Application of AFM-Based Techniques in Studies of Corrosion and Corrosion Inhibition of Metallic Alloys

corrosion and materials degradation

Review Application of AFM-Based Techniques in Studies of Corrosion and Corrosion Inhibition of Metallic Alloys

Kiryl Yasakau

Department of Materials and Ceramic Engineering, CICECO—Aveiro Institute of Materials, University of Aveiro, Campus of Santiago, 3810-193 Aveiro, Portugal; [email protected]

Received: 7 October 2020; Accepted: 5 November 2020; Published: 7 November 2020

Abstract: In this review several scanning probe microscopy techniques are briefly discussed as valuable assets for corrosionists to study corrosion susceptibility and inhibition of metals and alloys at sub-micrometer resolution. At the beginning, the review provides the reader with background of atomic force microscopy (AFM) and related techniques such as scanning Kelvin probe force microscopy (SKPFM) and electrochemical AFM (EC-AFM). Afterwards, the review presents the current state of corrosion research and specific applications of the techniques in studying important metallic materials for the aircraft and automotive industries. Different corrosion mechanisms of metallic materials are addressed emphasizing the role of intermetallic inclusions, grain boundaries, and impurities as focal points for corrosion initiation and development. The presented information demonstrates the importance of localized studies using AFM-based techniques in understanding corrosion mechanisms of metallic materials and developing efficient means of corrosion prevention.

Keywords: AFM; SKPFM; EC-AFM: localized corrosion; corrosion inhibition; metallic alloys

1. Introduction Corrosion of metallic materials and structures is a heavy burden for the economy which costs about 3.4% of the global GDP (2013) according to the Application and Economics of Corrosion Technology (IMPACT) study released by NACE International [1]. The study also acknowledged that corrosion prevention strategies could increase global savings by about 15–35% of the cost of the damage (NACE International, Houston, TX, USA, 2016). Exposure of metallic materials to corrosive environments is the main cause of corrosion. Earth’s natural oceanic waters and atmospheric moisture containing man-generated pollutants such as oxides of carbon, oxides of nitrogen, oxides of sulfur are the most typical corrosive environments [2]. Therefore, metallic structures such as ships, naval platforms, vehicles, airplanes, etc. operated in such environments suﬀer from corrosion attack. Corrosion monitoring and prevention are especially important for the transportation sector. Vehicles may be operated in a wide variety of environments favoring corrosion development which often leads to failure of critical structural parts. Strong safety concerns arise when corrosion impairs a critical component. Therefore predicting and understanding corrosion and building eﬃcient corrosion protection strategies are extremely important for engineers and scientists. Corrosion is the destructive attack of a material by reaction with its environment [3]. Electrochemical and chemical reactions contribute to the degradation of metals. The susceptibility of metals to corrosion depends on the nobility of the metals, the environment, the temperature, the pH of the aqueous solution, the presence of passive layers, and the possible electrochemical reactions occurring in the environment [2]. Thermodynamic data allows prediction of the equilibrium state of a system and theoretical corrosion susceptibility of metals which can be easily visualized on Eh-pH diagrams [4]. By following the oxidative–reduction potential and pH on the diagrams one can predict

Corros. Mater. Degrad. 2020, 1, 345–372; doi:10.3390/cmd1030017 www.mdpi.com/journal/cmd Corros. Mater. Degrad. 2020, 1 346 the occurrence of corrosion, passivity, and immunity of metals to corrosion. The kinetic characteristic such as exchange current at equilibrium defines the rate of degradation of a metal in an environment. There are many forms of corrosion and various factors leading to different forms of corrosion such as the composition of metallic materials, environment, mechanical stress, geometry, temperature, and time [3]. General (uniform) corrosion is the case in which all the metal surface is subjected to corrosion. Galvanic corrosion typically occurs in an aqueous environment when dissimilar metals, having different nobility, are electrically connected. The material with more noble electrode potential becomes the cathode and the material with the less noble potential becomes the anode and is dissolved. Localized corrosion is a collective term of several major corrosion cases. These cases are as follows: pitting corrosion, crevice corrosion, stress corrosion cracking, exfoliation corrosion, filiform corrosion, intergranular corrosion. Localized corrosion is typically concentrated in several spots on the metal surface, unlike the case of general corrosion where the entire surface is active. Metallic structures are typically made of alloys because pure metals possess poor mechanical properties, an example is a pure aluminum and its alloys [5]. Amongst engineering metallic alloys one can mention ferrous alloys, aluminum alloys, titanium alloys, nickel alloys. However, energy-saving directives [6] are promoting the race for new metallic materials. In this perspective magnesium alloys stand out as lucrative lightweight materials. Light alloys found their application as structural and functional materials in aerospace, automotive industries, and biomedicine [7,8]. Metallic alloys are engineered in such a way as to improve mechanical properties by alloying the main metal with other elements. In some circumstances the alloying improves corrosion susceptibility as well. For example, stainless steel uses the alloying elements of chromium, nickel, and molybdenum which can result in the formation of a stable oxide layer on the top surface, which protects the bottom metal from corrosion and prevents pitting corrosion [9]. In other situations, the alloying may increase the corrosion susceptibility of an alloy. A typical aerospace aluminum alloy such as AA2024 contains copper as the main alloying element. It has a much higher corrosion rate than pure aluminum due to the adverse effects of copper-containing intermetallics which induce high susceptibility to localized corrosion [10–13]. The automotive and aerospace sectors actively utilize engineered materials such as aluminum alloys, magnesium alloys, and zinc galvanized steel. Intermetallic phases existing in alloys normally have higher corrosion potential when they contain more noble elements compared with the base metal, for instance, Al3Fe, Al2Cu, Al7Cu2Fe in aluminum alloys [14–17]. A similar observation is made for intermetallics e.g., Mg17Al12, η-Al8Mn5 found in magnesium alloys [18]. Electron microscopy investigations help to elucidate localized corrosion events associated with intermetallics in the depth of the metallic matrix [14,15]. General electrochemical techniques such as electrochemical impedance spectroscopy (EIS) and DC-polarization are commonly employed to reveal interfacial characteristics and corrosion kinetics of different metallic phases and alloys [19–21]. However, general electrochemical measurements offer limited information coming only from the macroscopic surface. Although a deeper understanding of the mechanism of corrosion below macro-scale can be obtained using the scanning vibrating electrode technique (SVET), scanning electrochemical microscopy (SECM), the scanning Kelvin probe (SKP), a more local analysis is often needed to investigate at the level comparable with the size of intermetallics. In situ measurements using atomic force microscopy (AFM) and related methods help to elucidate various phenomena at microscale with sufficient resolution to observe surface changes even at the atomic scale. AFM-based methods are complementary to common methods of analysis such as electron microscopy and general corrosion investigation methods [22]. This review aims to provide an overview of the state of the art in applications of atomic force microscopy-based techniques for corrosion investigation of different metals and alloys. The first part of this review briefly addresses the principles of AFM, and AFM-based techniques such as scanning Kelvin probe force microscopy (SKPFM) and electrochemical AFM (EC-AFM). The next sections present relevant knowledge emphasizing how the above techniques are used for studying localized corrosion Corros. Mater. Degrad. 2020, 1 347 phenomena, the properties of the interfaces, and the surface evolution of metals and alloys. Various examples and approaches are presented. The review ends with concluding remarks.

2. Principles of Main AFM Based Techniques

2.1. AFM Atomic force microscopy (AFM) was developed in 1986 by Binnig, Quate, and Gerber who demonstrated for the first time the ideas of AFM which is based on a combination of the principles of the Scanning Tunneling Microscope (STM) and the stylus profilometer which used an ultra-small probe tip at the end of a cantilever [23]. Typical commercial AFM systems nowadays use a laser reflection method to detect the displacement of a cantilever. Figure1a displays a typical scheme of AFM consisting of several essential components: a photodetector, a very sharp tip attached to a cantilever, a piezoelectric tube (PZT) scanner, and control and feedback electronics. AFM operates by maintaining with high precision the force of interaction between the tip and the surface (Figure1b). When the tip comes close to the surface it experiences van der Waals, electrostatic, and magnetic forces that pull the tip towards the surface. The further the tip approaches the surface the more the repulsive forces start to act upon the tip. The force curve presented in Figure1b displays the three regions in which AFM is operated. The respective modes of AFM are called non-contact mode, intermittent contact (tapping) mode, and contact mode. In the former two modes, the cantilever is vibrated close to the resonance frequency of a cantilever with the help of a piezo drive and the vibration amplitude/frequency is controlled by the control circuit connected to the photodetector (Figure1a). In non-contact mode, the tip vibrates close to the surface and experiences attractive forces, so the amplitude of vibration and vibration frequency are lowered. A frequency modulation (FM) technique improves the sensitivity [24]. This method is applied to soft samples to minimize surface damage in imaging of live cells [25], measurements in thin liquid layers [26], and studies of point defects in oxides [27]. In an intermittent contact regime the tip gently taps the surface while experiencing different attractive and repulsive interactions. When the tip comes close to the surface it experiences repulsive forces and the frequency of vibration of the cantilever increases and its amplitude of vibration decreases. Aside from topography images, this method is sensitive to the mechanical properties of the surface such as adhesive forces, stiffness, or softness. The contact mode is much simpler than the previous two modes. In such a mode the tip stays in contact with the surface and the force of interaction is proportional to the deflection of the cantilever monitored via the photodetector. The contact mode allows mechanical properties to be measured such as the frictional force between the tip and the sample thanks to the lateral forces which the tip experiences during movement on the surface [28]. In all the modes the feedback circuit controls the force the tip experiences and adjusts the position of a PZT scanner to maintain the same force. The PZT scanner is composed of a tube made with piezoelectric materials arranged in a specific way which allows the tube to extend/contract and move in different directions depending on the applied voltage. A control scheme makes the scanner move in three dimensions therefore providing a topography image of the scanned surface.

2.2. SKPFM Scanning Kelvin probe force microscopy (SKPFM) is an extension of AFM. This technique was first described by Nonnenmacher and coworkers in 1991 [29]. It is based on a fundamental working principle derived from Lord Kelvin’s experiments. When two metals with different work functions are electrically connected, electrons flow from the higher Fermi level to the lower Fermi level until the Fermi levels equilibrate (Figure2). As a consequence, a surface charge develops on the probe and the sample, with a related potential difference is known as the contact potential difference (CPD) or Volta potential difference (VPD) (Figure2). VPD is related to the work function di fference as described in Corros. Mater. Degrad. 2020, 3 FOR PEER REVIEW 3 corrosion phenomena, the properties of the interfaces, and the surface evolution of metals and alloys. Various examples and approaches are presented. The review ends with concluding remarks.

2. Principles of Main AFM Based Techniques

2.1. AFM Atomic force microscopy (AFM) was developed in 1986 by Binnig, Quate, and Gerber who demonstrated for the first time the ideas of AFM which is based on a combination of the principles of the Scanning Tunneling Microscope (STM) and the stylus profilometer which used an ultra-small probe tip at the end of a cantilever [23]. Typical commercial AFM systems nowadays use a laser reflection method to detect the displacement of a cantilever. Figure 1a displays a typical scheme of AFM consisting of several essential components: a photodetector, a very sharp tip attached to a cantilever, a piezoelectric tube (PZT) scanner, and control and feedback electronics. AFM operates by maintaining with high precision the force of interaction between the tip and the surface (Figure 1b). When the tip comes close to the surface it experiences van der Waals, electrostatic, and magnetic forces that pull the tip towards the surface. The further the tip approaches the surface the more the repulsive forces start to act upon the tip. The force curve presented in Figure 1b displays the three regions in which AFM is operated. The respective modes of AFM are called non-contact mode, intermittent contact (tapping) mode, and contact mode. In the former two modes, the cantilever is vibrated close to the resonance frequency of a cantilever with the help of a piezo drive and the vibration amplitude/frequency is controlled by the control circuit connected to the photodetector (Figure 1a). In non-contact mode, the tip vibrates close to the surface and experiences attractive forces, so the amplitude of vibration and vibration frequency are lowered. A frequency modulation (FM) technique improves the sensitivity [24]. This method is applied to soft samples to minimize surface damage in imaging of live cells [25], measurements in thin liquid layers [26], and studies of point defects in oxides [27]. In an intermittent contact regime the tip gently taps the surface while experiencing different attractive and repulsive interactions. When the tip comes close to the surface it experiences repulsive forces and the frequency of vibration of the cantilever increases and its amplitude of vibration decreases. Aside from topography images, this method is sensitive to the mechanical properties of the surface such as adhesive forces, stiffness, or softness. The contact mode is much simpler than the previous two modes. In such a mode the tip stays in contact with the surface and the force of interaction is proportional to the deflection of the cantilever monitored via the photodetector. The contact mode allows mechanical properties to be measured such as the frictional force between the tip and the sample thanks to the lateral forces which the tip experiences during Corros. Mater. Degrad. 2020, 1 348 movement on the surface [28]. In all the modes the feedback circuit controls the force the tip experiences and adjusts the position of a PZT scanner to maintain the same force. The PZT scanner is composedEquation (1). of a If tube the work made function with piezoelectric of the tip is materials known then arranged it is easy in toa specific calculate way the which work functionallows the of tubethe sample. to extend/contract and move in different directions depending on the applied voltage. A control scheme makes the scanner moveeVVPD in= threee ψ sampledimensionsψprobe therefore= φtip providingφsample a topography image of the(1) scanned surface. − −

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Figure 1.1. A scheme of tapping mode atomic force microscopy (AFM) (a) andand force-distanceforce-distance curvecurve Corros.between Mater. Degrad. the AFM 2020 tip, 3 andFOR sample PEER REVIEW ((b).). 5

2.2. SKPFM Evac Evac Evac Scanning Kelvin probe force microscopy (SKPFM) is an extension of AFM. This technique was level level level E E E V first described by Nonnenmacher and coworkersE cpd in 1991 [29]. It is based on aΦ fundamentalmetal working Φmetal vac principle derived from Lord Kelvin's experiments. When twoΦprobe metals with different work Φfunctionsprobe EF Φprobe EF are electrically connected, electrons flow from Φthemetal higher Fermi level to the lower Fermi level until E the Fermi levels equilibrate (Figure 2). As a+++++ consequence,---- a surfaceF charge develops on the probe and EF EF the sample, with a related potential difference is known as the contact potential difference (CPD) or Volta potential difference (VPD) (Figure 2). VPD is related to the work function difference as described in Equation (1). If the work function of the tip is known then it is easy to calculate the work function of the sample. a) No connection b) c) connection 𝑒𝑉 =𝑒(𝜓 −𝜓 )=𝜙 −𝜙 V =∆ψ (VPD) DC (1) FigureInFigure SKPFM 2. 2.Electronic Electronicmeasurements states states of a the probeof aVolta probe and apotential sampleand a metalssample diffe beforerence metals connectionmeasured before connection( aat), aftereach electric point (a), after connectionon the electric surface contains(bconnection), andinformation after ( applyingb), and on after athe DC applying local bias VworkDC a DC= Vfunction biasVPD (VcDC). =difference VVPD (c). between the tip and the sample. The SKPFM technique includes the same components as in the standard AFM, operated in the tapping modeIn (Figure SKPFM 1). measurements A two-pass scan the technique Volta potential is commonly difference applied measured in SKPFM at each measurements. point on the The surface first containsscan acquires information a surface on thetopography local work using function the distandardfference tapping between mode the tip operation and the sample. mode. TheDuring SKPFM the techniquesecond scan includes the cantilever the same componentsis lifted off asthe in thesurface standard at a AFM,certain operated distance in and the tappingis no longer mode (Figurepiezoelectrically1). A two-pass driven scanat its techniquemechanical is resonance commonly frequency. applied in An SKPFM alternating measurements. current (AC) The voltage, first scanat the acquires resonance a surface frequency topography (ω) of the using cantilever, the standard induces tapping the oscillating mode operation electric force mode. (Fe) During (Equation the second(2)) between scanthe the cantileverAFM tip and is lifted the sample off the surfacesurface. at The a certain oscillating distance force and is nullified is no longer by applying piezoelectrically a direct current offset voltage VDC which equals the Volta potential difference established between the sample and the tip (Figure 3). The total voltage applied (ΔV) is a sum of the applied (DC) voltage and the VPD of the sample. Equation (3) is the expression of the total electrostatic force applied to the AFM tip. A lock-in amplifier is used to measure the VVPD at the frequency Fω (Equation (4)). More details on the principle of SKPFM can be found in [30–32].

