Galvanic Coupling Research Articles

Electrochemical study of the surface metal matrix composite developed on AA 2024-T351 by the friction stir process

ABSTRACTThis work is focused on the evaluation of the corrosion behaviour of an AA2024-T351/SiC surface composite produced by friction stir processing (FSP). The surfaces composite show a quite homogenous distribution of the SiC particles in the stir zone, together with a significant grain size reduction as Electron Backscatter Diffraction analysis confirms. The corrosion behaviour was evaluated in 3.5 wt-% NaCl solution using electrochemical techniques. The complete study shows that all processed samples have similar corrosion resistance at the stirred region and differ from the base material behaviour. The results indicate that the galvanic coupling metal matrix–SiC particles are less active than the galvanic coupling metal matrix–Cu-rich second phases. Although the initial corrosion resistance of the base aluminium alloy is the worst of all systems studied, for long immersion times the development of corrosion through grain boundaries was found to progressdeeper in the 2024Al/SiC surface produced by FSP.

Finite Element Modelling of Microelectrode Arrays of Mg and AA2024 Galvanic Couple

Aerospace aluminum alloys, such as AA2024-T3, are susceptible to microgalvanic corrosion and pitting due to its heterogeneous microstructure. Thus, they require additional corrosion protection methods. Magnesium rich primer coatings (MgRP) have emerged as an effective alternative to highly toxic chromate-based coatings. MgRP systems consist of an organic resin loaded with Mg pigment. The organic resin provides barrier protection, while metallic Mg pigment serves as a sacrificial anode by galvanic coupling with more noble Al alloy. The distance over which an active corrosion system can protect a defect exposing a metal surface is named galvanic throwing power and it is an important parameter used to evaluate the effectiveness of this type of protection method. Microelectrode array (MEA) is a technique commonly used to determine the galvanic throwing power. In this technique, wires of the metals of interest are mounted in a non-conductive matrix and the galvanic current distribution over the wires is measured. The wire diameter and the distance between the wires are important factors that affect the accuracy of MEA. While thin wires can provide finer spatial resolution, the experimental setup is challenging, and edge effects can decrease the representativeness of the measurement. Finite element analysis (FEA) is a tool used to predict both spatial and temporal potential and current distributions in electrochemical systems related to metal dissolution and cathodic reactions. FEA also can be used as an experimental design tool. In this work, FEA was used to assess important parameters and predict an optimal combination of wire diameters and distance between wires that result in higher accuracy of MEA. COMSOL Multiphysics (ver.5.4) was used to perform the finite element simulations. Potentiodynamic polarization scans of AA2024 and pure Mg immersed in 0.9 M NaCl solution were performed to generate E - i curves that were used as the electrochemical boundary conditions. Laplace equation was used to describe the current distribution that resulted from the galvanic coupling. The effect of the spacing between the wires and their diameters on the current distribution was studied. The results were compared to a reference solid plate configuration, where the two metals were in electrical contact.

Galvanic Corrosion of Automotive Mixed Metal Substrates: Fundamental Understanding

Dissimilar metal joining is of increasing interest in many structural applications due to its potential to enable light weighting. Light-weight substrates such as aluminum alloys, magnesium alloys and carbon-fiber reinforced polymer (CFRP) are increasingly used in place of conventional steels due to their high strength-to-weight ratios. Two of the main challenges associated with the increased usage of mixed metal systems are galvanic corrosion and challenges with classical joining methods. 1,2. Most joining methods such as riveting, mechanical fasteners, and welding, results in electrical connection between the joined metals, which poses galvanic corrosion issues. Even galvanic isolation approaches such as the use of adhesives can cause galvanic corrosion problems due to the complexity of mixed metal assemblies. In galvanic coupling scenarios, the less noble material experiences severe corrosion and more noble material is protected. It is important to build a fundamental understanding of this phenomenon relative to the metallurgy and electrochemistry of the substrates, as well as the coupling method, to provide effective corrosion mitigation solutions. Electrochemical test techniques such as zero resistance ammeter measurements, open circuit potential measurements, and potentiodynamic polarization were utilized to understand effect of substrate type and solution pH on galvanic corrosion. In addition to the substrate effect, the effect of fastener type on galvanic corrosion between substrate and fasteners was also investigated for selected systems. The influence of joining methods such as self-piercing rivets, adhesives, and mechanical fasteners on galvanic corrosion was also studied. Galvanic and self-coupled coupons were exposed to standard laboratory accelerated life cycle test methods. Wedging by corrosion products, scribe creep, and corrosion volume loss were each characterized to quantify the extent of galvanic corrosion after environmental testing. Preliminary results indicated that galvanic coupling enhanced the wedging of substrate by corrosion products when aluminum alloy 6111-T4 (AA 6111-T4) is coupled with cold rolled steel (CRS). In addition, the pitting corrosion of AA 6111-T4 increased when it is coupled with CRS (Figure 1). Although hot dip galvanized (HDG) based galvanic coupons showed no significant wedging by corrosion products, galvanic coupling of CRS with HDG aggravate the scribe creep of HDG. Galvanic isolation methods such as coating before joining or utilizing adhesives significantly reduced galvanic corrosion in the model mixed-material systems. Figure 1. Average pit depth of AA 6111-T4 substrate coupled with different mixed-metal substrates after various exposure times in neutral and acidic cyclic corrosion tests (CCT). Acknowledgements: Research was sponsored by the Army Research Laboratory, the Tank Automotive Research Design Engineering Center, and Drexel University under Cooperative Agreement W911NF-13-2-0046 and related sub awards.

