INVITED CRITICAL REVIEW

Atomic Emission Spectroelectrochemistry: Real-Time Rate Measurements of Dissolution, Corrosion, and Passivation

Kevin Ogle‡ Downloaded from http://meridian.allenpress.com/corrosion/article-pdf/75/12/1398/2656216/3336.pdf by guest on 29 September 2021

Atomic emission spectroelectrochemistry (AESEC) is a relatively novel technique that gives real-time elemental dissolution rates for a material/electrolyte combination, either reacting spontaneously or with electrochemical polarization. This methodology gives direct insight into questions such as how specific elements of an alloy interact with one another, or how specific additives in a surface treatment solution will affect different alloying elements or different phases. This paper discusses AESEC instrumentation and presents the basic quantitative relationships between the electrochemical and spectroscopic measurements. A wide range of applications are used to illustrate these relationships including the surface pretreatment of aluminum alloys (etching and deoxidation) and the passivation of Fe-Cr and Ni-Cr alloys. The focus is on the use of in-line inductively coupled plasma atomic emission spectroscopy (ICP-AES), although a brief discussion of similar techniques using in-line inductively coupled mass spectroscopy (ICP-MS) is included.

KEY WORDS: aluminum alloy, atomic emission spectroelectrochemistry (AESEC), copper, corrosion rate, inductively coupled mass spectroscopy (ICP-MS), inductively coupled plasma atomic emission spectroscopy (ICP-AES), molybdenum, nickel alloys, passivation, stainless steels

INTRODUCTION spontaneous reactions the changes in the electrochemical potential may be correlated with elemental reactivity and the queous metallic corrosion is an electrochemical phe- total faradaic current of conventional electrochemistry may be Anomenon and much of the theory and experimental tech- decoupled into partial dissolution reactions. In many situations, niques for predicting and assessing corrosion are based on the evolution of surface composition may be determined in the the application of electrochemical concepts and techniques (for form of elemental enrichment as oxides or dealloyed metallic example, see Kelly, et al.).1 Electrochemical measurements films by application of a mass and charge balance to the provide rich and meaningful kinetic information on the underlying AESEC data. faradaic processes, but are often insufficient to identify the Interest in AESEC has increased in recent years and chemical mechanisms of these processes, limiting their utility as related techniques have been developed. The objective of this predictive tools. As a complement to conventional electro- article is to serve as a basic tutorial in AESEC methodology. chemical information, we would like to know how do the individual The instrumentation is discussed and the basic quantitative elemental constituents of a material react with the environ- relationships between the electrochemical and spectroscopic ment? Can we identify and quantify the specific fate of each measurements are presented, illustrated with numerous recent element? What are the stoichiometries of dissolution, film examples. Although the focus is on the AESEC technique as formation, and electron exchange? Do alloy components dis- developed in the author’s laboratory, many of the principles will solve selectively, leaving behind other components in the form apply to other in-line techniques such as electrochemistry— of oxide films or dealloyed metallic layers? Can non-faradaic inductively coupled plasma mass spectrometry (ICP-MS). corrosion processes such as oxide dissolution or the release of intermetallic particles or metallic grains be detected and quantified? 1.1 | Brief History These and other questions may be addressed by a The term spectroelectrochemistry was first coined by relatively new technique, atomic emission spectroelectrochem- Kuwana2 to describe the coupling of electrochemistry with istry (AESEC). This technique involves coupling an electro- UV-visible spectroscopic chemical analysis of the electrolyte chemical flow cell to a downstream inductively coupled plasma to identify and quantify electrogenerated species in an optically atomic emission spectrometer (ICP-AES, also referred to as transparent thin layer cell (OTTLE). In this way, the transient optical emission spectroscopy or ICP-OES), so that the con- electrochemical measurements of potential and current (E, i) centrations of dissolved species may be followed as a function could be related to concentration transients in solution, of time yielding a direct and simultaneous measurement of yielding a precise mechanistic interpretation. AESEC provides the elemental dissolution rates in real time. In this way, for similar information via the coupling of an ICP-AES downstream

Submitted for publication: July 18, 2019. Revised and accepted: September 10, 2019. Preprint available online: September 10, 2019, https://doi.org/10.5006/3336. ‡ Corresponding author. E-mail: kevin.ogleatchimie-paristech.fr. Chimie ParisTech, PSL University, CNRS-IRCP 11, rue Pierre et Marie Curie, 75005 Paris, France.

ISSN 0010-9312 (print), 1938-159X (online) © 2019 NACE International. 1398 DECEMBER 2019 • Vol. 75 • Issue 12 Reproduction or redistribution of this article in any form CORROSIONJOURNAL.ORG is prohibited without express permission from the publisher. INVITED CRITICAL REVIEW

from an electrochemical flow cell (Figure 1). The electrolyte reaction cell from a separate compartment where the reference reacts with the material under investigation in the flow cell and (RE) and counter (CE) electrodes are placed. The porous the dissolved species released from the material are continu- membrane allows ionic conductivity while preventing bulk mixing ously analyzed downstream by ICP-AES. In this way, the of the electrolytes in the two compartments. concentration transients in the electrolyte may be compared with Fresh electrolyte enters at the bottom of the cell, reacts electrochemical data (E and i) as in the original OTTLE with the specimen, and is removed from the cell at the top experiment. containing all dissolved species coming from the specimen. The coupling of electrochemical dissolution with ICP The temperature of the reaction is controlled by placing the spectroscopy has been around for a long time, for example as a electrolyte reservoir in a thermal regulated water bath and by 3-4 detector of anodic stripping voltammetry. However, the pumping the water from the bath through a hollow copper block spectroelectrochemical application, that is to say, the measure- positioned on the back side of the specimen. ment of concentration transients correlated with electrochem- Interfacial reactivity may depend directly on the hydrody- istry in order to understand the electrochemical reactions, was namics of the electrolyte at the interface. Elementary processes fi 5-6 rst developed in the early 1990s at the Institut de Recherche such as corrosion product precipitation, pH changes, oxygen de la Sidérurgie (IRSID) at Saint Germain-en-Laye, France. The diffusion, etc. may be diminishedorenhancedbychangingelec- Downloaded from http://meridian.allenpress.com/corrosion/article-pdf/75/12/1398/2656216/3336.pdf by guest on 29 September 2021 development of time resolved emission spectroscopy had been trolyte velocity. Unfortunately, the range of possible flow rates is pioneered at IRSID by Roger Berneron, who had previously limited by the nebulization system of the ICP-AES. The flow rate fi developed depth pro ling by glow discharge optical emission of electrolyte (f) through the cell is usually controlled at around spectroscopy (GDOES) by combining an emission spectrometer 3 3 3 3 1cm/min to 5 cm /min (0.02 cm /s to 0.08 cm /s). Higher rates with a Grimm lamp as a plasma source,7 and Pierre DeGelis, who may be used by diverting a fraction of the electrolyte with a had developed a prototype for AESEC, a non-electrochemical, second pump as has been done for gas measurements.10 The open dissolution cell coupled to an ICP-AES.8-9 average diffusion layer thickness has been determined by 11 potentiostatic reduction of K3Fe(CN)6. The limiting diffusion INSTRUMENTATION current corresponded to a diffusion layer thickness of approx- 3 The AESEC instrumentation may be divided into three imately 200 μmto130μmatflow rates of 2.0 cm /min to 3 modules (Figure 1): (a) the electrochemical flow cell where the 5.0 cm /min. The diffusion layer thickness increased sharply reactions of interest occur, (b) the plasma and spectrometer in at lower flow rates. which the products of the reactions are analyzed, and (c) the data After reaction, the electrolyte is aspirated into the torch acquisition system which gives a real-time presentation of of an ICP-AES which is used to determine the elemental con- 12 elemental intensities as a function of time. A diagram of a flow cell centration from the emission spectrum of the plasma. This is commonly used in the author’s laboratory is shown to the left. similar in concept to a rotating ring-disk electrode or a double The sample or working electrode (WE) with an exposed surface channel flow cell in which the electrochemical detection at the area (A ≈ 1cm2) is exposed to a small volume (V ≈ 0.2 cm3)of ring or second electrode is replaced by the ICP-AES down continuously renewed electrolyte, the flow of which is maintained stream from the flow cell. Dissolved species are desolvated in by a peristaltic pump. A porous membrane separates the the plasma due to the high temperature (8,000 K to 10,000 K),

λ 6,000 K M Hollow copper I λ block 8,000 K M RE 10,000 K Quartz bonnet CE RF induction coil ν Capillary injection tube m = Cm /A O-ring Plasma tube

25 mm ν jm = nF m Sample (WE) Coolent tube Membrane ν Mg ν Tangential Ar flow Zn

Ar plasma gas inlet

From temperature regulated From electrolyte t /s water bath reservoir Small volume reaction flow cell ICP atomic emission spectrometer Real time data acquisition Three electrode electrochemical cell Concentrations of released elements Dissolution rates vs. time (a) (b) (c) FIGURE 1. Conceptual block diagram of AESEC showing three major components of the method: (a) the reaction flow cell, (b) the ICP torch and aspiration system, and (c) the data acquisition and quantification, illustrated for Zn and Mg. Flow cell design (a) modified after Shkirkiy and Ogle60 under CC-BY license and torch image (b) modified from Xvlun (commons.wikipedia.org).