1 𝑑𝐶 𝐹(𝑧) = 𝛥𝑉 (2) Figure 3. A scheme of amplitude modulation2 mode𝑑𝑧 scanning Kelvin probe force microscopy (SKPFM). 1 𝑑𝐶 𝐹= (𝑉 +𝑉 𝑠𝑖𝑛(𝜔𝑡) −𝑉 ) (3) 2 𝑑𝑧 The above discussion applies to metals.𝑑𝐶 However, when the studied surface has semiconducting 𝐹 = 𝑉 (𝑉 −𝑉 ) (4) properties the space charge layer on the surface𝑑𝑧 must be considered in SKPFM measurements [33]. Moreover, the bias applied in SKPFM measurements affects the measured signal [34] and a proper model has to be devised to correlate the measured signal with the dopant density in semiconductors. Several limitations of the SKPFM technique should be mentioned. The most important ones are resolution, speed, and environment in which SKPFM operates. The resolution of SKPFM depends on many factors, in amplitude modulation (AM) mode, features as small as a few ten nanometers in size can be detected [35]. By decreasing the height in the lift scan it is possible to achieve much better resolution and contrast in SKPFM, as has been demonstrated by Jacobs et al. [32]. The geometry of the AFM probe plays a significant role since both the tip and the cantilever have capacitive coupling with the sample surface. The measured VPD is a convoluted value which depends on the surrounding of the tip. When the surface is composed of different metallic phases the measured VPD becomes a weighted average over all potentials, derivatives of capacitance on the surface [32,35,36]. It has been so devised that the cantilever significantly contributes to the capacitive coupling with the substrate along with the tip, and its geometry and size influence the resolution and numerical VPD values [37–39]. The contribution to the electrostatic force due to the cantilever has been reported to be as high as 50% [38] or, as has been demonstrated by scientists from Bruker, the contribution from the cone of the tip can be as low as about 10% [40]. In this context, the frequency modulation (FM) SKPFM technique is an improvement on the standard amplitude modulation detection technique, which has a much better resolution and is faster than the two-pass scan technique [30]. The FM mode provides a sub-nanometer spatial resolution when the proper tip and environmental conditions are selected [30]. FM SKPFM works with the same speed as a standard AFM and is twice as fast compared

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driven at its mechanical resonanceEvac frequency. An alternatingEvac current (AC) voltage, at the resonanceEvac level frequencylevel (ω) of the cantilever, induces the oscillating electric forcelevel (Fe) (Equation (2)) between the E E E V E cpd Φmetal AFM tip andΦ themetal sample surface. The oscillatingvac force is nullified by applying a direct current offset Φ Φ voltage VDC which equalsΦ the Volta potential difference establishedprobe between the sample andprobe the tip EF probe Φ EF (Figure3). The total voltage applied ( ∆V) is a summetal of the applied (DC) voltage and the VPD of the E +++++ ---- F sample. Equation (3) is the expressionEF of the total electrostatic force applied to the AFM tip. AE lock-inF amplifier is used to measure the VVPD at the frequency Fω (Equation (4)). More details on the principle of SKPFM can be found in [30–32]. 1 dC F (z) = ∆V2 (2) e 2 dz a) No connection b) c) connection 1 dC 2 F = (V + V sin(ωt) V ) VDC=∆ψ (VPD) (3) 2 dz DC AC − VPD Figure 2. Electronic states of a probe anddC a sample metals before connection (a), after electric Fω = VAC(VDC VVPD) (4) connection (b), and after applying a DC biasdz VDC = VVPD (−c).

Figure 3.3. AA scheme scheme of amplitudeof amplitude modulation modulation mode mode scanning scanning Kelvin probeKelvin force probe microscopy force microscopy (SKPFM). (SKPFM). The above discussion applies to metals. However, when the studied surface has semiconducting propertiesThe above the space discussion charge applies layer onto metals. the surface However, must when be considered the studied in SKPFMsurface has measurements semiconducting [33]. Moreover,properties the space bias applied charge inlayer SKPFM on the measurements surface must abeffects considered the measured in SKPFM signal measurements [34] and a proper [33]. modelMoreover, has tothe be bias devised applied to correlatein SKPFM the measurements measured signal affects with the the measured dopant density signal in [34] semiconductors. and a proper modelSeveral has to limitationsbe devised ofto thecorrelate SKPFM the technique measured should signal bewith mentioned. the dopant The de mostnsity in important semiconductors. ones are resolution,Several speed, limitations and environment of the SKPFM in technique which SKPFM should operates. be mentioned. The resolution The most of important SKPFM dependsones are onresolution, many factors, speed, in and amplitude environment modulation in which (AM) SKPFM mode, operates. features The as resolution small as a fewof SKPFM ten nanometers depends on in sizemany can factors, be detected in amplitude [35]. By modulation decreasing (AM) the height mode, in features the lift scan as small it is possibleas a few toten achieve nanometers much in better size resolutioncan be detected and contrast [35]. By in decreasing SKPFM, as the has height been demonstratedin the lift scan byit is Jacobs possible et al. to [ 32achieve]. The geometrymuch better of theresolution AFM probe and contrast plays a significantin SKPFM, role as has since been both demo the tipnstrated and the by cantilever Jacobs et haveal. [32]. capacitive The geometry coupling of withthe AFM the sample probe plays surface. a significant The measured role since VPD isboth a convoluted the tip and value the cantilever which depends have capacitive on the surrounding coupling ofwith the the tip. sample When the surface. surface The is composed measured of VPD different is a metallic convoluted phases value the measured which depends VPD becomes on the a weightedsurrounding average of the over tip. allWhen potentials, the surface derivatives is compos of capacitanceed of different on themetallic surface phases [32,35 the,36 ].measured It has been VPD so devisedbecomes that a weighted the cantilever average significantly over all potentials, contributes deri tovatives the capacitive of capacitance coupling on with the thesurface substrate [32,35,36]. along withIt has the been tip, so and devised its geometry that the ca andntilever size influence significantly the resolutioncontributes and to the numerical capacitive VPD coupling values with [37– 39the]. Thesubstrate contribution along with to thethe electrostatictip, and its geometry force due an tod the size cantilever influence hasthe beenresolution reported and tonumerical be as high VPD as 50%values [38 [37–39].] or, as hasThe beencontribution demonstrated to the electrostati by scientistsc force from due Bruker, to the the cantilever contribution has frombeen reported the cone ofto thebe as tip high can as be 50% as low[38] asor, aboutas has 10%been [demonstrated40]. In this context, by scientists the frequency from Bruker, modulation the contribution (FM) SKPFM from techniquethe cone of is the an improvementtip can be as low on the as standardabout 10% amplitude [40]. In this modulation context, detectionthe frequency technique, modulation which has(FM) a muchSKPFM better technique resolution is an and improvement is faster than on the the two-pass standard scan amplitude technique modulation [30]. The FMdetection mode providestechnique, a sub-nanometerwhich has a much spatial better resolution resolution when and theis faster proper than tip the and two-pa environmentalss scan technique conditions [30]. are The selected FM mode [30]. FMprovides SKPFM a sub-nanometer works with the spatial same speedresolution as a standardwhen the AFMproper and tip is and twice environmental as fast compared conditions withAM are selected [30]. FM SKPFM works with the same speed as a standard AFM and is twice as fast compared

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with AM SKPFM working in a two-pass scan mode. However, the surface properties of the AFM tip play a very important contribution to the signal measured in FM mode. Even the slightest contamination of the tip can drastically affect the measured VPD signal. On the contrary, AM SKPFM is less sensitive to changes of the tip. However, artifacts commonly appear in VPD measurements due to degradation of the tip and the presence of high topographic features. Ideally, the studied surface should be flat. SKPFM measurements are normally done in air at low relative humidity to prevent the reaction of a metal surface with moisture. SKPFM cannot be performed in conductive solutions since uncontrollable electrochemical processes will hamper the measurements because of the high bias voltage applied between the tip and the sample.

2.3. EC-AFM

Corros.In Mater. 1991 Degrad. electrochemical2020, 1 AFM (EC-AFM) measurements of a gold surface in perchloric acid350 electrolyte and under potential control were reported [41]. Surface topography of the metal surface under the action of electrochemical polarization is the main characteristic studied by this technique. SKPFMEC-AFM working is routinely in a two-pass employed scan to mode.understand However, the properties the surface of properties passive films of the and AFM various tip play localized a very importantcorrosion phenomena contribution on to metallic the signal substrates measured in liquid in FM [42]. mode. Figure Even 4 thepresents slightest a typical contamination setup of the of EC- the tipAFM can system. drastically It includes affect theseveral measured parts, the VPD AFM signal. part Onis represented the contrary, by AMseveral SKPFM components: is less sensitive a scanner, to changesan AFM ofprobe, the tip. an optical However, system, artifacts a laser, commonly and a ph appearotodetector. in VPD An measurements AFM may be operated due to degradation in the three ofmodes the tip described and the earlier, presence both of highadvantages topographic and draw features.backs Ideally, of these the modes studied are surfacedescribed should in [42]. be The flat. SKPFMcontact measurementsand tapping modes are normally are successfully done in airused at lowfor imaging relative humiditythe metallic to prevent surfaces the [41]. reaction The peak of a metalforce tapping surface withmode moisture. is used when SKPFM extremely cannot be low performed forces between in conductive the tip solutions and the sample since uncontrollable are required electrochemicalfor measurements. processes The mode will hamperhas been the successfully measurements applied because in measuring of the high topography bias voltage changes applied at betweensurfaces under the tip electrochemical and the sample. polarization [43,44]. 2.3. EC-AFMThe electrochemical part is comprised of an electrochemical cell that includes a reference electrode (RE), a counter electrode (CE), and a sample surface (working electrode WE) which are connectedIn 1991 to electrochemicalthe outlets of a potentiostat. AFM (EC-AFM) The cell measurements is attached to of the a sample gold surface holder in and perchloric it is typically acid electrolytefilled with andan electrolyte under potential which controlis refreshed were wi reportedth a pumping [41]. Surface system. topography In EC-AFM of measurements the metal surface the undercurrent the or actionpotential of electrochemicalare not applied between polarization the tip is theand main the sample characteristic as it is conducted studied by in this EC-STM technique. [45], EC-AFMbut between is routinely the counter employed electrode to understandand the sample. the propertiesIt is important of passive to pay films attention and variousto the fact localized that at corrosionhigh current phenomena densities onthe metallic voltage substratesdrop (IR drop) in liquid between [42]. Figurethe sample4 presents and the a typical RE starts setup to ofmatter the EC-AFMespecially system. when the It includesconcentration several of parts,an electrolyte the AFM is partsmall. is Thus represented the reference by several electrode components: should be a scanner,placed close an AFM to the probe, sample an opticalsurface. system, It is advisa a laser,ble andto compare a photodetector. electrochemical An AFM measurements may be operated taken in thefrom three the modes sample described placed earlier, inside both the advantages EC-cell wi andth drawbacksthe measurements of these modes done arein describeda macro in scale [42]. Theelectrochemical contact and tappingcell. Another modes limitation are successfully of EC-AFM used foris the imaging rate of the corrosion. metallic surfacesThe speed [41 ].at The which peak a forcetopography tapping map mode is isacquired used when should extremely be comparable low forces with between the rate the of tip localized and the samplecorrosion are attack required on the for measurements.surface or the rate The at mode which has the been surface successfully changes. applied If the corrosion in measuring rate topographyis higher, AFM changes will not at surfaces be able underto effectively electrochemical track surface polarization changes. [ 43,44].

FigureFigure 4.4.A A scheme scheme of of electrochemical electrochemical AFM AFM (EC-AFM) (EC-AFM) liquid liquid cell containing cell containing main partsmain such parts as countersuch as electrodecounter electrode (CE), working (CE), electrodeworking electrode (WE), and (WE), reference and electrodereference (RE);electrode AFM (RE); scanning AFM system scanning containing system an AFM probe, a scanner, an optical system, a laser, and a photodetector; the external potentiostat controls the applied voltage between a counter and a working electrode.

The electrochemical part is comprised of an electrochemical cell that includes a reference electrode (RE), a counter electrode (CE), and a sample surface (working electrode WE) which are connected to the outlets of a potentiostat. The cell is attached to the sample holder and it is typically ﬁlled with an electrolyte which is refreshed with a pumping system. In EC-AFM measurements the current or potential are not applied between the tip and the sample as it is conducted in EC-STM [45], but between the counter electrode and the sample. It is important to pay attention to the fact that at high current densities the voltage drop (IR drop) between the sample and the RE starts to matter especially when the concentration of an electrolyte is small. Thus the reference electrode should be placed close to the sample surface. It is advisable to compare electrochemical measurements taken from the sample placed inside the EC-cell with the measurements done in a macro scale electrochemical cell. Another Corros. Mater. Degrad. 2020, 1 351 limitation of EC-AFM is the rate of corrosion. The speed at which a topography map is acquired should be comparable with the rate of localized corrosion attack on the surface or the rate at which the surface changes. If the corrosion rate is higher, AFM will not be able to eﬀectively track surface changes.

3. In Situ AFM Investigation of Metallic Surfaces In situ AFM has become an indispensable tool for the characterization of localized corrosion activity of metals and alloys during immersion in electrolyte solutions. The principles of EC-AFM operation are similar to AFM measurements in air. However, in liquid electrolytes active corrosion processes occurring on metallic surfaces significantly contribute to noise and hamper the measurements. For example, when a contact mode in in situ AFM is applied to study a metal corroding in a water solution the corrosion products, which are produced by the metal, may attach to the AFM tip and be dragged along the scan. This can greatly affect the measurements to the extent that the AFM maps become completely unreadable and all the features of the surface become obstructed by the noise. In circumstances where a heavy contact with the surface is needed, as was presented by Schmutz and Frankel [46], the contact of the AFM tip with the AA2024 surface has been used to observe localized corrosion development near intermetallic particles. The scratching with the AFM tip was used to stimulate local depassivation events near different intermetallic particles. It was found that the dissolution increases on a pure aluminum with a higher applied setpoint (higher force) to the AFM tip. Among the two types of inclusions such as Al-Cu-Mg and Al-Cu-(Fe, Mn) the former were easily depassivated and dissolved in the NaCl solution, while the latter showed etching and trenching due to dissolution of the aluminum matrix [46]. Similar trenching was observed on the AA5083 surface containing (Fe, Mn) intermetallics during immersion in NaCl solutions [47]. Polished metallic samples were studied by contact mode AFM in 0.005 M and 0.5 M NaCl solutions in a fluid cell. The surface was continuously scanned during the corrosion test and every 10 min a surface topography image was recorded. During the first 180 min of immersion in 0.005 M NaCl local corrosion attack was revealed on the tested zone (Figure5a). It appeared that the setpoint force might not have been sufficient to cause local corrosion attack on (Fe, Mn) containing intermetallics as was shown in Ref. [46]. To accelerate the corrosion process 0.5 M NaCl solution was pumped into the liquid cell. Evident trenching started to appear close to intermetallic particles after 130 min of exposure (Figure5b). A high concentration of chloride ions depassivated the surface oxide film which led to the dissolution of the matrix. During the following immersion, the pit expanded laterally and vertically (Figure5c,d). The evolution of the depth of the pit is shown in Figure5e. Two stages can be noticed; the first one being linear which may signify an active growth stage while the second one with the lower slope may be attributed to passivation of the pit [47]. Various intermetallics have different electrochemical properties. In the same work the corrosion susceptibility of another type of intermetallics i.e., Mg2Si was studied and the corrosion mechanism of the intermetallics was proposed based on in situ measurements of localized corrosion activity and microstructural analysis [47]. Such intermetallics were found to be anodic to the aluminum matrix [17] and were dissolving in a corrosive environment. Figure6 displays AFM images obtained from the polished surface containing Mg2Si intermetallics before and during in situ immersion in 0.005 M NaCl. The uncorroded surface (Figure6a) displays anodic intermetallics having shallow cavities of about 30 nm in depth which were formed during the polishing process. During immersion some sort of precipitates were formed above these intermetallics as can be observed on AFM maps and topography profiles (Figure6b,d). After immersion a larger dissolution was noticed at the intermetallics (Figure6c,d). Surface changes of Mg 2Si intermetallics have been ascribed to the different processes including chemical and electrochemical steps [47]. At the initial stage Mg2Si can be hydrolyzed by water with the formation of different types of silanes, in particular mono-silane (reaction 5), which then hydrolyze forming SiO2 (reaction 6).