Investigations of the Effects of Electrochemical, Environmental and Geometric Parameters on Galvanic Coupling through Finite Element Modeling

Experimental studying of the galvanic coupling between different alloys can quickly become a difficult task, because of the many parameters to consider. Many of these parameters are dependent on each other, forcing researchers to create a web of test matrices in order to isolate one parameter at a time. Both electrochemical and geometric parameters must be considered, along with a constant environmental storage of samples before and after tests to guarantee that there has been no change in the system. This complexity makes experimental testing slow, however it can produce thorough results. Finite element modeling (FEM) has become a useful tool in studying galvanic corrosion[1] mainly because of its ability to both efficiently assess the effects of specific parameters and to isolate changes in parameters in a way that may not be possible experimentally. A computational study using FEM can allow a vast portion of parameter space to be assessed, guiding experimental tests to regions of interest. This approach narrows the daunting sea of parameters which experimentalists swim in, especially in the field of galvanic corrosion. In real applications, these parameters can be changed in a variety of ways. For example, different coatings can affect the electrochemical kinetics of the alloys, humidity changes can affect the concentration of the electrolyte solution, and atmospheric pressure or environmental changes can affect the temperature of the solution. The focus of this research therefore was to study the effects of different parameters on the galvanic coupling of AA7075 and SS316. These two materials are both common in aerospace applications, particularly with AA7075 as the structural material and SS316 as the fastener holding the assembly together. SS316 has been shown to severely couple with AA7075[2], leading to the formation of intergranular corrosion fissures of 2mm in length inside of the fastener hole[3]. Knowing which parameters create a large impact on this galvanic corrosion, and which do not, is valuable information when trying to maintain the safety of structures. A detailed computational matrix was run in the model, analyzing the effects of cathodic and anodic kinetics, conductivity, and temperature on the overall current distributions. These effects were quantified to determine which parameter made the largest contribution to the galvanic current, and which were negligible. A study of the interactions between these parameters was also conducted, in order to determine which parameter would be easily isolated by experimental technique. The computations were first run on a simple geometry in the model with one cathode surface and one anode surface, to get a generalized idea of the importance of the parameters. Afterwards, an occluded cell geometry was utilized in the model, to determine how the geometric change effects the contributions of the different parameters on the current distributions. A comprehensive analysis of these parameters will be presented, along with ideas of how to use this analysis for practical applications. [1] C. Liu, V. N. Rafla, J. R. Scully, and R. G. Kelly, “Mathematical modeling of potential and current distributions for atmospheric corrosion of galvanic coupling in airframe components,” Corros. 2015, no. May 2016, 2015. [2] C. A. Matzdorf, W. C. Nickerson, B. C. Rincon Tronconis, G. S. Frankel, L. Li, and R. G. Buchheit, “Galvanic test panels for accelerated corrosion testing of coated al alloys: Part 1 - Concept,” Corrosion, vol. 69, no. 12, pp. 1240–1246, 2013. [3] R. J. Skelton and R. G. Kelly, “Investigation of Galvanic Coupling on AA7075-T6 Navair Plate inside Fastener Hole,” in ECS Meeting s, 2018.

Chemical Imaging of Reactive Surfaces Using Microsensors As Tips in Scanning Electrochemical Microscopy: Applications in Corrosion Research and Protection

