The scanning Kelvin probe; a new technique for the in situ analysis of the delamination of organic coatings

Corrosion reactions which take place during delamination in the vicinity of defects have not been understood up to now. In this paper the delamination of a simple model coating from clean steel and the delamination of technical coatings from petreated steel substrates are analysed with a scanning Kelvin probe. It is possible, with this technique, to follow the delamination in situ and to understand the basic corrosion mechanism at the metal/polymer interface.     

High-resolution Kelvin probe microscopy in corrosion science: Scanning Kelvin probe force microscopy (SKPFM) versus classical scanning Kelvin probe (SKP)

With the introduction of a Kelvin probe mode to atomic force microscopy, the so called scanning Kelvin probe force microscopy (SKPFM), the Kelvin probe technique finds application in a steadily increasing number of different fields, from corrosion science to microelectronics and biosciences. For many of these applications, high resolution is required as the relevant information lies in the sub-microscopic distribution of work functions or potentials, which explains the increasing interest in SKPFM. However, compared to the standard scanning Kelvin probe (SKP) technique SKPFM is prone to much more artefacts, which are often not taken into account in the interpretation of the results, as is also the case with the real physical nature of the measured data. A critical discussion of possible artefacts and on the interpretation of the data is presented in this paper, with the main focus on application in corrosion science.     

Failure modes in organic coatings studied by scanning acoustic microscopy

Scanning acoustic microscopy (SAM) has been found to be well suited to study physically and chemically induced changes and defects in polymer coatings exposed to corrosive environments. In this work, SAM was used to investigate sub-surface migration and blister formation in polymer coatings of different layer structure after exposure to a corrosive solution. Two model systems consisting of base coat and clear coat on steel substrates where studied. The time evolution of sub-surface migration fronts and blister initiation and their growth were investigated by analysing SAM images after different exposure times. Depending on the layer structure, it was possible to differentiate between transport of the electrolyte solution (i) through the coating and (ii) along the coating/substrate interface. Samples without clear coat typically showed randomly distributed blisters at the coating/substrate interface, irrespective of the location of initial defects. The random distribution of blisters is related to diffusion of the electrolyte solution through the coating layer followed by “nucleation” at weak spots of the substrate, at the interface between polymer and substrate or within the polymer. In contrast, samples with a clear coat acting as a diffusion barrier showed a sub-surface migration front of 2–4 μm height, propagating along the coating/substrate interface, starting at initial defects. The linear propagation of this front cannot be explained by Fickian diffusion and is discussed in terms of an accelerated diffusion or crack growth kinetics. Since blistering started only at regions, where the migration front has already passed, the presence of electrolyte solution or water at the coating/substrate interface was found to be a prerequisite for the nucleation of blisters.     

Studies of microhardness and mar resistance using a scanning probe microscope

Scanning probe microscopy (SPM) is in a period of rapid development. It shows great promise for characterizing coating surfaces. This paper describes modification of an SPM so that it can be used to mar the surfaces of coatings under controlled conditions and to characterize the mars. Mar resistance of coatings is analyzed in terms of a ‘three response, two mechanism model.' The three responses (fracture, elastic, and plastic) can be measured quantitatively using the SPM. Of the three responses, only two (fracture and plastic deformation) are marring mechanisms – elastic deformations recover instantaneously. In some cases mars resulting from plastic deformation may recover slowly with time or with immersion in water; this phenomenon is attributed to viscoelastic creep. Microhardness is also measured with the modified SPM. Some thermoset coatings appear to be substantially harder near their surfaces than in the mass of material, and such materials may respond quite differently to stress applied at different levels near the surface. This finding has important implications for all coating properties that are strongly influenced by the surface. A quantity called ‘micro mar resistance' is defined. It may be useful for comparing different coatings under specified conditions of marring. However, there can be no single quantity that expresses ‘mar resistance' of a coating under all conditions.     