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Polychromator Monochromator generator 2 N

Al-Iλ 167.081 nm

Mg-Iλ A/D 250 kHz buffer Downloaded from http://meridian.allenpress.com/corrosion/article-pdf/75/12/1398/2656216/3336.pdf by guest on 29 September 2021 279.553 nm Zn-I λ A/D Data 213.856 nm 250 kHz buffer acquisition Al-Iλ 167.081 nm A/D 250 kHz buffer

E j Other analog devicesOther analog Potentiostat devices

FIGURE 2. The AESEC spectrometer and data acquisition system illustrating the use of a Runge polychromator and a Czerny-Turner monochromator for true simultaneous measurements of elemental dissolution rates and electrochemical data. Torch image modified from Xvlun (commons.wikipedia.org). molecular bonds are broken, and usually, the constituent ideal for AESEC in that each element may be monitored in a elements are reduced to atomic species. Electronic excitation completely independent fashion by adjusting the sensitivity via and de-excitation occur with the emission of characterisitic the high voltage on each photomultiplier. The photomultiplier atomic line spectra. The intensity of the individual lines is has the large dynamic range needed for AESEC operation and directly proportional to the concentration of the element in the the elemental concentrations vs. time date are truly simultaneous plasma, which is proportional to the concentration in the without stepping from one element to the other and with electrolyte. no dead time between measures. The Czerny-Turner There are numerous advantages to using ICP-AES as a monochromator is used to select a single element that was detector. It is sensitive to most elements in the periodic table and not preinstalled on the polychromator system, or that requires fi quanti cation is straightforward using commercially available a higher spectral resolution, hence lower detection limit. standard solutions. It is extremely robust and stable over long The electrochemical data (four analog signals) are also routed periods of time even with complex electrolytes containing high into the same data acquisition system so that current and salt or total dissolved solid concentrations, or containing potential are measured on the same time base as the spectro- organics, oils, etc. The detection limits are low, on the order of scopic data. μ 3 a few ppb ( g/dm ) for most metallic elements, and the dynamic The AESEC system used in our laboratory is equipped fi fi range of quanti cation usually spans ve to six orders of with a rapid, simultaneous data acquisition system, originally magnitude making it possible to compare electrical current and developed by HORIBA Jobin Yvon for GDOES depth profiling dissolution rates over a wide range of conditions. Interfer- analysis. The signals from the phototubes, monochromator, ences between elements are rare due to the narrow linewidth of and polychromator are monitored simultaneously by the the emission spectra and the high resolution of the spec- † Quantum™ software and data acquisition electronics. Three trometer used, and when they exist they may be readily 16-bit A/D converters operating at a frequency of 250 kHz are accounted for and corrected by standard methods. An im- used to continually monitor the output of 31 photomultipliers, portant advantage of ICP-AES is the near absence of matrix 30 on the polychromator and 1 on the monochromator. effects. This is due to the destruction of almost all molecular The data are transferred to the computer after averaging over a bonds in the plasma by the high temperature (≈8,000 K to fi 10,000 K) and their decomposition into atoms. The drawback user-de ned integration period, usually set for 1 s. This means to this is that all molecular information is lost: molecular speci- that each measured value of intensity corresponds to the av- ation and elemental oxidation states must be determined or erage of 250,000 16-bit data points giving an extremely large inferred using other information. dynamic range. Much faster data acquisition rates are possible A more detailed diagram of the spectrometer in our for certain applications, for example the measurement of laboratory is given in Figure 2. The Paschen Runge polychro- particle release (Section 4.3) was performed at a rate of mator system with photomultiplier tube (PMT) detectors is 10 ms per point. Alternative detectors and/or optical mounts are available, † Trade name. namely semiconductor detectors, such as charge-coupled

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device (CCD) or the charge injection device (CID) and/or CM = ðIλ − Iλ°Þ=kλ,M (1) echelle type optics. These systems have the advantages of being less expensive, may in principle measure more elements, and where Iλ° and κλ,M are the background intensity and the are usually more compact if echelle systems are used. The major sensitivity factor for element M, respectively, for a given drawback is a limited dynamic range: the sensitivity for a specific wavelength (λ). These values are determined by conventional element is controlled by optimizing the integration time for that ICP-AES calibration methods and of course depend upon the element, making it difficult to measure several elements simul- specific conditions of the plasma and nebulization system and taneously and truly correlating the signals. The CCD detection must be determined regularly. The concentration is in turn systems are also less sensitive in the ultraviolet wavelength converted into dissolution rate (νM): range. An ICP-AES system with CCD detection system and a quasi- fl identical electrochemical ow cell was used by Mercier, et al., to νM =fCM=A (2) investigate the corrosion13-15 and anodization16-17 of Al, although acqusition was limited to the analysis of a single element, Al. where f is the flow rate of the electrolyte and A is the exposed geometric surface area of the electrode. The flow rate is

2.1 | Inductively Coupled Plasma Mass Spectrometry measured to <1% uncertainty for each experiment. For conve- Downloaded from http://meridian.allenpress.com/corrosion/article-pdf/75/12/1398/2656216/3336.pdf by guest on 29 September 2021 nience, the dissolution rate may be presented as an equivalent Although this article concerns atomic emission spectros- faradaic current density (j ) by application of Faraday’s law: copy, it is of interest to briefly review the use of inductively coupled M plasma mass spectrometry (ICP-MS) for similar experiments. A ν recent review of the electrochemistry coupled to plasma mass jM =nMF M (3) spectrometry and optical spectroscopy has been presented by 18 where n is the number of electrons exchanged in the disso- Cherevko and Meyrhofer. Their technique was first developed by M 19-21 22-24 lution reaction and F is the Faraday constant where ν is consid- Homozava, et al., and further developed by Ott, et al. M ered to be in mol·s−1·cm−2. Note that by convention the rates are Their technique involved a noncontinuous operation in which ali- normalized to the geometrical surface area. quots were taken and analyzed at regular intervals. Continuous The plot of the elemental dissolution rates as a function of in-line experiments have been conducted using ICP-MS in partic- time for a given specimen-electrolyte composition is referred to ular using a scanning droplet cell(SDC)asdevelopedattheMax 25-28 as a dissolution profile. Direct insight into the mechanisms of Planck Institute in Düsselorf by Klemm, et al. Using either dissolution may be obtained from the dissolution profile. First and the SDC or a conventional flow cell, ICP-MS has been used to 29-34 perhaps foremost, the stoichiometry of the dissolution is investigate the corrosion of Mg, Al, and Ni alloys, the kinetics 26-27,35-38 obtained directly by comparing the ratio of the dissolution rates of the degradation of heterogeneous catalysts, and to 39-44 (electrolyte concentrations) with the composition of the ma- detect partial currents during anodization. Recently, Lopes, 45 terial. This reveals whether or not the dissolution occurs con- et al., moved away from the flow cell technique to directly gruently for all elements or preferentially for some elements. It sample the electrolyte in the vicinity of a rotating disk electrode with also offers a direct measurement of the faradaic yield of an anodic transfer to an ICP-MS to investigate the dissolution of Pt single dissolution process by comparing the electrical current density crystals. measured by the electrometer of the potentiostat (j ) with the sum ICP-MS is similar in concept to emission spectroscopy e of the elemental dissolution current densities (jΣ = Σ j ). except that ions drawn out of the plasma are usually detected M M with a quadrapole mass spectrometer. ICP-MS has the ad- 46 vantage of very low detection limits (on the order of parts per 3.1.1 | Example 1: Anodic Dissolution of MgZn2 trillion [ppt]) and is sensitive to most elements in a single The MgZn2 phase occurs in Al alloys containing Zn and measurement. The drawbacks include less versatility as com- Mg, i.e., the 7000 series with ≈3% Zn and Mg, and in the Zn-Al-Mg pared to ICP-AES in terms of electrolyte compatibility, being coatings of galvanized steel, usually with ≈ 1% to 3% of Al and limited to relatively dilute solutions and acids. Plasma stability for Mg. It is a very reactive phase and is frequently the most anodic ICP-MS is also an issue and usually an internal standard is phase when present in a multiphase system. Figure 3 shows required for long operating times. ICP-MS is also very prone to the dissolution profile obtained during the anodic dissolution of a matrixeffects:astheionsmoveawayfromtheplasma,theymay MgZn2 pure phase intermetallic in a slightly alkaline electrolyte. + regroup into molecules creating mass interferences such as ArO The figure gives the system response to an applied potential (Eap) + 56 + 52 + and ArC which interfere with Fe and Cr . These species of −0.8 VSCE, applied at t = 0, approximately 200 mV anodic to may be reduced by reaction with a secondary gas in a “dynamic the spontaneous corrosion potential. As shown in Han and reaction cell.”27 Ogle,46 cathodic reactions are negligible at this potential. Note that the current density is presented as je and je, the latter being BASIC MEASUREMENT PRINCIPLES determined by numerical convolution of the former described in detail later in the Time Resolution and Convolution section. 3.1 | Dissolution Rate Measurement This example clearly illustrates the basic quantitative AESEC offers a direct measurement of the instantaneous principles of AESEC. The distribution of the faradaic electrical dissolution rate of a material on an element by element basis. The current density (je) between the dissolution of the two alloy electrolyte carrying the dissolved reaction products is aspi- components, Zn and Mg was nearly congruent and faradaic. rated into the inductively coupled plasma and the dissolved Congruent dissolution is indicated by the ratio of the elemental species are atomized and excited by collisions with electrons dissolution rates, jZn/jMg = 1.92±0.03, very close to the MgZn2 and argon ions. The relaxation of the excited atoms gives rise to stoichiometry, and faradaic dissolution is indicated by the ratio the emission of radiation at different wavelengths associated of elemental dissolution to the anodic current, jΣ/je = 0.97±0.02, with different elements. Through calibration with elemental very close to 1. The elemental dissolution rates are consistent standards, the emission intensity at a specific wavelength (Iλ)is with the stoichiometry of the material: x-ray diffraction (XRD) and converted into concentration: electron backscatter diffraction (EBSD) revealed an essentially

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Blank E = –0.8 VSCE

400 je

je*

300 jΣ (Zn + Mg) Zn ) 2 200 /cm

A

μ Mg

j ( 100

0 Downloaded from http://meridian.allenpress.com/corrosion/article-pdf/75/12/1398/2656216/3336.pdf by guest on 29 September 2021

–100 –100 0 100 200 300 400 500 600 1,300 1,400 Time (s)

FIGURE 3. Dissolution profile obtained during the potentiostatic anodic dissolution of a pure phase MgZn2 intermetallic in 30 mM NaCl, pH = 10.1. The applied potential was E = −0.8 V at t = 0. Elemental dissolution rates are presented as equivalent currents assuming aqueous Zn(II) and Mg(II). Modified after Han and Ogle46 under CC-BY license.

