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Surface Analytical Methods Applied to Magnesium Corrosion Understanding magnesium alloy corrosion is of primary concern, and scanning probe techniques are becoming key analytical characterization methods for that purpose. This Feature presents recent trends in this field as the progressive substitution of steel and aluminum car components by magnesium alloys to reduce the overall weight of vehicles is an irreversible trend. Philippe Dauphin-Ducharme and Janine Mauzeroll* Laboratory for Electrochemical Reactive Imaging and Detection of Biological Systems, Department of Chemistry, McGill University, Montreal, Quebec Canada, H3A 0G4 requirement of many industries including the automotive industry. In addition to these requirements, households can invest, according to the automotive protective agency of Canada,4 up to $1000 during a vehicle lifetime, in corrosion prevention treatments. Moving forward, corrosion will still remain an important challenge as the automotive industry is trying to reduce the vehicle mass to improve fuel efficiency and the maximum range of electric vehicles. Recent statistics reported that a 10% decrease in vehicle mass would result in a 6−8% increase in fuel efficiency for internal combustion engines.5 Strong candidates for inclusion in automotive design and achieving weight reduction are fibers, polymers, and lightweight alloys.6 Mg and its alloys provide an excellent balance in terms of mechanical and lightweight properties. Automotive manufacturers have introduced Mg components, such as cradle, inner door module, or lift gates, in their vehicles, Figure 1A.6,7 However, in 2004, the amount of Mg inserted in vehicle’s body remained low (1−35 lbs).6 By 2020, the United States Automotive Materials Partnership aims to use 340 lbs of Mg rather than 630 lbs of ferrous and aluminum components, a 290 lbs economy (∼10% of an average car weight). This aggressive goal of substantially increasing the Mg content in the automobile structure is still limited by technical issues such as long-term functional durability in fatigue and impact8 as well as its poor corrosion properties. Artwork created by François Ducharme Equations 1−3 describe the general corrosion of Mg when immersed in an aqueous solution where an oxidation of the INTRODUCTION AND WHAT TO KNOW ABOUT base metal generates Mg2+ (Eo = −2.38 V vs NHE) ions (eq 1) MAGNESIUM CORROSION and a reduction of the solvent (eq 2). Equation 3 describes the major corrosion product formed that precipitates on the Corrosion is defined as the interaction of a material with its material surface, Mg(OH)2 (Ksp = 5.61 × 10−12).9 environment which leads to its oxidation and the reduction of a species in the surrounding environment (H2O, O2, H+) Mg(s) → Mg 2 +(aq) + 2e− (1) resulting in deterioration of the material and its properties.1 It has been estimated that 3−4% of the gross domestic product 2H 2O(aq) + 2e− ⇆ 2OH−(aq) + H 2(aq,g) (2) of each country is directly related to the cost of corrosion.1 For example, in 2010, $1.8 trillion has been spent in the United Mg 2 +(aq) + 2OH−(aq) ⇆ Mg(OH)2 (s) States2 and $46.4 billion in Canada in 20033 to remedy the (3) effects of corrosion. Usual strategies to undertake corrosion Mg alloys can experience accelerated corrosion rates due to involve (1) material replacement, (2) surface treatments, (3) processes such as stress corrosion, fretting corrosion, high application of protective coatings, (4) cathodic protection, and temperature oxidation, pitting corrosion, and of most (5) use of chemical inhibitors, all resulting in additional costs. pertinence to automotive applications, galvanic corrosion.10 In harsh environments, such as coastal regions and in more Galvanic corrosion is defined as an electrochemical reaction northern regions of the globe, higher salt concentration in air significantly influences the corrosion of materials due to the corrosive properties of chlorides. Corrosion protection is thus a
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must be determined. Surface analytical techniques able to track these events on a surface both on the micrometer and millimeter scale with good spatial resolution are therefore required.
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USING SCANNING PROBE TECHNIQUES TO STUDY MAGNESIUM CORROSION Scanning probe techniques (SPTs) are bringing analytical insights and quantitative analyses that are mandatory to tackle some of the biggest challenges in corrosion, from identifying corrosion initiation sites, to predicting propagation, to smart inhibition of the corrosion processes, to proper material selection and lifetime predictions of automotive materials used in service. In the particular case discussed in this Feature, Mg is a very reactive material, which as soon as immersed creates a substantial amount of H2, which affect the natural convection of the solution and in turn, the electrochemical reactivity probed. In addition, the overall analysis time is an important variable because Mg(OH)2 precipitates as the solution pH increases and further alters the electrochemical activity of the material. Throughout this Feature, we will highlight the advantages and challenges faced by SPTs to characterize Mg corrosion, as this remains a complex task. SPTs Quantifying Mg Galvanic Corrosion. Scanning vibrating electrode technique (SVET) or local electrochemical impedance spectroscopy (LEIS) can characterize the potential gradients existing between galvanically coupled dissimilar materials. SVET uses a three electrode setup to measure and map potential gradients generated at an immersed electrochemically active surface (working electrode) through the vibration of an electrode (counter electrode, normally made of Pt−Ir alloy (>15 μm)) in solution, as illustrated in Figure 2A. This technique was initially developed for extracellular measurements on biological samples17 and due to its benefits of spatially resolved processes, SVET was introduced into corrosion science.18−25 SVET can measure current densities as small as 5 μA cm−2 through the oscillation of the tip at a specific vibration amplitude (A in m). A potential drop (ΔE in V) is measured and can be converted to current densities (j in A m−2), assuming knowledge of the solution conductivity (σ in S m−1) (eq 4).
Figure 1. (A) Different Mg vehicle components currently employed in the automotive industry (from left to right, cradle (General Motors Corvette); door inner module (Aston Martin); and lift gate (Volkswagen))6 (Adapted from United States Automotive Materials Partnership Magnesium Vision 2020: A North American Automotive Strategic Vision for Magnesium, 2004); Corrosion mechanisms; (B) galvanic corrosion sets when two materials are placed in electrical contact; (C) microgalvanic corrosion is a localized process where the secondary phases of an alloy is galvanically coupled with the metal matrix initiating its dissolution.