Scanning Kelvin probe force microscopy study of chromium nitrides in 2507 super duplex stainless steel—Implications and limitations

The effect of chromium nitrides on localized corrosion resistance of 2507 super duplex stainless steel was investigated in this study. The Volta potential difference measured with scanning Kelvin probe force microscopy (SKPFM) indicates that chromium nitrides with the size range of 80–230 nm precipitated isothermally at the ferrite/austenite phase boundaries may detrimentally affect the corrosion resistance due to the observed local Volta potential drop at the phase boundaries. Small quenched-in nitrides with the size range of 50–100 nm formed in the centre of the ferrite phase, on the other hand, may have small or no adverse effect on the corrosion resistance since they showed no difference in Volta potential relative to the matrix.     

Thin films of amphiphilic polyelectrolytes. Soft materials characterized by Kelvin probe force microscopy

Hydrophobically modified amphiphilic polyelectrolyte films derived from poly (maleic anhydride-alt-styrene) containing hydrophobic aryl-alkyl type side chains such as phenyl-ethyl, phenyl-butyl, naphthyl-ethyl and naphthyl-butyl were studied by Kelvin probe force microscopy. These films were adsorbed from polyelectrolyte solutions at 0.001 mol/L and 0.1 mol/L NaCl onto silicon wafers modified with 3-aminopropyltrimethoxysilane. At high ionic strength, the work function was dependent on the hydrophobic character of the side chain. At low ionic strength this behavior was determined by the spacer group in the side chain. The fractal analysis of the films indicated self-affinity surfaces whereas the fractal dimensions of the surface topography follow a similar trend as the electronic work function with the ionic strength. This behavior can be explained by the increasing hydrophobic character of the side chain with naphthyl moieties. Relationship between the molecular structure and the fractal dimensions with the work function of the adsorbed polyelectrolytes was found.     

Potential sensitivities in frequency modulation and heterodyne amplitude modulation Kelvin probe force microscopes

In this paper, the potential sensitivity in Kelvin probe force microscopy (KPFM) was investigated in frequency modulation (FM) and heterodyne amplitude modulation (AM) modes. We showed theoretically that the minimum detectable contact potential difference (CPD) in FM-KPFM is higher than in heterodyne AM-KPFM. We experimentally confirmed that the signal-to-noise ratio in FM-KPFM is lower than that in heterodyne AM-KPFM, which is due to the higher minimum detectable CPD dependence in FM-KPFM. We also compared the corrugations in the local contact potential difference on the surface of Ge (001), which shows atomic resolution in heterodyne AM-KPFM. In contrast, atomic resolution cannot be obtained in FM-KPFM under the same experimental conditions. The higher potential resolution in heterodyne AM-KPFM was attributed to the lower crosstalk and higher potential sensitivity between topographic and potential measurements.     

Use of a scanning thermal microscope to examine corrosion protective coatings in exposure

As the capabilities for control of coatings properties at the nanoscale advance, characterization of these properties at these same length scales is needed. One set of organic coatings properties that is very important to understand and control at these length scales are the thermomechanical properties of the coatings. A new hybrid instrument, a scanning thermal microscope was used to examine the morphology and the thermal property simultaneously for two component polyurethane coatings under exposure to corrosive environments. Through the morphology measurement, the surface appearance on the micro-level was monitored and the influence of different corrosive factors such as wet/dry cycling, thermal cycling and condensation on the appearance was studied. At the same time, the micro-scale thermal measurements and concurrent FTIR testing provided the information on the relationship between the change of the structure and the properties of the coating. The information gained from these measurements on the changes in local coatings properties will be presented and correlated to other studies in our lab on nanoscale properties as well as global coating property changes due to exposure.     

Ag/AgCl/KCl micro-electrodes as O2-insensitive reference tips for dynamic scanning Kelvin probe measurement

The metals commonly used as reference tip in scanning Kelvin probe (SKP) devices experience a change in their work function when initially present in inert atmosphere and then exposed to oxygen. This change in work function biases the signal measured on a substrate submitted to a variation in environmental O₂ partial pressure and prohibits a wide range of kinetic investigations on reactional mechanisms involving oxygen. In this paper, reference probes based on the Ag/AgCl/KCl reference electrode structure are described and validated in regards to their use for SKP measurements in humid atmosphere. A validation example is presented where such a silver reference tip is used to measure on a gold substrate the potential change induced by introduction of oxygen in the investigation atmosphere.     