Mg Si + 4H O 2Mg(OH) + SiH (5) 2 2 → 2 4 Corros. Mater. Degrad. 2020, 3 FOR PEER REVIEW 7

containing an AFM probe, a scanner, an optical system, a laser, and a photodetector; the external potentiostat controls the applied voltage between a counter and a working electrode.

3. In Situ AFM Investigation of Metallic Surfaces In situ AFM has become an indispensable tool for the characterization of localized corrosion activity of metals and alloys during immersion in electrolyte solutions. The principles of EC-AFM operation are similar to AFM measurements in air. However, in liquid electrolytes active corrosion processes occurring on metallic surfaces significantly contribute to noise and hamper the measurements. For example, when a contact mode in in situ AFM is applied to study a metal corroding in a water solution the corrosion products, which are produced by the metal, may attach to the AFM tip and be dragged along the scan. This can greatly affect the measurements to the extent that the AFM maps become completely unreadable and all the features of the surface become obstructed by the noise. In circumstances where a heavy contact with the surface is needed, as was presented by Schmutz and Frankel [46], the contact of the AFM tip with the AA2024 surface has been used to observe localized corrosion development near intermetallic particles. The scratching with the AFM tip was used to stimulate local depassivation events near different intermetallic particles. It was found that the dissolution increases on a pure aluminum with a higher applied setpoint (higher force) to the AFM tip. Among the two types of inclusions such as Al-Cu-Mg and Al-Cu-(Fe, Mn) the former were easily depassivated and dissolved in the NaCl solution, while the latter showed etching and trenching due to dissolution of the aluminum matrix [46]. Similar trenching was observed on the AA5083 surface containing (Fe, Mn) intermetallics during immersionCorros. Mater. Degrad.in NaCl 2020 solutions, 3 FOR PEER [47]. REVIEW Polished metallic samples were studied by contact mode AFM in8 0.005 M and 0.5 M NaCl solutions in a fluid cell. The surface was continuously scanned during the corrosionFigure test 5. Topographyand every 10evolution min a surfaceof AA5083 topograp surfacehy during image in wassitu AFMrecorded. measurements During the showing first 180 pit min of immersioninitiation andin 0.005 growth: M NaCl(a) topography local corrosion after 180 attack min of was immersion revealed in 0.005on the M tested NaCl, (zoneb) pit (Figure initiation 5a). It appearedafter 130that min the of setpoint immersion force in might0.5 M NaCl, not have (c) pit been growth sufficient after 220 to mincause in local0.5 M corrosionNaCl and (attackd) image on (Fe, after immersion for 450 min in 0.5 M NaCl; the kinetics of the pit depth propagation (e). Reprinted Mn) containing intermetallics as was shown in Ref. [46]. To accelerate the corrosion process 0.5 M from [47] Copyright (2020), with permission from Elsevier. NaCl solution was pumped into the liquid cell. Evident trenching started to appear close to intermetallic particles after 130 min of exposure (Figure 5b). A high concentration of chloride ions Various intermetallics have different electrochemical properties. In the same work the corrosion depassivated the surface oxide film which led to the dissolution of the matrix. During the following Corros.susceptibility Mater. Degrad. of another2020, 1 type of intermetallics i.e., Mg2Si was studied and the corrosion mechanism352 immersion, the pit expanded laterally and vertically (Figure 5c,d). The evolution of the depth of the of the intermetallics was proposed based on in situ measurements of localized corrosion activity and pit is shown in Figure 5e. Two stages can be noticed; the first one being linear which may signify an microstructural analysis [47]. Such intermetallics were found to be anodic to the aluminum matrix active growth stage while the secondSiH + onemH withO theSiO lowernH Oslope+ 4H may be attributed to passivation(6) of [17] and were dissolving in a corrosive4 environmen2 → t.2· Figure2 6 displays2↑ AFM images obtained from the pit [47]. the polished surface containing Mg2Si intermetallics before and during in situ immersion in 0.005 M NaCl. The uncorroded surface (Figure 6a) displays anodic intermetallics having shallow cavities of about 30 nm in depth which were formed during the polishing process. During immersion some sort of precipitates were formed above these intermetallics as can be observed on AFM maps and topography profiles (Figure 6b,d). After immersion a larger dissolution was noticed at the intermetallics (Figure 6c,d). Surface changes of Mg2Si intermetallics have been ascribed to the different processes including chemical and electrochemical steps [47]. At the initial stage Mg2Si can be hydrolyzed by water with the formation of different types of silanes, in particular mono-silane (reaction 5), which then hydrolyze forming SiO2 (reaction 6).

Mg2Si + 4H2O → 2Mg(OH)2 + SiH4 (5)

SiH4 + mH2O → SiO2·nH2O + 4H2↑ (6)

Simultaneously the electrochemical dissolution of magnesium from the Mg2Si intermetallics occurs enriching the latter in SiO2 (reaction 7).

Mg2Si + 2H2O → 2Mg2+ + SiO2 + 4H+ + 8e- (7) The in situ AFM experiments were in good agreement with the proposed behavior of the Mg-Si phase.Figure During 5. Topography the first stage evolution of imme of AA5083rsion a thin surface layer during of deposits in situ AFM (SiO measurements2·nH2O or Mg(OH) showing2) impaired pit furtherinitiation dissolution and growth: at the (sitesa) topography of magnesium-rich after 180 min intermetallics of immersion (Figure in 0.005 M6b). NaCl, After (b immersion) pit initiation a layer enrichedafter 130in Si min and of O immersion was observed in 0.5 Mon NaCl, the surface (c) pit growth of the afterintermetallics 220 min in [47]. 0.5 M Although NaCl and AFM (d) image has high capabilitiesafter immersion in studying for 450 localized min in 0.5 corrosion M NaCl; theprocesses kinetics in of theaqueous pit depth solutions, propagation other (e ).surface Reprinted analysis methodsfrom [are47] Copyrightnecessary (2020), to understand with permission chemical from changes Elsevier. of intermetallics and metallic phases after corrosion.

Figure 6. AFM images of the alloy surface: (a) before in situ immersion, (b) after 25 min of in situ immersion and (c) after the in situ experiment followed by the removal of the corrosion deposits (performed in air); (d) comparison of the topography proﬁles across the black line on the AFM images. Reprinted from [47] Copyright (2020), with permission from Elsevier.

Simultaneously the electrochemical dissolution of magnesium from the Mg2Si intermetallics occurs enriching the latter in SiO2 (reaction 7).

2+ + Mg Si + 2H O 2Mg + SiO + 4H + 8e− (7) 2 2 → 2 Corros. Mater. Degrad. 2020, 1 353

The in situ AFM experiments were in good agreement with the proposed behavior of the Mg-Si phase. During the first stage of immersion a thin layer of deposits (SiO nH O or Mg(OH) ) impaired 2· 2 2 further dissolution at the sites of magnesium-rich intermetallics (Figure6b). After immersion a layer enriched in Si and O was observed on the surface of the intermetallics [47]. Although AFM has high capabilities in studying localized corrosion processes in aqueous solutions, other surface analysis methods are necessary to understand chemical changes of intermetallics and metallic phases after corrosion. AFM measurements can help to visualize the corrosion inhibiting action of different compounds on metallic alloys. Iannuzzi and Frankel studied corrosion inhibition of aluminum alloy 2024-T3 by vanadates in situ using a contact mode AFM [48]. The samples exposed to 0.1 M NaCl with no inhibitor developed a trench-like attack at the periphery of large Al-Cu-Fe-Mn-(Si) intermetallic phases. The addition of dilute metavanadate solution markedly reduced the kinetics of the corrosion attack. Besides, most of the S-phase (Al2CuMg) intermetallics remained free of attack. The higher concentrations of metavanadate enhanced corrosion protection of the substrate to the extent that only at high forces (high setpoint) could pitting corrosion be induced. Vanadate species seem to act as an adsorption inhibitor and to not form precipitates on the surface of AA2024. Decavanadate species were proven to be poor corrosion inhibitors. The surface of AA2024 quickly developed significant localized corrosion attack in the solution containing decavanadate species which obstructed long term AFM measurements. Corrosion inhibitors that form precipitates, such as lanthanides, were studied in [11]. It is well known that the S-phase, which is the intermetallic phase typically present in Al-Cu-Mg alloys such as AA2024, has very strong localized corrosion [10–13,46]. The localized corrosion develops into pitting corrosion and is a major concern for the aerospace industry utilizing such types of alloys. Figure7 displays the kinetics of topography changes of an S-phase intermetallic during corrosion in the presence of an inhibitive additive such as the soluble salts of lanthanides, i.e., La(NO3)3. At the beginning the substrate was immersed for 5 h and 23 min in the electrolyte containing 0.005 M NaCl and 0.5% of La(NO3)3 (Figure7a,c). Localized corrosion was very weak at that point. Afterward, the electrolyte with a higher concentration of chlorides (0.5 M) and the same concentration of the inhibitor was introduced into the liquid cell. Two orders of magnitude increase in chloride concentration led to a higher speed of lanthanum hydroxide deposition (Figure7b,c). The cathodic process of oxygen reduction (reaction 8) produces hydroxyls ions that react with lanthanum cations forming an insoluble hydroxide precipitate (reaction 9) [11]. Figure7d shows the kinetics of La(OH) 3 precipitate formation across the profile images. A linear kinetic law describes the rate of the deposit formation. The estimated rate of the deposit growth is about 0.4 nm/min in the weak chloride-based electrolyte. However, the rate is increased by almost 50 times reaching 20 nm/min in the concentrated solution (Figure7d). AFM provides a valuable means for studying the kinetics of processes occurring at metal/electrolyte interfaces and is routinely used for corrosion studies.

O + 2H O + 4e− 4OH− (8) 2 2 → 3+ La + 3OH− La(OH) (9) → 3 ↓ Corros. Mater. Degrad. 2020, 1 354 Corros. Mater. Degrad. 2020, 3 FOR PEER REVIEW 10

Figure 7. AFM images of the alloy surface (a) at the beginning of immersion in 0.5% La(NO3)3 + Figure 7. AFM images of the alloy surface (a) at the beginning of immersion in 0.5% La(NO3)3 + 0.005 0.005 M NaCl, (b) at the end of immersion in 0.5% La(NO3)3 + 0.5 M NaCl; (c) Evolution of topography M NaCl, (b) at the end of immersion in 0.5% La(NO3)3 + 0.5 M NaCl; (c) Evolution of topography across the profiles depicted in (a,b) in 0.5% La(NO3)3 solution with 0.005 M or 0.5 M NaCl; (d) Rate across the profiles depicted in (a,b) in 0.5% La(NO3)3 solution with 0.005 M or 0.5 M NaCl; (d) Rate of of the lanthanum hydroxide precipitation. Reprinted with permission from [11] Copyright (2020) the lanthanum hydroxide precipitation. Reprinted with permission from [11] Copyright (2020) American Chemical Society. American Chemical Society. Corrosion of steel has been an active subject of research. Various publications show the performance Corrosion of steel has been an active subject of research. Various publications show the of different inhibitors in corrosion protection of steel substrates using AFM. For example, chromeno performance of different inhibitors in corrosion protection of steel substrates using AFM. For pyrimidine compounds were tested as potential corrosion inhibitors for N80 steel in 15% hydrochloric example, chromeno pyrimidine compounds were tested as potential corrosion inhibitors for N80 steel acid solution [49], and thiazolyl imines were investigated in acidic corrosion of mild steel [50]. Corrosion in 15% hydrochloric acid solution [49], and thiazolyl imines were investigated in acidic corrosion of inhibition of steel in NaCl solutions at pH 4 using salicylaldehyde–Chitosan Schiff Base was studied mild steel [50]. Corrosion inhibition of steel in NaCl solutions at pH 4 using salicylaldehyde–Chitosan in [51]. X-ray photoelectron spectroscopy revealed adsorption of the inhibitors on the surface of the Schiff Base was studied in [51]. X-ray photoelectron spectroscopy revealed adsorption of the steel. The presence of these inhibitors in the corrosive medium resulted in a decrease of the corrosive inhibitors on the surface of the steel. The presence of these inhibitors in the corrosive medium resulted attack and roughness of the surface measured by AFM. Corrosion inhibiting and passivation processes in a decrease of the corrosive attack and roughness of the surface measured by AFM. Corrosion in alkali media have been studied on carbon steel and AISI 304 steel using AFM, electron and optical inhibiting and passivation processes in alkali media have been studied on carbon steel and AISI 304 microscopy, and electrochemical methods [52,53]. In the former work Quaternium-green ionic liquid steel using AFM, electron and optical microscopy, and electrochemical methods [52,53]. In the former was used for corrosion inhibition of carbon steel in a simulated concrete pore solution. In the latter work Quaternium-green ionic liquid was used for corrosion inhibition of carbon steel in a simulated work AFM was used to study the formation of the surface passive film after immersion in NaOH concrete pore solution. In the latter work AFM was used to study the formation of the surface passive solution at pH 13. It was demonstrated that the roughness of the surface significantly decreases either film after immersion in NaOH solution at pH 13. It was demonstrated that the roughness of the due to the presence of the inhibitor or due to the growth of the passive film in the alkali solutions. surface significantly decreases either due to the presence of the inhibitor or due to the growth of the Changing the composition of steel is an alternative way to improve the corrosion performance passive film in the alkali solutions. of ferrous materials in comparison with the corrosion inhibition strategy discussed above. Handoko Changing the composition of steel is an alternative way to improve the corrosion performance et al. investigated the effects of small additions of Cr to a high carbon low alloy steel with the of ferrous materials in comparison with the corrosion inhibition strategy discussed above. Handoko dual-phase structure of martensite and austenite [54]. In situ AFM images were recorded from different et al. investigated the effects of small additions of Cr to a high carbon low alloy steel with the dual- samples containing 0.10–0.18% Cr; 0.60–0.80% Cr; and 1.80–2.50% Cr during immersion in 0.1 M NaCl phase structure of martensite and austenite [54]. In situ AFM images were recorded from different solution (Figure8). The observed preferential attack on the austenite microstructure was related to the samples containing 0.10–0.18% Cr; 0.60–0.80% Cr; and 1.80–2.50% Cr during immersion in 0.1 M NaCl different Cr content that affected the shape, size, and distribution of each grain boundary interface. solution (Figure 8). The observed preferential attack on the austenite microstructure was related to It appeared that the addition of a low quantity of Cr increased the stabilization of the retained austenite, the different Cr content that affected the shape, size, and distribution of each grain boundary improving corrosion resistance. In this work an assumption was made that the modifications of surface interface. It appeared that the addition of a low quantity of Cr increased the stabilization of the roughness could be related to the dissolution and preferential attack of the austenitic microstructure. retained austenite, improving corrosion resistance. In this work an assumption was made that the modifications of surface roughness could be related to the dissolution and preferential attack of the austenitic microstructure. Thus the weight loss was obtained and the results were fitted with a

Corros. Mater. Degrad. 2020, 1 355

ThusCorros. the Mater. weight Degrad. loss 2020 was, 3 FOR obtained PEER REVIEW and the results were ﬁtted with a polynomial function [54]. It 11was demonstrated that the corrosion rate of the steel was reduced when the stability of austenite was polynomial function [54]. It was demonstrated that the corrosion rate of the steel was reduced when enhancedthe stability by alloying of austenite the steelwas enhanc with Cr.ed by alloying the steel with Cr.

FigureFigure 8. 8.In In situ situ AFM AFM three-dimensional three-dimensional topography topography images images of of high high carbon carbon martensiticmartensitic steelsteel surfacesurface at (i)at 0 min;(i) 0 (min;ii) 60 (ii) min; 60 min; and (andiii) 105(iii) min,105 min, the exposurethe exposure to 0.1 to M0.1 NaCl M NaCl solution. solution. Sample Sample A (0.10–0.18A (0.10–0.18 wt% Cr),wt% Sample Cr), Sample B (0.60–0.80 B (0.60–0.80 wt% Cr), wt% Sample Cr), Sample C (0.10–0.18 C (0.10–0.18 wt% wt% Cr). ReprintedCr). Reprinted from from [54]. [54].