(2) pure MgZn2 intermetallic phase and ICP-AES measurements wt% 75.8%Cu-21.0% Zn alloy (CW724R brass [UNS C69300 ]) in an independent laboratory confirmed the 2:1 Zn:Mg atomic during the imposition of a fixed galvanostatic current density of ratio (=1.98±0.02).47 40 μA/cm2. The dissolution profile shows that the faradaic current density is distributed between the dissolution of Zn 3.2 | Mass-Charge Balance and of Cu, as for MgZn2 (Figure 3), however in this case, the faradaic yield of dissolution is only 42% assuming Zn(II) and Cu(II) The reaction of a material with an electrolyte may involve are the dissolved oxidation states. The charge involved in the faradaic processes which do not result in the immediate release “hidden” faradaic process, in this case the formation of oxides, of dissolved species and these processes are therefore un- may be calculated by integrating Equation (4), indicated by the detectable by ICP-AES. Cathodic reactions and the formation of oxides are among the electrochemical processes which may pink shading in Figure 4. not be monitored by AESEC. In the absence of surface charging, Following their formation, the oxides were selectively these processes may be quantified indirectly by a mass- dissolved at open-circuit in a deaerated 0.2 M citrate buffer, charge balance. The faradaic current due to the undetectable pH = 4.9, shown to the right of Figure 4. The quantities of Cu and Zn dissolving during this step corresponded to the total processes is given by the difference current density (jΔ): amount of oxide formed during the preceding anodic disso- jΔ =je − jΣ (4) lution. (The quantity of accumulated Zn oxides was very low and essentially independent of time and was therefore ignored in where the * denotes the numerical process of convolution, the following analysis.) This is demonstrated in Figure 5, which described in the Time Resolution and Convolution section. For compares the integral of the total current density (Qe =je Δt/F) a faradaic dissolution yield of 100%, jΔ = 0. For jΔ ≠ 0, it is with the sum of dissolved Cu and Zn in the tap water solution and sometimes possible to attribute jΔ to either a specific cathodic the citrate buffer as shown. The results for similar experiments or anodic process making possible a further breakdown of the with a Cu reference material are also shown. electrical current density into elementary processes. Oxide From Figure 5, n values of 0.89 and 1.1 were determined for formation may be determined when jΔ > 0 and the cathodic the formation of the Cu-oxide films on Cu-Zn and pure Cu, re- 48-52 current density is negligible, or the cathodic current spectively, consistent with a mechanism in which Cu is oxidized density may be determined when jΔ < 0 and oxide formation to form insoluble Cu(I)-oxide and soluble Cu(II) as majority species. can be ignored.53-54 In separate postmortem experiments, the presence of Cu2Owas confirmed by Raman spectroscopy and XRD for pure copper,56 3.2.1 | Example 2: Anodic Dissolution of Brass in brass,55,57 and bronze.58 Synthetic Tap Water55 This experiment also demonstrates the high surface sen- The quantitative determination of oxide formation by sitivity of the technique. The integrated quantity of oxide dissolved mass-charge balance is well exemplified by the anodic dissolu- in the citrate buffer yields an approximate film thickness of (1) 3 tion of Cu and Cu-Zn (Figure 4) in a synthetic tap water. 60 nm assuming the standard density of Cu2O(6.0g/cm)and Galvanostatic dissolution is shown to the left of the figure for a auniformfilm over the 1 cm2 of geometric surface area. This would be significantly less if the true surface area was used in the (1) The synthetic tap water composition was MgSO4·7H2O (31.9 mg/L), NaHCO3 calculation considering surface roughness. The estimated − − (36.0 mg/L), CaCl2·7H2O (102.9 mg/L), Na2SO4 (275.5 mg/L). average oxide formation rate is 0.25 mmol·s 1·cm 3, (2) UNS numbers are listed in Metals & Alloys in the Unified Numbering System, published by the Society of Automotive Engineers (SAE International) and which under the same assumptions, corresponds to a rate of cosponsored by ASTM International. 0.06 nm/s.

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Free jap = Free –2 –1 corrosion 40 μA·cm ·s corrosion 0.1

Synthetic tap water Citrate buffer 0 )

0.25 SCE –0.1 E (V

) 0.2 –2 –0.2 ·cm

–1 0.15 Cu O 2 Cu /4 je /2F 0.1 j*e /2F Cu(aq) Downloaded from http://meridian.allenpress.com/corrosion/article-pdf/75/12/1398/2656216/3336.pdf by guest on 29 September 2021

Rate (nmol·s 0.05 Zn(aq) Zn /4 jΣ (Zn + Cu) 1,200 s 0

0 500 1,000 1,500 2,500 3,000 Time (s) FIGURE 4. Elemental dissolution rates during the galvanostatic anodic dissolution of a wt% 75.8Cu-21Zn alloy at 40 μA/cm2 in aerated synthetic tap water, pH = 7.5 (left), followed by oxide dissolution in a deaerated 0.2 M citrate buffer, pH = 4.9 (right). The shaded areas represent the determination of oxide formed during the galvanostatic step, indirectly by mass-charge balance on the left, and directly by dissolution on the right. Note that current density is divided by 2F and the dissolution rates in citrate buffer are divided by 4. Modified after Zhou, et al.,55 with permission.

Brass Pure Cu 250 600 n = 0.89 E n = 1.1 1 Cu/Zn = 0.26 (0.27 bulk) 240 ) 500 ) –1 0.8 SCE ·s 230 ) 2 400 Q –2 Q e e 0.6 220 E (mV 300 j /2F 210 Cu (citrate) Cu (citrate) 0.4 200

Q (nmol/cm 2x Cu(aq) Zn 2x Cu(aq) 0.2 100 2x Zn(aq) Rate (nmol·cm Cu x6 0 0 FIGURE 5. 300

Mass-charge balance from the results of Figure 4 (right) ) and an identical experiment for pure Cu (left). The total charge (Q )is 2 e 200 Cu(I) - oxide shown in blue, and the elemental analysis of dissolution in citrate (green) plus dissolved Cu (red) and Zn (gray) during the anodic 100 Cu(0) dissolution. The results demonstrate that Cu oxide formation is con- sistent with a Cu(I) species, presumably Cu2O, in both cases. After 0 data presented in Zhou, et al.55-56 Q (nmol Cu/cm –100 –200 0 200 400 600 800 1,000 1,200 Mechanistic detail is apparent in the dissolution profile. Time (s) The early stages of dissolution show a significant Zn dissolution rate which rapidly passes through a maximum while Cu dis- FIGURE 6. A complete kinetic analysis for the dealloying of Cu-42Zn at μ 2 solution increases slowly. This early Zn dissolution correlates 80 A/cm in synthetic tap water. The Cu and Zn dissolution rates, with a dip in the open-circuit potential and may be interpreted Cu(I)-oxide formation, and the formation of the Cu(0) dealloyed layer as the early stages of dealloying which do not seem to be were obtained by application of Equations (4) through (7). Note the rapid onset of Zn dissolution and the more gradual increase in Cu affected by the formation of the Cu O film, followed by Cu 2 dissolution indicative of dealloying in the early stages of the reaction. dissolution as Cu(II). After data presented in Zhou, et al.55

3.3 | Mass Balance surface enrichments via a mass balance. The total quantity of Frequently, one or more components of an alloy or dissolved M at any time (QM(t)) may be calculated by integrating complex material will undergo selective dissolution, leaving be- the dissolution rate of M: hind a surface layer enriched in the other components. The ð surface enrichment may be either an oxide in the case of pas- t ν ð Þ sivation or a metal film in the case of dealloying. The AESEC QM = M t dt (5) method can sometimes be used to indirectly determine these 0

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E ap = E –1.60 V –1.00 V oc 400

300 Form Zn(0) layer Al Zn (s) 200 j 420 nmol Al 5.2 Al 5.2Al(III) + Zn(0) (s) + 15.6 e– ≈81 nmol Zn 100 ) 2 /cm

0

A 80 μ ( M j 60 63 nmol Zn

≈ Downloaded from http://meridian.allenpress.com/corrosion/article-pdf/75/12/1398/2656216/3336.pdf by guest on 29 September 2021 78% yield Dissolve Zn(0) layer 40 Direct to 2+ – j Zn(0) (s) Zn + 2e Zn –1.0 V 20 jZn x4

0 0 500 1,000 1,500 2,000 Time (s) FIGURE 7. The formation and dissolution of metallic dealloyed Zn by AESEC using a double step potentiostatic sequence. The specimen was a α-phase Zn-Al in 0.03 M NaCl, pH = 10.1. The selective dissolution of Al occurs during the first step at −1.6 V (left) to form aqueous Al(III) and residual Zn(0). The accumulated dealloyed Zn(0) dissolves during the second step at −1.00 V (right). Also shown is the effective Zn dissolution current density when the alloy is stepped directly to −1.00 V without dealloying at −1.4 V, in which no Zn dissolution was detected. Modified after Han and Ogle59 under CC-BY license.

If the bulk composition of the material is known and resist dealloying. In this case, the enrichment of Cu(0) (ΘCu(0)) may assumed to be uniform in depth, it is possible to calculate the be determined as a function of time taking into account the total quantity of accumulated M enriched at the surface (ΘM) repartition of excess Cu between dealloyed Cu(0) and insoluble relative to another element (N) which is assumed to be Cu(I) corrosion products, presumably Cu2O: completely soluble: ΘCuð0Þ =xQZn − QCu − ΘCuðIÞ (8) ΘM =xQN − QM (6) fi where x = (%M/%N) in either wt% or at% depending on the units The dissolution pro le is shown in the upper curve and of ν (t) in Equation (5). The first term of Equation (6), xQ , gives the variation of the interfacial composition is shown in the lower M N fi the hypothetical dissolution rate of M assuming congruent dis- gure. The quantity of accumulated solid Cu(I) oxidation pro- Θ solution with N. If the dissolution of M and N were perfectly ducts ( Cu(I)) was determined by the mass-charge balance of Equation (4). congruent, ΘM =0. This measurement of the accumulated M by Equation (6) does not distinguish between insoluble oxidation products and 3.3.2 | Example 4: Cathodic Dealloying of an Al-Zn Alloy dealloyed metallic films. The distinction may be made by The Cu(0) enrichment in Example 3 was very small, on the considering the charge balance. The accumulated quantity of order of a few nm, and proved difficult to confirm by a post- insoluble oxidation products may be determined in units of mortem analysis. This was achieved, however, for the wt% Zn- Θ fi charge ( e), and expressed in terms of a speci c element M 68Al system (Al5.2Zn) which underwent a mechanism of cathodic Θ 59 ( M(z)) as follows: ð dealloying in 0.03 M NaCl at pH = 10.1. The quantitative t analysis of this reaction was obtained with a double-step Θe = jΔðtÞdt 0 potentiostatic experiment, shown in Figure 7. The dealloying reaction (left) occurs at −1.60 VSCE (Equation [9]) where only Al ΘMðzÞ = Θe=zF (7) dissolution was observed. where z is the presumed oxidation state of element M (mol/cm2) in the solid oxidation product. 3þ − Al5.2ZnðsÞ → 5.2Al þ Znð0ÞðsÞþ15.6e ð−1.6VÞ (9)

3.3.1 | Example 3: Brass Dealloying Kinetics Znð0ÞðsÞ → Zn2þ þ 2e− ð−1.0 VÞ (10) To illustrate these relationships, Figure 6 presents a complete kinetic analysis of the anodic dissolution of wt% The anodic dissolution of the excess Zn(0) occurred at − Cu-42Zn in synthetic tap water at 80 μA/cm2. This alloy 1.00 V (right, Equation [10]). The quantity of Zn dissolved undergoes dealloying much more readily as compared to the at this potential was 78% of that determined by mass balance. wt% Cu-21Zn (Figure 4)(3), which was specially engineered to Also shown is the effective Zn dissolution current density when the alloy was stepped directly to −1.00 V without dealloying at (3) −1.6 V; no Zn dissolution was observed, indicating that the The synthetic tap water composition was MgSO4·7H2O (31.92 mg/L), NaHCO3 (36.00 mg/L), CaCl2·7H2O (102.9 mg/L), Na2SO4 (275.5 mg/L). Zn of the pure phase Al5.2Zn has a more positive dissolution

