Handbook of Modern Coating Technologies

Other localized techniques

In this last section other localized techniques are briefly presented and put in perspective with SVET.

• Scanning reference electrode technique

The SRET is the predecessor of SVET. A reference electrode scans the solution close to the metal surface and measures the potential referred to another reference electrode placed in a

FIGURE 1-17 Detection of defects and porosity using organic solvents and applying current: (A) silane film on tinplate after 2 days of immersion in 0.05 M NaCl, (B) SVET response in 0.05 M NaCl, (C) response in ethanol saturated with NaCl and a current of -2 pA and (D) with a current of 2 pA passing in the cell, (E) Al₂O₃ layer on LM24 aluminum alloy, and (F) porosity revealed by passing a current of 2 pA through the sample in ethanol saturated with NaCl. SVET, Scanning vibrating electrode technique.

stationary position. In some designs the two reference electrodes are mounted with fixed dis­tance between them and scan together the area of interest. The local difference in potential is related to the local current density by Eq. (1.4). The results can be presented either as
maps of potential or maps of current density. Compared with SRET, SVET brought noise reduction and higher sensitivity. SRET has been used to analyze many corrosion problems [33], including coated systems [136].

• Potentiometric microelectrodes

SVET and SRET do not identify the chemical species involved in the measured potentials and currents. For this, potentiometric microelectrodes can be used, detecting and quantify­ing important chemical species involved in the corrosion process, such as the metal cations originated in the oxidation half-reaction, corrosion inhibitors, and electrolyte ions. These microelectrodes are not new. Ammann refers the existence of pH microelectrodes already in (1927) [137] and their use in the life sciences has several decades of experience [138—140]. However, nowadays they are often presented as an application of scanning electrochemical microcopy (SECM) [141 — 143]. Examples in corrosion research can be found in reviews [83,144,145] and many times they appear together with SVET [51,95,115,116,118,120,127, 146,147].

• Voltammetric/amperometric microelectrodes

Similarly to potentiometric microprobes, inert microelectrodes (Pt and Au) can be used as voltammetric or amperometric sensors for species relevant to corrosion, like reactants (O₂ [51,148]) or products (metal cations from the substrate oxidation [148] or H₂ generated in the cathodic process of magnesium alloys corrosion [149,150]). The inert electrode is polarized at a potential where the species of interest is oxidized or reduced and the mea­sured current (positive or negative) is proportional to the concentration of the species.

• Scanning electrochemical microscopy

This technique appeared at the end of the 1980 decade [151,152] and since then was applied in many areas of research [153]. SECM in its original form is an imaging technique with a bipotentiostat to independently control the potential of the substrate and the potential of the tip, with a reversible redox mediator in between. For corrosion and coatings characterization the most important mode of operation is the substrate generation/tip collection mode, in which the microelectrode works as a sensor, like the potentiometric and amperometric microelectrodes described above. SECM has the advantage of high (micrometer) resolution and has been applied to many corrosion studies [83,144,145].

• Localized electrochemical impedance spectroscopy

Another powerful technique with still high potential ahead is localized electrochemical impedance spectroscopy (LEIS) [154,155]. It measures local ionic currents while a small AC potential stimulation is applied to the substrate. The AC potential signal divided by the local AC current gives the impedance response of that local point of the sample. Often, the areas where the impedance is lower correspond to the areas of higher current density in SVET maps. One of the most striking features to expect from LEIS is the ability to acquire separate kinetic parameters of the oxidation and reduction reactions involved in the corrosion pro­cess. An important related breakthrough would be the capability to analyze and spatially resolve the individual reactions occurring underneath polymeric paints.

• Alternate current scanning electrochemical microscopy

A different experimental approach for LEIS is the alternating current-scanning electrochemi­cal microscopy (AC-SECM), where the AC potential perturbation is applied to the SECM tip and no mediator is added to solution [156—158]. A lock-in amplifier determines the tip cur­rent magnitude and its phase shift with respect to the AC potential. AC-SECM acquires topo­graphic images and collects local electrochemical information of the substrate. A recent modification is the scanning electrochemical impedance microscopy [159—162] where local impedance spectra are acquired through multifrequency AC-SECM in each point of mea­surement. The technique has been employed to study the action of corrosion inhibitors [163—165], defects [166], and blistering [167] in coatings and localized corrosion processes [149,159,168,169].