Chemical imaging of reactive surfaces with high spatial resolution has become available with the introduction of scanning electrochemical microscopy (SECM). In fact, this technique has become a powerful tool in the study of a wide range of corrosion processes. This presentation provides a brief review on the applications of scanning electrochemical microscopy (SECM) in measuring, characterising and evaluating corroding systems as developed in our research group at the University of La Laguna (Spain). Localized corrosion processes and electrochemical activity distributions in surfaces can thus be investigated in real time with high spatial resolution. The SECM is a unique near-field scanning technique that is electrochemically integrated as to detect chemical and electrochemical activities in electrochemical heterogeneous systems such as those operating in corrosion research. The SECM can be used in a variety of ways, which can be broadly classified into amperometric and potentiometric modes, depending on the type of the sensing probe, namely an ultramicroelectrode (UME) and an ion-selective microelectrode, respectively. Because the corroding metal usually involves local changes in ionic concentrations, as they produce Mz+ at the anodic sites transferred under diffusion control, the system can be studied by amperometric SECM. This feature has been exploited to image the generation of specific metal cations for the visualization of metastable pits on austenitic steel [1], and the detection of metal dissolution either from inclusions in alloys [2], or from defects in polymer-coated metals [3]. Additionally, in a neutral aqueous media, the reaction of dissolved oxygen from the electrolyte at the cathodic areas may be analyzed through the monitoring of the subsequent depletion of oxygen content in the solution volume adjacent to the cathodic sites [4]. However, for the detection of concentration distributions in certain corroding samples, especially for metals with sufficiently negative redox potentials in aqueous environments, the use of Pt microelectrodes is limited by the onset of oxygen reduction and hydrogen evolution reactions [5]. Though less employed, these physicochemical parameters may be studied using ion-selective electrodes (ISEs) as tips. In this case, the SECM is operated in the potentiometric mode, which gives greater chemical selectivity. In this respect, the applicability of Zn2+ [6] and Mg2+ ion-selective microelectrodes [7] with SECM has been established, although the onset of high electric fields in the electrolyte phase associated to galvanic corrosion [8] requires new tip assemblies for accurate monitoring [9]. In this contribution, the operation modes of the instrument are briefly described together with typical experiments selected to illustrate their application in sensing localized corrosion. Selected examples regarding the characterization of corrosion processes using scanning electrochemical microscopy are presented, including passivity breakdown and pit initiation, galvanic coupling, and blistering and delamination processes in coated metals. [1] Y. González-García, G.T. Burstein, S. González, R.M. Souto. Imaging metastable pits on austenitic stainless steel in situ at the open-circuit corrosion potential. Electrochemistry Communications 6, 637-642 (2004). [2] J. Izquierdo, S. González, R.M. Souto. Application of AC-SECM in corrosion science: Local visualization of heterogeneous chemical activity in AA2024 surfaces. International Journal of Electrochemical Science 7, 11377-11388 (2012). [3] R.M. Souto, Y. González-García, S. González. In situ monitoring of electroactive species by using the scanning electrochemical microscope. Application to the Investigation of degradation processes at defective coated metals. Corrosion Science 47, 3312-3323 (2005). [4] J.J. Santana, J. González-Guzmán, L. Fernández-Mérida, S. González, R.M. Souto. Visualization of local degradation processes in coated metals by means of scanning electrochemical microscopy in the redox competition mode. Electrochimica Acta 55, 4488-4494 (2010). [5] R.M. Souto, Y. González-García, D. Battistel, S. Daniele. In situ SECM detection of metal dissolution during zinc corrosion by means of mercury sphere-cap microelectrode tips. Chemistry, A European Journal 17, 230-236 (2011). [6] J. Izquierdo, L. Nagy, Á. Varga, I. Bitter, G. Nagy, R.M. Souto. Scanning electrochemical microscopy for the investigation of corrosion processes: measurement of Zn2+ spatial distribution with ion selective microelectrodes. Electrochimica Acta 59, 398-403 (2011). [7] J. Izquierdo, A. Kiss, J.J. Santana, L. Nagy, I. Bitter, H.S. Isaacs, G. Nagy, R.M. Souto. Development of Mg2+ ion-selective microelectrodes for potentiometric Scanning Electrochemical Microscopy monitoring of galvanic corrosion processes. Journal of The Electrochemical Society 160, C451-C459 (2013). [8] A. Kiss, D. Filotás, R.M. Souto, G. Nagy. The effect of electric field on potentiometric Scanning Electrochemical Microscopic imaging. Electrochemistry Communications 77, 138-141 (2017). [9] D. Filotás, B.M. Fernández-Pérez, A. Kiss, L. Nagy, G. Nagy, R.M. Souto. Double barrel microelectrode assembly to prevent electrical field effects in potentiometric SECM imaging of galvanic corrosion processes. Journal of The Electrochemical Society 165, C270-C277 (2018).