InIn all all the the cases cases surface surface topography topography was was usedused as the principal principal parameter parameter in in determining determining localized localized corrosioncorrosion activity activity and and the the kinetics kinetics of of the the corrosioncorrosion ofof metallic materials. materials. However, However, by by itself, itself, the the AFM AFM methodmethod is is not not able able to to predict predict thethe corrosion susceptibility susceptibility of of the the metallic metallic phases phases in the in thealloy alloy matrix matrix at atthe micro-level micro-level without without damaging damaging the surface. the surface. Below, Below,the application the application of SKPFM of relevant SKPFM to corrosion relevant to corrosionresearch research is discussed. is discussed. Various examples Various are examples given where are given SKPFM where measurements SKPFM measurements offer important oﬀ er importantinformation information regarding regarding the corrosion the susceptibility corrosion susceptibility of metallic and of metalliccoated substrates and coated at micrometer substrates at micrometerresolution. resolution.

4. Application4. Application of of SKPFM SKPFM for for Studying Studying LocalizedLocalized Corrosion Susceptibility Susceptibility SKPFMSKPFM became became a a useful useful technique technique inin corrosioncorrosion re researchsearch of of metallic metallic materials materials due due to tohigher higher resolutionresolution compared compared with with the standardthe standard scanning scanni Kelvinng Kelvin probe (SKP)probe technique (SKP) technique and powerful and powerful capabilities in studiescapabilities of local in studies corrosion of local susceptibility corrosion ofsusceptibility metallic materials, of metallic distribution materials, of distribution charge in semiconductors, of charge in semiconductors, and properties of organic and inorganic thin films [31]. However, in and properties of organic and inorganic thin ﬁlms [31]. However, in electrochemistry the corrosion electrochemistry the corrosion potential is the relevant parameter for determining the electrochemical potential is the relevant parameter for determining the electrochemical nobility of metals in aqueous nobility of metals in aqueous solutions. The pioneering work of Schmutz and Frankel [55] addressed solutions. The pioneering work of Schmutz and Frankel [55] addressed the question of how to make the question of how to make a correlation between the work function and the corrosion potential of a correlation between the work function and the corrosion potential of a metal. They performed a metal. They performed measurements of open circuit potential (OCP) of various metals in 0.5 NaCl measurementssolution and ofcompared open circuit them potentialwith VPD (OCP) measurements of various in metalsair on the in 0.5surface NaCl of solution the immersed and compared metals them(Figure with 9). VPD A correlation measurements was found in air between on the surface VPD an ofd theOCP immersed measurements. metals This (Figure result9). is A crucial correlation and wasdemonstrates found between the VPDpowerful and OCPcapabilities measurements. of SKPFM This inresult the characterization is crucial and demonstrates of the susceptibility the powerful of capabilitiesmetals and of alloys SKPFM to corrosion. in the characterization The measured VPD of the values susceptibility were inverted of metals to compare and alloys the output to corrosion. VPD Thesignal measured with VPDthe electrochemical values were inverted nobility to of compare metals which the output is the VPD necessary signal withprocedure the electrochemical due to the nobilityspecifics of metals of the which nulling is thetechnique necessary in procedurewhich the due voltage to the speciﬁcsis applied of theto the nulling AFM technique tip in SKPFM in which themeasurements. voltage is applied The todiscussion the AFM concerning tip in SKPFM the measurements. polarity in SKPFM The discussionmeasurements concerning can be thefound polarity in

Corros. Mater. Degrad. 2020, 3 FOR PEER REVIEW 12

[55,56]. Cook et al. employed electrolyte dosing experiments to compare SKPFM results with corrosion potential and SKP measurements [57]. They obtained a linear relationship between Volta potential measured by SKPFM and corrosion potential and Volta potential measured by SKP. Having

Corros.said Mater. that, Degrad. the resolution2020, 1 and artifacts discussed in Section 2.2 must be kept in mind when making356 a quantitative comparison between different small intermetallics/phases and properties of bulk macro surfaces. in SKPFMIn measurementsquantitative measurements can be found inof [VPD55,56 ].it Cookis impo etrtant al. employed to employ electrolyte a reference dosing material experiments to ensure to comparereproducibility SKPFM of results results. with The corrosion use of potentialstandard andreference SKP measurements materials permits [57]. Theythe comparison obtained a of linearmaterials relationship with others between in the Volta scientific potential literature measured [58]. byThe SKPFM nickel surface and corrosion has been potential proposed and as Volta a stable potentialand reproducible measured by substrate SKP. Having to be said used that, as thea reference resolution for and calibration artifacts discussedof the AFM in Sectiontip work 2.2 functionmust be kept[46,55]. in mindHighly when oriented making pyrolytic a quantitative graphite comparison (HOPG) between substrate diﬀ erentalso is small a stable intermetallics and reproducible/phases andreference properties material of bulk [59]. macro surfaces.

0.2 0.0 Ni Pt -0.2 Cu -0.4 -0.6 -0.8 Al Fe -1.0 -1.2 -1.4 VPD vs. Ni (V) -1.6 Mg -1.8 -1.5 -1.0 -0.5 0.0 OCP vs. SCE (V) FigureFigure 9. VPD 9. VPD after after immersion immersion in 0.5 in M 0.5 NaCl M solutionNaCl solution vs. corrosion vs. corrosion potentials potentials measured measured in 0.5 M NaClin 0.5 M solutionNaCl (a),solution the Figure (a), the was Figure adapted was fromadapted [55]. from [55].

In quantitativeMetallic alloys measurements contain different of VPD intermetallics it is important and heterogeneous to employ a reference phases. materialIn corrosion to ensure research reproducibilitythe SKPFM technique of results. has The become use of standard the state reference of the ar materialst method permitsfor studies the of comparison microgalvanic of materials corrosion withsusceptibility others in the of scientificintermetallics. literature Microgalvanic [58]. The corro nickelsion surface in magnesium has been alloys proposed has been as a stablethe subject and of reproduciblemany research substrate works to [36,60–67]. be used as A a referenceclear relationsh for calibrationip was observed of the AFM between tip work the precipitation function [46, 55of]. Al- Highlyrich phases oriented and pyrolytic the Volta graphite potential. (HOPG) The Volta substrate potential also increased is a stable with and the reproducible aluminum content reference in the materialphase. [ 59The]. β-Mg17Al12 phase and the Al8Mn5 phase both work as local cathodes when coupled to the magnesiumMetallic alloys phase contain under di ffatmosphericerent intermetallics weathering and heterogeneousconditions [18]. phases. Coy Inet corrosional. made researchsystematic themeasurements SKPFM technique of VPD has on become the surface the state of four of the differen art methodt Mg alloys for studies such as of ZE41 microgalvanic as-cast, WE43-T6 corrosion sand- susceptibilitycast, WE43-T6 of intermetallics. wrought, and WE54-T6 Microgalvanic wrought corrosion [62]. Th inis magnesiumwork provided alloys important has been insights the subject into the of manyrelationship research between works the [36 ,VPD60–67 measured]. A clear on relationship top of different was observed intermetallics between and the localized precipitation corrosion of Al-richactivity phases of intermetallics. and the Volta Immersion potential. Thetests Voltawere potentialdone in 3.5 increased wt% NaCl with for the different aluminum times content from 5 into 60 themin. phase. Those The βmicro-consti-Mg17Al12 phasetuents andwhich the Alpossess8Mn5 phasehigh VPD both vs. work the as Mg local matrix cathodes usually when demonstrate coupled to thesignificant magnesium microgalvanic phase under activity. atmospheric Figure weathering10 presents conditionsthe AFM and [18 ].VPD Coy maps et al. taken made from systematic the ZE41 measurementssample. The oflargest VPD VPD on the was surface attributed of four to Zr-rich different (Zr Mg4Zn) alloys phases such and as T-phase ZE41 as-cast, (Mg7Zn WE43-T63RE) which sand-cast,had VPD WE43-T6 of about wrought, +180 ± 10 and mV WE54-T6and +100 wrought± 5 mV respectively [62]. This workvs. Mg provided matrix (Figure important 10a,b) insights [62]. On intothe the contrary, relationship grain between boundaries the VPD had measureda lower VPD on topof about of diff erent−80 ± intermetallics5 mV vs. Mg andmatrix the (Figure localized 10c). corrosionMicrostructural activity of observations intermetallics. after Immersion corrosion tests revealed were done a significant in 3.5 wt% localized NaCl for dissolution different times at the from grain 5 toboundaries 60 min. Those which micro-constituents was intensified whichby T-phase possess (Figure high VPD10d,e). vs. Zr-rich the Mg phases matrix usuallyrevealed demonstrate a tendency to significantlocalize microgalvanicpitting corrosion activity. and more Figure severe 10 presents corrosi theon. AFM WEXX and alloys VPD mapsrevealed taken significant from the cathodic ZE41 sample.activity The of largestthe Zr-rich VPD phases was attributed with VPD to reaching Zr-rich (Zr +2504Zn) ± 10 phases mV vs. and Mg T-phase matrix (Mg [62].7Zn Although3RE) which Y-rich had VPD of about +180 10 mV and +100 5 mV respectively vs. Mg matrix (Figure 10a,b) [62]. intermetallics like Mg(RE,Y)± showed relatively± high VPD contrast, about 50 ± 5 mV vs. Mg matrix, On the contrary, grain boundaries had a lower VPD of about 80 5 mV vs. Mg matrix (Figure 10c). they did not promote galvanic corrosion during 60 min immersion− ± in the corrosive solution [62]. This Microstructuralwork revealed observations that despite after relatively corrosion high revealed VPD, some a significant intermetallic localized phases dissolution do not reveal at the localized grain boundariescorrosion which attack wasduring intensified immersion. by T-phaseTherefore, (Figure care must10d,e). be Zr-richtaken when phases correlating revealed athe tendency VPD level to localize pitting corrosion and more severe corrosion. WEXX alloys revealed significant cathodic activity of the Zr-rich phases with VPD reaching +250 10 mV vs. Mg matrix [62]. Although Y-rich ± intermetallics like Mg(RE,Y) showed relatively high VPD contrast, about 50 5 mV vs. Mg matrix, they ± did not promote galvanic corrosion during 60 min immersion in the corrosive solution [62]. This work Corros. Mater. Degrad. 2020, 1 357 revealedCorros. Mater. that Degrad. despite 2020 relatively, 3 FOR PEER high REVIEW VPD, some intermetallic phases do not reveal localized corrosion 13 attack during immersion. Therefore, care must be taken when correlating the VPD level and corrosion and corrosion activity of metallic phases. Studies of the localized corrosion susceptibility of alloys activity of metallic phases. Studies of the localized corrosion susceptibility of alloys using SKPFM using SKPFM should be accompanied by an extensive microstructural analysis of the surface after should be accompanied by an extensive microstructural analysis of the surface after corrosion. corrosion.

FigureFigure 10. 10.SKPFM SKPFM study study of of the the as-cast as-cast ZE41 ZE41 alloy: alloy: ( a(a)) topography topography map,map, ((bb)) surfacesurface potentialpotential map,map, and (c) line-proﬁle(c) line-profile analysis analysis of relative of relative Volta Volta potential potential through through an anα α-Mg-Mg dendrite;dendrite; SEM SEM study study of of the the as-cast as-cast ZE41ZE41 after after immersion immersion in in 3.5 3.5 wt%wt% NaClNaCl solution:solution: ( (dd)) 60 60 min min (surface) (surface) and and (e ()e 60) 60 min min (cross-section). (cross-section). ReprintedReprinted from from [62 [62]] Copyright Copyright (2020), (2020), withwith permissionpermission from from Elsevier. Elsevier.

AluminumAluminum alloys alloys have been been one one of ofthe themost most studied studied materials materials by the bySKPFM the SKPFMtechnique technique due to duethe to diverse the diverse microstructure microstructure and high and activity high of activity the intermetallic of the intermetallic phases in local phases corrosion in local initiation corrosion initiation[22,46,55,68–72]. [22,46,55 ,For68– 72example,]. For example, in the work in theof Zhu work et ofal. ZhuVolta et potentials al. Volta were potentials measured were on measured many ondifferent many diindependentfferent independent intermetallics intermetallics in as-polished in as-polished Al-Cu-Li Al-Cu-Lialloy AA2070-T8 alloy AA2070-T8 [22]. The [22 ]. electrochemical nobility and activity of intermetallics were assessed by DC-polarization and OCP The electrochemical nobility and activity of intermetallics were assessed by DC-polarization and measurements and correlated with the changes of Volta potential before and after immersion. OCP measurements and correlated with the changes of Volta potential before and after immersion. Amongst many intermetallics, Al65Cu20Fe15 and Al37Cu2Fe12 compositions were found to be the noblest Amongst many intermetallics, Al Cu Fe and Al Cu Fe compositions were found to be the based on OCP and SKPFM measurements.65 20 The15 tendency37 to2 trenching12 between the intermetallics and noblest based on OCP and SKPFM measurements. The tendency to trenching between the intermetallics the Al matrix was characterized by the higher content of copper and aluminum elements in the andcompositions the Al matrix of the was intermetallics. characterized It by should the higher be noted content that the of copperdirect comparison and aluminum between elements the OCP in the compositionsand electrochemical of the intermetallics. activity of intermetallics It should is be no notedt entirely that correct, the direct because comparison some intermetallics between the (e.g., OCP andAl electrochemical2Cu) may supply activity higher currents of intermetallics even though is not the entirely OCP is correct,lower than because that of some others intermetallics [17]. However, (e.g., Al2theCu) report may by supply Mallinson higher et currentsal. on the even localized though corrosion the OCP susceptibility is lower than in aluminum that of others alloy [ 17AA7075]. However,-T6 thepointed report out by Mallinsona different ettrend al. on[71] the. The localized intermetallics corrosion such susceptibility as Al12Fe3Si, (Al,Cu) in aluminum6(Fe,Cu) alloy and Al AA7075-T67Cu2Fe, pointedhad a outsimilar a diff VPDerent measured trend [71]. in The air intermetallics after polishin suchg. However as Al12Fe they3Si, (Al,Cu)revealed6(Fe,Cu) different and localized Al7Cu2 Fe, hadcorrosion a similar susceptibility VPD measured and in corrosion air afterpolishing. current densities However which they increased revealed diinff erentthe range: localized Al12Fe corrosion3Si > susceptibility(Al,Cu)6(Fe,Cu) and corrosion> Al7Cu current2Fe. The densities particle which (Al,Cu) increased6(Fe,Cu) in thedisplayed range: Alsignificant12Fe3Si > (Al,Cu)sub-surface6(Fe,Cu) > Alintergranular7Cu2Fe. The corrosion particle at (Al,Cu) the particle/matrix6(Fe,Cu) displayed interface significant as well as sub-surface signs of the intergranular onset of trenching. corrosion at the particleAA2024/matrix is an interface aluminum as alloy well aswith signs copper of the as the onset main of trenching.alloying element. It is considered a good modelAA2024 system is anfor aluminum the investigation alloy with of localized copper ascorrosion the main processes alloying and element. inhibition It is[73]. considered Typical a goodabundant model intermetallic system for the particles investigation have the of following localized composition corrosion processes Al2CuMg. and Such inhibition intermetallics [73]. Typical are very susceptible to localized corrosion attack in aqueous NaCl solutions which makes them common abundant intermetallic particles have the following composition Al2CuMg. Such intermetallics are verymodel susceptible objects tofor localized studying corrosion corrosion attack susceptibilit in aqueousy and NaCl corrosion solutions inhibition which makesat the themmicro-scale. common Initially AFM images of topography and VPD show well-defined independent intermetallics that model objects for studying corrosion susceptibility and corrosion inhibition at the micro-scale. Initially have high Volta potential vs. the Al matrix (Figure 11a,b,e). After 5 h of immersion in 0.05 M NaCl

Corros. Mater. Degrad. 2020, 1 358

AFMCorros. imagesMater. Degrad. of topography 2020, 3 FOR PEER and REVIEW VPD show well-deﬁned independent intermetallics that have high 14 Volta potential vs. the Al matrix (Figure 11a,b,e). After 5 h of immersion in 0.05 M NaCl solution, trenchingsolution, trenching was evident was nextevident to thenext intermetallic to the intermetallic inclusion inclusion marked marked with the with proﬁle the profile (Figure (Figure 11b,e). The11b,e). intermetallics The intermetallics displayed displayed on the Voltaon the potential Volta potential map became map became less resolved less resolved with unclear with bordersunclear betweenborders between the intermetallics the intermetallics and the and matrix the (Figure matrix (F11igurec–e). The11c,d,e). Volta The potential Volta potential level increased level increased towards towards200 mV −200 vs. mV Ni (Figurevs. Ni (Figure 11e) which 11e) which is a consequence is a consequence of the of copperthe copper redeposition redeposition process process which which is − well-knownis well-known [10 [10,11,46,74].,11,46,74].