where two dissimilar metals are in electrical contact leading to establishment of an anode and cathode, with the anode corroding preferentially. A general schematic of this corrosion mechanism is presented in Figure 1B. Specifically, Mg sits at the bottom of the electrochemical series11 and when placed in electrical contact with a second metal it will act as an anode (oxidation of Mg (eq 1), right side metal in Figure 1B) while the more noble material (left side metal in Figure 1B) will support the cathodic half reaction, i.e., water reduction (eq 2). Galvanic corrosion, while usually driving the overall corrosion, can occur also on the microscale and is defined as microgalvanic corrosion leading to self-corrosion of the material. Microgalvanic corrosion refers to the formation of galvanic couples between microstructures within a heterogeneous alloy surface, possibly enhancing corrosion of the alloy in a self-destructive process. As illustrated in Figure 1C, Mg alloys commonly have heterogeneous microstructures due to the formation of secondary phases as a result of solubility limitations of alloying elements in Mg solid solution (metal matrix). Many of these phases are more noble than the Mg matrix (α-phase) and can form microgalvanic couples.12 Increasingly detrimental to Mg alloy performance are the presence of impurities (Fe, Cu, Ni), which can further enhance microgalvanic corrosion.13 While these are the general guidelines for Mg corrosion, there is still an important debate in the literature to explain how, where, and which corrosion products or electrochemical fluxes come in play.14 One of the major dissonant points is the negative difference effect, a particular behavior that results in an enhancement of the cathodic reactivity (i.e., evolution of H2) while under anodic polarization.15,16 This leads to localized corrosion events that can cause severe damage to a material, where the causes of initiation and propagation of such events
ΔE (4) A On the basis of its success in monitoring corrosion processes on more stable materials, the SVET has been applied in the investigation of corroding Mg surfaces.26−35 An example of SVET application in Mg corrosion was an AE44 Mg alloy galvanically coupled to an AA6063 Al alloy.36 The Mg alloy (AE44; contains mostly Mg with less than 4 wt % Al and a 4 wt % mixture of rare earths elements such as Nd, La, and Ce), being less noble, will act as the anode in the couple with water reduction reaction occurring at the more noble Al alloy surface (AA6063; contains mostly Al with less than 0.6 wt % silicon carbide, less than 0.9 wt % Mg and less than 0.35 wt % Fe with other trace elements). A potential difference exists between both materials (∼0.2 V), generating a peak current density of 18 A m−2 at the galvanic couple interface, Figure 2B. The observed potential gradient decays with increasing distance from the galvanic interface thus resulting in lower current densities, which can successfully be mapped with SVET. The j = −σ
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Figure 2. (A) SVET setup schematic representation. (B) The initial spatial current density variation obtained from the SVET experiments for AE44AA6063 galvanic couple (squares) vs calculated (full line).36 (Reprinted from Corr. Sci., 52, Deshpande, K.B., Validated numerical modeling of galvanic corrosion for couples: Magnesium alloy (AE44)-mild steel and AE44-aluminum alloy (AA6063) in brine solution, 3514−3522, (2010), with permission from Elsevier). (C) Schema of the 3D simulation domain: (D) experimental SVET current density (dot) and current density calculated in 2D model when the SVET probe is located at 150 μm above from the surface (full line).37 Reprinted from ref 37. Copyright 2012 American Chemical Society.
using profilometry, a moving interface calculated with Faraday’s law (eq 5) was implemented in the predictive numerical model:36,37
galvanic corrosion susceptibility of Mg materials is further enhanced depending on the nature of the second material. Because of its more noble position in the electrochemical series, mild steel (MS; contains Fe with less than 0.25 wt % carbon) will act as a cathode when galvanically coupled to Mg. Figure 2B,D shows an increase of the material interface current densities (up to 43 A m−2) measured when comparing AE44AA6063 (squares in Figure 2B) and AE44-MS (squares in Figure 2D) galvanic couples.36 Investigation of the interaction of dissimilar materials with SVET is imperative to allow development of numerical models to predict lifetime corrosion behaviors. An example of this model development was based on previous work reported by Deshpande36 and was implemented in COMSOL Multiphysics to investigate the galvanic corrosion of an AE44-MS galvanic couple.37 This model takes in consideration contribution from edge and tip-to-substrate effects, which returns, with respect to the experimental values, more accurate corrosion rate values. The experimental potentiodynamic polarization curves of AE44 and MS, recorded separately, were used as boundary conditions in a finite element algorithm solving the Laplace equation for the electric field in the solution domain, Figure 2C. As expected, the corrosion models predict large current density of 55 A m−2 at the galvanic interface (Figure 2D), which corroborates the interfacial current density of 43 A m−2 measured by SVET. In order to translate damages suffered by the surfaces during galvanic corrosion, which were recorded
r=
jM zFρ
(5)
where r is the local corrosion rate (m s−1), j the local current density (A m−2), F is the Faraday constant (96 485.34 C mol−1), M is the atomic mass of Mg (26.82 g mol−1), ρ is the mass density (1820 kg m−3) for the AE44 alloy, and z is the number of electron transferred during the galvanic corrosion process. For the above reasons, SVET remains indispensable for galvanic corrosion measurements due to its ability of expressing surface damages in terms of local corrosion rates that can be addressed by predictive numerical models and compared to bulk corrosion rates. LEIS is another SPT used to investigate corroding surfaces (Figure 3A) pioneered by Isaacs.38 In LEIS, a five electrode setup is used where a sinusoidal voltage perturbation, ΔÈ(ω), is applied between a corroding sample (the working electrode) and a reference electrode. This stimulus is maintained without modifying the reference electrode by passing the current through a separate counter electrode. Independently, a bielectrode, which contains two reference electrodes (separated by a fixed “d” in Figure 3A), is maintained at a constant distance above the working electrode (represented in Figure 3A as “h”). C
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Figure 3. (A) LEIS schematic representation; (B) Nyquist plots of the local electrochemical impedance response of the AZ91 alloy after 2 h immersion in 1 mM Na2SO4 solution at the corrosion potential. The spectra were recorded close to the electrode center at various locations over the alloy.49 (C) LEIS image of the AZ91 alloy after 24 h immersion in 1 mM Na2SO4 solution at the corrosion potential. The map was acquired by moving the probe over the alloy surface at a fixed frequency (100 Hz).49 (Reprinted from Corr. Sci., 51, Galicia, G., Pebere, N., Tribollet, B., Vivier, V., Local and global electrochemical impedances applied to the corrosion behavior of an AZ91 magnesium alloy, 1789−1794, (2009), with permission from Elsevier).