Inhibition of corrosion-driven organic coating disbondment on galvanised steel by smart release group II and Zn(II)-exchanged bentonite pigments

Bentonite pigments exchanged with either zinc or group II cations are characterised as inhibitors of corrosion-driven cathodic disbondment of model polyvinylbutyral (PVB) coatings adherent to the intact zinc surface of hot dip galvanised steel. An in situ scanning Kelvin probe (SKP) technique is used to quantify rates of coating delamination as a function of pigment volume fraction (?_pt) and draw up a ranking order of inhibitor efficiency. Group II cation-exchanged bentonites show a moderate degree of inhibition, where rates of coating disbondment are reduced by up to 60-70% compared to the unpigmented case. In contrast, bentonite pigments containing exchangeable Zn²⁺ ions are markedly more effective, and no delamination is observed over periods of up to 24 h when ?_pt ≥ 0.1. The efficiency of in-coating Zn²⁺ is attributed to the ability to block underfilm cathodic oxygen reduction by reinforcing a pre-existing zinc (hydr)oxide layer.     

In situ scanning tunneling microscopy study of selective dissolution of Au3Cu and Cu3Au (0 0 1)

We present an electrochemical study of Au₃Cu (0 0 1) single crystal surfaces in 0.1 mol dm⁻³ H₂SO₄ and 0.1 mol dm⁻³ H₂SO₄ + 0.1 mmol dm⁻³ HCl, and of Cu₃Au (0 0 1) in 0.1 mol dm⁻³ H₂SO₄. The focus is on in situ scanning tunneling microscopy experiments. The changes of the surface morphology, which are time- and potential-dependent, have been observed, clearly resolving single atomic steps and mono-atomic islands and pits. Chloride additives enhance the surface diffusion and respective morphologies are observed earlier. All surfaces have shown considerable roughening already in the passive region far below the critical potential.     

In situ infrared spectroscopic and scanning Kelvin probe measurements of water and ion transport at polymer/metal interfaces

Scanning ATR-spectroscopy and scanning Kelvin probe studies are introduced as new analysis techniques for the study of water and ion transport at polymer/metal interfaces. The new approach of scanning ATR spectroscopic analysis of water transport at the adhesive/metal interface is combined with scanning measurements in transmission mode of water transport along adhesive joints and non-scanning ATR-measurements of the water transport in a vertical direction through the adhesive. D₂O was chosen in some cases instead of water due to its excellent traceability in the infrared spectra. A scanning Kelvin probe was chosen for the detection of the transport kinetics of hydrated alkali ions along the adhesive/metal interface based on the local measurement of interfacial electrode potentials.The complimentary FT-IR techniques showed that the interfacial diffusion of water, in the case of epoxy adhesives on iron, is about two orders of magnitude faster that the transport through the adhesive itself. Similar transport kinetics at the interface is also shown by hydrated ions. Moreover, the here presented FT-IR-ATR and SKP results reveal more information on how adhesion promoting organosilane layers and organosilanes as additives act at polymer/metal interfaces in the presence of water incorporated in the interphase zone.     

Oxidative and carbonaceous patterning of Si surface in an organic media by scanning probe lithography

A simple top-down fabrication technique that involves scanning probe lithography on Si is presented. The writing procedure consists of a chemically selective patterning in mesitylene. Operating in an organic media is possible to perform local oxidation or solvent decomposition during the same pass by tuning the applied bias. The layer deposited with a positively biased tip with sub-100-nm lateral resolution consists of nanocrystalline graphite, as verified by Raman spectroscopy. The oxide pattern obtained in opposite polarization is later used as a mask for dry etching, showing a remarkable selectivity in SF₆ plasma, to produce Si nanofeatured molds.     