Figure 11. Topography (a), (b) and Volta potentialpotential mapsmaps ((c),), ((d)) obtainedobtained atat thethe samesame placesplaces ofof 20242024 aluminum alloy before ((a),a), ((c)c) and after ((b),b), ((d)d) 5 h of immersion in 0.05 M NaCl solution; topography and VPD proﬁlesprofiles (ee)) taken taken from from maps maps a,b,c,d. a,b,c,d. Reprinted Reprinted from from [73] [73] Copyright (2020), with permission from Elsevier.Elsevier.

The surface of AA2024 had a completely didifferentfferent appearance after immersion in NaCl solution containing corrosion inhibitor. FigureFigure 1212a,ba,b displays the initially polished surface with clearly visible intermetallics onon the VPD map. The The uncorroded uncorroded intermetallics intermetallics reveal reveal a a notably notably similar similar Volta Volta potential (about 550 mV) compared with the previous case (Figures 11e and 12e). After 5 h of immersion in a (about −550 mV) compared with the previous case (Figures 11e and 12e). After 5 h of immersion in a solution containingcontaining 0.50.5 gg/L/L of of salicylaldoxime salicylaldoxime in in 0.05 0.05 M M NaCl NaCl there there were were no no substantial substantial changes changes to the to topographythe topography and and VPD VPD on the on surface the surface (Figure (Figure 12b,d,e). 12b,d, Onlye). Only the VPD the VPD profile profile displayed displayed an off setan tooffset lower to potentiallower potential values values which which may suggest may suggest adsorption adsorption of the of inhibitor the inhibitor on the on metallic the metallic surface surface (Figure (Figure 12e). The12e). above The above results results demonstrate demonstrate a typical a approach typical appr for studyingoach for thestudying efficiency the of efficiency corrosion of inhibition corrosion at ainhibition local scale at before a local and scale after before treatment and after in di treatmenfferent solutions.t in different The depletionsolutions.of The the depletion more active of the elements more dueactive to dealloyingelements due can to be dealloying effectively measuredcan be effectively by comparing measured the Volta by comparing potential di thefference Volta before potential and afterdifference immersion. before and after immersion. SKPFM is capable of resolving the electrochemical activity of independent phases at the microscale and to follow the delamination processes of organic layers from metals. Rohwerder et al. applied SKPFM to study the delamination of synthetic organic layers on model gold surfaces [75]. In this paper SKPFM was proven to probe Volta potentials at the metal/coating interfaces with submicron resolution and follow the kinetics of delamination. However, such a study requires the preparation of special model samples coated with ultrathin polymer coatings, since on thick coatings it was not possible to achieve submicron resolution with SKPFM [75]. The plasma polymer was synthesized from hexamethyldisilane (HMDS) in an argon carrier gas in a microwave plasma reactor and deposited on the gold surface with a final thickness of about 100 nm. The ultrathin plasma polymer coating was proven to delaminate according to a purely cathodic delamination behavior by SKPFM and time of flight secondary mass spectrometry.

Corros. Mater. Degrad. 2020, 3 FOR PEER REVIEW 14 solution, trenching was evident next to the intermetallic inclusion marked with the profile (Figure 11b,e). The intermetallics displayed on the Volta potential map became less resolved with unclear borders between the intermetallics and the matrix (Figure 11c,d,e). The Volta potential level increased towards −200 mV vs. Ni (Figure 11e) which is a consequence of the copper redeposition process which is well-known [10,11,46,74].

Figure 11. Topography (a), (b) and Volta potential maps (c), (d) obtained at the same places of 2024 aluminum alloy before (a), (c) and after (b), (d) 5 h of immersion in 0.05 M NaCl solution; topography and VPD profiles e) taken from maps a,b,c,d. Reprinted from [73] Copyright (2020), with permission from Elsevier.

The surface of AA2024 had a completely different appearance after immersion in NaCl solution containing corrosion inhibitor. Figure 12a,b displays the initially polished surface with clearly visible intermetallics on the VPD map. The uncorroded intermetallics reveal a notably similar Volta potential (about −550 mV) compared with the previous case (Figures 11e and 12e). After 5 h of immersion in a solution containing 0.5 g/L of salicylaldoxime in 0.05 M NaCl there were no substantial changes to the topography and VPD on the surface (Figure 12b,d,e). Only the VPD profile displayed an offset to lower potential values which may suggest adsorption of the inhibitor on the metallic surface (Figure 12e). The above results demonstrate a typical approach for studying the efficiency of corrosion inhibition at a local scale before and after treatment in different solutions. The depletion of the more activeCorros. Mater.elements Degrad. due2020 ,to1 dealloying can be effectively measured by comparing the Volta potential359 difference before and after immersion.

Figure 12. Topography (a), (b) and Volta potential maps (c), (d) obtained at the same places of 2024

aluminum alloy before (a), (c) and after (b), (d) 5 h of immersion in 0.5 g/L solution of salicylaldoxime with 0.05 M NaCl; topography and VPD proﬁles (e) taken from maps a,b,c,d. Reprinted from [73] Copyright (2020), with permission from Elsevier.

The application of the SKPFM technique to study filiform corrosion (FFC) on industrially relevant aluminum alloy AA2017-T4 was reported by Senöz and Rohwerder [69]. The aluminum alloy was artificially aged at different temperatures to study the effects of the distribution of intermetallic particles in the matrix on the development of filiform corrosion. For the deposition of organic coatings a hexamethyldisiloxane (HMDSO)–argon (80:20) precursor mixture was used. FFC was initiated by dispensing a few droplets of 1.0 M aqueous HCl along the scratch made on the coated samples. To make certain that the excess water had evaporated, the sample was held at ambient conditions for about 60 min and then was positioned in the environment chamber of SKPFM, where the relative humidity was held at ca. 85%. The as-received alloy, without any heat treatment, did not show well-defined FFC and constant propagation of localized corrosion during testing. The authors proposed that the density of intermetallics was insufficient to cause severe FFC corrosion. The alloy which was aged at 450 ◦C for 45 min contained the highest density of intermetallics with compositions of Al6(Si, Mn, Fe, Cu) and Al2Cu in the matrix. Figure 13 shows AFM images and SKPFM maps of the Volta potential acquired from the coated sample in places of filiform corrosion attack. Intermetallics appear as darker spots on the VPD images. Similarly, the filaments display cathodic dark parts and anodic bright parts as is clearly shown on images at 2 h and 4 h 15 min (Figure 13). Part (a) of the Figure shows that the filament proceeds in jumps towards the cathodic intermetallics indicated on the images. In part (b) the activity of the filament seems to decrease but the head seems to continue, slowly creeping towards the intermetallic marked with red circles (Figure 13b). Figure 14 displays the mechanism of filiform corrosion development. The active head “jumps” towards the cathodically active intermetallics. The authors suggested that the cathodic activity at the intermetallics weakens the bonds between the coating and the metal, thus letting the anodic head easily propagate along with the weakened interface. Corros. Mater. Degrad. 2020, 1 360 Corros. Mater. Degrad. 2020, 3 FOR PEER REVIEW 16

Figure 13.13. Example ofof filamentfilament propagation propagation (100 (100µ mμm 100× 100µm) μm) observed observed on on sample sample coated coated with with 344 nm344 × hexamethyldisiloxanenm hexamethyldisiloxane (HMDSO) (HMDSO) film at relativefilm athumidity relative aroundhumidity 85%. around Part (a )85%. presents Part observations (a) presents of theobservations surface from of the 2 hrs surface until 4from hrs 152 hrs min, until part 4 (bhrs) presents 15 min, observationspart (b) presents of the observations surface from of 8 the hrs surface 30 min untilfrom 128 hrs hrs 30 25 min min. until Note 12 that hrs the 25 topographic min. Note that images the ontopographic the top have images a full on scale the range top have of 6 µ am full and scale the surfacerange of potential 6 μm and (SP) the mapssurface at potential the bottom (SP) have maps a fullat the scale bottom of 1.3 have V. The a full contour scale of lines 1.3 V. are The positioned contour tolines show are thepositioned slight expansion to show the in slight the borders expansion of the in filamentthe borders when of the surface filament potential when andsurface topography potential imagesand topography are compared. images Reprinted are compared. from [69 Reprinted] Copyright from (2020), [69] Copyright with permission (2020), from with Elsevier. permission from Elsevier.

Corros. Mater. Degrad. 2020, 1 361 Corros. Mater. Degrad. 2020, 3 FOR PEER REVIEW 17

FigureFigure 14. SchematicSchematic illustration illustration of of the the “j “jumps”umps” during during the the propagation propagation of the of the active active head. head. Note Note that thatthe nearest the nearest intermetallic intermetallic particles particles are shown are shown in red. in red. After After incorporation incorporation into intothe thefilament filament they they are areshown shown in ruby. in ruby. The Thetop sequence top sequence shows shows the propagatio the propagationn of the offiligree the filigree in the first in the 4.5 first h and 4.5 the h and bottom the bottomsequence sequence shows showsthe further the further propagation propagation imaged imaged in the in thefollowing following 17.5 17.5 h. h. Reprinted Reprinted from from [[69]69] CopyrightCopyright (2020),(2020), withwith permissionpermission fromfrom Elsevier.Elsevier.

InIn anotheranother exampleexample thethe eeffectsffects ofof annealingannealing temperaturetemperature onon thethe microstructuremicrostructure andand corrosioncorrosion behaviorbehavior ofof duplexduplex stainlessstainless steelsteel 25072507 werewere investigatedinvestigated employingemploying variousvarious techniquestechniques includingincluding AFM,AFM, magneticmagnetic forceforce microscopymicroscopy (MFM),(MFM), andand SKPFMSKPFM [[76].76]. TheThe as-polishedas-polished steelsteel exhibitedexhibited ferriteferrite phasephase ((αα)) andand austeniteaustenite phasephase ((γγ)) whichwhich isis seenseen inin FigureFigure 1515(a1).(a1). With the rise ofof thethe annealingannealing temperaturetemperature thethe volume volume fraction fraction of theof austenitethe austenite phase phase decreased decreased and that and of thethat ferrite of the phase ferrite increased phase (Figureincreased 15 (a2)).(Figure Before 15(a2)). the Before annealing the annealing treatment, trea thetment, average the average Volta potential Volta potential difference difference was 65 was mV between65 mV between the austenite–ferrite the austenite–ferrite phases. phases. After theAfter annealing the annealing treatment treatment VPD VPD between between the ferrite the ferrite and austeniteand austenite phases phases increased increased and then and decreased then decrease with temperature:d with temperature: 1050 ◦C 1101050 mV °C (Figure 110 mV 15 (b1,b2)),(Figure 110015(b1,b2)),◦C 210 1100 mV, °C 1150 210◦C mV, 80 mV.1150 Figure °C 80 15mV.a–f Figure shows 15 thea–f in shows situ AFM the in images situ AFM of the images surface of topography the surface fortopography the steel for samples the steel annealed samples at annealed 1050 ◦C duringat 1050 immersion°C during immersion in 0.3 M hydrochloric in 0.3 M hydrochloric acid solution. acid Unlikesolution. the Unlike case of the the case austenite of the phase, austenite the ferrite phase, was the susceptible ferrite was to susceptible dissolution. to The dissolution. corrosion rateThe measurementscorrosion rate measurements revealed that the revealed sample that annealed the samp at 1100le annealed◦C had at the 1100 highest °C had corrosion the highest depth corrosion and the greatestdepth and weight the greatest loss in weight the ferrite loss phase, in the whichferrite meansphase, which that it hadmeans the that highest it had corrosion the highest rate. corrosion On the contrary,rate. On the the contrary, sample annealed the sample at 1150annealed◦C had at 1150 the lowest°C had corrosion the lowest rate. corrosion The greatest rate. The VPD greatest difference VPD betweendifference the between ferrite andthe ferrite austenite and phases austenite was phases observed was in observed the sample in the annealed sample at annealed 1100 ◦C dueat 1100 to the °C highestdue to the diff highesterence indifference nickel content in nickel in the content two phases. in the two This phases. observation This observation was supported was by supported the highest by susceptibilitythe highest susceptibility to corrosion to of corrosion the ferrite of phase the ferrite according phase to according in situ AFM to in experiments situ AFM experiments and the highest and corrosionthe highest rate corrosion determined rate determ by weightined loss by analysis.weight loss analysis.

Corros. Mater. Degrad. 2020, 1 362 Corros. Mater. Degrad. 2020, 3 FOR PEER REVIEW 18

FigureFigure 15. InIn situ situ AFM AFM images images of of corrosion corrosion behavior behavior of of the the sample sample annealed annealed at at 1050 1050 °C◦C in in 0.3 0.3 M M hydrochlorichydrochloric acid solution: (a) t = 0, ( b) t = 2020 min, min, ( c) t = 5050 min, min, ( d) t = 8080 min, min, ( e) t = 100100 min, min, ( f) t = 120120 min; MagneticMagnetic ((a1a1,a2,a2)) and and VPD VPD ( b1(b1,b2,b2,c1,c1,c2,c2) measurements) measurements and and proﬁles profiles across across diﬀ differenterent ferrite ferrite and andaustenite austenite zones. zones. Reprinted Reprinted from from [76] [76] Copyright Copyright (2020), (2020), with with permission permission from from Elsevier. Elsevier.

5.5. In In Situ Situ Corrosion Corrosion Investigation Investigation by by EC-AFM EC-AFM ElectrochemicalElectrochemical Atomic Atomic Force Force Microscopy Microscopy (EC-AFM) (EC-AFM) has the has capabilities the capabilities to study electrode– to study electrolyteelectrode–electrolyte interfaces at interfaces equilibrium at equilibrium and non-equi andlibrium non-equilibrium electrode potential electrode conditions. potential The conditions. method hasThe found method wide has foundapplications wide applications in material in science material [77], science biological [77], biological science science[78], physical [78], physical [79], and [79], chemistryand chemistry [80]. In [80 corrosion]. In corrosion research research EC-AFM EC-AFM is mainly is applied mainly to applied metallic toelectrodes metallic and electrodes conductive and coatingsconductive [42]. coatings Essentially [42]. EC-AFM Essentially operates EC-AFM following operates the following principles the of principles AFM described of AFM in describedSection 2.1. in AllSection operating 2.1. All modes operating such modesas contact, such semi-contact as contact, semi-contact,, and non-contact and non-contact modes can modes be used can in be EC-AFM used in measurements.EC-AFM measurements. However, However, it is important it is important to assess to whether assess whether the selected the selected mode is mode acceptable is acceptable to study to thestudy metallic the metallic surface surface at the selected at the selected electroche electrochemicalmical conditions conditions in a corrosive in a corrosive electrolyte. electrolyte. ForFor example,example, polyanilinepolyaniline (PANI) (PANI) coatings coatings have have been been the subject the subject of research of research in studies in ofstudies corrosion of corrosionprotection protection of diﬀerent of metallic different materials metallic [ 81material,82]. Electrochemicals [81,82]. Electrochemica mechanismsl mechanisms of solvent-borne of solvent- alkyd composite coating containing 1.0 wt% polyaniline (PANI) and 1.0 wt% CeO nanoparticles (NPs) were borne alkyd composite coating containing 1.0 wt% polyaniline (PANI)2 and 1.0 wt% CeO2 nanoparticlesstudied for carbon (NPs) steel were in studied 3.0 wt% for NaCl carbon solution steel [in81 ].3.0 The wt% results NaCl of solution EC-AFM [81]. combined The results with of cyclic EC- AFMvoltammetry combined (CV) with demonstrated cyclic voltammetry a volume (CV) change demonstrated of the PANI a volume NPs upon change reduction of the PANI and oxidation NPs upon at reductionapplied potentials and oxidation on the at coating. applied Alkyd potentials coatings on the with coating. 1 wt% Alkydp-toluene coatings sulfonic with acid 1 wt% (PTSA) p-toluene doped sulfonicPANI were acid prepared (PTSA) doped and the PANI mechanisms were prepared of electrochemical and the mechanisms activity of and electrochemical self-healing properties activity and of self-healingthe composite properties coating wereof the investigated composite bycoating EC-AFM were [82 investigated]. The results by revealed EC-AFM high [82]. electrochemical The results revealedactivity of high the dopedelectrochemical PANI in the activity composite of the coating doped as wellPANI as in reversible the composite redox reactions coating betweenas well theas reversibleemeraldine redox salt (ES)reactions and leuco between emeraldine the emeraldine base (LB) sa forms.lt (ES) and leuco emeraldine base (LB) forms. MetallicMetallic electrodes electrodes undergo undergo surface surface changes changes du duringring electrochemical electrochemical polarization polarization in in different diﬀerent electrolyteselectrolytes and and are are typical typical subjects subjects of of EC-AFM EC-AFM re research.search. For For instance, instance, a a platinum platinum catalyst catalyst undergoes undergoes aa complex complex deterioration deterioration process process during during its its operation operation as as a a cathode cathode in in a proton exchange membrane fuel cell [83]. By using in situ electrochemical atomic force microscopy (EC-AFM) with super-sharp