A novel semiairborne frequency-domain controlled-source electromagnetic system: Three-dimensional inversion of semiairborne data from the flight experiment over an ancient mining area near Schleiz, Germany

We have developed a novel semiairborne frequency-domain electromagnetic (EM) system and successfully tested it within the DESMEX project. The semiairborne approach relies on the fact that part of the system is positioned on the ground and the rest is airborne. This allows us to take advantage of ground and airborne techniques. In particular, a high-moment transmitter can be installed on the earth’s surface, which enables us to inject and induce strong EM fields in the subsurface. Moreover, galvanic coupling is possible, which is an advantage if additional ground stations are deployed. The airborne receivers allow easier, significantly faster, and more uniform spatial coverage of the study area than the ground receivers. In our implementation, transmitters and electric field receivers are installed on the ground. Magnetic field sensors, such as commercially available fluxgate, total field magnetometers, and newly developed induction coils, are installed on a helicopter-towed bird. First, we describe the results of a semiairborne survey performed in a selected area with ancient mining located in the Saxothuringian zone near Schleiz, Germany. A 3D semiairborne inversion model represents several conductive anomalies, which agree well with the outcrop of alum shale formations at the surface. In addition, the shallow parts of the semiairborne model are compared with the result of an independent helicopter-borne survey, which consists of stepwise 1D models.

Corrosion Behaviour of Mg Alloys in Alkaline Solutions and in Alkali-Activated Binders

The reprocessing of spent fuels from UNGG (Uranium Natural Graphite Gas) nuclear reactors in France generates cladding wastes mainly composed of Mg alloys. The management of these wastes involves their immobilization in hydraulic binders with high pH interstitial solutions (12.5<pH<13.7). In the case of magnesium corrosion, the choice of these materials for immobilization and storage is not straightforward due to the characteristic reactivity of these alloys, which is caused by several aspects [1]. First, magnesium corrosion is strongly influenced by the pH and the composition of the electrolyte (the pore solution of hydraulic binder). Furthermore, the electrochemical potential of Mg places this metal as an anode with respect to several materials [2] (e.g. residual graphite materials initially present in the fuel assemblies or steel from the container), which can create galvanic couplings and accelerate corrosion. Previous work has found low corrosion rates of Mg-Zr alloys and low H2 releases produced when Mg is immobilized in a Na-geopolymer mortar in the presence of fluorides (magnesium corrosion inhibitor) [3]. Similarly, there was a low release of H2 from corrosion during the galvanic coupling between Mg-Mn and graphite, as well as the formation of thinner corrosion product layers when Mg-Mn is immobilized in an alkali-activated slag mortar [4]. These results have contributed to the choice of these two materials as the most appropriates for embedding these wastes. Nevertheless, the electrochemical mechanisms responsible for this weaker corrosion, compared to conventional cementitious matrices, must be identified in these two reference scenari. The purpose of this work is to investigate the corrosion processes that occur once the Mg alloys are embedded in their immobilization matrices. The goal is to understand the phenomena that lead to low corrosion rates in these materials, in the case of general and galvanic corrosions. From these guidelines, a study of the cementitious matrices (porosity, resistivity and gas diffusion) and its interstitial solutions (pH, ionic species and conductivity) was carried out in order to identify the main parameters that can limit the corrosion rate of magnesium. The corrosion behavior of Mg alloys against these conditions has been tested in model solutions and in each cementitious binders using electrochemical techniques (OCP, ZRA, EIS and polarization curves), gravimetry and surface characterization (XRD, SEM/EDS and FIB/STEM). Both matrices are relevant for the storage of Mg wastes and have a thermodynamically favorable pH for magnesium passivation due to the precipitation of Brucite (Mg(OH)2) on its surface. The high resistivity of the alkali-activated slag and the possible consumption of O2 by the sulphides of the pore solution are favorable aspects to reduce galvanic corrosion rate. In the case of the Na-geopolymer/NaF, the oxygen concentration can also limit the rates of galvanic corrosion, since, during storage, oxygen access could be restrained. However, the presence of corrosion inhibitors (silicates and fluorides) is mainly responsible for the reduction of Mg corrosion, because of a protective film formed at the MgZr/Na-geopolymer interface. This corrosion rate may vary with the concentration of fluoride available in solution and is probably related to the nature of the corrosion product formed on the magnesium surface. General and galvanic corrosion in the Na-geopolymer/NaF mortar were quantified by gravimetric measurements and in both cases the nature of the corrosion products was characterized by scanning electron microscopy (SEM/EDS), X-ray diffraction (XRD) and FIB/STEM (Focused Ion Beam/Scanning Transmission Electron Microscope). The results revealed the stratification of the corrosion products found at the Mg-Zr/Na-geopolymer interface and were in good agreement with the experiments performed in the model alkaline solution. Magnesium corrosion in the Na-geopolymer/NaF mortar is rather limited by the chemistry of the interstitial solution. [1] K. W. Guo , “A Review of Magnesium/Magnesium Alloys Corrosion and its Protection”, Recent Patents on Corrosion Science, vol. 2, pp. 13-21, 2010. [2] G. L. Song and A. Atrens, “Corrosion Mechanisms of Magnesium Alloys”, Adv. Eng. Mater., vol. 1, no. 1, pp. 11–33, 1999. [3] D. Chartier, B. Muzeau, L. Stefan, J. Sanchez-Canet and C. Monguillon, “Magnesium alloys and graphite wastes encapsulated in cementitious materials: Reduction of galvanic corrosion using alkali hydroxide activated blast furnace slag”, Journal of Hazardous Materials, vol. 326, pp. 197-210, 2017. [4] D. Lambertin, F. Frizon and F. Bart, “Mg-Zr alloy behaviour in basic solutions and immobilization in Portland cement and Na-geopolymer with sodium fluoride inhibitor”, Surface and Coating Technology, vol. 206, pp. 4567-4573, 2012.