1404 DECEMBER 2019 • Vol. 75 • Issue 12 CORROSIONJOURNAL.ORG INVITED CRITICAL REVIEW ) potential than that of the dealloyed, metallic Zn(0) formed by (d) 2 2.5 Equation (9). (a) 0.05 2

3.4 | Time Resolution and Convolution 0 1.5

The original idea of spectroelectrochemistry was to )

Cu (ICP) (mC/cm 1 determine a direct relationship between the electrochemical NHE –0.05 121.5 2.5 current density and the concentration of electrogenerated Cu (pstat) (mC/cm2) E (V species so as to interpret the instantaneous current density –0.1 in terms of chemical reaction rates. For AESEC, the instan- taneous dissolution rates are directly related to the instanta- –0.15 neous downstream concentrations by mass balance (Equation [2]). However, transport in the capillary system leads to (b) 0.06 a time offset between the instantaneous concentrations and 0.05 electrochemical measurement (Δt), while mixing and diffusion 0.04 β = 1.10 ± 0.005 within the flow cell leads to a broadening of the concentration τ ± Downloaded from http://meridian.allenpress.com/corrosion/article-pdf/75/12/1398/2656216/3336.pdf by guest on 29 September 2021 0.03 = 9.47 s 0.02 transients. 0.02 The time offset is easily accounted for by simply shifting h(t) (s) the spectroscopic data with respect to the electrochemical data by 0.01 Δ t, an easily measurable value. The broadening of the con- 0 centration transients, however, presents a more complex problem. –0.01 When the dissolution rate is changing rapidly, a rigorous com- –20 0 20 40 60 80 parison of electrical current density and dissolution rate requires 0.06 Time (s) “smoothing” the electrochemical data (je) so that it has the same (c) time resolution as the dissolution rate data, je. For example, the use 0.05 of Equation (4) for the determination of oxide during electro- 0.04 chemical transients requires that the electrical current density (j ) e 0.03 τ be convoluted with the residence time distribution function (h(t)) so that they have identical time resolution. This is illustrated in 0.02 h(t) (s) Figures 3, 4,and6 where the je and je differ significantly during 0.01 fi the rst seconds of the transient and only become equal around 0 120 s. Failure to use the convoluted current density in Equa- tion (4) would lead to a significant overestimation of the quantity of –0.01 oxide formed during this time period. If the time evolution of 110100 current density and dissolution rate are slow on the time scale of Time (s) the experiment, convolution is not necessary. FIGURE 8. Experimental determination of the residence time distribu- The principle of convolution is straightforward. For a tion. (a) The variation of potential as a function of time showing the given element, the true interfacial dissolution rate (νM°(t)) is 1.5 s potentiostatic pulse at t = 0 simulating a delta function of Cu related to the measured elemental concentration transient dissolution, followed by the return to the open-circuit potential. (b) The 6,60 (νM(t)) via a convolution integral: normalized concentration transient of dissolved Cu downstream from the electrochemical flow cell. The dashed curve represents a nonlin- ð fi fi t ear least square t to Equation (14) with the tting parameters given in νMðtÞ = νM°ðτÞhðt − τÞdτ (11) the figure. (c) The data of (b) expressed on a log (t) axis to demonstrate 0 the log-normal nature of the residence time distribution. (d) Integral of the Cu concentration transient (expressed as charge assuming n = 1) 61-62 where h(t) is the residence time distribution (RTD) of the versus the integral of the electrochemical current transient. This result electrochemical flow cell. τ is simply a variable of integration. confirms the n = 1 dissolution mechanism. Reprinted from Shkirskiy, The idea is entirely equivalent to the transfer function between et al.,60 under CC-BY license. the electrochemical and spectroscopic measurements proposed 63 by Gabreilli, et al. application of a delta function of dissolution at the working The ideal situation would be to solve Equation (11) electrode/electrolyte interface: directly for νM°(t) and deconvolution routines for the dissolution ð 15,53 ∞ rate data have been proposed. However, due to the hðtÞ =CðtÞ= CðtÞdt (13) magnification of error implicit in such calculations, they have not 0 been used routinely. In practice, it is necessary to perform a A typical potentiostatic pulse experiment for Cu/HCl is numerical convolution of the experimental current data set j (t). e shown in Figure 8. The 1.5 s anodic pulse was considered As the electrochemical current measurement may be con- sufficient to approximate a delta function. Numerical simula- sidered instantaneous on the time scale of these experiments, tions have demonstrated a good agreement with the experi- the convolution integral is as follows: fl ð mental data by only considering the electrolyte ow patterns in t ð Þ ðτÞ ð − τÞ τ the electrochemical cell. Empirically, it has been demonstrated je t = je h t d (12) fl 0 that h(t) closely follows a log-normal function for the ow cell design in Figure 1: The RTD will be specific for each flow cell design and rffiffiffiffiffiffiffiffi β must be determined experimentally. This may be obtained by − 1 −β 2 t hðtÞ = e 4βe ln (14) measuring the concentration transient (C(t)) following the πτ2 τ

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je* Type II 2 je jZn Type I 2

jΔ 0.2 mA/cm

2 mA/cm 1.5 mV/s

1.0 mV/s

0.5 mV/s Current Density Downloaded from http://meridian.allenpress.com/corrosion/article-pdf/75/12/1398/2656216/3336.pdf by guest on 29 September 2021

0.25 mV/s

–1,600 –1,400 –1,200 –1,000 –800 –1,600 –1,400 –1,200 –1,000 –800

E (mVAg/AgCl) E (mVAg/AgCl) (a) (b) FIGURE 9. (a) AESEC polarization curves of pure Zn (electrodeposited coatings) in 0.1 M NaOH, cathodic to anodic, at different sweep rates as indicated. (b) The rate of ZnO formation expressed in current density (jΔ) showing two different forms of oxide determined from the data of (a). Modified after Mokaddem, et al.,48 with permission.

τ β where value of and are empirically determined parameters, SURFACE TREATMENT KINETICS which may be construed as measures of the time resolution of the system. A fundamental problem in the investigation of corrosion processes is that they occur spontaneously without passage of 3.4.1 | Example 5: The Formation of Zn Oxides electrical current through an external circuit. Therefore, the use of dynamic electrochemical methods requires perturbing Much effort has been made to identify the different oxide the system and extrapolating the electrical current to the species that form during anodic dissolution of Zn in alkaline zero-current potential (E ). It is possible, however, to measure electrolytes.64 To this end, AESEC was used to distinguish two j=0 the potential of a metal or an alloy during free corrosion, and forms of Zn oxide in the AESEC polarization curves (Figure 9)of on occasion, the value of the potential may be used to diagnose nominally pure Zn (electrogalvanized steel) in a 0.1 M NaOH the state of the system by comparison with thermodynamic solution. Figure 9(a) shows the overlay of j ,j , and j at different e e Zn calculations65 or with polarization curves. This is of particular sweep rates as indicated. The difference between j and j e Zn interest for monitoring the progress of industrial surface throughout the active domain increased markedly with increasing treatment processes such as pickling, etching, or conversion sweep rate, which at a glance might be interpreted as oxide coating where the reactions may occur very rapidly without formation. The convoluted current density (je), however, overlays any real steady-state occurring. Surface treatments are often jZn throughout the active domain, demonstrating that the performed by simple immersion in which the material reacts effect was mostly due to the residence time distribution of the spontaneously with the electrolyte and the only electrochemical fl ow cell. Figure 9(b) gives jΔ, which may be interpreted as the measurement available is the open-circuit potential, which of fi formation of an insoluble Zn(II) oxide/hydroxide lm. It has been course gives no direct information on the interfacial reactions. previously proposed that two types of oxide formed during As a classic example, Ghali and Potvin (1972)66 may be cited linear scan voltammetry experiments and this hypothesis is on the mechanism of phosphating of steel, and a more recent confirmed by the AESEC-LSV data. For 0.5 mV/s and above, an example of Schoukens, et al., concerning Zr-based conver- oxide formation peak (Type I oxide) was observed at −1,210 mV sion coatings on aluminum alloys.67 and this peak became increasingly resolved from background noise as the sweep rate increased consistent with a surface reaction. This was followed by a second peak at −1,100 mV (Type II oxide), perturbed by the nonstationary peak immediately 4.1 | Aluminum Alloy Surface Treatment Kinetics following the maximum region. In this way, the AESEC tech- AESEC provides a window into these processes by nique distinguished two types of oxide based on the kinetics of directly measuring the dissolution rates of the different alloying their formation. The existence of the two forms of oxide was elements and correlating them with potential changes. further confirmed by potentiostatic experiments. The pretreatment of Al alloys prior to conversion coating or

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) fi Commercial Etching Solution 2 Figure 10). Note that Mg is multiplied by a factor of ve to be on 0 –0.70 the same scale as the Al dissolution rate: its dissolution would g/cm make a negligible contribution to the total mass loss in a typical μ 10 gravimetric measurement of the etching rate.

) These results also demonstrate the importance of time b c

SCE 20 resolution. Following the dissolution of Mg, a peak of Mn –0.75

(V dissolution was observed, here multiplied by 100 to be on

oc the same scale as Al. This clearly correlates with the maximum E 30 (“b” in Figure 10)inE . Beyond this, a relatively stable μ oc 0.85 m/h Al Mass Change ( a dissolution rate was obtained with a slow, steady increase in

–0.80 Eoc, perhaps due to the enrichment of metallic Cu on the 0.15 surface. Obviously, a detailed interpretation of these results would require additional knowledge of the metallurgy of the system: for example, how the different elements are distributed 0.10 Al x1

between phases at the surface and how the elements are Downloaded from http://meridian.allenpress.com/corrosion/article-pdf/75/12/1398/2656216/3336.pdf by guest on 29 September 2021

) distributed in depth in the material. Nevertheless, the elemental –2 fi 0.05 dissolution rates reveal a ngerprint of the mechanistic

·cm processes occurring during the surface treatment that –1 underlies the fine structure of the Eoc profile. g·s μ μ 0.00 The robustness of AESEC is also illustrated by the results Mg x5 of Figure 10. The commercial etching solution used was an 0.10 Mn x100 extremely complex, nearly opaque electrolyte containing a high total dissolved solid content including transition metal 0.05 cations, surfactants, etc. Despite the complex matrix, excellent

Etching Rate ( detection limits for the dissolving elements were readily obtained. 0.00