• Scanning Kelvin microscopy

One of the few techniques able to give substantial information about the processes beneath organic coatings is the scanning Kelvin probe (SKP) [170,171], which was first applied to cor­rosion by Stratmann in 1987 [172]. SKP measures the volta potential difference between the metallic probe and the conductive substrate through insulating media (air or polymer film). The volta potential difference is usually considered to be proportional to the corrosion potential [170,171].

• Scanning Kelvin probe force microscopy

The resolution of SKP can be improved by applying the concept to atomic force microscopy (AFM) devices, in an arrangement called scanning Kelvin probe force microscopy (SKPFM). Similar to SKP, SKPFM provides topographic and potential maps on a sample surface [170,173—175]. The higher spatial resolution is due to the smaller probe size and the shorter distance to the surface. A resolution of 100 nm is possible, which allows studying samples with heterogeneous microstructure. On the other hand, AFM scanners are typically limited to 100 pm and many relevant processes in corrosion take place on a larger scale.

• Microcapillary and microdroplet cells

A different methodology to obtain localized information is by delimiting very small areas of the working electrode with the so called microdroplet or microcapillary cells [176,177]. Areas of 10—100 pm in diameter can be analyzed. Different grains of a metal or alloy can be stud­ied separately without being short-circuited by the electrolyte solution. The main drawback is the small cell size leading to high uncompensated resistance, fast contamination of the solution, and blockage by corrosion products and gas bubbles. The problems were overcome by redesigning the cell to allow recirculation of solution. These microcells are not so suited to study organic coatings (exceptionally at defects) but they can be used to study cut-edges, surface treatments and metallic coatings, being of particular interest if different phases and other heterogeneities are present at the surface.

• Wire beam electrodes

Another way to analyze the electrochemical response of small areas is to use small electrodes assembled together to imitate a large electrode. This is called wire beam electrode and was developed by Tan [178]. Typically 4—100 wires are assembled in a beam. The electrochemi­cal response (open circuit potential, polarization curves, electrochemical noise, EIS, etc.) can be obtained for the entire beam of selected wires. Examples of application to coatings can be found in Ref. [179].

The high resolution attained by several techniques (SECM, AFM, and SKPFM) is not always an advantage. It requires small-sized maps (50—100 pm side or less) and the probe close to the surface (10 pm or less). This is good for grain boundaries, intermetallic particles, inclusions, and small pits but cannot be applied with large, rough, or very active surfaces. The samples analyzed in this chapter would not be satisfactorily characterized by SECM, AFM, or SKPFM (at least taking the advantage of the high resolution) because of their size and the rapid formation of corrosion products. SECM, and amperometric or potentiometric sensors in general, show the distribution of a single chemical species, evidencing part of the corrosion process that is taking place. SVET, on the other hand, gives the distribution of the whole electrochemical process, which cannot be provided by any other technique.

The localized techniques in this section are more complements than competitors, since they provide, not the same type of data, but different pieces of information which, together, offer a more complete picture of the system under analysis. The combination of various techniques is possible, desirable and, in fact, often reported. These localized techniques and their methods are continuously under development, with improvements in resolution, sensitivity, selectivity, and response time. Future will bring an increasing compatibility between techniques, with different sensors scanning together the same target. Progress in dedicated computer analysis and model­ing will expand the information that can be extracted from the experimental data.

• Conclusions

This chapter presented an overview of SVET and its application to coatings characterization. The SVET shows the global view of the electrochemical activity at the metal surface in a single image. No other technique is able to provide such information. SVET has been applied to the study of many coating systems, being well suited to detect pores and defects on nonconductive coatings, characterize the corrosion of metallic coatings, and study the electrochemical activity of any conductive surface. Examples were presented to highlight both the capabilities and the limitations of the technique in the characterization of modern coatings.