersion times. Moreover, zero resistant ammeter results demonstrated that the few nanoampere galvanic currents are attributed to microstructural and morphological differences between these two regions. Therefore, the welding procedure could not deteriorate the general corrosion resistance of the restored damaged NAB parts operating in marine environments.

Corrosion behaviour assessment of cast and HIPed Stellite 6 alloy in a chloride-containing environment

Cobalt-base (Stellite) alloys have seen extensive use in wear environments mainly due to their high strength, corrosion resistance and hardness. Co-base superalloys rely primarily on carbides formed in the Co matrix and at grain boundaries, for their strength and wear resistance. The distribution, size and shape of carbides depend on processing conditions. Currently, the use of Stellite alloys has extended into various industrial sectors (e.g. pulp and paper processing, oil and gas processing, pharmaceuticals, chemical processing) and the need for improved information regarding corrosion of Stellite alloys has increased. It has been recognized that processing changes, which affect the microstructure of Stellite alloys, most affect corrosion resistance.In this work the corrosion behaviour of Stellite 6 alloy in the as-cast and the HIPed consolidated forms is compared and contrasted using DC and AC electrochemical techniques in static saline conditions. The results show that there is a significant difference in the corrosion performance of HIP consolidated Stellite 6 and it is possible to link the corrosion behaviour to the microstructure. The benefits of using HIPing as a manufacturing process for the corrosion performance of Stellite 6 are discussed.

Imitating seasonal temperature fluctuations for the H2S corrosion of 304L and 316L austenitic stainless steels

Temperature fluctuations are inevitable in sour oil and gas production. In this study, the H2S corrosion of 304L and 316L alloys was investigated at pH 3 and temperatures of 20–60 °C using DC and AC electrochemical techniques. Two-fold increases in the corrosion rates of both alloys were reported with increases in temperature to 60 °C. In the 304L alloy, the surface layer was observed to be 3% rougher and 34% thicker than that of the 316L alloy. The two alloys exhibited different corrosion behaviors in the temperature ranges of 20–40 °C and 40–60 °C. Although the 316L alloy revealed a greater corrosion resistance at the free potential condition, the passivation on the 304L alloy was significantly greater than that of the 316L alloy at 40 °C and 15 ppm H2S. The FeS2 and combined FeS2-MoS2 compounds contributed to the surface layer constituents in the 304L and 316L alloys, respectively. The increase in temperature kinetically provided more favorable conditions for FeS2 than MoS2 formation, i.e. it had a relatively constructive effect on the 304L alloy passivation.

Effect of operational parameters on the performance of PEMFC assembled with Au-coated Ni-foam

An innovative proton exchange membrane fuel cell was assembled using Au-coated nickel foam instead of the conventional flow field (carbon plate). The effect of operational parameters on the performance of this cell was investigated by DC polarization and electrochemical impedance spectroscopy techniques. Parameters such as cell operating temperature, cathode humidification temperature, and cathode-gas stoichiometry were of concern.Our results indicate that the testing temperature is increased from 40 °C to 60 °C, the power density increases, charge-transfer resistance decreases. When the cathode humidification temperature increases from 50 °C to 70 °C, the power density decreases. This is ascribed to an increase in mass-transfer resistance of the cell from 0.009 to 0.032 Ω, possibly because of partial flooding. The cell also performs better if cathode-gas stoichiometry is increased from 2 to 5. A maximum power density of 791.6 mW/cm2 was realized under the following optimal conditions: a testing temperature of 60 °C, cathode humidification temperature of 50 °C, and cathode-gas stoichiometry of 5.

Anodic reaction characteristics of tungsten in basic phosphate solutions

DC polarization and electrochemical impedance spectroscopy techniques were used to investigate the pH dependent anodic behaviour of tungsten in basic solutions. Anodic currents of W were pH independent between pH 8 and 11. Above pH 12 as pH increased the anodic currents of W also increased. The diffusion control in pH independent regime was observed to arise from slow diffusion of the solubilized species. At higher pH levels, however, OH − ion diffusion from bulk of solution was observed to control the anodic process. Hydrate layer formation, which was observed with low frequency capacitive loop formation, accelerated with increase in pH.