coatings.39−42 In addition, LEIS can help describe charge transfer resistance, diffusion across porous surface deposits, etc., which are frequency dependent. Overall, LEIS is a powerful technique to investigate kinetics and deconvolute the rates of the different electrochemical fluxes that form the basis of all mechanistic analyses. As such, it has been employed in several areas of corrosion research.38,43−48 An example of LEIS capabilities was demonstrated when mapping the corrosion of a cylinder of 99.9% Mg galvanically coupled to a cylindrical layer of pure Al.47 Mg was again assumed to act as the anode. LEIS results show an increase in local admittance (inverse of the local impedance) at the edge of pure Mg and pure Al at a constant frequency of 1 Hz. Higher admittances were correlated to higher current densities (calculated using the Laplace equation for the electric field in the solution domain), leading the authors to conclude that higher corrosion rates were expected at this interface. This was further confirmed by ex situ microscopy that showed enhanced corrosion rates at the contour of the Mg cylinder. In comparison to EIS, LEIS has thus the ability to evaluate the extent of this galvanic couple on the local scale to predict the dissolution kinetics of Al2CuMg particles, found in certain Al alloys. From the results obtained using either SVET or LEIS, it is possible to understand that the geometry of the interface between galvanically coupled materials (linear or cylindrical) will greatly affect the amount of localized corrosion damages. In addition, these techniques can help determine the polarity of a galvanic couple to choose the best material to couple Mg alloys with, in order to prevent major corrosion events. SPTs to Characterize Microgalvanic Corrosion. The high spatial resolution garnered by LEIS (∼1 μm), although dependent on the positioning system, can be used to probe Mg
This bielectrode probes the solution potential distribution over a short distance of d (ΔEprobe(ω)). Consequently, if the solution resistance is known, a measure of the local current densities near the surface of interest is derived. In turn, this allows for quantification of electrochemical processes. The potential difference recorded across the two electrodes forming the bielectrode probe thus enables the local impedance (z(ω)) to be recorded (eq 6). z(ω) =
ΔE(̀ ω) ΔE(̀ ω)d = σ ΔEprobe(ω) j(ω)
(6)
where ΔÈ(ω) represents the alternating potential difference between the working and reference electrode and ΔEprobe(ω) is the potential difference across the two electrodes in the bielectrode. z(ω) represents both an absolute value and phase shift between the current and the potential reliant on several different parameters including reaction kinetics that often can be deconvoluted according to their frequency dependence (ω). The heterogeneous nature of corroding electrodes makes surface reaction kinetics analysis using standard large scale EIS difficult. This is overcome in LEIS since the examined area is greatly reduced and therefore represents better localized corrosion. To assess the heterogeneities of an electrode, which includes passive or highly reactive regions, the LEIS probe is attached to a three axis positioning system that allows the tip to be rastered across the surface at a given frequency (chosen using impedance spectra at known location in order to probe a specific corrosion process, while still permitting the creation of a contrast map depending on regions scanned). Following point-to-point impedance measurements and imaging by scanning electron microscopy (SEM), the impedance maps allow corrosion rates to be extracted, making it very useful in the study of aging and failure of protective D
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Analytical Chemistry alloy corrosion.47,49−51 Figure 3C presents an estimated corrosion rate map obtained from a scan performed at constant frequency over the different phases of a corroding AZ91 alloy (contains less than 9 wt % Al and 1 wt % Zn). The blue regions are representative of lower admittance regions which have been correlated to an ex situ electron micrograph of the alloy and shows that the secondary phase, β-phase (Mg17Al12), is less corroded compared to the α-Mg matrix.49 To associate qualitatively and get a more representative overview of an admittance map in terms of local corrosion rates, the authors performed a local electrochemical impedance measurement over these phases and showed that the time constant at high frequency was 10 times lower for the α-phase in comparison to the β-phase, Figure 3B. The high frequency capacitive loop was associated with the charge transfer resistance, leading the authors to conclude that, in this frequency regime, the α-phase was corroding faster than the β-phase. In summary, LEIS remains an indispensable method to perform a mechanistic study of microgalvanic kinetic processes over an immersed protected or unprotected sample. When LEIS is performed with a fully immersed Mg alloy, long-range coupling between microstructural features and the α-Mg phase occurs. To remove long-range coupling effects, the corrosion behavior of individual microstructures can be investigated through isolation of the features in a small volume of solution using a microelectrochemical cell originally designed by Suter and Bohni.52,53 Many variations of this technique are available, including the scanning droplet cell (SDC) previously applied on other corroding systems.23,54−58 In SDC (Figure 4A), the electrochemical cell is confined into a microcapillary filled with an electrolyte solution that is placed in contact with the substrate material, which serves as the working electrode. Interestingly, in certain designs, the electrolyte solution can also flow through the microcapillary, allowing fresh solution to be brought to the substrate surface.59 The reference and counter or combined reference/counter electrodes are then inserted at the open end of the microcapillary. SDC remains versatile toward electrochemical techniques that can be performed locally such as potentiodynamic polarization or voltammetry to extract local kinetics through accurate corrosion modeling. The wide variety of electrochemical techniques also include local chronoamperometry that can probe ions released during the corrosion process57,60 or even perform local modification of surfaces.61,62 There are several examples where effective removal of longrange coupling has been achieved in Mg corrosion,12,30,63−65 including AZ91 Mg alloy, which presents heterogeneous microstructures (1−5 in Figure 4B).66 Microcapillaries of 5− 10 μm diameter were positioned to record potentiodynamic polarization scans between −2000 and −1300 mV vs Ag|AgCl66 at the numbered locations, Figure 4C. The β phase was reported to be the most corrosion resistant providing the lowest currents in the scans. This shows the ability of SDC to collect electrochemical information from microstructural features without interference from the bulk of the alloy. Compiling the previous information gathered using these different SPTs, Deshpande constructed a microgalvanic bimetallic corrosion numerical predictive tool.67 Bulk electrochemical potentiodynamic polarization curves were obtained over as cast α-Mg (from an AM30 Mg alloy) and β-phase (based on their known chemical compositions) and then used as inputs to solve the Laplace equation of the electric field in the solution domain, as seen in the galvanic couple example
Figure 4. (A) SDC schematic representation; (B) optical micrograph of an AZ91 Mg alloy following surface etching where sites of local potentiodynamic polarization curves have been highlighted (1−5). (C) Local polarization curves recorded at 16.6 mV/s using a 10 μm diameter capillary filled with an electrolyte solution of 0.1 M NaClO4.66 (Reprinted from Corr. Sci., 53, Krawiec, H. and Stanek, S. and Vignal, V. and Lelito, J. and Suchy, J. S., The use of microcapillary techniques to study the corrosion resistance of AZ91 magnesium alloy at the microscale, 3108−3113, (2011), with permission from Elsevier).
listed previously. This produced calculated microgalvanic currents flowing between the different phases. The author then verified that the results of the numerical simulation were in agreement with the current densities measured by SVET. Local corrosion rates are calculated using the measured local current densities (eq 5) that are on the order of 0.45 A m−2, to provide the material withdrawal rate. By extracting numerically a β-phase distribution using an electron micrograph, the material withdrawal rate can be implemented as a moving interface in COMSOL Multiphysics to observe where and how the corrosion initiates and evolves. Again, development of such a model would not be possible without information gained using high-resolution electrochemical probe techniques. Scanning Kelvin probe force microscopy (SKPFM) (represented in Figure 5A) measures the work function difference between a substrate and a reference probe to acquire high-spatial mapping of the surface potential distribution68 (higher resolution is achieved in SKPFM compared to SKP while prone to much more artifacts), which has a strong impact on microgalvanic corrosion. Because of the differences in elemental composition of the tip and the sample, there is a contact potential difference (VCPD) that exists between the two materials as defined by VCPD = E
Φs Φ Φ − t = q q q
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Figure 5. (A) SKPFM schematic and representation of the electronic levels of the tip and sample when disconnected (i), connected (ii), and connected with an applied bias between the tip and sample (iii); (B) Volta potential distribution of an area of interest of an AZ91D Mg alloy. (C) Mg (black curve) and Al (red curve) elemental composition determined using energy dispersive X-ray spectroscopy electron spectroscopy across Line 1. (D) Volta potential magnitude recorded across Line 1.77 (Reproduced by permission of ECS − The Electrochemical Society).