Galvanic Corrosion Processes of Aluminum Coupled to Iron in Chloride Solutions at Near-Neutral pH

Multimaterial structures are effective for weight reduction of automobiles, which contributes to the prevention of global warming and the decrease in CO2 emission. Steel-aluminum joints have many advantages such as low cost, high strength, and high corrosion resistance. However, galvanic corrosion was expected to occur because the corrosion potential of aluminum alloys is usually different from those of iron and steels. Considering the galvanic corrosion processes in chloride-containing near-neutral pH solutions, the existence of surface oxides is an important factor affecting the initiation of corrosion. In this research, the galvanic corrosion behavior of aluminum alloy coupled to iron and the steel in near-neutral pH solutions was investigated. Pure iron, AISI 1045 carbon steel, pure aluminum, and A5083-O(Al-Mg alloy) were used as the specimens in this research. The surfaces of pure iron and AISI 1045 were polished down to 1 µm. Pure aluminum and A5083-O were polished down to 0.25 µm with a diamond paste and cleaned ultrasonically in ethanol before electrochemical measurements. The electrode area was ca. 1 cm2. Ag/AgCl (3.33 M KCl) was used as the reference electrode. In diluted synthetic seawater (200mg/L [Cl-], pH 8.2, naturally aerated, galvanic current was measured for 20 h using zero resistance ammeter technique. The distance between two electrodes was 1 cm. Fig. 1(a) shows the time variation of the galvanic current and electrode potential of pure aluminum coupled to pure iron. From the beginning of the immersion to 30 ks, many current transients were generated. However, after 30 ks, the galvanic current became a constant value of approximately 0.15 µA cm-2. The potential gradually decreased with time. The pH of the electrolyte decreased from 8.2 to 6.7 by galvanic corrosion. Fig. 1(b) and 1(c) shows the electrode surfaces of pure iron and pure aluminum after galvanic current measurements. Rust was observed on pure iron, and filiform corrosion was generated on pure aluminum. Anodic polarization curves were measured in diluted synthetic seawater (200mg/L [Cl-], deaerated), and the pH was adjusted to 8.2, 6.7, 5.2, 3.7, and 2.2 with NaOH or H2SO4. Potentiodynamic polarization was started from corrosion potential, and the scanning rate was 23 mV min-1. The electrode potential of pure aluminum under galvanic coupling conditions was found to be lower than the pitting potentials of pure aluminum. But, corrosion was initiated on the aluminum surface (see Fig. 1(b, c)). The galvanic corrosion behavior of pure aluminum and aluminum alloy AA5083 were discussed in terms of the surface properties of the specimens and solution chemistry. Figure 1

Microstructure and corrosion behavior of AlCoCrFeNiSi0.1 high-entropy alloy

The microstructure and corrosion behavior of AlCoCrFeNiSi0.1 high-entropy alloy are investigated. The alloy was prepared by vacuum arc melting and subsequent injection casting. Two different phases, including one disordered BCC (A2) phase and one ordered BCC (B2) phase, are identified in this alloy. The alloy shows a typical equiaxed dendritic microstructure, both of the dendritic and interdendritic regions have an (Al, Ni)-rich matrix with B2 structure and (Cr, Fe)-rich precipitates with A2 structure. The precipitates present different morphologies in the dendritic and interdendritic regions, whereas the chemical composition and crystal structure are very similar. With regard to the corrosion behavior in deaerated 3.5 wt% NaCl solution, the pits were formed at the dendrites, and the selective dissolution of the dendrites could be mainly ascribed to the lower Cr content and galvanic coupling between the dendrites and the interdendrites. The alloy exhibits an active-passive corrosion behavior in deaerated 0.5 mol/L H2SO4 solution. The preferential dissolution of the A2 phase is more severe than the B2 phase. The correlation between chemical composition, microstructure, and corrosion behavior is well illustrated.