Corros. Mater. Degrad. 2020, 1 363 fuel cell [83]. By using in situ electrochemical atomic force microscopy (EC-AFM) with super-sharp probes, Khalakhan et al. quantitatively described the roughening of platinum thin films by analyzing the height–height correlation functions (HHCFs) computed from AFM images during electrochemical cycling voltammetry (CV) to different upper potentials (EU). The results showed that at a critical upper potential EU = 1.0 V the surface of the Pt film did not change significantly. When EU increased to 1.3 V and 1.5 V the Pt morphology evolved significantly with the number of cycles [83]. The corrosion processes of AA2024 were analyzed via the combined approach employing Raman spectroscopy and EC-AFM [84]. To get a deeper insight into the morphology changes, a three-electrode in situ electrochemical AFM cell was constructed and chronocoulometric charging of AA2024 was performed in time steps of 10 s. The impact of this charging was reflected in AFM topography images and extracted parameters such as surface roughness Sq and kurtosis Sku. The results of in situ measurements revealed a gradual increase in the surface roughness and dynamics of the formation of the passivation layer [84]. In this example the researchers paid attention to the fact that the tapping mode was more beneficial since it did not disturb the formed corrosion product film on the surface. Shi et al. employed in situ EC-AFM to investigate the localized corrosion of the AlxCoCrFeNi high-entropy alloys (HEAs) [85]. Surface topography changes on the micro/sub-micron scale were monitored at different applied anodizing potentials in 3.5 wt% NaCl solution. Due to the microstructural variations in the AlxCoCrFeNi HEAs, localized corrosion processes started after the breakdown of the passive film and changed from pitting to phase boundary corrosion at higher content of Al in the alloys [85]. Pitting and general corrosion are typical corrosion attacks found on many metallic materials. Initial stages of localized corrosion of model Zn (Z) and Zn-Al-Mg (ZM) galvanized coatings have been studied in 0.005 M NaCl to get a better insight into the mechanisms of localized corrosion at galvanized steel cut-edges [86]. To promote anodic dissolution of the galvanized coatings, a potential of +40 mV vs. the corrosion potential of the sample was applied. This potential difference was similar to the polarization of zinc coatings by steel as follows from the polarization measurements presented in (Figure 16F). Figure 16 shows the topography of the Z sample before (Figure 16B) and during the polarization (Figure 16B–D). The black line profiles are drawn across the same place. The arrow indicated in Figure 16B shows the moment during the scan, going from the bottom to the top, when the polarization started. The upper AFM image reveals immediate localized pitting corrosion of the Z surface. During immersion, the local pits grow deeper into the zinc surface (Figure 16E). The localized corrosion activity of the ZM sample on a selected zone is presented in Figure 17. Different metallic phases appear at the ZM sample surface such as zinc solid solution, binary or ternary eutectic solution of zinc, aluminum, and MgZn2. The ZM surface before immersion was analyzed by AFM and SKPFM methods to distinguish the properties of different phases (Figure 17A,B). The area in the middle of the images belongs to a ternary eutectics region. This place has a lower Volta potential level (dark area) compared to the surrounding zone of zinc solid solution separated by contour profiles (light area) (Figure 17B). The AFM map after 4 min of immersion at polarization reveals the evident dissolution of the metal matrix (Figure 17C). The AFM topography profiles before and during the immersion were leveled relative to the zero level located on the uncorroded zinc surface (Figure 17F). These profiles show the visible dissolution of eutectic phases and also local corrosion attack at the zone of zinc solid solution (Figure 17F). The topography maps (c,d,e) clearly show corrosion attacks at both the eutectic and the solid solution zones. However, the dissolution process did not progress uniformly on the zinc solid solution zones. After 16 min of immersion the dissolution of zinc solid solution became visible on the left side of the image (Figure 17C–F). With longer immersion (22 min) the AFM profile shows evidence of zinc solid solution dissolution on the right side (Figure 17F). Corros. Mater. Degrad. 2020, 3 FOR PEER REVIEW 19 probes, Khalakhan et al. quantitatively described the roughening of platinum thin films by analyzing the height–height correlation functions (HHCFs) computed from AFM images during electrochemical cycling voltammetry (CV) to different upper potentials (EU). The results showed that at a critical upper potential EU = 1.0 V the surface of the Pt film did not change significantly. When EU increased to 1.3 V and 1.5 V the Pt morphology evolved significantly with the number of cycles [83]. The corrosion processes of AA2024 were analyzed via the combined approach employing Raman spectroscopy and EC-AFM [84]. To get a deeper insight into the morphology changes, a three- electrode in situ electrochemical AFM cell was constructed and chronocoulometric charging of AA2024 was performed in time steps of 10 s. The impact of this charging was reflected in AFM topography images and extracted parameters such as surface roughness Sq and kurtosis Sku. The results of in situ measurements revealed a gradual increase in the surface roughness and dynamics of the formation of the passivation layer [84]. In this example the researchers paid attention to the fact that the tapping mode was more beneficial since it did not disturb the formed corrosion product film on the surface. Shi et al. employed in situ EC-AFM to investigate the localized corrosion of the AlxCoCrFeNi high-entropy alloys (HEAs) [85]. Surface topography changes on the micro/sub-micron scale were monitored at different applied anodizing potentials in 3.5 wt% NaCl solution. Due to the microstructural variations in the AlxCoCrFeNi HEAs, localized corrosion processes started after the breakdown of the passive film and changed from pitting to phase boundary corrosion at higher content of Al in the alloys [85]. Pitting and general corrosion are typical corrosion attacks found on many metallic materials. Initial stages of localized corrosion of model Zn (Z) and Zn-Al-Mg (ZM) galvanized coatings have been studied in 0.005 M NaCl to get a better insight into the mechanisms of localized corrosion at galvanized steel cut-edges [86]. To promote anodic dissolution of the galvanized coatings, a potential of +40 mV vs. the corrosion potential of the sample was applied. This potential difference was similar to the polarization of zinc coatings by steel as follows from the polarization measurements presented in (Figure 16F). Figure 16 shows the topography of the Z sample before (Figure 16B) and during the polarization (Figure 16B–D). The black line profiles are drawn across the same place. The arrow indicated in Figure 16B shows the moment during the scan, going from the bottom to the top, when Corros. Mater. Degrad. 2020, 1 364 the polarization started. The upper AFM image reveals immediate localized pitting corrosion of the Z surface. During immersion, the local pits grow deeper into the zinc surface (Figure 16E).

Corros. Mater. Degrad. 2020, 3 FOR PEER REVIEW 20

topography across the black lines profiles; Figure (F) presents the cathodic polarization curve on steel and the anodic polarization curves on Zn (Z) and Zn-Al-Mg (ZM) samples. Reprinted from [86] Copyright (2020), with permission from Elsevier.

The localized corrosion activity of the ZM sample on a selected zone is presented in Figure 17. Different metallic phases appear at the ZM sample surface such as zinc solid solution, binary or ternary eutectic solution of zinc, aluminum, and MgZn2. The ZM surface before immersion was analyzed by AFM and SKPFM methods to distinguish the properties of different phases (Figure 17A,B). The area in the middle of the images belongs to a ternary eutectics region. This place has a lower Volta potential level (dark area) compared to the surrounding zone of zinc solid solution separated by contour profiles (light area) (Figure 17B). The AFM map after 4 min of immersion at polarization reveals the evident dissolution of the metal matrix (Figure 17C). The AFM topography profiles before and during the immersion were leveled relative to the zero level located on the uncorroded zinc surface (Figure 17F). These profiles show the visible dissolution of eutectic phases and also local corrosion attack at the zone of zinc solid solution (Figure 17F). The topography maps (c,d,e) clearly show corrosion attacks at both the eutectic and the solid solution zones. However, the dissolution process did not progress uniformly on the zinc solid solution zones. After 16 min of immersion the dissolution of zinc solid solution became visible on the left side of the image (Figure 17C–F).Figure With 16. longerAFM topography immersion maps (22 of min) polished the ZAFM galvanized profile coating shows before evidence immersion of zinc (A) andsolid during solution dissolutionimmersionFigure on16. AFMthe in 0.005right topography M side NaCl: (Figure maps 3 min of17F). (polishedB), 22 min Z ga (Clvanized), 35 min coating (D); Figure before ( Eimmersion) presents ( theA) and evolution during ofTheseimmersion topography experiments in 0.005 across M confirmed the NaCl: black 3 min lines the ( proﬁles;Blocalized), 22 min Figure nature(C), 35 (F )ofmin presents corrosion (D); Figure the attack cathodic (E) presents on polarization Z and the ZM evolution samples. curve on of The corrosionsteel andprocess the anodic develops polarization similarly curves as it onwas Zn determ (Z) andined Zn-Al-Mg in the (ZM)cut-edges samples. [86]. Reprinted More specifically, from [86] the corrosionCopyright locus (2020), can be with located permission at the fromzinc Elsevier.solid solution (Z sample) and both the eutectic phases and zinc solid solution in the case of the ZM sample.

Figure 17. AFMAFM topography ( (AA)) and and SKPFM ( B) maps of polished ZM galvanized coating before immersion andand during during immersion immersion in in 0.005 0.005 M NaCl:M NaCl: 4 min 4 (minC), 9(C min), 9 ( Dmin), 22 (D min), 22 (E );min Figure (E); ( FFigure) presents (F) thepresents evolution the evolution of topography of topography across the across black the lines black proﬁles. lines profiles. Reprinted Reprinted from [86 from] Copyright [86] Copyright (2020), with(2020), permission with permission from Elsevier. from Elsevier.

It is important to emphasize that the above AFM measurements revealed surface changes with 5–7 min of delay. There is a gap between what can be seen in the picture and the real state of the surface. It essentially limits the applicability of in situ AFM measurements for studies of metallic systems that rapidly evolve. The study made by Moore and co-workers highlights the application of

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These experiments conﬁrmed the localized nature of corrosion attack on Z and ZM samples. The corrosion process develops similarly as it was determined in the cut-edges [86]. More speciﬁcally, the corrosion locus can be located at the zinc solid solution (Z sample) and both the eutectic phases and zinc solid solution in the case of the ZM sample. It is important to emphasize that the above AFM measurements revealed surface changes with 5–7 min of delay. There is a gap between what can be seen in the picture and the real state of the surface. It essentially limits the applicability of in situ AFM measurements for studies of metallic systems that rapidly evolve. The study made by Moore and co-workers highlights the application of high-speed Corros. Mater. Degrad. 2020, 3 FOR PEER REVIEW 21 AFM (HS-AFM) for the study of localized corrosion events occurring on thermally sensitized AISI 304 stainlesshigh-speed steel in AFM an aqueous (HS-AFM) solution for the of 1%study sodium of loca chloridelized corrosion [87]. The events AFM occurring measurements on thermally were made alongsensitized the grain AISI boundaries 304 stainless which steel contained in an aqueou chromiums solution carbides. of 1% sodium Figure chloride18 shows [87]. before The AFM and after topographicmeasurements images were of an made intergranular along the grain pit forming boundaries at awhich grain contained boundary chromium (GB) during carbides. a galvanostatic Figure scan at18 ashows current before of 0.05 and mA after and topographic a sample surfaceimages of area an ofintergranular 0.13 cm2. Thepit forming timeframe at a atgrain which boundary the pit was formed(GB) is during less than a galvanostatic 0.5 s and this scan is at less a current than the of 0.05 rate mA at which and a sample HS-AFM surface was area acquiring of 0.13 cm topographic2. The images.timeframe Electron at microscopy which the pit observations was formed revealedis less than a much0.5 s and more this detailed is less than picture the ofrate localized at which corrosion. HS- The intergranularAFM was acquiring pits appeared topographic to be images. openings Electron for a larger microscopy network observations of voids underneath revealed a much the surface more [87]. detailed picture of localized corrosion. The intergranular pits appeared to be openings for a larger These results suggest that standalone AFM observations are not capable of resolving the full picture of network of voids underneath the surface [87]. These results suggest that standalone AFM localizedobservations corrosion. are not capable of resolving the full picture of localized corrosion.

Figure 18. Topographic maps showing a grain boundary (GB) in sensitized AISI 304 stainless steel Figure 18. Topographic maps showing a grain boundary (GB) in sensitized AISI 304 stainless steel within 1% aqueous NaCl: (a) before the formation of the intergranular pit, (b) after the formation of within 1% aqueous NaCl: (a) before the formation of the intergranular pit, (b) after the formation of the the intergranular pit, and (c) the full intergranular pit formed. Reprinted from [87]—Published by The intergranularRoyal Society pit, of and Chemistry. (c) the full intergranular pit formed. Reprinted from [87]—Published by The Royal Society of Chemistry. A quite interesting approach has been recently presented by Zhang et al. [88,89]. The researchers Astudied quite anodizing interesting processes approach on hasAA6060 been and recently AA7075-T5 presented alloys by in Zhang situ electrochemical et al. [88,89]. atomic The researchers force studiedmicroscopy anodizing combined processes with onelectrochemical AA6060 and impeda AA7075-T5nce spectroscopy. alloys by The in EC-AFM situ electrochemical revealed detailed atomic forcemorphological microscopy combined changes at withthe intermetallics electrochemical during impedance the anodizing spectroscopy. of AA6060 alloy The in EC-AFM 0.2 M Na revealed2SO4 detailedsolution morphological [88]. AlFeSi changesparticles remained at the intermetallics stable during during the anodizing the anodizing up to 8 V. of However, AA6060 alloyparticular in 0.2 M localized dissolution was observed on Mg2Si particles during the anodizing step in parallel to anodic Na2SO4 solution [88]. AlFeSi particles remained stable during the anodizing up to 8 V. However, Al oxide (AAO) formation. The Mg2Si particles may transform into Si-enriched particles due to the preferential anodic dissolution of Mg. On the other hand, AA7075-T5 alloy contains different intermetallics such as Al7Cu2Fe, S-phase Al2CuMg, Mg2Si, and MgZn2 [89]. Figure 19 displays in situ AFM topography images taken from the surface at OCP and during anodizing at different voltages in 0.2 M Na2SO4 solution. Figure 19a shows a typical surface containing intermetallic particles such as Mg2Si selected by a blue arrow and Al2CuMg encased in a square. During anodizing at low potential (Figure 19b–f) a crater was formed in place of the Al2CuMg particle. This evidences

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particular localized dissolution was observed on Mg2Si particles during the anodizing step in parallel to anodic Al oxide (AAO) formation. The Mg2Si particles may transform into Si-enriched particles due to the preferential anodic dissolution of Mg. On the other hand, AA7075-T5 alloy contains different intermetallics such as Al7Cu2Fe, S-phase Al2CuMg, Mg2Si, and MgZn2 [89]. Figure 19 displays in situ AFM topography images taken from the surface at OCP and during anodizing at different voltages in 0.2 M Na2SO4 solution. Figure 19a shows a typical surface containing intermetallic particles such as Mg2Si selected by a blue arrow and Al2CuMg encased in a square. During anodizing at low potential (Figure 19b–f) a crater was formed in place of the Al2CuMg particle. This evidences dissolution of the S-phase type intermetallics during anodizing. Dissolution was reported also for MgZn2 particles Corros. Mater. Degrad. 2020, 3 FOR PEER REVIEW 22 which had anodic potential compared to the Al matrix. The particle on the top part of the image, Mg2Si, haddissolution a top layer of the of S-phase corrosion type productsintermetallics which during dissolved anodizing. at Dissolution 2 V and 4was V (Figurereported 19alsoc–e). for At higher MgZn2 particles which had anodic potential compared to the Al matrix. The particle on the top part voltages some deposits e.g., Al(OH)3 formed on the top of the arrow marked particle (Figure 19f). of the image, Mg2Si, had a top layer of corrosion products which dissolved at 2 V and 4 V (Figure During anodizing, Al7Cu2Fe remained stable but showed peripheral dissolution around the boundary. 19c–e). At higher voltages some deposits e.g., Al(OH)3 formed on the top of the arrow marked particle The presented(Figure works 19f). During highlighted anodizing, important Al7Cu2Fe remained localized stable corrosion but showed mechanisms peripheral duringdissolution anodic around film growth on aluminumthe boundary. alloys. The presented works highlighted important localized corrosion mechanisms during anodic film growth on aluminum alloys.