Role of intermetallic phases in localized corrosion of AA5083

The presence of intermetallic inclusions very often plays a crucial role for the susceptibility of different aluminium alloys to localized corrosion attack. The intimate details of localized corrosion of 5083 aluminium alloy have been studied in the present work. Local techniques such as scanning Kelvin probe force microscopy, in situ atomic force microscopy, scanning electron microscopy coupled with energy dispersive spectroscopy were used to investigate the mechanisms and the kinetics of localized corrosion attack. The importance of iron-rich and Mg₂Si intermetallic phases in the initiation of corrosion processes is demonstrated in the paper.The Mg₂Si phase has a potential lower relatively to the matrix. Moreover, the high reactivity of magnesium leads to the dissolution and consequently to the fast dealloying of these intermetallics. However, hydroxide (Mg(OH)₂ and SiO₂·nH₂O) deposits formed during corrosion act as an additional diffusion barrier hindering the deep propagation of pits.The iron containing intermetallics have the potential higher with respect to the aluminum matrix playing the role of effective cathodic centers for oxygen reduction causing anodic polarization and pitting in the surrounding alloy matrix. Dealloying of such intermetallics with subsequent iron enrichment was also revealed. Pitting initiation seems to be statistical and independent of the composition of Fe-rich intermetallics. However, the active growth of the pits prevents initiation of localized corrosion attack in nearby sites. A new pit can start to grow only when a neighbor one becomes passivated.     

Volta potential of second phase particles in extruded AZ80 magnesium alloy

Magnesium alloys show strong susceptibility to localized corrosion when immersed in aggressive solutions (e.g. chlorides). The existence of second phase particles in the microstructure might represent initiation sites for localized corrosion. This is due to the formation of galvanic couples between the particles and the matrix. Extruded AZ80 magnesium alloy has been investigated by means of scanning Kelvin probe force microscopy (SKPFM) in order to measure the Volta potential of different phases relative to the matrix. The phases present in the alloy have been identified by optical microscopy and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDXS). Three different phases were observed: Al₈Mn₅, Mg₂Si and Mg₁₇Al₁₂ (β phase). All phases exhibited positive Volta potentials relative to the matrix indicating a cathodic behaviour. The Volta potential depends on the composition of second phase particles. The Al₈Mn₅ intermetallics showed the strongest cathodic behaviour. Based on the SKPFM results, it is expected that the cathodic phases are effective sites for the initiation of localized corrosion in extruded AZ80 magnesium alloy.     

Imaging of conductive filler networks in heterogeneous materials by scanning Kelvin microscopy

This article reports a novel application of scanning Kelvin microscopy for exclusively revealing the distribution of a percolated conductive filler network in heterogeneous materials. The materials under investigation are carbon black and carbon nanotube‐filled epoxies with a highly inhomogeneous conductivity distribution due to their fabrication. The Kelvin method is demonstrated to be especially suitable for resolving the resistive particle network in these kinds of composite materials with sample resistance levels in the megaohm range. Transmission optical microscopy reveals matches between the scanning Kelvin images and the sample morphologies, whereas the percolating backbone cannot be distinguished in the optical micrographs. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 3381–3386, 2001     

Visualisation of the local electrochemical activity of thermal sprayed anti-corrosion coatings using scanning electrochemical microscopy

Scanning electrochemical microscopy was used to study the electrochemical activity of anti-corrosion coatings formed from Inconel 625, a Ni-Cr-Mo alloy commonly used in engineering applications. The coatings were formed using a high velocity oxygen fuel thermal spraying technique. Upon spraying the alloy onto mild steel substrates, clear splat boundaries were formed at the interface between adjacent droplets as they cooled on the substrate surface. Scanning electrochemical microscopy in the feedback mode, employing ferrocenemethanol as redox mediator, was used to study the local electrochemical activity of samples of the wrought alloy, the sintered alloy and the thermal sprayed coating. The wrought and sintered materials showed responses typical of that expected for a purely insulating material. However, SECM approach curve data showed that the electrochemical activity of the thermal sprayed material was higher than that of the bulk alloy. Local variations in the coating's electrochemical activity were then visualised using SECM imaging, which appear to be related to the splat boundaries formed during the thermal spray process.     