where Φs and Φt are the work function of the sample and tip and q is the elementary charge. When the tip and the sample are in close proximity, an electrostatic force is observed (VCPD in Figure 5A-i). When unconnected, a difference in Fermi levels exists between the tip and sample and as soon as they are connected to one another, the Fermi levels of both materials align (EF in Figure 5A-ii). This creates a flow of electron from the material with the lower work function to the one with the higher work function leading to a charging effect and electrostatic force. To nullify this force, an external bias between the tip and sample is applied (VCPD in Figure 5A-iii referred to the Volta potential or electrostatic potential). In SKPFM, the tip is an atomic force microscopy cantilever, oscillating with respect to the sample in a noncontact fashion. Simultaneous collection of topography and Volta potential remains experimentally challenging. Normally, to acquire Volta potential maps, the topography is first acquired using the tapping mode and the reflection of a laser at the back of the cantilever. A second pass of the tip is then acquired where the electrostatic force is nullified by applying VCPD between the tip and sample. In principle, 1 nm resolution can be achieved using this technique. SKPFM is a powerful tool in corrosion and materials research as it can provide an ex situ approximation of
surface potential distribution between different regions of a heterogeneous microstructure, which could lead to microgalvanic or localized corrosion attack, and thus has been widely applied in the Mg corrosion literature.30,69−80 Figure 5B is the representative Volta potential map recorded across an AZ91 Mg alloy in air. Figure 5C presents the Al and Mg distribution (red and black curve, respectively) recorded using energy-dispersive X-ray spectroscopy across line 1 of Figure 5B. Figure 5D shows a Volta potential line scan (recorded across line 1) measured over different phases and showed a direct correlation with their respective chemical composition. The higher the Al content was, the less negative the Volta potentials were. Similar trends between SKP and SKPFM were recorded where values of ∼−700 mV and −900 mV vs SHE for the AlMn intermetallics precipitates and βphase, respectively, have been measured.76,77 To link the Volta potentials measured in air with the in situ corrosion potentials, the authors performed a Volta potentials calibration curve of pure metals (i.e., Mg, Al, Fe, etc.) as a function of their electrochemical corrosion potentials, in an aqueous solution, to extrapolate the corrosion potentials of these phases.76 The Volta potential measured by SKPFM was thus transferred to corrosion potential values with this calibration curve in order to F
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Analytical Chemistry obtain a local corrosion potential map useful for local corrosion characterization. Consequently, AlMn intermetallics and βphase are expected to corrode less in comparison to the αphase Mg-rich matrix and be sites to sustain the cathodic half reaction (eq 2).76 SPTs Used for Spatiotemporal Tracking of Chemical Fluxes at the Corroding Interface. Scanning electrochemical microscopy (SECM) is a highly relevant electroanalytical method to quantify local chemical fluxes and map the heterogeneous electrochemical activity of corroding surfaces. This surface analytical technique was first reported in 198981 and has the ability of acquiring high-resolution in situ images where topographical and electrochemical reactivity can be decoupled. The standard SECM experimental setup involves four electrodes (two working electrodes, microelectrode probe or ME and substrate or sample; counter electrode, CE; reference electrode, REF) controlled by a bipotentiostat. SECM measures electrochemical fluxes in solution when a microelectrode is placed in close proximity to a sample. In corrosion studies, SECM can use indirect redox mediators in feedback mode or direct detection of ionic or molecular fluxes from the corroding interface. A redox mediator is a species that exhibits stable and reversible electrochemical oxidation/reduction at a standard potential. Common mediators used in aqueous based SECM studies are hexaammineruthenium(III) chloride and ferrocenemethanol (FcMeOH). During the electrolysis of the dissolved redox mediator in a high ionic strength solution, the ME, often made with Pt, behaves as a local electrochemical probe. In quiescent solutions, steady-state currents are established within the millisecond time scale.82 Far away from the sample surface, the faradaic current recorded at the ME is driven by diffusion processes according to eq 9.83 iss = 4βnFDC*a
Figure 6. SECM schematic representations: (A) negative feedback, (B) positive feedback, (C) SG/TC, (D) potentiometric mode. “O” and “R” represent the oxidized and reduced form of the redox mediator. [I+] is the ionic species probed in potentiometric mode.
For example, corrosion product deposition affects the SECM feedback regeneration of the redox mediator FcMeOH when an AM60 Mg alloy (contains less than 6 wt % Al and 0.5 wt % Mn) is immersed in a NaCl solution. Using the SECM feedback, Liu et al. proposed that upon initial immersion, the FcMeOH was not being regenerated from the Mg alloy surface due to the presence of an oxide (MgO) protective film.87 As immersion times increased, the authors proposed that the Cl− finally breaks the protective film (referred to as active spots) and allows the redox mediator oxidized at the tip to be regenerated. This example shows the ability of SECM to map locally activated events on a corroding surface. In SECM, direct detection of chemical fluxes from the corroding interface is achieved using sample-generation/tipcollection mode (SG/TC). Alternatively, local corrosion initiation can also be studied using tip-generation/substratecollection (TG/SC). As illustrated in Figure 6C, the ME collects the generated species diffusing from the corroding interface. Before carrying out a SECM SG/TC (or TG/SC) experiment, species are not yet generated, resulting in a negligible measured current (when the ME tip is biased at the species oxidation/reduction potential). Molecular H2 is an example of a species generated in corrosion processes that helps visualizing the evolution of cathodic sites on the millimeter scale using SVET.32 To improve the spatial resolution, H2 can be oxidized at a Pt ME tip using the SG/TC mode of SECM.86,88−90 Recently, this strategy was used to observe the rapid time-dependence behavior of an AM50 die cast Mg alloy (contains less than 5 wt % Al and 0.5 wt % Mn) and identify cathodic sites.86,88 To observe this variation of molecular H2 fluxes evolution, repetitive approach curves over the same position were performed. Within this contribution, COMSOL Multiphysics was used to numerically simulate the molecular H2 diffusion and its collection at the polarized ME tip depending on the fluxes and size of the site responsible for evolving molecular H2. It was shown that molecular H2 fluxes on the order of 7 mmol m−2 s−1 with an active site size of 15 μm were initially measured.88 The molecular H2 fluxes then decreased with time while the active site size increases and is associated with the
(9)