4.2 | Accumulation of Cu and Other Elements –0.05 A simulation of a complete pretreatment sequence is –100 0 100 200 300 400 500 shown in Figure 11, in this case involving NaOH/water rinse/HNO3 Time (s) under the conditions indicated in the figure.69 The results FIGURE 10. Typical open-circuit AESEC measurement during surface demonstrate the intense Al dissolution in alkaline solution, fol- treatment: In this case, a 6000 series aluminum alloy was treated lowed by the dissolution of residual Cu and the passivation of with a commercial acidic pickling solution. Courtesy of Novelis Global the Al in the nitric acid step. The quantitative aspects of AESEC Research and Technology Center (NGRTC), modified after Beck, et al.68 are well illustrated: Al dissolution was rapid in the alkaline solution corresponding to an etching rate of approximately 160 μg/h. The Cu dissolution rate, on the other hand, was three orders of magnitude lower. Under these conditions, Cu disso- lution would have made a negligible contribution to the overall anodization is a case in point. The pretreatment often involves mass change and could not have been detected by conventional two steps: (1) alkaline or acid etching to remove the mechanically gravimetric methods. altered layer, followed by (2) a nitric acid rinse to remove The situation changes when the surface is exposed to residual elements, especially Cu. Figure 10 gives an example of a nitric acid: the Cu dissolution rate was approximately 3× larger typical open-circuit AESEC measurement during a surface 68 than the Al dissolution rate. It was also observed that Al and Cu treatment. In this case, a 6000 series aluminum dissolution peaks did not have the same shape. Al dissolution alloy was treated with a commercial acidic etching solution. occurred in two peaks, the first which overlays with the time The open-circuit potential vs. time and the average mass constant distribution of the flow cell here shown as a dashed loss (upper curves, Figure 10) are the standard in-house red curve. measurements for monitoring the progress of the reaction. The selective dissolution of Al during the alkaline In this case, the standard mass loss was determined to be treatment is apparent in Figure 11. This leaves behind a surface μ −2 −1 40 g·cm ·s , corresponding to an etching rate of approxi- layer enriched in metallic Cu, the growth and dissolution of μ fi fi mately 0.85 m/h and the Eoc pro le shows considerable ne which may be determined indirectly via a mass balance. This is structure: an early anodic dip, followed by a slow rise through a showninFigure12 which gives the Al mass loss and the maximum, and then a steady rise toward the end of the buildup of Cu gain as a function of time. These results also experiment. Alone, these results would be almost meaningless as demonstrate that under the conditions of these experi- they cannot be directly related to either the corrosion rate as ments, Cu dissolution was not complete during the nitric predicted by a hypothetical Evans diagram, or the equilibrium acid rinse. model of the Pourbaix diagram. AESEC offers a new dimension Note that the measurements of Figures 10, 11, and 12 to our understanding: by following the elemental dissolution would be difficult with CCD detection because the Cu and Al are rates, one can see that the potential variations correlate with on very different orders of magnitude, or with ICP-MS because the dissolution rate variations of specific elements. Al dissolution of the high Al dissolution rate for Figure 11 and the complex was initially slow while Mg dissolution passed through an early electrolyte containing high dissolved solids and surfactants for maximum. This maximum correlates with the anodic dip (“a” in Figure 10.

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0.5 1.2 M NaOH H2O 25°C 2.8 M HNO3, 25°C 60°C 0 ) –0.5 SCE –1 E (V RTD –1.5 20 Al x1 Reaction )

–2 Cu x10

·cm 15 –1 g·s μ μ Cu x500

10 Al x10 Downloaded from http://meridian.allenpress.com/corrosion/article-pdf/75/12/1398/2656216/3336.pdf by guest on 29 September 2021

5

Dissolution Rate ( 0

–5 0 200 400 600 800 1,000 Time (s) FIGURE 11. Dissolution profile for the spontaneous reaction of AA2024-T3 during the deoxidation pretreatment sequence of a 1.2 M NaOH followed by 2.8 M HNO3. The red dashed curve shows the RTD for the flow cell under the conditions of these experiments. Modified from Gharbi, et al.,69 under CC-BY-NC-ND license.

4.3 | Non-Faradaic Processes: Detection of Particle particles, AESEC was used to monitor the dissolution of the 73 Release alloying elements during the anodization of AA7050-T74. The corrosion of Al alloys may involve the undermining Based on a mass balance it was possible to identify the disso- and release of constituent particles, which are typically inter- lution rates of the different particles. It was found that MgZn2 metallic compounds.70 As a non-faradaic process, this phe- completely dissolved, whereas Al2Cu, Al2CuMg, and coarse Al Cu Fe intermetallic compounds underwent a more complex nomenon cannot be detected by electrochemistry and makes a 7 2 dissolution process leading to the accumulation of copper on the negligible contribution to the mass loss. The high time reso- surface and the formation of holes in the anodized layer. The lution of the AESEC method and the ability to correlate the kinetics of dealloying of the latter pure phases to form nano- elemental signals makes it possible to detect and identify the porous Cu films was investigated as a function of potential.50 released particles. Constituent particle release was observed during the ELEMENTAL POLARIZATION CURVES AND alkaline etch of Figure 11 as indicated by the sharp spikes of Cu dissolution. The detection of individual particles is more ap- PASSIVATION KINETICS parent in Figure 13, which was obtained from a similar alkaline The polarization curve may be used to predict the elec- etch with enhanced time resolution of 10 points per second. trochemical behavior and corrosion rates over a wide range of Particles released into the electrolyte were carried to the plasma potentials74 and the rate laws for the elementary anodic and giving rise to very sharp peaks of a single point. As each point cathodic reactions, the input for most numerical simulations of represents the average emission intensity over the data col- corrosion, and are usually derived from polarization curves.75 lection period, the intensity of the point will be directly pro- For multicomponent and multiphase materials, however, it is portional to the data collection frequency. Previous work dem- necessary to know specifically which elements are dissolving onstrated that the duration of the individual particle transients and which elements remain behind, becoming enriched on the was <10 ms.71 Of particular interest is that the composition of the surface, if we wish to understand what is really happening. particles may be obtained from this data. In the figure, it is clear Prediction also requires knowledge of the “steady-state” polar- that Cu, Fe, and Mn peaks correlate with each other as do Cu and ization curve, which in general is not available and arguably Mg. A complete statistical analysis of the particle distribution is does not exist, as the interfacial reactions that occur during also given for this experiment.72 corrosion induce changes in both the material and the envi- ronment and preclude the existence of a true steady-state. For example, dissolved species formed by the anodic and ca- 4.4 | Constituent Particle Dissolution During thodic reactions alter the nature of the environment with time and Anodization may generate films of corrosion products. Selective dissolu- The selective dissolution of constituent particles during tion of one element may lead to the formation of accumulated the anodization of Al alloys leads to the formation of defects metallic films on the surface such that even the nature of the in the final oxide film. To evaluate the fate of the constituent alloy changes with time.

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Dry 1.25 M NaOH, 60°C H2O, 23°C 2.8 M HNO3, 23°C 1,000 250

0 200 ) ) 2 2 –1,000 c 150 g/cm g/cm μ μ –2,000 100

–3,000 b 50 –4,000 Downloaded from http://meridian.allenpress.com/corrosion/article-pdf/75/12/1398/2656216/3336.pdf by guest on 29 September 2021 Al Mass Dissolved ( a Excess Surface Cu (

0 –5,000

–6,000 –50 –100 0 100 200 300 400 500 600 700 800 900 1,000 1,100 1,200 Time (s)

abc

4 μm μ 4 m 4 μm

FIGURE 12. The integrated dissolution profile showing Al mass loss and the accumulation of Cu as determined by mass balance. Also shown are electron micrograph cross sections of the residual film obtained at various etching times. Modified from Gharbi, et al.69 under CC-BY-NC-ND license.

5.1 | Stainless Steel and the “Cathodic Loop” observed to coincide precisely with the dissolution of this 79 To illustrate the importance of the elemental dissolution excess Cu. rates, first consider the polarization curve of a stainless steel in Taken together, these results strongly support the idea sulfuric acid. This system may be considered an archetypal that the enhanced catalytic reaction is due to the buildup of example of a standardized electrochemical measurement and is excess Cu during the active peak as suggested by Hermas, 81 commonly used in the stainless steel industry.76 A typical et al. This example demonstrates the effect that alloying example of a Type 304 austenitic stainless steel (UNS S30400) in elements may have on the polarization curve such that the 77 2MH2SO4 is shown in Figure 14, redrawn after Ogle, et al. history of the material is critical. Cu buildup may have important Note that the je vs. E characteristic curve shows two Ej=0 points, consequences in some applications, for example, resulting defining two cathodic and two anodic domains. The second in an anodic polarization of the material due to galvanic cou- cathodic domain is referred to as a “cathodic loop,”78-81 the origin pling between steel and Cu. In some cases, this may lead to 79 of which has been debated. an enhancement of the passive film. The elemental polarization curves, shown below the je–E characteristic, yield a precise interpretation of the origin of this loop. First, all of the alloy components, with the exception of 5.2 | The Formation and Dissolution of Passive Films Cu, dissolved congruently during the first anodic peak.(4) The on Stainless Steel dissolution in the active peak was faradaic, as indicated by the The dissolution of a multicomponent material is often close correspondence between je and jΣ. However, the absence selective for certain elements leaving behind either the more of Cu dissolution in this potential range suggests that metallic noble metals as a metallic film as illustrated for Cu/stainless Cu built up on the surface due to the selective dissolution of the steel above, or as insoluble oxides. In both cases, the residual film other elements. Further, the end of the cathodic loop was may have a profound effect on the reactivity of the underlying substrate. The Cr-rich passive film on the surface of Fe-Cr and (4) The integrated results for elemental dissolution in mass % gives 16.7 Cr, 9.0 Ni-Cr alloys is a case in point. Only a few nanometers thick, its Ni, 1.3 Mn, and 0.11 Mo, Cu which is reasonably close to the bulk composition of 17.6 Cr, 9.03 Ni, 1.37 Mn, and 0.17 Mo. The total quantity of dissolved Cu measurement normally requires ex situ surface analysis, for gives 0.17% as compared to the bulk composition of 0.19% Cu. example, by XPS. As discussed in the Mass Balance section it

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Cu-Fe, Mn 0.00 0.10 0.20 0.000 0.010 0.020 Cu-Mg Mg1 0.00 0.06 0.12

Cu Mg alone

Cu alone 0.00 0.10 0.20 Mn 0.00 0.04 0.08 0.020

Fe2 Cu 0.0000.00 0.010 0.04 0.08 0.12 0.00 0.04 0.08 Downloaded from http://meridian.allenpress.com/corrosion/article-pdf/75/12/1398/2656216/3336.pdf by guest on 29 September 2021 (arbitrary unit)