Figure 19. In-situ EC-AFM topography images of an actively dissolving region of the AA7075 in 0.2 Figure 19. In-situ EC-AFM topography images of an actively dissolving region of the AA7075 in 0.2 M M Na2SO4 solution at (a) open circuit potential (OCP), (b) 1 V, (c) 2 V, (d) 4 V fist scan, (e) 4 V second Na2SO4 solutionscan, and at (f () a8) V open vs. Ag/AgCl, circuit potentialrespectively. (OCP), The inset (b) of 1 V,each (c )image 2 V, (isd )the 4 Venla ﬁstrgement scan, of (e )the 4 Varea second scan, and (f) 8 Vmarked vs. Ag with/AgCl, a square. respectively. Reprinted from The [89] inset Copyright of each (2020), image with isperm theission enlargement from Elsevier. of the area marked with a square. Reprinted from [89] Copyright (2020), with permission from Elsevier. The applications of EC-AFM extend to studies of lithium-ion batteries (LIBs) which were explicitly discussed in the review of Zhao et al. [90] and others [91,92]. A proper study of LIBs requires the use of a glove box because of the high reactivity of lithium towards oxygen [93]. Various reports provide in situ observation of surface transformations of electrode materials and solid electrolyte

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The applications of EC-AFM extend to studies of lithium-ion batteries (LIBs) which were explicitly discussed in the review of Zhao et al. [90] and others [91,92]. A proper study of LIBs requires the use of a glove box because of the high reactivity of lithium towards oxygen [93]. Various reports provide in situ observation of surface transformations of electrode materials and solid electrolyte interface (SEI) employed in LIBs [94–98]. Among the exotic application of EC-AFM one can mention a technique which combines scanning electrochemical microscopy (SECM) and AFM (AFM-SECM) which utilizes the microelectrode built in the AFM tip [99]. AFM-SECM can sense ionic species such as Cu2+ ions by electrochemically reducing them at the AFM tip-integrated electrode followed by subsequent re-dissolution of Cu into the bulk solution by anodic stripping voltammetry [100,101]. This technique is highly specialized and is applied only by a small research community due to several issues such as cost, reliability, and durability of the combined AFM-SECM probes.

6. Conclusions This review has provided information on the main principles of AFM, SKPFM, and EC-AFM. Various cases have been discussed emphasizing several specific applications of the techniques in corrosion research of metallic materials. AFM methods are capable of providing important information at the resolution levels not achievable by general methods of analysis. However, to properly interpret information obtained from AFM, SKPFM, and EC-AFM measurements the preliminary knowledge of the surface microstructure, the chemistry/composition, and the general electrochemical behavior of metals and alloys are required. Other methods of research such as electron microscopy, surface analysis methods, and electrochemical methods are essential to obtain a complete picture and gain a solid understanding of corrosion processes at metal/aqueous interfaces. Many examples can be given where several AFM based methods have been used in sequence to characterize metallic materials. This happens because the techniques are complementary. For example, the topography and Volta potential maps can locate different phases on the metallic surface and reveal their susceptibility to corrosion while in situ analysis further clarifies the kinetics of corrosion processes at the intermetallics. This of course can be achieved by performing careful calibration, especially in the case of the SKPFM technique, and paying attention to various artifacts and limitations of the techniques. Future perspectives are encouraging especially in light of the appearance of the more advanced AFM-based methods having a higher resolution than the older techniques and offering more diverse information.

Funding: K.A. Yasakau thanks the Portuguese Foundation for Science and Technology for the Researcher grant (IF/01284/2015). A part of this work was developed within the scope of the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020 & UIDP/50011/2020, financed by national funds through the Portuguese Foundation for Science and Technology/MCTES. Project FUNCOAT: “Development and design of novel multiFUNctional PEO COATings” Grant Agreement 82394, H2020-MSCA-RISE-2018 is acknowledged. Conflicts of Interest: The author declares no conflict of interest.

References

1. The NACE International IMPACT Study. Available online: http://impact.nace.org/economic-impact.aspx (accessed on 27 September 2020). 2. Shreir, L.L.; Burstein, G.T.; Jarman, R.A. Corrosion, 3rd ed.; Volume 1 Metal/Environment Reactions; Butterworth-Heinemann: Linacn House, Jordan Hill, Oxford, UK, 1994; pp. 2:3–2:163. 3. Roberge, P.R. Handbook of Corrosion Engineering; McGraw-Hill: New York, NY, USA, 2000. 4. Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous Solutions; National Association of Corrosion Engineers: Houston, TX, USA, 1974. 5. Totten, G.E.; MacKenzie, D.S. Handbook of Aluminum: Volume 1: Physical Metallurgy and Processes; Marcel Dekker, Inc.: Basel, Switzerland, 2003. 6. EU. The Energy Eﬃciency Directive 2012/27/EU. Oﬀ. J. Eur. Union 2012, L 315/1. Corros. Mater. Degrad. 2020, 1 368

7. Lightweight alloys for aerospace application. In Proceedings of the Symposium Sponsored by the Non-Ferrous Metals Committee of the Structural Materials Division (SMD) of TMS (The Minerals, Metals & Materials Society), the TMS Annual Meeting, New Orleans, LA, USA, 12–14 February 2001; TMS: Warrendale, PA, USA, 2001. 8. Kainer, K.U.; Kaiser, F.; Materialkunde, D.G.f. Magnesium Alloys and Technology; DGM Wiley-VCH: Weinheim, Germany, 2003. 9. Uijl, N.J.D.; Carless, L.J. 3—Advanced metal-forming technologies for automotive applications. In Advanced Materials in Automotive Engineering; Rowe, J., Ed.; Woodhead Publishing: Cambridge, UK, 2012; pp. 28–56. 10. Boag, A.; Hughes, A.E.; Glenn, A.M.; Muster, T.H.; McCulloch, D. Corrosion of AA2024-T3 Part I: Localised corrosion of isolated IM particles. Corros. Sci. 2011, 53, 17–26. [CrossRef] 11. Yasakau, K.A.; Zheludkevich, M.L.; Lamaka, S.V.; Ferreira, M.G.S. Mechanism of Corrosion Inhibition of AA2024 by Rare-Earth Compounds. J. Phys. Chem. B 2006, 110, 5515–5528. [CrossRef][PubMed] 12. Boag, A.; Taylor, R.J.; Muster, T.H.; Goodman, N.; McCulloch, D.; Ryan, C.; Rout, B.; Jamieson, D.; Hughes, A.E. Stable pit formation on AA2024-T3 in a NaCl environment. Corros. Sci. 2010, 52, 90–103. [CrossRef] 13. Hughes, A.; Muster, T.H.; Boag, A.; Glenn, A.M.; Luo, C.; Zhou, X. Thompson, G.E.; McCulloch, D. Co-operative corrosion phenomena. Corros. Sci. 2010, 52, 665–668. [CrossRef] 14. Birbilis, N.; Cavanaugh, M.K.; Buchheit, R.G. Electrochemical behavior and localized corrosion associated with Al7Cu2Fe particles in aluminum alloy 7075-T651. Corros. Sci. 2006, 48, 4202–4215. [CrossRef] 15. Meng, Q.; Frankel, G.S. Effect of Cu Content on Corrosion Behavior of 7xxx Series Aluminum Alloys. J. Electrochem. Soc. 2004, 151, B271. [CrossRef] 16. Buchheit, R.G. A Compilation of Corrosion Potentials Reported for Intermetallic Phases in Aluminum Alloys. J. Electrochem. Soc. 1995, 142, 3994–3996. [CrossRef] 17. Birbilis, N.; Buchheit, R.G. Electrochemical Characteristics of Intermetallic Phases in Aluminum Alloys. J. Electrochem. Soc. 2005, 152, B140. [CrossRef] 18. Jönsson, M.; Thierry, D.; LeBozec, N. The influence of microstructure on the corrosion behaviour of AZ91D studied by scanning Kelvin probe force microscopy and scanning Kelvin probe. Corros. Sci. 2006, 48, 1193–1208. [CrossRef] 19. Uan, J.-Y.; Li, C.-F.; Yu, B.-L. Characterization and Improvement in the Corrosion Performance of a Hot-Chamber Diecast Mg Alloy Thin Plate by the Removal of Interdendritic Phases at the Die Chill Layer. Metall. Mater. Trans. A 2008, 39, 703–715. [CrossRef] 20. Wang, H.; Song, Y.; Yu, J.; Shan, D.; Han, H. Characterization of Filiform Corrosion of Mg–3Zn Mg Alloy. J. Electrochem. Soc. 2017, 164, C574–C580. [CrossRef] 21. Li, J.; Birbilis, N.; Buchheit, R.G. Electrochemical assessment of interfacial characteristics of intermetallic phases present in aluminium alloy 2024-T3. Corros. Sci. 2015, 101, 155–164. [CrossRef] 22. Zhu, Y.; Sun, K.; Garves, J.; Bland, L.G.; Locke, J.; Allison, J.; Frankel, G.S. Micro- and nano-scale intermetallic phases in AA2070-T8 and their corrosion behavior. Electrochim. Acta 2019, 319, 634–648. [CrossRef] 23. Binnig, G.; Quate, C.F.; Gerber, C. Atomic Force Microscope. Phys. Rev. Lett. 1986, 56, 930–933. [CrossRef] [PubMed] 24. Albrecht, T.R.; Grütter, P.; Horne, D.; Rugar, D. Frequency modulation detection using high-Q cantilevers for enhanced force microscope sensitivity. J. Appl. Phys. 1991, 69, 668–673. [CrossRef] 25. Seifert, J.; Rheinlaender, J.; Novak, P.; Korchev, Y.E.; Schäffer, T.E. Comparison of Atomic Force Microscopy and Scanning Ion Conductance Microscopy for Live Cell Imaging. Langmuir 2015, 31, 6807–6813. [CrossRef] 26. Checco, A.; Cai, Y.; Gang, O.; Ocko, B.M. High resolution non-contact AFM imaging of liquids condensed onto chemically nanopatterned surfaces. Ultramicroscopy 2006, 106, 703–708. [CrossRef] 27. Setvín, M.; Wagner, M.; Schmid, M.; Parkinson, G.S.; Diebold, U. Surface point defects on bulk oxides: Atomically-resolved scanning probe microscopy. Chem. Soc. Rev. 2017, 46, 1772–1784. [CrossRef] 28. Breakspear, S.; Smith, J.R.; Campbell, S.A. AFM in Surface Finishing: Part III. Lateral Force Microscopy and Friction Measurements. Trans. IMF 2003, 81, B68–B70. [CrossRef] 29. Nonnenmacher, M.; O’Boyle, M.P.; Wickramasinghe, H.K. Kelvin probe force microscopy. Appl. Phys. Lett. 1991, 58, 2921–2923. [CrossRef] 30. Melitz, W.; Shen, J.; Kummel, A.C.; Lee, S. Kelvin probe force microscopy and its application. Surf. Sci. Rep. 2011, 66, 1–27. [CrossRef] Corros. Mater. Degrad. 2020, 1 369

31. Rohwerder, M.; Turcu, F. High-resolution Kelvin probe microscopy in corrosion science: Scanning Kelvin probe force microscopy (SKPFM) versus classical scanning Kelvin probe (SKP). Electrochim. Acta 2007, 53, 290–299. [CrossRef] 32. Jacobs, H.O.; Leuchtmann, P.; Homan, O.J.; Stemmer, A. Resolution and contrast in Kelvin probe force microscopy. J. Appl. Phys. 1998, 84, 1168–1173. [CrossRef] 33. Hudlet, S.; Jean, M.S.; Roulet, B.; Berger, J.; Guthmann, C. Electrostatic forces between metallic tip and semiconductor surfaces. J. Appl. Phys. 1995, 77, 3308–3314. [CrossRef] 34. Baumgart, C.; Helm, M.; Schmidt, H. Quantitative dopant profiling in semiconductors: A Kelvin probe force microscopy model. Phys. Rev. B 2009, 80, 085305. [CrossRef] 35. Jacobs, H.O.; Knapp, H.F.; Müller, S.; Stemmer, A. Surface potential mapping: A qualitative material contrast in SPM. Ultramicroscopy 1997, 69, 39–49. [CrossRef] 36. Yasakau, K.A.; Höche, D.; Lamaka, S.L.; Ferreira, M.G.S.; Zheludkevich, M.L. Kelvin microprobe analytics on iron-enriched corroded magnesium surface. Corrosion 2017, 73, 583–595. [CrossRef] 37. Koley, G.; Spencer, M.G.; Bhangale, H.R. Cantilever effects on the measurement of electrostatic potentials by scanning Kelvin probe microscopy. Appl. Phys. Lett. 2001, 79, 545–547. [CrossRef] 38. Elias, G.; Glatzel, T.; Meyer, E.; Schwarzman, A.; Boag, A.; Rosenwaks, Y. The role of the cantilever in Kelvin probe force microscopy measurements. Beilstein J. Nanotechnol. 2011, 2, 252–260. [CrossRef] 39. Cherniavskaya, O.; Chen, L.; Weng, V.; Yuditsky, L.; Brus, L.E. Quantitative Noncontact Electrostatic Force Imaging of Nanocrystal Polarizability. J. Phys. Chem. B 2003, 107, 1525–1531. [CrossRef] 40. Bruker Application Note #140: PeakForce Kelvin Probe Force Microscopy. Available online: https://www.bruker.com/fileadmin/user_upload/8-PDF-Docs/SurfaceAnalysis/AFM/ApplicationNotes/ AN140-RevA1-PeakForce_KPFM-AppNote.pdf (accessed on 25 September 2020). 41. Manne, S.; Massie, J.; Elings, V.B.; Hansma, P.K.; Gewirth, A.A. Electrochemistry on a gold surface observed with the atomic force microscope. J. Vac. Sci. Technol. B Microelectron. Nanometer Struct. Process. Meas. Phenom. 1991, 9, 950–954. [CrossRef] 42. Chen, H.; Qin, Z.; He, M.; Liu, Y.; Wu, Z. Application of Electrochemical Atomic Force Microscopy (EC-AFM) in the Corrosion Study of Metallic Materials. Materials 2020, 13, 668. [CrossRef] 43. Wu, H.; Feng, X.; Kieviet, B.D.; Zhang, K.; Zandvliet, H.J.W.; Canters, G.W.; Schön, P.M.; Vancso, G.J. Electrochemical atomic force microscopy reveals potential stimulated height changes of redox responsive Cu-azurin on gold. Eur. Polym. J. 2016, 83, 529–537. [CrossRef] 44. Xu, K.; Sun, W.; Shao, Y.; Wei, F.; Zhang, X.; Wang, W.; Li, P. Recent development of PeakForce Tapping mode atomic force microscopy and its applications on nanoscience. Nanotechnol. Rev. 2018, 7, 605. [CrossRef] 45. Mudali, U.K.; Padhy, N. Electrochemical scanning probe microscope (EC-SPM) for the in situ corrosion study of materials: An overview with examples. Corros. Rev. 2011, 29, 73. [CrossRef] 46. Schmutz, P.; Frankel, G.S. Corrosion Study of AA2024-T3 by Scanning Kelvin Probe Force Microscopy and In Situ Atomic Force Microscopy Scratching. J. Electrochem. Soc. 1998, 145, 2295–2306. [CrossRef] 47. Yasakau, K.A.; Zheludkevich, M.L.; Lamaka, S.V.; Ferreira, M.G.S. Role of intermetallic phases in localized corrosion of AA5083. Electrochim. Acta 2007, 52, 7651–7659. [CrossRef] 48. Iannuzzi, M.; Frankel, G.S. Inhibition of Aluminum Alloy 2024 Corrosion by Vanadates: An In Situ Atomic Force Microscopy Scratching Investigation. Corrosion 2007, 63, 672–688. [CrossRef] 49. Singh, A.; Ansari, K.R.; Chauhan, D.S.; Quraishi, M.A.; Kaya, S. Anti-corrosion investigation of pyrimidine derivatives as green and sustainable corrosion inhibitor for N80 steel in highly corrosive environment: Experimental and AFM/XPS study. Sustain. Chem. Pharm. 2020, 16, 100257. [CrossRef] 50. Singh, A.K.; Chugh, B.; Thakur, S.; Pani, B.; Lgaz, H.; Chung, I.-M.; Pal, S.; Prakash, R. Green approach of synthesis of thiazolyl imines and their impeding behavior against corrosion of mild steel in acid medium. Colloids Surf. A Physicochem. Eng. Asp. 2020, 599, 124824. [CrossRef] 51. Ansari, K.R.; Chauhan, D.S.; Quraishi, M.A.; Mazumder, M.A.J.; Singh, A. Chitosan Schiff base: An environmentally benign biological macromolecule as a new corrosion inhibitor for oil & gas industries. Int. J. Biol. Macromol. 2020, 144, 305–315. [CrossRef] 52. Sliem, M.H.; Radwan, A.B.; Mohamed, F.S.; Alnuaimi, N.A.; Abdullah, A.M. An efficient green ionic liquid for the corrosion inhibition of reinforcement steel in neutral and alkaline highly saline simulated concrete pore solutions. Sci. Rep. 2020, 10, 14565. [CrossRef] Corros. Mater. Degrad. 2020, 1 370