Uses of scanning electrochemical microscopy for the characterization of thin inhibitor films on reactive metals: The protection of copper surfaces by benzotriazole

Scanning electrochemical microscopy (SECM) was used to study the film formation of benzotriazole towards corrosion of copper. SECM was operated in the feedback mode by using ferrocene-methanol as redox mediator, and the sample was left unbiased at all times to freely attain its open circuit potential in the test environment. Following exposure to aggressive electrolytes the anticorrosion abilities of the layers were characterized by image analysis and by an electrochemical method derived from the experimental approach curves. Changes in the shape of the approach curves were clearly observed during the inhibitor film formation process. They showed the transition from an active conducting behaviour towards ferrocinium reoxidation typical of unprotected copper, to a surface exhibiting insulating characteristics when the metal was covered by a surface film containing the inhibitor. This supports that SECM is a practical technique in the investigation of corrosion inhibitor performance. However, a consistent tendency for the characterization of inhibitor film formation using SECM measurements in the positive feedback mode for the copper-benzotriazole system was only found if the experiments were conducted when the inhibitor molecule was not present in the test solution. That is, inhibitor molecules were found to interact not only with the copper surface during the monitoring process, but with the SECM tip as well, this effect being significantly enhanced when chloride ions were present in the electrolyte. Finally, a procedure to image the chemical activity of copper surfaces partially covered with the inhibitor film with SECM is proposed.     

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

This article presents for the first time an in situ high-resolution study of the interaction between the active head in filiform corrosion (FFC) and intermetallic particles within an aluminium alloy. For the first time direct evidence will be provided that the intermetallic particles directly determine the so far seemingly random course of the filaments. Both the segments of active filaments and the intermetallic particles (IMPs) were successfully imaged in a humid air (ca. 85% RH) environment by scanning Kelvin probe force microscopy (SKPFM) through a plasma polymer coating of about 340 nm thickness. In order to be able to do that, the experimental parameters need to be adjusted in such a way, that the width of the filaments is small enough to be well within the scan window of SKPFM (100 μm × 100 μm). Also it is important that the small IMPs can still be mapped by SKPFM through the coating. This was successfully achieved by use of a HDMSO plasma polymer film. Surface potential values in the head region of the propagating filaments were found to be 200 mV lower than the interface between intact plasma polymer and the aluminium alloy, indicating the active region. On the other hand, the surface potential values in the trailing filament tail are found to be about 250 mV higher than background, pointing out the cathodic site and superpassivation due to the accumulated corrosion products in this region. It was found that the direction of the filament is determined by the location of the IMPs nearest to the active head.     

Electrodeposition of Ti from TiCl4 in the ionic liquid l-methyl-3-butyl-imidazolium bis (trifluoro methyl sulfone) imide at room temperature: study on phase formation by in situ electrochemical scanning tunneling microscopy

Titanium was electrodeposited from a nominal 0.24 M TiCl₄ in l-methyl-3-butyl-imidazolium bis (trifluoro methyl sulfone) imide ([BMIm]BTA) at room temperature on a Au(1 1 1) substrate. The process of electrodeposition was studied by cyclic voltammetry, chrono amperometry and in situ scanning tunneling microscopy (STM). In a first step TiCl₄ is reacted to TiCl₂, which is subsequently reduced to metallic Ti. Two dimensional (2D) clusters form preferentially on the terraces in the under potential deposition range. 2D clusters presumably of TiCl₃ precipitates grow and coalesce to cover the whole substrate with a 2D film at a substrate potential below −1.1 V versus ferricenium/ferrocene ([Fc]⁺/[Fc]) redox couple. At a potential of −1.8 V a dense layer of three dimensional (3D) clusters of titanium of 1-2 nm thickness is formed. The features of the I-U tunneling spectra and the relative reduction of the effective tunneling barrier by 0.8 eV with respect to gold clearly indicate the metallic character of Ti deposits. Observation of circular holes on the Au(1 1 1) substrate after dissolution of the deposited Ti indicates the formation of Au-Ti surface alloying.