Electrocatalytic activation of Ni electrode for hydrogen production by electrodeposition of Co and V species

Hydrogen is considered to be the most promising candidate as a future energy carrier. One of the most used technologies for the electrolytic hydrogen production is alkaline water electrolysis. Electrode materials used in alkaline water electrolysis are mainly made from Ni or Ni-based alloys due to their desirable mechanical and chemical stability in hot and alkaline solution. Considerable research effort has been conducted on enhancement of electrocatalytic activity of Ni electrodes. Electrocatalytic activity of Ni electrode for the hydrogen evolution reaction (HER) in alkaline solution was compared to the electrode made from electrodeposition of Co and Co–V species on Ni support electrode using ac and dc electrochemical techniques. Analysis of Tafel polarization curves for the HER in alkaline solution reveals an increased electrocatalytic activity of the electrodeposited electrodes if compared to the Ni. This was additionally proved by measuring energy input (overpotential) for the given hydrogen production (fixed current density) and vice versa. Electrochemical impedance spectroscopy was applied in order to investigate the obtained effect of electrodeposited electrodes. It was found that the apparent electrochemical surface of investigated electrodes was significantly increased by the process of electrodeposition.

LBW of Similar and Dissimilar Skin-Stringer Joints Part II: Electrochemical Characterization

The durability of metal structures depends most upon the corrosion behaviour of its materials as well as the on the electrochemical behaviour of joints and the technology employed to construct the joints itself. This work describes the effect of Laser Beam Welding (LBW) technology on the electrochemical and corrosion behaviour of parent materials and bead of several aluminium alloy joints. Investigation was carried out by using dc electrochemical techniques (open circuit potential monitoring, OCP, and anodic polarization) on selected micro areas of parent materials and bead by means of a suitable electrochemical microcell. With regards to the corrosion behaviour of heat affected zone (HAZ), no data has been presented due to the small area of HAZ when compared to the microcapillary area adopted in this study. Alloys investigated in this work are the AA 2139 and AA 6156 as skin in the form of rolled sheets, and AA 2139 and PA 765 as stringer, in form of extruded parts. In addition, the AA 4047 has been used as filler material in form of wire. Results obtained in this investigation indicated that no major corrosion concerns arise from the LBW of both laminated AA 2139 skin with extruded AA 2139 stringer and laminated AA 6156 skin with extruded AA 2139 stringer. In the former case, the bead exhibits corrosion potential nobler than that exhibited by the skin and stringer of circa 100 mV for the AA 6156 skin with extruded AA 2139 joint. This result is attributed to the presence of Si in the bead due to the process parameters adopted for such a weld. Quite similar corrosion behaviour was observed for the skin, stringer and bead in the latter case. Attention must be paid when laminated AA 2139 skin with extruded PA 765 stringer, and laminated AA 6156 skin with extruded PA 765 stringer are joined by LBW. In this case low potential and high surface activity were observed on the stringer and bead and attributed to the excess of Zn in the PA 765 alloy.

Using DC electrochemical techniques to assess the relative corrosiveness of water-based coatings and their ingredients

It has been common to assess corrosion inhibitors by ranking their relative corrosion currents at some arbitrary potential above the open circuit potential via DC potentiodynamic anodic scans. Groseclose et al.1 demonstrated that when bare steel panels are subjected to such scans, the difference in mV between the open circuit potential and breakdown potential is a sensitive measure of the panels’ tendency to corrode once coated and exposed to a corrosive environment. The present authors show that a combination of the two DC electrochemical techniques can be used to assess the relative corrosion (or inhibition) contribution of various ingredients in water-based coatings, and that those results translate into the same order of behavior when coatings utilizing those ingredients were applied to steel and tested in a corrosive environment. This technique is also shown to be capable of ranking finished liquid paints for their subsequent corrosion resistance as dried coatings on steel, and verifying the batch consistency of liquid paints in production.