Microstructure and corrosion performance of AlFeCoNiCu high entropy alloy coatings by addition of graphene oxide

High entropy alloys (HEAs) have gained attention due to their exceptional properties. Only a few studies on microstructure-electrochemical property correlation in electrodeposited HEA coatings have been reported so far. Here, AlFeCoNiCu HEA coating containing different amounts of graphene oxide (GO) waselectrodeposited on mild steel substrate and studied. Motivation behind the addition of GO into HEA coatings was to further enhance their corrosion resistance behavior arising primarily from the chemical inertness and impermeability of GO to the passage of corrosive media. As-deposited HEA-GO composite coatings exhibited a granular morphology which became finer and relatively more compact with increase in the GO amount. Structural characterization revealed the presence of mixture of BCC and FCC phase with fraction of the FCC phase increasing with the amount of GO. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) showed that the corrosion rate of HEA-GO coatings progressively decreased with increase in the GO content. Transmission electron microscopy revealed the presence of highly Al-rich matrix phase and Co, Ni, Cu and Fe containing dendritic phase in the coating microstructure. It was noticed that with the addition of GO, the coating microstructure progressively became more compositionally homogeneous. Al distribution between the matrix and dendritic phases became more uniform. Microstructural homogenization which reduced the extent of mutual galvanic coupling between phases and uniform distribution of Al which can form stable and protective alumina phase along with the impermeability properties of GO enhanced corrosion resistance performance of the HEA-GO coatings when compared to pristine HEA coating.

Corrosion behavior of metastable AISI 321 austenitic stainless steel: Investigating the effect of grain size and prior plastic deformation on its degradation pattern in saline media

The role of grain size and strain rate on the corrosion behavior of plastically-deformed Ti-stabilized austenitic stainless steel (AISI 321) in saline media was investigated. The as-received coarse-grained alloy (CG: ~37 µm) was subjected to thermomechanical processing to develop fine (FG: ~3 µm) and ultrafine (UFG: ~0.24 µm) grained structures. These samples were deformed under high (dynamic) and low (quasi-static) strain-rate conditions to a similar true strain of ~0.86. Microstructural analyses on specimens after deformation prior to corrosion study suggests a shift from the estimated stacking fault energy value of the steel. Electrochemical tests confirm the highest corrosion resistance for UFG specimens due to the formation of the most stable adsorbed passive film. This is followed by FG and CG specimens in that order. For the three grain sizes, the corrosion resistance of specimen deformed under quasi-static loading condition is higher than that subjected to dynamic impact loading while the corrosion resistance of undeformed samples is the least. This work also confirms the non-detrimental effect of TiCs in AISI 321 austenitic stainless steel on its corrosion resistance. However, TiNs were observed to be detrimental by promoting pitting corrosion due to galvanic coupling of TiNs with their surrounding continuous phase. The mechanism of pitting in AISI 321 in chloride solution is proposed.

Body-Guided Galvanic Coupling Communication for Secure Biometric Data

In a world dominated by the wearable IoT devices, malicious security threats have become a common concern in system design. Guided by this notion, we propose a secure transmission system through body-guided channels using Galvanic Coupling (GC). The GC-method injects weak electrical current into human tissue, primarily propagating through the skin. The proposed design provides impermeability to malicious attacks, (e.g. side-channel sniffing) when sending biometric data, as the body behaves as a natural waveguide. The following contributions are: 1) the analytical formulation and empirical verification of a 3D tissue equivalent circuit model for GC-signal propagation of the human arm-wrist-palm channel; 2) the simulation study of numerous modulation schemes, drawn from the validated results of the GC-channel model; 3) the design and implementation of a transceiver prototype using optimal communication parameters (modulation, frequency, power) for transmission on a dielectrically equivalent tissue phantom; and 4) through the experimental trials, we demonstrate the eavesdropping susceptibility of the GC-signals, and similar body communication techniques, over-the-air and while in direct contact with the medium. The performance results of the GC-transceiver prototype yield a bit error rate of 10−6 with a transmit power of −2 dBm, in addition to over 7 $\times$ reduction of signal radiation outside the body compared to capacitive coupling.

Pb-PbOHCl Composite Nanowires Synthesized by Galvanic Deposition in Template.

In this paper, we report a detailed study on the synthesis of composite nanowires of Pb-PbOHCl via galvanic deposition into the pores of a membrane acting as a template. PbOHCl deposition quantitatively occurs as the solution pH exceeds the value of about 4.12. Simultaneously, owing to the galvanic coupling, electro-deposition of lead occurs, so composite nanowires were formed. The role of different parameters controlling the kinetic evolution of the process, such as oxygen bubbling, solution pH, surface area and type of sacrificial anode were investigated one at a time. The results suggest that every modification accelerating the alkalization of the solution inside the template pores favors growth of composite nanowires richer in PbOHCl. Alumina and polycarbonate membranes were investigated as template, and the best results were obtained for polycarbonate membrane, which can be easily removed by dissolution in CHCl₃ avoiding the collapse and successive loss of the nanowires.