M Cu No Cu I 85% 15%

Cu Cu (Al Cu) Mg (Mg Si) Mg 80% only 20% 2 90% 2

Cu-Mg Fe (Al CuMg) (Al3Fe) 5 s 44% 2 5%

Cu-Fe (Al Cu Fe) Mg-Mn (Al3Mn 19% 7 2 Mg Si) Fe 5% 2

Mn Cu-Mn-Fe (Al-Cu-Mn-Fe) 17%

Time (s) FIGURE 13. (Left) Transient data for Cu, Mg, Fe, and Mn during the reaction of AA2024-T351 with 1.2 M NaOH at 60°C obtained at 10 points per second. The sharp peaks (single points) correspond to particle release. Different types of particles are detected including those that contain Cu and Mg (dashed lines) and Cu, Fe, and Mn (solid lines). Particles containing only Cu are indicated with a “*” and only Mg by a “+.” Modified from Gharbi, et al.,69 under CC-BY-NC-ND license. (Right) Statistical analysis of the elemental correlations, modified from from Gharbi, “In situ investigation of elemental corrosion reactions during the surface treatment of Al-Cu and Al-Cu-Li alloys,” Ecole Nationale Supérieure de Chimie de Paris, 2016, with permission.72 is often possible to indirectly determine the quantity of residual between the two curves. The formation of the Cr enrichment elements remaining on the surface of the dissolving material caused the system to transition into the passive state, clearly through a mass balance via Equation (6).82-84 indicated by the decreased dissolution rate and negative A recent example is the cyclic activation and passivation open-circuit potential, following the release of the potential of Type 304 stainless steel in 2 M H2SO4 in Figure 15. Shown are (+412 mVSCE). ν ν 0 the Fe and Cr dissolution rates, Fe and Cr, respectively, where The activation step (A in Figure 15) involves the cathodic the apostrophe indicates normalization to the bulk composition dissolution of the passive film. The dissolution reaction in as follows: this case was complex but most likely due to the reduction of the Cr(III) enriched passive film to give a soluble Cr(II) ν 0 ð = Þν Cr = %Fe %Cr Cr (15) species: In this way, the dissolution profile obtained during the cyclic experiments reveals at a glance much about the CrðIIIÞðoxide − hydroxideÞþe− → Cr2þðaqÞ (17) mechanisms of dissolution. The open-circuit (active) period is characterized by congruent dissolution of Cr and Fe as indicated by the equality of the normalized Cr and Fe dissolution rates, The removal of the passive film caused the system to ν 0 ν Cr = Fe. In this period, the rates of dissolution were proportional transition into the active state indicated by the markedly en- to the bulk composition and no detectable film formation hanced dissolution rate and a more negative open-circuit occurred. potential (322 mVSCE). The dissolution of the Cr enrichment The passivation step involved the formation of a Cr-rich occurred during this step, indicated in Figure 15 by the excess passive film, inferred from the dissolution of excess Fe ν 0 ν Cr going into solution ( Cr > Fe). The quantity of dissolved Cr in (ν 0 < ν ): fi ν 0 Cr Fe the passive lm was determined by the difference between Cr and ν , indicated by the blue shaded area. − → 2þð Þþ ð Þ − þð þ Þ − Fe Fe Crx Fe aq xCr III oxide 2 3x e (16) The kinetics of passive film formation and dissolution may The total quantity of the Cr(III) enrichment was estimated be obtained from the mass balance equations (Equation [6]) from a mass balance and is indicated by the pink shaded area which give the accumulated excess Cr (ΘCr) as a function of

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–1 Free Corrosion (active) Free Corrosion (passive) C1 500 ) –2 P +412 mV SCE

) A1 2 –3 0 E (applied) –322 mV

–4 E (mV E (open circuit)

(A/cm C2 | –500 j

| –5 A2 A

Log j 0.15 –6 e Cr* ) Excess –7 –2 Cr dissolving Fe

·cm 0.1

–8 Σ –1 jM

2 Excess

(ng·s 0.05 Fe dissolving M A/cm ν μ Downloaded from http://meridian.allenpress.com/corrosion/article-pdf/75/12/1398/2656216/3336.pdf by guest on 29 September 2021 200 0 0 100 200 300 400 500 Time (s) FIGURE 15. Dissolution profile for a single active–passive cycle for an

Fe austenitic stainless steel (Cr 18, Ni 15, Si 4) in 2 M H2SO4 T=25°C.Shown Cr x2 are the Fe and normalized Cr dissolution rates. The shaded areas Ni x2 Mo x40 represent periods of either Cr (green) or Fe (pink) selective dissolution. Original data described in Ogle, et al.,83 reprinted with permission.

Cu x200 5.3 | Spontaneous Passivation on Ni-Cr-Mo Alloys –600 –350 –100 150 400 650 It is of interest to compare the elemental polarization curve of stainless steel with that of a Ni-Cr-Mo alloy, E (mVSCE) Figure 17.85 The polarization curve in the upper part of the FIGURE 14. Elemental polarization curve of a Type 304 stainless steel figure shows a similar form with clearly defined active peaks, 2 sample in 2 M H2SO4 at 25°C. Electrode surface area = 0.5 cm and although this alloy does not show any cathodic loop. The potential sweep rate = 0.5 mV/s. (a) Conventional polarization curve elemental dissolution, however, is quite different: the active peak giving the electrochemical current density (j ) as a function of poten- e of stainless steel shows nearly congruent dissolution with a tial; (b) comparison of j and Σj showing nearly faradaic dissolution in e M near 100% faradaic efficiency, while that of Ni-Cr-Mo reveals the active peak; and (c) partial elementary dissolution current densi- elemental dissolution prior to the active peak, but minimal ties (jM) for M = Fe, Cr, Ni, Mo, and Cu. The values have been multiplied by arbitrary factors as indicated for clarity. Modified after Ogle, et al.,77 dissolution during the active peak. This suggests that the ma- with permission. jority of the oxidation, indicated by the total anodic current density, was due to the formation of insoluble species. The absence of Mo dissolution in this domain suggests that Mo time. The simultaneous measurement of the corrosion rate and builds up on the surface during the active to passive transition film quantity allows the examination of the effect of the film on and may play a role in the passivation mechanism. A mass- the corrosion rate in situ. Typical results are shown in charge balance during the active peak (a2 in Figure 17) yields Figure 16 for a sequence of four active/passive cycles. These approximately 2.5 mC/cm2 of undetected oxidation (blue results demonstrate the inverse relationship between the shaded area), which is a factor of 100 less than was obtained for soluble corrosion rate, jdiss (= jΣ), measured as the sum of the pure Ni. This would correspond to the formation of a nano- elemental dissolution rates, and ΘCr. Passive film formation metric film on the order of 2 nm to 3 nm assuming a standard under these conditions involves the formation of an oxide film density for Cr2O3. containing approximately 0.4 μg Cr/cm2, corresponding to The cyclic activation–passivation of a Ni-Cr-Mo alloy is approximately 0.7 nm assuming a uniform Cr2O3 surface film given for both Cr and Mo in Figure 18. A unique feature of this of ordinary density. This may seem very small; however, alloy as compared to stainless steel (Figure 16) is the spon- it should be borne in mind that this is also a very short time taneous passivation which occurred during the open-circuit time passivation corresponding to only the first steps of passive period labeled SP in the figure. Following the cathodic removal

film growth. of the passive film (A), ΘCr began to increase as soon as the Information on the kinetics of passivation are also potential was released. This spontaneous formation of the Cr available from results like Figure 16: activation (passive film enriched passive film continued until the passive potential (P) was dissolution) occurs very rapidly with the dissolution peak applied. From the ΘCr versus time profile, the kinetics of corresponding approximately to the residence time distribution repassivation may be directly obtained. in the cell. Dissolution may be even faster than the experi- The ΘMo vs. time profile indicates that Mo enrichment mental RTD due to the formation of hydrogen gas which facil- only occurred during the spontaneous passivation step, con- itates mixing in the cell. Passivation (passive film formation) sistent with the elemental polarization curves of Figure 17 in occurs more slowly and continues even after the applied which no Mo dissolution was observed below 0 V. Partial Mo potential is released. dissolution was observed during the potentiostatic step to the

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Passive Active P2 P1 E P3 P4 500 oc ) Eoc

SCE 0

–500 E (mV A1 A2 A3 400 600 ) –2 ) ·cm 2 400 –1 200 (ng·s (ng/cm 200 Downloaded from http://meridian.allenpress.com/corrosion/article-pdf/75/12/1398/2656216/3336.pdf by guest on 29 September 2021 Cr diss Θ ν

0 0 50 2,150 2,650 3,150 3,650 4,150 4,650 Time (s) FIGURE 16. The accumulation and dissolution of excess surface Cr determined by mass balance during a series of active (A)-passive (P) cycles of Figure 15, for a Type 304 stainless steel in 2 M H2SO4. The relationship between the excess Cr (blue) and the total elemental dissolution rate (red) is clearly indicated, showing the effect of passivation and activation. Original data described in Ogle, et al.,83 reprinted with permission.

passive potential, and complete dissolution occurred during indicated by a drop in pH from 7.4 at the flow cell entrance, to activation. This complex behavior of Mo may offer a partial approximately 3 to 4 at the exit. Obviously a more intense explanation as to why it has been difficult to elucidate the acidification would be expected at the interface. These results mechanism by which Mo improves the corrosion resistance of suggest a mechanism of Mo enrichment and release that could Ni-Cr alloys: whether or not Mo is enriched in the film depends play a significant role in the repassivation of a Ni-Cr-Mo alloy in on the potential at which passivation occurs. neutral chloride electrolytes, for example, during crevice corrosion. 5.4 | Transpassive Enrichment of Mo: Ni-Cr-Mo Alloys in Neutral Chloride Electrolyte Mo enrichment during transpassive–passive cycles was ALTERNATING CURRENT AND POTENTIAL investigated for a series of Ni-Cr-Mo alloys in a neutral saline TECHNIQUES solution (1.0 M NaCl, pH = 7.4, T = 75°C) using a similar cyclic potentiostatic method.86 The idea was to simulate the pitting/ Electrochemical impedance spectroscopy (EIS) is an repassivation behavior that might be observed during crevice important technique in the arsenal of the corrosionist. For corrosion. The transpassive–passive experiments were con- steady-state corroding systems, the EIS spectrum may in some cases be used to estimate the corrosion rate without ducted by stepping the potential from the passive domain to significant electrochemical perturbation, and the EIS spec- the transpassive domain and then returning to the passive trum itself is often considered a fingerprint for specific domain. mechanisms revealing different kinetic processes over a It was found that Mo enrichment occurred during trans- wide range of time constants. It is straightforward to perform passive dissolution and redissolved when the potential returned EIS simultaneously during an AESEC experiment and to the passive domain as indicated by the normalized disso- Cwalina, et al.,87 used high-frequency AC electrochemical lution profiles of Figure 19. The enrichment of Mo during the measurements to follow the growth of oxide films for the transpassive step is indicated by the low dissolution rate Ni-Cr-Mo system during conventional DC experiments (normalized to Ni) during this period. The quantity of enriched coupled with ICP-MS measurement of elemental dissolu- Θ Mo ( Mo) may be determined from the difference between tion rates. Ni and Mo dissolution rates, indicated by the yellow shaded It is also possible to monitor the oscillating elemental area. When the potential was potentiostatically returned to the dissolution rates and to decompose the global EIS data into passive domain, the excess Mo dissolved. The quantity of elemental components at least for low frequencies. In this way Mo dissolved (QMo) may be determined from the shaded blue the EIS measurement may be obtained on an element by element Θ area. The inset to Figure 19 gives a plot of Mo vs. QMo for a series basis and the AC response of oxide formation or the cathodic of experiments performed at different hold times as indicted in reaction obtained by consideration of jΔ as in the previous the transpassive domain. sections. Of particular importance is that dynamic systems From these results, it was proposed that Mo precipitation may be treated directly, by analyzing data in the time domain. The and redissolution are driven by local pH changes. Even though a latter idea was demonstrated for the Mg/1 M NaCl system in flow cell was used, a diffusion zone of about 130 μm to 200 μm which EIS-AESEC was used to pin down the origin of the low- was present at the material/electrolyte interface (Instrumentation frequency inductive loop commonly observed and the origin of section). During transpassive polarization the acidification was which has been debated.88 This work revealed that the inductive