53. Benaioun, N.E.; Maafa, I.; Florentin, A.; Denys, E.; Hakiki, N.E.; Moulayat, N.; Bubendorff, J.L. Time dependence of the natural passivation process on AISI 304 in an alkaline medium: Atomic force microscopy and scanning Kelvin probe force microscopy as additional tools to electrochemical impedance spectroscopy. Appl. Surf. Sci. 2018, 436, 646–652. [CrossRef] 54. Handoko, W.; Pahlevani, F.; Sahajwalla, V. The Effect of Low-Quantity Cr Addition on the Corrosion Behaviour of Dual-Phase High Carbon Steel. Metals 2018, 8, 199. [CrossRef] 55. Schmutz, P.; Frankel, G.S. Characterization of AA2024-T3 by Scanning Kelvin Probe Force Microscopy. J. Electrochem. Soc. 1998, 145, 2285–2295. [CrossRef] 56. Örnek, C.; Leygraf, C.; Pan, J. On the Volta potential measured by SKPFM–fundamental and practical aspects with relevance to corrosion science. Corros. Eng. Sci. Technol. 2019, 54, 185–198. [CrossRef] 57. Cook, A.B.; Barrett, Z.; Lyon, S.B.; McMurray, H.N.; Walton, J.; Williams, G. Calibration of the scanning Kelvin probe force microscope under controlled environmental conditions. Electrochim. Acta 2012, 66, 100–105. [CrossRef] 58. Efaw, C.; da Silva, T.; Davis, P.; Li, L.; Graugnard, E.; Hurley, M. Improving the Relative Calculations of Volta Potential Differences Acquired from Scanning Kelvin Probe Force Microscopy (SKPFM) from Comparing an Inert Material to First-Principle Calculations. ECS Trans. 2018, 85, 701–713. [CrossRef] 59. Beerbom, M.M.; Lägel, B.; Cascio, A.J.; Doran, B.V.;Schlaf, R. Direct comparison of photoemission spectroscopy and in situ Kelvin probe work function measurements on indium tin oxide films. J. Electron. Spectrosc. Relat. Phenom. 2006, 152, 12–17. [CrossRef] 60. Kalb, H.; Rzany, A.; Hensel, B. Impact of microgalvanic corrosion on the degradation morphology of WE43 and pure magnesium under exposure to simulated body fluid. Corros. Sci. 2012, 57, 122–130. [CrossRef] 61. Ascencio, M.; Pekguleryuz, M.; Omanovic, S. An investigation of the corrosion mechanisms of WE43 Mg alloy in a modified simulated body fluid solution: The influence of immersion time. Corros. Sci. 2014, 87, 489–503. [CrossRef] 62. Coy, A.E.; Viejo, F.; Skeldon, P.; Thompson, G.E. Susceptibility of rare-earth-magnesium alloys to micro-galvanic corrosion. Corros. Sci. 2010, 52, 3896–3906. [CrossRef] 63. Kirkland, N.T.; Birbilis, N.; Staiger, M.P. Assessing the corrosion of biodegradable magnesium implants: A critical review of current methodologies and their limitations. Acta Biomater. 2012, 8, 925–936. [CrossRef] 64. Mohedano, M.; Blawert, C.; Yasakau, K.A.; Arrabal, R.; Matykina, E.; Mingo, B.; Scharnagl, N.; Ferreira, M.G.S.; Zheludkevich, M.L. Characterization and corrosion behavior of binary Mg-Ga alloys. Mater. Charact. 2017, 128, 85–99. [CrossRef] 65. Woo, S.K.; Blawert, C.; Yasakau, K.A.; Yi, S.; Scharnagl, N.; Suh, B.-C.; Kim, Y.M.; Sun You, B.; Dong Yim, C. Effects of combined addition of Ca and Y on the corrosion behaviours of die-cast AZ91D magnesium alloy. Corros. Sci. 2020, 166, 108451. [CrossRef] 66. Yang, J.; Blawert, C.; Lamaka, S.V.; Yasakau, K.A.; Wang, L.; Laipple, D.; Schieda, M.; Di, S.; Zheludkevich, M.L. Corrosion inhibition of pure Mg containing a high level of iron impurity in pH neutral NaCl solution. Corros. Sci. 2018, 142, 222–237. [CrossRef] 67. Hurley, M.F.; Efaw, C.M.; Davis, P.H.; Croteau, J.R.; Graugnard, E.; Birbilis, N. Volta potentials measured by scanning kelvin probe force microscopy as relevant to corrosion of magnesium alloys. Corrosion 2015, 71, 160–170. [CrossRef] 68. de Wit, J.H.W. Local potential measurements with the SKPFM on aluminium alloys. Electrochim. Acta 2004, 49, 2841–2850. [CrossRef] 69. Senöz, C.; Rohwerder, M. Scanning Kelvin probe force microscopy for the in situ observation of the direct interaction between active head and intermetallic particles in filiform corrosion on aluminium alloy. Electrochim. Acta 2011, 56, 9588–9595. [CrossRef] 70. Lafouresse, M.C.; Charvillat, C.; Blanc, C.; de Bonfils-Lahovary, M.-L.; Laffont, L. Role of hydrogen in intergranular corrosion of 2024 aluminum alloy: An SKPFM Study. In CORROSION 2017; NACE International: New Orleans, LA, USA, 2017; p. 11. 71. Mallinson, C.F.; Yates, P.M.; Baker, M.A.; Castle, J.E.; Harvey, A.; Watts, J.F. The localised corrosion associated with individual second phase particles in AA7075-T6: A study by SEM, EDX, AES, SKPFM and FIB-SEM. Mater. Corros. 2017, 68, 748–763. [CrossRef] Corros. Mater. Degrad. 2020, 1 371

72. Esfahani, Z.; Rahimi, E.; Sarvghad, M.; Rafsanjani-Abbasi, A.; Davoodi, A. Correlation between the histogram and power spectral density analysis of AFM and SKPFM images in an AA7023/AA5083 FSW joint. J. Alloy. Compd. 2018, 744, 174–181. [CrossRef] 73. Lamaka, S.V.; Zheludkevich, M.L.; Yasakau, K.A.; Montemor, M.F.; Ferreira, M.G.S. High eﬀective organic corrosion inhibitors for 2024 aluminium alloy. Electrochim. Acta 2007, 52, 7231–7247. [CrossRef] 74. Buchheit, R.G.; Martinez, M.A.; Montes, L.P. Evidence for Cu Ion Formation by Dissolution and Dealloying

the Al2CuMg Intermetallic Compound in Rotating Ring-Disk Collection Experiments. J. Electrochem. Soc. 2000, 147, 119. [CrossRef] 75. Rohwerder, M.; Hornung, E.; Stratmann, M. Microscopic aspects of electrochemical delamination: An SKPFM study. Electrochim. Acta 2003, 48, 1235–1243. [CrossRef] 76. Guo, L.Q.; Li, M.; Shi, X.L.; Yan, Y.; Li, X.Y.; Qiao, L.J. Effect of annealing temperature on the corrosion behavior of duplex stainless steel studied by in situ techniques. Corros. Sci. 2011, 53, 3733–3741. [CrossRef] 77. Maurice, V.; Marcus, P. Application of Surface Science to Corrosion. Surf. Interface Sci. 2020, 799–825. [CrossRef] 78. Eliaz, N. Corrosion of Metallic Biomaterials: A Review. Materials 2019, 12, 407. [CrossRef] 79. Yang, X.; Chen, Z.; Zhou, D.; Zhao, W.; Qian, X.; Yang, Q.; Sun, T.; Shen, C. Ultra-low Au–Pt Co-decorated TiO2 nanotube arrays: Construction and its improved visible-light-induced photocatalytic properties. Sol. Energy Mater. Sol. Cells 2019, 201, 110065. [CrossRef] 80. Brummel, O.; Waidhas, F.; Khalakhan, I.; Vorokhta, M.; Dubau, M.; Kovács, G.; Aleksandrov, H.A.; Neyman, K.M.; Matolín, V.; Libuda, J. Structural transformations and adsorption properties of PtNi nanoalloy thin film electrocatalysts prepared by magnetron co-sputtering. Electrochim. Acta 2017, 251, 427–441. [CrossRef] 81. Li, J.; Ecco, L.; Ahniyaz, A.; Pan, J. Probing electrochemical mechanism of polyaniline and CeO2 nanoparticles in alkyd coating with in-situ electrochemical-AFM and IRAS. Prog. Org. Coat. 2019, 132, 399–408. [CrossRef] 82. Li, J.; Huang, H.; Fielden, M.; Pan, J.; Ecco, L.; Schellbach, C.; Delmas, G.; Claesson, P.M. Towards the mechanism of electrochemical activity and self-healing of 1 wt% PTSA doped polyaniline in alkyd composite polymer coating: Combined AFM-based studies. RSC Adv. 2016, 6, 19111–19127. [CrossRef] 83. Khalakhan, I.; Choukourov, A.; Vorokhta, M.; Kúš, P.; Matolínová, I.; Matolín, V. In situ electrochemical AFM monitoring of the potential-dependent deterioration of platinum catalyst during potentiodynamic cycling. Ultramicroscopy 2018, 187, 64–70. [CrossRef] 84. Kreta, A.; Rodošek, M.; Slemenik Perše, L.; Orel, B.; Gaberšˇcek,M.; Šurca Vuk, A. In situ electrochemical AFM, ex situ IR reflection–absorption and confocal Raman studies of corrosion processes of AA 2024-T3. Corros. Sci. 2016, 104, 290–309. [CrossRef] 85. Shi, Y.; Collins, L.; Balke, N.; Liaw, P.K.; Yang, B. In-situ electrochemical-AFM study of localized corrosion of AlxCoCrFeNi high-entropy alloys in chloride solution. Appl. Surf. Sci. 2018, 439, 533–544. [CrossRef] 86. Yasakau, K.A.; Kallip, S.; Lisenkov, A.; Ferreira, M.G.S.; Zheludkevich, M.L. Initial stages of localized corrosion at cut-edges of adhesively bonded Zn and Zn-Al-Mg galvanized steel. Electrochim. Acta 2016, 211, 126–141. [CrossRef] 87. Moore, S.; Burrows, R.; Picco, L.; Martin, T.L.; Greenwell, S.J.; Scott, T.B.; Payton, O.D. A study of dynamic nanoscale corrosion initiation events using HS-AFM. Faraday Discuss. 2018, 210, 409–428. [CrossRef] [PubMed] 88. Zhang, F.; Nilsson, J.-O.; Pan, J. In Situ and Operando AFM and EIS Studies of Anodization of Al 6060: Influence of Intermetallic Particles. J. Electrochem. Soc. 2016, 163, C609–C618. [CrossRef] 89. Zhang, F.; Örnek, C.; Nilsson, J.-O.; Pan, J. Anodisation of aluminium alloy AA7075—Influence of intermetallic particles on anodic oxide growth. Corros. Sci. 2020, 164, 108319. [CrossRef] 90. Zhao, W.; Song, W.; Cheong, L.-Z.; Wang, D.; Li, H.; Besenbacher, F.; Huang, F.; Shen, C. Beyond imaging: Applications of atomic force microscopy for the study of Lithium-ion batteries. Ultramicroscopy 2019, 204, 34–48. [CrossRef] 91. Zhu, J.; Zeng, K. Studying the Localized Electrochemical Phenomena in Rechargeable Li-Ion Batteries by Scanning Probe Microscopy Techniques. In Nanotechnology for Sustainable Energy; Hu, Y.H., Burghaus, U., Eds.; American Chemical Society: Washington, DC, USA, 2013; Volume 1140, pp. 23–53. 92. Tripathi, A.M.; Su, W.-N.; Hwang, B.J. In situ analytical techniques for battery interface analysis. Chem. Soc. Rev. 2018, 47, 736–851. [CrossRef] Corros. Mater. Degrad. 2020, 1 372

93. Aurbach, D.; Cohen, Y. The Application of Atomic Force Microscopy for the Study of Li Deposition Processes. J. Electrochem. Soc. 1996, 143, 3525–3532. [CrossRef] 94. Vidu, R.; Quinlan, F.T.; Stroeve, P. Use of in Situ Electrochemical Atomic Force Microscopy (EC-AFM) to Monitor Cathode Surface Reaction in Organic Electrolyte. Ind. Eng. Chem. Res. 2002, 41, 6546–6554. [CrossRef] 95. Shen, C.; Hu, G.; Cheong, L.-Z.; Huang, S.; Zhang, J.-G.; Wang, D. Direct Observation of the Growth of Lithium Dendrites on Graphite Anodes by Operando EC-AFM. Small Methods 2018, 2, 1700298. [CrossRef] 96. Alliata, D.; Kötz, R.; Novák, P.; Siegenthaler, H. Electrochemical SPM investigation of the solid electrolyte interphase ﬁlm formed on HOPG electrodes. Electrochem. Commun. 2000, 2, 436–440. [CrossRef] 97. Ramdon, S.; Bhushan, B.; Nagpure, S.C. In situ electrochemical studies of lithium-ion battery cathodes using atomic force microscopy. J. Power Sources 2014, 249, 373–384. [CrossRef] 98. Liu, X.; Wang, D.; Wan, L. Progress of electrode/electrolyte interfacial investigation of Li-ion batteries via in situ scanning probe microscopy. Sci. Bull. 2015, 60, 839–849. [CrossRef] 99. Shi, X.; Qing, W.; Marhaba, T.; Zhang, W. Atomic force microscopy—Scanning electrochemical microscopy (AFM-SECM) for nanoscale topographical and electrochemical characterization: Principles, applications and perspectives. Electrochim. Acta 2020, 332, 135472. [CrossRef] 100. Izquierdo, J.; Fernández-Pérez, B.M.; Eifert, A.; Souto, R.M.; Kranz, C. Simultaneous atomic force—scanning electrochemical microscopy (AFM-SECM) imaging of copper dissolution. Electrochim. Acta 2016, 201, 320–332. [CrossRef] 101. Izquierdo, J.; Eifert, A.; Kranz, C.; Souto, R.M. In situ investigation of copper corrosion in acidic chloride solution using atomic force—Scanning electrochemical microscopy. Electrochim. Acta 2017, 247, 588–599. [CrossRef]

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