Correlation between microstructure and corrosion behaviour of SnBi-graphene oxide composite coatings

In the present study, microstructure-electrochemical property correlation in electrodeposited SnBi-GO composite coatings containing different amounts of graphene oxide has been investigated. The SnBi-GO coatings exhibited uniform and compact morphology for lower GO concentrations, whereas for higher GO concentrations the morphological defects appeared. X-ray diffraction analysis showed the presence of Sn rich and a Bi rich phase in the coatings. Crystallite size of Bi-rich phase decreased whereas for Sn-rich phase it remained nearly unaffected with increasing GO addition. Growth texture analysis showed that the incorporation of GO enhanced growth along the low index planes for the Sn-rich grains and along the high index planes for the Bi-rich grains. Corrosion properties of coatings examined by the potentiodynamic polarisation and impedance spectroscopy methods revealed that the corrosion rate of the coatings decreased with increase in the GO content till an “optimum” addition of GO, after which the corrosion rate significantly increased with further GO addition. Microstructural analysis conducted using the electron backscatter diffraction (EBSD) technique showed that the coating with “optimum” GO amount which yielded the minimum corrosion rate contained highest fraction of low energy low angle grain boundaries which was due to the lowering of Bi dissolution in the Sn grains. Initial decrease in the corrosion rate was attributed to coating compactness, growth of grains along low energy index planes and GO impermeability. Increase in the corrosion rate after the optimum was attributed to increase in the morphological defects and possible galvanic coupling between the GO and the metals in the coatings. The enhanced corrosion properties of SnBi-GO coatings make them a viable material for interconnections used in the electronic components.

Galvanic interaction of pyrite with Cu activated sphalerite and its effect on xanthate adsorption

In the presence of Cu ions, a packed bed electrochemical reactor (PBER) was employed to deliberately avoid or induce galvanic coupling between pyrite and Cu‐activated sphalerite. The effect of galvanic interaction on Cu ions uptake and xanthate adsorption were investigated. Solution chemistry and surface chemistry studies (ethylenediaminetetraacetic acid extraction and time of flight secondary ion mass spectrometry) have observed that when sphalerite and pyrite were galvanically coupled, Cu ions migrated from the pyrite surface to the surface of the sphalerite. Along with the marked decrease in the adsorption of Cu ions on pyrite, xanthate adsorption on the minerals also dramatically dropped. The pseudo‐adsorption rate constant for the minerals in the mixed mode is only 0.0583 s‒1, much less than that in the decoupled mode, which is 0.1368 s ‒1. This testing program shows that the galvanic coupling of minerals contributes to more copper transfer and Cu ions preferentially adsorbed by sphalerite rather than pyrite. This affects the pyrite surface and causes it to become xanthate unflavoured.

Microstructure-corrosion property correlation in electrodeposited AlCrFeCoNiCu high entropy alloys-graphene oxide composite coatings

AlCrFeCoNiCu high entropy alloys (HEA) composite coating with and without graphene oxide (GO) were electrodeposited over mild steel substrate. Potentiodynamic polarization and electrochemical impedance spectroscopy tests performed in 3.5 wt% NaCl aqueous solution showed that the corrosion resistance of the HEA-GO composite coatings was higher than the HEA coating without GO. The corrosion resistance gradually increased with increase in the GO amount in the coating. Detailed microstructural characterization performed using transmission electron microscopy technique revealed that addition of GO facilitated microstructural and compositional homogeneity which eliminated localized corrosive attack due to elemental segregation induced galvanic coupling thereby increasing the corrosion resistance for the HEA-GO composite coatings.

Mineralogical controls on drainage quality during the weathering of waste rock

Mineralogical controls on waste-rock weathering rates are important for mine water quality predictions, as the reactivity of minerals is impacted not only by its chemical composition, but also by its texture, arrangement, and association. In this study, seven-year on-site field barrel kinetic tests were conducted with different types of waste rock from the Antamina mine, Peru. Quantitative automated mineralogy was applied to characterize mineral texture, liberation, and association during weathering and used to examine mobilization and attenuation mechanisms. The weathering of reactive intrusive waste rock with low neutralization potential led to mobilization of Cu, Fe, and Zn, and was associated with >75% liberated Fe-, Cu-, or Zn-bearing sulfide minerals and galvanic coupling of pyrite with chalcopyrite and sphalerite. In contrast, the mobilization of Cu, Fe, and Zn was less effective in skarn waste rock types with high neutralization potential, where <70% Cu-, Fe-, and Zn-bearing sulfide minerals were liberated and passivation of pyrite surfaces occurred. Finally, waste rock weathering and drainage quality trends were simulated using a reactive transport model, and calibration of sulfide dissolution rates quantitatively affirmed the observed changes in mineralogical waste rock associations and liberation. This study thus demonstrates that accounting for mineralogical parameters using automated mineralogy may improve practical predictive models for mine waste water management.