where iss is the steady-state current, β a geometric factor, n the number of electrons involved in the electrochemical process, F is the Faraday constant, D is the diffusion coefficient of the redox mediator, C* is the redox mediator concentration, and a is the radius of the ME active surface. The ME current becomes distance sensitive when the tip-to-sample distance is within a few ME radii multiples (
Surface Analytical Methods Applied to Magnesium Corrosion Understanding magnesium alloy corrosion is of primary concern, and scanning probe techniques are becoming key analytical characterization methods for that purpose. This Feature presents recent trends in this field as the progressive substitution of steel and aluminum car components by magnesium alloys to reduce the overall weight of vehicles is an irreversible trend. Philippe Dauphin-Ducharme and Janine Mauzeroll* Laboratory for Electrochemical Reactive Imaging and Detection of Biological Systems, Department of Chemistry, McGill University, Montreal, Quebec Canada, H3A 0G4 requirement of many industries including the automotive industry. In addition to these requirements, households can invest, according to the automotive protective agency of Canada,4 up to $1000 during a vehicle lifetime, in corrosion prevention treatments. Moving forward, corrosion will still remain an important challenge as the automotive industry is trying to reduce the vehicle mass to improve fuel efficiency and the maximum range of electric vehicles. Recent statistics reported that a 10% decrease in vehicle mass would result in a 6−8% increase in fuel efficiency for internal combustion engines.5 Strong candidates for inclusion in automotive design and achieving weight reduction are fibers, polymers, and lightweight alloys.6 Mg and its alloys provide an excellent balance in terms of mechanical and lightweight properties. Automotive manufacturers have introduced Mg components, such as cradle, inner door module, or lift gates, in their vehicles, Figure 1A.6,7 However, in 2004, the amount of Mg inserted in vehicle’s body remained low (1−35 lbs).6 By 2020, the United States Automotive Materials Partnership aims to use 340 lbs of Mg rather than 630 lbs of ferrous and aluminum components, a 290 lbs economy (∼10% of an average car weight). This aggressive goal of substantially increasing the Mg content in the automobile structure is still limited by technical issues such as long-term functional durability in fatigue and impact8 as well as its poor corrosion properties. Artwork created by François Ducharme Equations 1−3 describe the general corrosion of Mg when immersed in an aqueous solution where an oxidation of the INTRODUCTION AND WHAT TO KNOW ABOUT base metal generates Mg2+ (Eo = −2.38 V vs NHE) ions (eq 1) MAGNESIUM CORROSION and a reduction of the solvent (eq 2). Equation 3 describes the major corrosion product formed that precipitates on the Corrosion is defined as the interaction of a material with its material surface, Mg(OH)2 (Ksp = 5.61 × 10−12).9 environment which leads to its oxidation and the reduction of a species in the surrounding environment (H2O, O2, H+) Mg(s) → Mg 2 +(aq) + 2e− (1) resulting in deterioration of the material and its properties.1 It has been estimated that 3−4% of the gross domestic product 2H 2O(aq) + 2e− ⇆ 2OH−(aq) + H 2(aq,g) (2) of each country is directly related to the cost of corrosion.1 For example, in 2010, $1.8 trillion has been spent in the United Mg 2 +(aq) + 2OH−(aq) ⇆ Mg(OH)2 (s) States2 and $46.4 billion in Canada in 20033 to remedy the (3) effects of corrosion. Usual strategies to undertake corrosion Mg alloys can experience accelerated corrosion rates due to involve (1) material replacement, (2) surface treatments, (3) processes such as stress corrosion, fretting corrosion, high application of protective coatings, (4) cathodic protection, and temperature oxidation, pitting corrosion, and of most (5) use of chemical inhibitors, all resulting in additional costs. pertinence to automotive applications, galvanic corrosion.10 In harsh environments, such as coastal regions and in more Galvanic corrosion is defined as an electrochemical reaction northern regions of the globe, higher salt concentration in air significantly influences the corrosion of materials due to the corrosive properties of chlorides. Corrosion protection is thus a
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must be determined. Surface analytical techniques able to track these events on a surface both on the micrometer and millimeter scale with good spatial resolution are therefore required.
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USING SCANNING PROBE TECHNIQUES TO STUDY MAGNESIUM CORROSION Scanning probe techniques (SPTs) are bringing analytical insights and quantitative analyses that are mandatory to tackle some of the biggest challenges in corrosion, from identifying corrosion initiation sites, to predicting propagation, to smart inhibition of the corrosion processes, to proper material selection and lifetime predictions of automotive materials used in service. In the particular case discussed in this Feature, Mg is a very reactive material, which as soon as immersed creates a substantial amount of H2, which affect the natural convection of the solution and in turn, the electrochemical reactivity probed. In addition, the overall analysis time is an important variable because Mg(OH)2 precipitates as the solution pH increases and further alters the electrochemical activity of the material. Throughout this Feature, we will highlight the advantages and challenges faced by SPTs to characterize Mg corrosion, as this remains a complex task. SPTs Quantifying Mg Galvanic Corrosion. Scanning vibrating electrode technique (SVET) or local electrochemical impedance spectroscopy (LEIS) can characterize the potential gradients existing between galvanically coupled dissimilar materials. SVET uses a three electrode setup to measure and map potential gradients generated at an immersed electrochemically active surface (working electrode) through the vibration of an electrode (counter electrode, normally made of Pt−Ir alloy (>15 μm)) in solution, as illustrated in Figure 2A. This technique was initially developed for extracellular measurements on biological samples17 and due to its benefits of spatially resolved processes, SVET was introduced into corrosion science.18−25 SVET can measure current densities as small as 5 μA cm−2 through the oscillation of the tip at a specific vibration amplitude (A in m). A potential drop (ΔE in V) is measured and can be converted to current densities (j in A m−2), assuming knowledge of the solution conductivity (σ in S m−1) (eq 4).
Figure 1. (A) Different Mg vehicle components currently employed in the automotive industry (from left to right, cradle (General Motors Corvette); door inner module (Aston Martin); and lift gate (Volkswagen))6 (Adapted from United States Automotive Materials Partnership Magnesium Vision 2020: A North American Automotive Strategic Vision for Magnesium, 2004); Corrosion mechanisms; (B) galvanic corrosion sets when two materials are placed in electrical contact; (C) microgalvanic corrosion is a localized process where the secondary phases of an alloy is galvanically coupled with the metal matrix initiating its dissolution.