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E (V ) Cycle 1 Cycle 2 Cycle 3 SCE 0.5 –0.6 –0.4 –0.2 0.0 0.2 0.4 0.6 0.8 A0 SP1 P1 A1 SP2 P2 A2 SP3 P3 A3 SP4 10–1 ) 0.0 10–2 (a) SCE –0.5 E (V 10–3

) –1.0 2 –4 10 250 2 Cr 10–5 j (A/cm 200 133 ng/cm 10–6 a2 15 150 )

–7 2 10 )

j (b) 2 a1 e 10 Downloaded from http://meridian.allenpress.com/corrosion/article-pdf/75/12/1398/2656216/3336.pdf by guest on 29 September 2021 100 –8 A/cm 10 Σj 5 μ

M ( M (ng/cm j

0 M 50 Θ

Ni 0 Mo Cr 3

) 0 500 1,000 1,500 2,000 2,500 2 Mo 3 Time (s) A/cm

μ FIGURE 18. The enrichment of Cr and Mo Ni-Cr-Mo alloy (Hastalloy †

Fe 5 10 ( G22 21%Cr, 13%Mo) in 2 M H2SO4 alloy at 25°C as a function of time for a series of three potentiostatic cycles between passivation − Mn 20 (+0.3 VSCE) and activation cycles ( 0.8 VSCE). Reprinted from Li, et al.,85 under CC-BY license.

0 500 1,000 1,500 2,000 2,500 3,000 Time (s) FIGURE 17. (a) Polarization curve of C22† alloy (UNS N06022) in 2 M

H2SO4 alloy at 25°C, from −1VSCE at 0.5 mV/s and ambient tempera- ture. (b) Electrical current density (je) overlaid with the sum of the AESEC elemental currents (ΣjM). Single elemental current densities (jM) are shown below with offset. The dotted line below each equivalent Trans- Open circuit Passive passive Passive elemental current density indicates the zero value. Some elemental (Passive)

current densities were multiplied by a constant for clarity. Reprinted 25 85 0.35 from Li, et al., under CC-BY license. Slope = 1.2 240 20 1.0 0.30 15 (s) 120 0.8 Ni Mo 10 Θ 0.25 ) Cr 5 ) 60 0.6 2 Fe loop was most likely due to a catalytic enhancement of the 0

0.20 Ag/AgCl dissolution rate at low frequency. 0 5 10 15 20 25 0.4 g/cm QMo (aq) An example of the use of EIS-AESEC is to distinguish μ ( 0.15 situations in which the oxidation of the metal leads directly to M 0.2 ν Mo the formation of dissolved ions or passes through a slightly 0.10 soluble or insoluble intermediate.53 In the former case, it was 0.0 Potential (V possible to measure fundamental electrokinetic parameters 0.05 –0.2 in a single experiment. For example, Figure 20(a) shows the fi dissolution pro le for Zn dissolution 0.1 M NH4Cl solution. 0.00 –0.4 The profile is divided into four time periods: I – measurement 0 200 400 600 of the background signal in the electrolyte alone; II – open- Time (s) circuit dissolution; III – measurement of potentiostatic EIS data FIGURE 19. A direct measurement of the retention and release of Mo from high to low frequency; and IV – return to open-circuit † during a Ni-Cr-Mo alloy (Hastalloy G30 [UNS N06030]) in 1 M NaCl at dissolution. The excellent correlation between the AC com- 75°C, during a potentiostatic cycle of passive (+0.3 VSCE) → transpas- ponents of the electrical current density and the zinc dis- sive (t = 0, +1.0 VSCE) → passive (+0.3 VSCE) → open circuit. The solution rate in the lower frequency range demonstrates that dissolution rates are normalized against the bulk composition. The anodic dissolution occurred directly without the formation of inset shows the relationship between Mo enrichment in the transpas- aninsolubleintermediateonthetime scale of the experiment. sive domain and Mo release in the passive domain. The shaded areas Analysis of the dissolution rate and total current density show the integrated data used to calculate Mo enrichment (yellow) and transients as a function of potential revealed the Mo release (blue). Modified after Henderson, et al.,86 with permission.

CORROSIONJOURNAL.ORG DECEMBER 2019 • Vol. 75 • Issue 12 1413 INVITED CRITICAL REVIEW ) 1 Hz ATOMIC EMISSION SPECTROELECTROCHEMISTRY 0.01 Hz 0.005 Hz –1.05 COMBINED WITH GRAVIMETRIC AND VOLUMETRIC

Ag/AgCl TECHNIQUES E –1.10 E (V The stoichiometric relationship between current and I II III IV dissolution obtained by AESEC may give considerable informa- tion concerning the mechanisms of interfacial reactions. jZn However, in many situations this is not sufficient to completely 0.2 )

2 describe the reaction and additional real-time techniques are of interest. 0.1 jcorr In situ gravimetry may yield additional insight into interfacial processes which involve simultaneous film formation j (mA/cm 0.0 and substrate dissolution, such as corrosion product for- je –0.1 mation or conversion coating chemistry. The electrochemical 0 1,000 2,000 3,000 4,000 5,000 6,000 quartz crystal microbalance (EQCM) is a technique that Downloaded from http://meridian.allenpress.com/corrosion/article-pdf/75/12/1398/2656216/3336.pdf by guest on 29 September 2021 (a) monitors the mass of the working electrode as a function of time: the test material is deposited on a thin quartz crystal ) 1 Hz wafer, the resonance frequency of which, under optimum 0.01 Hz 0.005 Hz –1.00 conditions, is inversely proportional to the mass of the –1.05 Ag/AgCl electrode. The coupling of EQCM–AESEC therefore provides E E (V information on the mass changes and dissolution simulta- I II III IV neously. This coupling was achieved via a special flow cell 0.3 replacing the working electrode of AESEC with a quartz

) electrode. It has been used to investigate the anodic dissolution

2 0.2 of Cu and the formation of phosphate90 and chromate91 j 0.1 Zn conversion coatings on zinc. Figure 22 gives EQCM–AESEC profiles for the sponta- j (mA/cm 0.0 neous, open-circuit reaction of a Zn electrode with a 0.2 M j e phosphoric acid solution (a) alone and (b) containing 1.37 g/L –0.1 KNO . The upper curves shows the AESEC dissolution profile, 0 1,000 2,000 3,000 4,000 5,000 6,000 3 Time (s) and the lower curve gives the calibrated QCM frequency transient expressed in “mass” superimposed with the integral of (b) the ICP transient. Also shown is the “precipitation profile,” FIGURE 20. Open-circuit dissolution of zinc (galvanized steel) being the difference between the total mass change from the

(period II) followed by a potentiostatic EIS experiment (period III) QCM (ΔMQCM) and the dissolution mass change from the ICP with AC potential variation from 10 kHz to 0.00398 Hz, in: (a) 0.1 M (ΔMICP). This difference (ΔMfilm) corresponds to the growth of NH4Cl and (b) 0.5 M NaCl. Period I is a measurement of background the Zn–phosphate film. The results of Figure 22 demonstrate electrolytewithnosamplecontact,andperiodIVisthereturnto that film formation was enhanced by the addition of nitrate, fi 53 open-circuit potential. Modi ed after Shkirskiy and Ogle with justifying its frequent use as an accelerator in conversion permission. coating formulations. For the nitrate-containing solution, the

ΔMfilm traced a sigmoidal curve with a saturation level of approximately 250 μg/cm2, roughly 10× that observed in the phosphoric acid solution without nitrate. Film formation was dominant only during the first ≈100s,whileZndissolution anodic and cathodic Tafel slopes for this system. In contrast, occurred throughout the exposure. At longer times, ΔMICP for Zn/0.5 M NaCl (Figure 20[b]), electrochemical oxidation closely followed ΔMQCM, demonstrating that, under the condi- leads to the formation of an intermediate corrosion product tions of these experiments, the frequency of the QCM is a film with subsequent dissolution. As a consequence, no reliable measure of mass loss. variation of the elemental dissolution rate was observed Gas evolution often occurs simultaneously with anodic despite the alternating electrochemical current. This type dissolution, for example during the negative difference effect of behavior has been previously seen for silicate-based (NDE) of Mg,92-95 the cathodic dissolution of Al,59,96-97 and conversion coatings using cyclic large amplitude DC anodization of Al alloys.98 A method of independently measuring potentiostatic cycles and is an interesting approach gas evolution and thereby the stoichiometry of gas formation to quantifying the stability of oxide films used as permits a kinetic decoupling of gas evolution, anodic dissolution, conversion coatings.52 film formation, and charge transfer. The first attempt to Different elements of an alloy react differently with combine a volumetric method with AESEC was obtained by the environment and this may sometimes be observed in Lebouil, et al., who, obtained the full stoichiometry of Mg the elemental impedance behavior. A fascinating example is dissolution, during the NDE.92-93 The technique she developed the Zn-Al-Mg system in a pH = 10.5 ammonium buffer, shown used an in-line high-speed video camera to measure H2 bubble in Figure 21.89 For this system, Zn and Mg dissolve in volume by image analysis after breaking the bubbles up in a phase with the AC current while Al dissolution is shifted microfluidic capillary system. More recently, Han and Ogle10 out of phase by 180°. This clearly indicates a different have combined AESEC with a downstream gravimetric mea- mechanism for Al. The origin of this shift is the subject of surement of hydrogen gas based on Archimedes’ principle as current research. proposed by Curioni.94

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je Zn x1 200 s 2 0.1 Hz 0.0046 Hz A/cm 0.2 μ 100 0.15 Al je 0.1 Mg x12.5 200 s 2 ) 2 0.05 Zn A/cm μ (mA/cm

M 0 j 100 Mg –0.05 i*

jZn jMg jAl je E je conv ≈ –0.1 Al x5 200 s je 800 1,300 1,800 2,300 2,800 3,300 3,800 180° Downloaded from http://meridian.allenpress.com/corrosion/article-pdf/75/12/1398/2656216/3336.pdf by guest on 29 September 2021

Time (s) 2 A/cm μ 100

+ FIGURE 21. EIS-AESEC profile for a Zn-Al-Mg coating (3.7% Al, 3.0% Mg) on steel in an ammonium buffer (0.1 M NH4 /0.09 M NH3 + 0.01 M − HCO3 electrolyte, pH 10). Right: Expanded scale of low-frequency period showing that Zn and Mg dissolve in phase with the electrical current, while Al is shifted out of phase by 180°. Redrawn after Shkirskiy, et al.,89 with permission.