Development of new criteria for evaluating the effectiveness of Zn-rich primers in protecting Al-Mg alloys

One inorganic and three organic Zn-rich primers (ZRPs) without a pretreatment or a topcoat were evaluated on highly sensitized aluminum alloy 5456-H116 in 0.6 M NaCl for their ability to suppress intergranular corrosion (IGC) and intergranular stress corrosion cracking (IG-SCC) based on the achievement and maintenance of protective potentials under simulated galvanic coupling conditions. These evaluations utilized a combination of existing criteria (e.g., the need to establish an intermediate cathodic potential) and additional new criteria based on fast anodic response and low polarizability. Ethyl silicate, epoxy, epoxy polyamide, and polyurethane resins were considered. Accelerated electrochemical cycle testing in full 0.6 M NaCl immersion demonstrated that anodic charge usage in the candidate ZRPs had a greater dependence on the pore resistance than on theoretical anodic charge capacity. Electrochemical impedance spectroscopy modeling of cycle testing data demonstrated that the ZRPs with low pore resistance also had the fastest anodic response time. Galvanostatic pulse testing demonstrated that the ZRPs with the highest anodic charge usage and low pore resistance were also the least polarizable. These analyses propose relevant metrics to evaluate the effectiveness of Zn-rich primers in a complex challenge heretofore not considered: suppressing IGC/IG-SCC on highly sensitized 5456-H116 in aggressive alternate immersion environments.

Effect of additive particle size on the CuO-accelerated formation of LaFeO3 perovskite conversion coatings in molten carbonate baths

An original iron conversion coating process has been recently developed to enhance the functional properties of ferritic stainless steels for use as Solid Oxide Fuel Cell interconnects. The conversion process produces a dense LaFeO3 perovskite layer grown above a spinel oxide underlayer by an immersion treatment in specially-formulated molten carbonate baths at around 600 °C. Galvanic coupling with a coarse CuO powder added to the bath has been earlier proved to be an effective approach to significantly reduce conversion times and coating thicknesses onto a 18Cr Type K41 ferritic stainless steel substrate. Further investigations on the CuO acceleration effects on the K41 steel are reported in the present work focusing on the effect of CuO particle size on the conversion times and coating structure. Studies carried out with different concentrations and particle sizes of the CuO additive have indicated that conversion coating kinetics is strongly affected by the CuO particle size highlighting thus the fact that CuO did not fully dissolve in the carbonate bath and that galvanic coupling effects took place prevalently with CuO particles dispersed in the molten bath. Dramatic reduction in conversion times could be obtained through CuO nanoparticle additions to the bath. The shortest conversion time of <3 h was achieved by adding 6 mol% nano CuO, at 610 °C. As consequence, perovskite coatings with thicknesses well below 10 μm could be produced due to minimal substrate corrosion and spinel underlayer growth during the short conversion processes in the nano CuO-containing salt baths. This structural refinement could play an important role for improving dimensional stability and functional properties of perovskite coatings in SOFC interconnect applications.

Shadow corrosion: Experiments and modeling

Shadow corrosion has long been an issue in BWRs (Boiling Water Reactors): the use of Ni-based grids in close connection to Zry-2 cladding induces locally an accelerated corrosion which can in some instance lead to low remaining metal thickness at the contact point.It is understood since at least 20 years that it is a galvanic coupling phenomenon. However, some important aspects are still debated, due in particular to the fact that it cannot be reproduced without irradiation. Under (neutron) irradiation and at reactor temperature, several parameters are not known accurately (for example the electrical conductivity of the coolant and of the oxide, or the Open Circuit Voltage and current of the metal parts). The role of the contact point between the grid and the cladding is also poorly understood.To improve our understanding, a photoelectrochemical cell, simulating the conditions of Light Water Reactors was designed, developed and validated. Electrochemical methods, performed in-situ, were used to study the electrochemical behavior of Zr-based and Ni-based alloys. Out of pile experiments with or without UV light were performed to measure the OCV (Open Circuit Voltage), the exchange current densities as well as the Tafel slopes (transfer coefficients) of Ni- and Zr-electrodes. The influence of the irradiation flux allowed extrapolations in order to attempt to assess what could happen under neutron irradiation.A finite element modeling based on the Butler-Volmer equation allowed to take simultaneously into account the thermal and galvanic corrosion. Thermal feedback due to the thermal insulating property of the oxide is also included. Parametric studies allowed deriving order of magnitudes of the unknown parameters. To our knowledge, this is currently the most advanced shadow corrosion modeling.