where two dissimilar metals are in electrical contact leading to establishment of an anode and cathode, with the anode corroding preferentially. A general schematic of this corrosion mechanism is presented in Figure 1B. Specifically, Mg sits at the bottom of the electrochemical series11 and when placed in electrical contact with a second metal it will act as an anode (oxidation of Mg (eq 1), right side metal in Figure 1B) while the more noble material (left side metal in Figure 1B) will support the cathodic half reaction, i.e., water reduction (eq 2). Galvanic corrosion, while usually driving the overall corrosion, can occur also on the microscale and is defined as microgalvanic corrosion leading to self-corrosion of the material. Microgalvanic corrosion refers to the formation of galvanic couples between microstructures within a heterogeneous alloy surface, possibly enhancing corrosion of the alloy in a self-destructive process. As illustrated in Figure 1C, Mg alloys commonly have heterogeneous microstructures due to the formation of secondary phases as a result of solubility limitations of alloying elements in Mg solid solution (metal matrix). Many of these phases are more noble than the Mg matrix (α-phase) and can form microgalvanic couples.12 Increasingly detrimental to Mg alloy performance are the presence of impurities (Fe, Cu, Ni), which can further enhance microgalvanic corrosion.13 While these are the general guidelines for Mg corrosion, there is still an important debate in the literature to explain how, where, and which corrosion products or electrochemical fluxes come in play.14 One of the major dissonant points is the negative difference effect, a particular behavior that results in an enhancement of the cathodic reactivity (i.e., evolution of H2) while under anodic polarization.15,16 This leads to localized corrosion events that can cause severe damage to a material, where the causes of initiation and propagation of such events
ΔE (4) A On the basis of its success in monitoring corrosion processes on more stable materials, the SVET has been applied in the investigation of corroding Mg surfaces.26−35 An example of SVET application in Mg corrosion was an AE44 Mg alloy galvanically coupled to an AA6063 Al alloy.36 The Mg alloy (AE44; contains mostly Mg with less than 4 wt % Al and a 4 wt % mixture of rare earths elements such as Nd, La, and Ce), being less noble, will act as the anode in the couple with water reduction reaction occurring at the more noble Al alloy surface (AA6063; contains mostly Al with less than 0.6 wt % silicon carbide, less than 0.9 wt % Mg and less than 0.35 wt % Fe with other trace elements). A potential difference exists between both materials (∼0.2 V), generating a peak current density of 18 A m−2 at the galvanic couple interface, Figure 2B. The observed potential gradient decays with increasing distance from the galvanic interface thus resulting in lower current densities, which can successfully be mapped with SVET. The j = −σ
B
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Figure 2. (A) SVET setup schematic representation. (B) The initial spatial current density variation obtained from the SVET experiments for AE44AA6063 galvanic couple (squares) vs calculated (full line).36 (Reprinted from Corr. Sci., 52, Deshpande, K.B., Validated numerical modeling of galvanic corrosion for couples: Magnesium alloy (AE44)-mild steel and AE44-aluminum alloy (AA6063) in brine solution, 3514−3522, (2010), with permission from Elsevier). (C) Schema of the 3D simulation domain: (D) experimental SVET current density (dot) and current density calculated in 2D model when the SVET probe is located at 150 μm above from the surface (full line).37 Reprinted from ref 37. Copyright 2012 American Chemical Society.
using profilometry, a moving interface calculated with Faraday’s law (eq 5) was implemented in the predictive numerical model:36,37
galvanic corrosion susceptibility of Mg materials is further enhanced depending on the nature of the second material. Because of its more noble position in the electrochemical series, mild steel (MS; contains Fe with less than 0.25 wt % carbon) will act as a cathode when galvanically coupled to Mg. Figure 2B,D shows an increase of the material interface current densities (up to 43 A m−2) measured when comparing AE44AA6063 (squares in Figure 2B) and AE44-MS (squares in Figure 2D) galvanic couples.36 Investigation of the interaction of dissimilar materials with SVET is imperative to allow development of numerical models to predict lifetime corrosion behaviors. An example of this model development was based on previous work reported by Deshpande36 and was implemented in COMSOL Multiphysics to investigate the galvanic corrosion of an AE44-MS galvanic couple.37 This model takes in consideration contribution from edge and tip-to-substrate effects, which returns, with respect to the experimental values, more accurate corrosion rate values. The experimental potentiodynamic polarization curves of AE44 and MS, recorded separately, were used as boundary conditions in a finite element algorithm solving the Laplace equation for the electric field in the solution domain, Figure 2C. As expected, the corrosion models predict large current density of 55 A m−2 at the galvanic interface (Figure 2D), which corroborates the interfacial current density of 43 A m−2 measured by SVET. In order to translate damages suffered by the surfaces during galvanic corrosion, which were recorded
r=
jM zFρ
(5)
where r is the local corrosion rate (m s−1), j the local current density (A m−2), F is the Faraday constant (96 485.34 C mol−1), M is the atomic mass of Mg (26.82 g mol−1), ρ is the mass density (1820 kg m−3) for the AE44 alloy, and z is the number of electron transferred during the galvanic corrosion process. For the above reasons, SVET remains indispensable for galvanic corrosion measurements due to its ability of expressing surface damages in terms of local corrosion rates that can be addressed by predictive numerical models and compared to bulk corrosion rates. LEIS is another SPT used to investigate corroding surfaces (Figure 3A) pioneered by Isaacs.38 In LEIS, a five electrode setup is used where a sinusoidal voltage perturbation, ΔÈ(ω), is applied between a corroding sample (the working electrode) and a reference electrode. This stimulus is maintained without modifying the reference electrode by passing the current through a separate counter electrode. Independently, a bielectrode, which contains two reference electrodes (separated by a fixed “d” in Figure 3A), is maintained at a constant distance above the working electrode (represented in Figure 3A as “h”). C
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Figure 3. (A) LEIS schematic representation; (B) Nyquist plots of the local electrochemical impedance response of the AZ91 alloy after 2 h immersion in 1 mM Na2SO4 solution at the corrosion potential. The spectra were recorded close to the electrode center at various locations over the alloy.49 (C) LEIS image of the AZ91 alloy after 24 h immersion in 1 mM Na2SO4 solution at the corrosion potential. The map was acquired by moving the probe over the alloy surface at a fixed frequency (100 Hz).49 (Reprinted from Corr. Sci., 51, Galicia, G., Pebere, N., Tribollet, B., Vivier, V., Local and global electrochemical impedances applied to the corrosion behavior of an AZ91 magnesium alloy, 1789−1794, (2009), with permission from Elsevier).