With nitrate Without nitrate 1 E ) 1 E ) –2 0.8 –2 0.8 –1,100 –1,100 ·cm

ν ) 0.6 ·cm Zn

–1 0.6 –1

0.4 SCE g·s 0.4 ν g·s μ

Zn –1,200 μ

( –1,200

0.2 ( 0.2 Zn E (mV Zn ν 0 ν 0 –0.2 –1,300 –0.2 –1,300 Δ 300 Mfilm 40 Δ Mfilm ) ) 200 Δ 20 2 M 2 QCM 0 100 ΔM –20 QCM g/cm g/cm

μ 0 μ μ Δ –40 MICP

Q ( –100 Q ( –60 Δ MICP –80 –200 –300 –50 50 150 300 450 –50 50 150 300 450 t (s) t (s) (a) (b)

FIGURE 22. Reaction profile for Zn in 0.2 M phosphoric acid solution: (a) without and (b) with nitrate. Upper curve: (νZn) ICP concentration transient expressed as dissolution rate; (E) open-circuit potential; lower curve: integral of νZn (ΔMICP); QCM frequency transient expressed in mass (ΔMQCM); precipitation mass (ΔMfilm = ΔMQCM − ΔMICP). The dashed line indicates the point at which the reactive electrolyte enters the cell (t = 0 s). Before this the cell was filled with water. Reprinted from Ogle, et al.,11 with permission.

∗ An example of the latter work is given in Figure 23 showing current density with a ratio of jjej=jAl = 1.7 0.1, consistent with the cathodic corrosion of an AA2024 (UNS A92024) alloy in a the previously cited work. This ratio was always >1 due to neutral electrolyte was investigated at an imposed potential of the formation of excess hydroxide which diffuses away from − 10 1.8 VSCE. The expected overall stoichiometry was: the interface. The stoichiometry of the hydrogen evolution reaction (HER) is displayed in the lower curve showing the þ þ − → ð Þ− þ Al 4H2O e Al OH 4 2H2 (18) integrals of the dissolved Al, QAl, the inversed net charge, –Qe, and the evolved H2,nH2. The latter is compared with the ∗ The upper curve gives the dissolution profile for je and jAl. hypothetical quantity of evolved H2, calculated from the mass The Al dissolution rate mirrors the slow onset of the cathodic balance of Reaction (4), nH2(mb) = (3QAl – Qe)/2. The ratio of the

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questions of how specific elements interact with each other in E Eap = –1.80 VSCE E oc oc an alloy, or how specific additives in a surface treatment solution (a) 5 will affect different alloying elements or different phases. The

) j 2 Al AESEC technique will no doubt prove useful for the development of more stable alloy materials and improved surface treatment 0 solutions. Although this work has been focused on metallic

(mA/cm materials, AESEC may be used to study the reactivity of other M j materials such as polymer and paint films, controlled inhibitor –5 je* release, or the selective dissolution of minerals during processing. ➣ In this work, the basic quantitative relationships were (b) 80 Q developed and illustrated by numerous examples including 70 e n (mb) the spontaneous reactivity of Al alloys with commercial 60 H2

mol) n surface treatment solutions and the dissolution and passiv- μ 50 H2

40 ation of Fe-Cr and Ni-Cr-Mo alloys. In the case of Al surface Downloaded from http://meridian.allenpress.com/corrosion/article-pdf/75/12/1398/2656216/3336.pdf by guest on 29 September 2021 30 treatment, the extremely fast etching of Al may be simulta- 20 neously measured by the extremely slow dissolution pro-

Quantity ( 10 –QAl cess of Cu and other alloying elements. The compatibility with 0 complex, concentrated electrolytes was also demonstrated. 0 500 1,000 1,500 2,000 2,500 AESEC detects non-faradaic processesaswellsuchasin- termetallic particle release. This type of experiment may Time (s) prove useful for the development of new surface treatment FIGURE 23. Simultaneous AESEC and volumetric determination of technologies. hydrogen evolution during the athodic dissolution of an AA2024 alloy ➣ By mass balance, the surface accumulation of small at Eap = –1.80 VSCE in a 30 g/L NaCl, pH = 6.3 solution. (a) Al dissolution quantities of various elements may be readily detected at the rate as equivalent current density (jAl) and je, and (b) quantity of sub-nanometer level. These may be metallic elements in the dissolved Al, Q , inversed net charge, −Q , released hydrogen, n , Al e H2 case of a dealloying phenomenon or oxide films in the case of and expected hydrogen by mass balance, n (mb). Reprinted from H2 passivation. The relationship between the corrosion rate and Han and Ogle,10 under CC-BY license. the accumulation of surface Cr yields new information on the relationship between surface composition/structure and re- activity. Such experiments may prove useful for the development of new alloy materials. measured to the hypothetical quantity of evolved hydrogen ➣ The objective of this work has been to demonstrate that was found to be n /n (mb) = 0.94 at the end of the potential H2 H2 AESEC has great potential to validate many hypotheses re- pulse, showing a fairly good agreement with the stoichiometry of garding corrosion and electrochemistry. A few selected Equation (18). applications of AESEC would be to understand the role of The coupling of other in-line methods of electrolyte analysis different alloying elements on passive film formation, opti- with AESEC is quite straightforward although it has not been mizing the formulation of additives during surface treatment, extensively exploited. Only a small portion (≈5%) of the elec- quantifying the impact of corrosion inhibitors on different trolyteisactuallyaspiratedintothe plasma, the remainder being elements and phases of an alloy, quantifying the kinetics of pumped away to a waste container. Downstream pH measure- cathodic dissolution such as is observed for Mg, Al, or ments have been used frequently. For example the variation of passive film dissolution during cathodic cycles of bipolar plate pH during linear scan polarization of Al alloys was presented in operation. A number of methodological ideas remain unde- Serdechnova, et al.51 Other techniques are possiblesuchasdown- veloped or underdeveloped. As previously mentioned, other stream electrochemical or UV-visible detection. For example, techniques may be coupled with the flow system such as electrochemical detection was used to quantify the dissolution rates electrochemical detection to determine oxidation states of of Cu(I) and Cu(II) during the anodic dissolution of Cu-Zn alloys in a dissolved ions or UV-visible spectroscopy to measure or- channel flow double electrode by Hoshi, et al.,99-100 and UV ganic species. Numerical modeling of the flow cell would allow detection was used to monitor xanthate adsorption and de- one to better understand the chemical and hydrodynamic sorption during a simulated electrochemical flotation of chalcopyrite conditions of the surface and perhaps optimize the geometry of with a flow spectroelectrochemical system by Walker, et al.101 the flow cell. The methodology could be extended to include There is much to be developed along these lines. gas analysis for the investigation of catalytic reactions and real- time video imaging of the surface. These and other possi- CONCLUDING REMARKS bilities are under consideration. ➣ The direct, time resolved measurement of elemental dis- solution rates yields insight into the reactivity of complex, ACKNOWLEDGMENTS multi-element materials, unavailable by conventional electro- chemical, gravimetric, or ex situ spectroscopic measurements. I would like to express my gratitude to my former coworkers This direct approach to interfacial kinetics removes some of Paul Lodi and Sophie Weber at the Institut de Recherche de la the limitations imposed by electrochemical methods, such as Siderurgie, who assisted in the early development of this the necessity of a steady-state reaction, the uncertainty of the technique. I am immensely grateful to all my former students and specific faradaic reactions occurring at the interface, or the collaborators whose works are cited herein, and to my col- guess work associated with interpreting open-circuit potential league Professor Polina Volovitch. I thank Patrick Chapon and measurements. This methodology gives direct insight into the Alice Stankova of Horiba France for technical assistance and

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NOMENCLATURE

AESEC Atomic emission spectroelectrochemistry Eoc Open-circuit or spontaneous corrosion potential ICP-AES Inductively coupled plasma atomic emission Ej=0 Zero current potential from a polarization curve spectroscopy nM Assumed number of electrons transferred during ICP-MS Inductively coupled plasma mass spectrometry the dissolution of M OTTLE Optically transparent thin layer cell jM Dissolution rate of element M expressed as an GDOES Glow discharge optical emission spectrometry equivalent current density SDC Scanning droplet cell jΣ Sum of elemental current densities RTD Residence time distribution, also h(t) jdiss Soluble corrosion rate expressed as current NDE Negative difference effect density, normally = jΣ if all elements are QCM Quartz crystal microbalance measured CM Concentration of element M downstream from je Electrical current measured by the electrometer flow cell of the potentiostat Iλ Emission intensity at wavelength (λ) je After convolution with residence time distribution Iλ° Background intensity at wavelength (λ) jΔ Faradaic current for phenomena not detected by κλ,M Sensitivity factor relating emission intensity at ICP-AES; jΔ =j*e – jΣ wavelength (λ) to concentration of M nH2 moles of hydrogen measured by in-line volumetry νM Measured dissolution rate of element M normal- nH2(mb) moles of hydrogen determined by stoichiometric ized to the surface area mass balance of AESEC data ν° True dissolution rate of element M QM The total quantity of element M dissolved in a ν 0 fi M Measured dissolution rate of element M, de ned time period normalized for the bulk concentration of the Qe Total charge, integral of je/F material ΘM Surface enrichment of element M determined by Eap Applied potential during potentiostatic mass balance experiments

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Θe Quantity of accumulated charge, integral h(t) Functional form of the residence time distribution of jΔ/F, used in the determination of oxide (RTD) formation τ, β Empirical factors of log-normal residence time ΘM(z) Surface enrichment of M, oxidation state z, usually distribution used in convolution in the form of oxide Δt Time shift between electrochemical measure- ΔMQCM Mass change of the working electrode as deter- ments and ICP-OES measurements due to elec- mined by QCM trolyte transport between the flow cell and plasma ΔMICP Mass change of the working electrode as deter- f Flow rate mined by ICP A Geometrical area of specimen exposed to the ΔMfilm Mass change due to film formation on working electrolyte in the flow cell electrode, = ΔMQCM – ΔMICP Downloaded from http://meridian.allenpress.com/corrosion/article-pdf/75/12/1398/2656216/3336.pdf by guest on 29 September 2021

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