coatings.39−42 In addition, LEIS can help describe charge transfer resistance, diffusion across porous surface deposits, etc., which are frequency dependent. Overall, LEIS is a powerful technique to investigate kinetics and deconvolute the rates of the different electrochemical fluxes that form the basis of all mechanistic analyses. As such, it has been employed in several areas of corrosion research.38,43−48 An example of LEIS capabilities was demonstrated when mapping the corrosion of a cylinder of 99.9% Mg galvanically coupled to a cylindrical layer of pure Al.47 Mg was again assumed to act as the anode. LEIS results show an increase in local admittance (inverse of the local impedance) at the edge of pure Mg and pure Al at a constant frequency of 1 Hz. Higher admittances were correlated to higher current densities (calculated using the Laplace equation for the electric field in the solution domain), leading the authors to conclude that higher corrosion rates were expected at this interface. This was further confirmed by ex situ microscopy that showed enhanced corrosion rates at the contour of the Mg cylinder. In comparison to EIS, LEIS has thus the ability to evaluate the extent of this galvanic couple on the local scale to predict the dissolution kinetics of Al2CuMg particles, found in certain Al alloys. From the results obtained using either SVET or LEIS, it is possible to understand that the geometry of the interface between galvanically coupled materials (linear or cylindrical) will greatly affect the amount of localized corrosion damages. In addition, these techniques can help determine the polarity of a galvanic couple to choose the best material to couple Mg alloys with, in order to prevent major corrosion events. SPTs to Characterize Microgalvanic Corrosion. The high spatial resolution garnered by LEIS (∼1 μm), although dependent on the positioning system, can be used to probe Mg
This bielectrode probes the solution potential distribution over a short distance of d (ΔEprobe(ω)). Consequently, if the solution resistance is known, a measure of the local current densities near the surface of interest is derived. In turn, this allows for quantification of electrochemical processes. The potential difference recorded across the two electrodes forming the bielectrode probe thus enables the local impedance (z(ω)) to be recorded (eq 6). z(ω) =
ΔE(̀ ω) ΔE(̀ ω)d = σ ΔEprobe(ω) j(ω)
(6)
where ΔÈ(ω) represents the alternating potential difference between the working and reference electrode and ΔEprobe(ω) is the potential difference across the two electrodes in the bielectrode. z(ω) represents both an absolute value and phase shift between the current and the potential reliant on several different parameters including reaction kinetics that often can be deconvoluted according to their frequency dependence (ω). The heterogeneous nature of corroding electrodes makes surface reaction kinetics analysis using standard large scale EIS difficult. This is overcome in LEIS since the examined area is greatly reduced and therefore represents better localized corrosion. To assess the heterogeneities of an electrode, which includes passive or highly reactive regions, the LEIS probe is attached to a three axis positioning system that allows the tip to be rastered across the surface at a given frequency (chosen using impedance spectra at known location in order to probe a specific corrosion process, while still permitting the creation of a contrast map depending on regions scanned). Following point-to-point impedance measurements and imaging by scanning electron microscopy (SEM), the impedance maps allow corrosion rates to be extracted, making it very useful in the study of aging and failure of protective D
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Analytical Chemistry alloy corrosion.47,49−51 Figure 3C presents an estimated corrosion rate map obtained from a scan performed at constant frequency over the different phases of a corroding AZ91 alloy (contains less than 9 wt % Al and 1 wt % Zn). The blue regions are representative of lower admittance regions which have been correlated to an ex situ electron micrograph of the alloy and shows that the secondary phase, β-phase (Mg17Al12), is less corroded compared to the α-Mg matrix.49 To associate qualitatively and get a more representative overview of an admittance map in terms of local corrosion rates, the authors performed a local electrochemical impedance measurement over these phases and showed that the time constant at high frequency was 10 times lower for the α-phase in comparison to the β-phase, Figure 3B. The high frequency capacitive loop was associated with the charge transfer resistance, leading the authors to conclude that, in this frequency regime, the α-phase was corroding faster than the β-phase. In summary, LEIS remains an indispensable method to perform a mechanistic study of microgalvanic kinetic processes over an immersed protected or unprotected sample. When LEIS is performed with a fully immersed Mg alloy, long-range coupling between microstructural features and the α-Mg phase occurs. To remove long-range coupling effects, the corrosion behavior of individual microstructures can be investigated through isolation of the features in a small volume of solution using a microelectrochemical cell originally designed by Suter and Bohni.52,53 Many variations of this technique are available, including the scanning droplet cell (SDC) previously applied on other corroding systems.23,54−58 In SDC (Figure 4A), the electrochemical cell is confined into a microcapillary filled with an electrolyte solution that is placed in contact with the substrate material, which serves as the working electrode. Interestingly, in certain designs, the electrolyte solution can also flow through the microcapillary, allowing fresh solution to be brought to the substrate surface.59 The reference and counter or combined reference/counter electrodes are then inserted at the open end of the microcapillary. SDC remains versatile toward electrochemical techniques that can be performed locally such as potentiodynamic polarization or voltammetry to extract local kinetics through accurate corrosion modeling. The wide variety of electrochemical techniques also include local chronoamperometry that can probe ions released during the corrosion process57,60 or even perform local modification of surfaces.61,62 There are several examples where effective removal of longrange coupling has been achieved in Mg corrosion,12,30,63−65 including AZ91 Mg alloy, which presents heterogeneous microstructures (1−5 in Figure 4B).66 Microcapillaries of 5− 10 μm diameter were positioned to record potentiodynamic polarization scans between −2000 and −1300 mV vs Ag|AgCl66 at the numbered locations, Figure 4C. The β phase was reported to be the most corrosion resistant providing the lowest currents in the scans. This shows the ability of SDC to collect electrochemical information from microstructural features without interference from the bulk of the alloy. Compiling the previous information gathered using these different SPTs, Deshpande constructed a microgalvanic bimetallic corrosion numerical predictive tool.67 Bulk electrochemical potentiodynamic polarization curves were obtained over as cast α-Mg (from an AM30 Mg alloy) and β-phase (based on their known chemical compositions) and then used as inputs to solve the Laplace equation of the electric field in the solution domain, as seen in the galvanic couple example
Figure 4. (A) SDC schematic representation; (B) optical micrograph of an AZ91 Mg alloy following surface etching where sites of local potentiodynamic polarization curves have been highlighted (1−5). (C) Local polarization curves recorded at 16.6 mV/s using a 10 μm diameter capillary filled with an electrolyte solution of 0.1 M NaClO4.66 (Reprinted from Corr. Sci., 53, Krawiec, H. and Stanek, S. and Vignal, V. and Lelito, J. and Suchy, J. S., The use of microcapillary techniques to study the corrosion resistance of AZ91 magnesium alloy at the microscale, 3108−3113, (2011), with permission from Elsevier).
listed previously. This produced calculated microgalvanic currents flowing between the different phases. The author then verified that the results of the numerical simulation were in agreement with the current densities measured by SVET. Local corrosion rates are calculated using the measured local current densities (eq 5) that are on the order of 0.45 A m−2, to provide the material withdrawal rate. By extracting numerically a β-phase distribution using an electron micrograph, the material withdrawal rate can be implemented as a moving interface in COMSOL Multiphysics to observe where and how the corrosion initiates and evolves. Again, development of such a model would not be possible without information gained using high-resolution electrochemical probe techniques. Scanning Kelvin probe force microscopy (SKPFM) (represented in Figure 5A) measures the work function difference between a substrate and a reference probe to acquire high-spatial mapping of the surface potential distribution68 (higher resolution is achieved in SKPFM compared to SKP while prone to much more artifacts), which has a strong impact on microgalvanic corrosion. Because of the differences in elemental composition of the tip and the sample, there is a contact potential difference (VCPD) that exists between the two materials as defined by VCPD = E
Φs Φ Φ − t = q q q
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Figure 5. (A) SKPFM schematic and representation of the electronic levels of the tip and sample when disconnected (i), connected (ii), and connected with an applied bias between the tip and sample (iii); (B) Volta potential distribution of an area of interest of an AZ91D Mg alloy. (C) Mg (black curve) and Al (red curve) elemental composition determined using energy dispersive X-ray spectroscopy electron spectroscopy across Line 1. (D) Volta potential magnitude recorded across Line 1.77 (Reproduced by permission of ECS − The Electrochemical Society).
where Φs and Φt are the work function of the sample and tip and q is the elementary charge. When the tip and the sample are in close proximity, an electrostatic force is observed (VCPD in Figure 5A-i). When unconnected, a difference in Fermi levels exists between the tip and sample and as soon as they are connected to one another, the Fermi levels of both materials align (EF in Figure 5A-ii). This creates a flow of electron from the material with the lower work function to the one with the higher work function leading to a charging effect and electrostatic force. To nullify this force, an external bias between the tip and sample is applied (VCPD in Figure 5A-iii referred to the Volta potential or electrostatic potential). In SKPFM, the tip is an atomic force microscopy cantilever, oscillating with respect to the sample in a noncontact fashion. Simultaneous collection of topography and Volta potential remains experimentally challenging. Normally, to acquire Volta potential maps, the topography is first acquired using the tapping mode and the reflection of a laser at the back of the cantilever. A second pass of the tip is then acquired where the electrostatic force is nullified by applying VCPD between the tip and sample. In principle, 1 nm resolution can be achieved using this technique. SKPFM is a powerful tool in corrosion and materials research as it can provide an ex situ approximation of
surface potential distribution between different regions of a heterogeneous microstructure, which could lead to microgalvanic or localized corrosion attack, and thus has been widely applied in the Mg corrosion literature.30,69−80 Figure 5B is the representative Volta potential map recorded across an AZ91 Mg alloy in air. Figure 5C presents the Al and Mg distribution (red and black curve, respectively) recorded using energy-dispersive X-ray spectroscopy across line 1 of Figure 5B. Figure 5D shows a Volta potential line scan (recorded across line 1) measured over different phases and showed a direct correlation with their respective chemical composition. The higher the Al content was, the less negative the Volta potentials were. Similar trends between SKP and SKPFM were recorded where values of ∼−700 mV and −900 mV vs SHE for the AlMn intermetallics precipitates and βphase, respectively, have been measured.76,77 To link the Volta potentials measured in air with the in situ corrosion potentials, the authors performed a Volta potentials calibration curve of pure metals (i.e., Mg, Al, Fe, etc.) as a function of their electrochemical corrosion potentials, in an aqueous solution, to extrapolate the corrosion potentials of these phases.76 The Volta potential measured by SKPFM was thus transferred to corrosion potential values with this calibration curve in order to F
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Analytical Chemistry obtain a local corrosion potential map useful for local corrosion characterization. Consequently, AlMn intermetallics and βphase are expected to corrode less in comparison to the αphase Mg-rich matrix and be sites to sustain the cathodic half reaction (eq 2).76 SPTs Used for Spatiotemporal Tracking of Chemical Fluxes at the Corroding Interface. Scanning electrochemical microscopy (SECM) is a highly relevant electroanalytical method to quantify local chemical fluxes and map the heterogeneous electrochemical activity of corroding surfaces. This surface analytical technique was first reported in 198981 and has the ability of acquiring high-resolution in situ images where topographical and electrochemical reactivity can be decoupled. The standard SECM experimental setup involves four electrodes (two working electrodes, microelectrode probe or ME and substrate or sample; counter electrode, CE; reference electrode, REF) controlled by a bipotentiostat. SECM measures electrochemical fluxes in solution when a microelectrode is placed in close proximity to a sample. In corrosion studies, SECM can use indirect redox mediators in feedback mode or direct detection of ionic or molecular fluxes from the corroding interface. A redox mediator is a species that exhibits stable and reversible electrochemical oxidation/reduction at a standard potential. Common mediators used in aqueous based SECM studies are hexaammineruthenium(III) chloride and ferrocenemethanol (FcMeOH). During the electrolysis of the dissolved redox mediator in a high ionic strength solution, the ME, often made with Pt, behaves as a local electrochemical probe. In quiescent solutions, steady-state currents are established within the millisecond time scale.82 Far away from the sample surface, the faradaic current recorded at the ME is driven by diffusion processes according to eq 9.83 iss = 4βnFDC*a
Figure 6. SECM schematic representations: (A) negative feedback, (B) positive feedback, (C) SG/TC, (D) potentiometric mode. “O” and “R” represent the oxidized and reduced form of the redox mediator. [I+] is the ionic species probed in potentiometric mode.
For example, corrosion product deposition affects the SECM feedback regeneration of the redox mediator FcMeOH when an AM60 Mg alloy (contains less than 6 wt % Al and 0.5 wt % Mn) is immersed in a NaCl solution. Using the SECM feedback, Liu et al. proposed that upon initial immersion, the FcMeOH was not being regenerated from the Mg alloy surface due to the presence of an oxide (MgO) protective film.87 As immersion times increased, the authors proposed that the Cl− finally breaks the protective film (referred to as active spots) and allows the redox mediator oxidized at the tip to be regenerated. This example shows the ability of SECM to map locally activated events on a corroding surface. In SECM, direct detection of chemical fluxes from the corroding interface is achieved using sample-generation/tipcollection mode (SG/TC). Alternatively, local corrosion initiation can also be studied using tip-generation/substratecollection (TG/SC). As illustrated in Figure 6C, the ME collects the generated species diffusing from the corroding interface. Before carrying out a SECM SG/TC (or TG/SC) experiment, species are not yet generated, resulting in a negligible measured current (when the ME tip is biased at the species oxidation/reduction potential). Molecular H2 is an example of a species generated in corrosion processes that helps visualizing the evolution of cathodic sites on the millimeter scale using SVET.32 To improve the spatial resolution, H2 can be oxidized at a Pt ME tip using the SG/TC mode of SECM.86,88−90 Recently, this strategy was used to observe the rapid time-dependence behavior of an AM50 die cast Mg alloy (contains less than 5 wt % Al and 0.5 wt % Mn) and identify cathodic sites.86,88 To observe this variation of molecular H2 fluxes evolution, repetitive approach curves over the same position were performed. Within this contribution, COMSOL Multiphysics was used to numerically simulate the molecular H2 diffusion and its collection at the polarized ME tip depending on the fluxes and size of the site responsible for evolving molecular H2. It was shown that molecular H2 fluxes on the order of 7 mmol m−2 s−1 with an active site size of 15 μm were initially measured.88 The molecular H2 fluxes then decreased with time while the active site size increases and is associated with the
(9)
where iss is the steady-state current, β a geometric factor, n the number of electrons involved in the electrochemical process, F is the Faraday constant, D is the diffusion coefficient of the redox mediator, C* is the redox mediator concentration, and a is the radius of the ME active surface. The ME current becomes distance sensitive when the tip-to-sample distance is within a few ME radii multiples (
