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The galvanic corrosion of an iron/zinc pair immersed in aqueous 0.1 M Na 2 SO 4 solution has been investigated by using the scanning vibrating electrode technique (SVET). In this way, investigations in the micrometer range of the progress of the electrochemical reactions involved in galvanic process were performed. The anodic oxidation process is observed to be initiated on the zinc sample in a localized manner, whereas the cathodic reaction involving the electroreduction of dissolved oxygen is homogeneously distributed over the iron sample. This later process is the rate determining step in the overall corrosion process, as demonstrated by the changes in the ionic and galvanic currents measured in the system when the area of the iron specimen is varied relative to that of zinc. The occurrence of coupled chemical reactions in the solution phase involving the products of the corrosion reactions could also be deduced from the integration of the ionic currents measured for each half-reaction during a SVET scan. Thus, the corrosion processes involved in the galvanic coupling of iron and zinc have been further understood by using this microelectrochemical technique appropriately, helping to better interpret large scale measurements.

Electrochemical impedance spectroscopy (EIS) has been employed in the investigation of the corrosion characteristics of polyurethane polymeric film applied on metallic substrates during immersion in 0.5 M NaCl aqueous solution. Galvanized steel and mild steel coupons were studied. EIS has made it possible to monitor the penetration of the surrounding electrolyte into the organic coating and determine the extent of the coating breakdown. The EIS spectra were modeled in terms of equivalent electric circuits to explain the impedance results. The time course of the impedance parameters was used to establish the effectiveness of the anticorrosive properties of the various paint-substrate systems. Pull-off adhesion and microhardness tests were conducted on the samples both before and after exposure to the corrosive environment, in order to investigate the existence of a relationship between the changes in mechanical properties of the coating/metal system and the degradation process as monitored by EIS measurements. Polyurethane coating shows higher adhesion and enhanced anticorrosion protection characteristics when applied on carbon steel than on galvanized steel.

The work makes use of the scanning vibrating electrode technique (SVET) and the scanning electrochemical microscope (SECM) to investigate microscopic aspects of the electrochemical reactions that occur in an iron–zinc galvanic couple immersed in aqueous sodium chloride solution. Detection of the corrosion processes was made by sensing the phenomena occurring in solution. The SVET provided information on the distribution of ionic currents arising from the metal surface, whereas the SECM measured the concentration of chemical species relevant to the corrosion processes. The two techniques had comparable sensitivity for the corrosion of iron but significant differences were observed concerning the detection of corrosion of zinc.

The scanning electrochemical microscope (SECM) is a non-optical scanning microscopic instrument capable of imaging highly localized electrical currents associated with charge transfer reactions occurring in the vicinity of the surface of coated metals. The SECM tip operates as working microelectrode in an electrochemical cell under potentiostatic control. Microelectrode current and position is recorded as the tip is scanned very near a coated sample at a constant Z height, and variations in the faradaical current measured at selected tip potential values can be related to changes in the local concentration of electroactive species involved in corrosion reactions occurring at the substrate. Carbon steel samples coated with polyurethane in which a holiday was produced prior to immersion in different aqueous electrolytes were investigated with this technique. The release of Fe(II) ionic species into the solution phase from local anodic sites, as well as the consumption of dissolved oxygen at the corresponding cathodic locations was successfully monitored.

The application of scanning electrochemical microscopy (SECM) for corrosion studies of organic coatings on reactive metals is presented. In this work, the SECM was used to monitor the electrochemical processes at painted galvanized steel substrates immersed in 0.1 M NaCl aqueous solution. The coatings were investigated both as obtained and scribed, to simulate a defect across the coating, down to the metal-substrate surface. The SECM was operated under either the feedback or a variation of the sample generation/tip collection modes. In the first case the variations in the feedback current measured at the SECM-tip when scanned at constant height over the sample allowed imaging of the topography of the coated surface with high local resolution. In the second case, chemical species participating in the corrosion reactions at the sample are detected at the SECM-tip by applying appropriate potential values to the microelectrode. Local concentration variations could be assessed in this way.

The corrosion behaviour of coil-coated steel (CCS) sheets with different intermediate zinc-based coatings was examined by electrochemical impedance spectroscopy (EIS). Corrosion tests were performed in a 0.5 M NaCl aqueous solution in contact with the air, at ambient temperature. The results of pull-off adhesion test are also presented. The impedance data show that the improvement in the protective behaviour of the composite system is connected with the higher resistance values for the samples in which aluminium was added to the galvanized layer, and the best results are shown by CCS with aluzinc (Zn-55Al). These effects have been related to differences in the adhesion of the organic coating to the underlying metal substrate, as determined from both non-exposed samples and samples exposed to the electrolyte solution.

Damage to paint coatings caused by immersion in aqueous electrolyte solution has been observed in situ by scanning electrochemical microscopy (SECM). SECM was used to image the surface topography of coated metal samples as a function of time in chloride and sulphate solutions. The degradation is observed as a swelling of the polyester coating as a function of time of immersion in chloride solution. The aggressive effect of chloride ions towards coating degradation was established after very short exposure times. No indications of coating failure could be observed by either conventional electrochemical techniques or visual observation. In contrast, blistering could not be observed in a chloride-free sulphate solution. Chloride enhances degradation of the coating at a very early stage.

Scanning electrochemical microscopy has been used to detect the redox species involved in a corrosion process at the open-circuit potential. During the immersion of pure iron in a neutral aqueous solution, the potential of the ultra microelectrode (tip) was set to either detect the Fe2+ ions originating from the electrodissolution of iron at the anodic sites on the metal surface, or the depletion of oxygen in the electrolyte in the proximity of the corresponding cathodic sites. Keywords: Scanning electrochemical microscopy, Corrosion, Iron dissolution, Oxygen reduction, Localized electrochemical measurements

Images describing metastable pits on 304 stainless steel obtained in situ in chloride solution at the open-circuit corrosion potential are presented. These images were obtained by scanning electrochemical microscopy and represent the oxidation of Fe2+ emanating from the metastable pits as the probe tip passes over them. Each anodic transient on the tip is succeeded by a transient in the cathodic direction. We ascribe this to an indirect observation of the cathodic reaction. Reduction of oxygen as the cathodic reaction causes peroxide to be produced adjacent to the propagating pits. This reacts with Fe2+ produced by the background passive current locally around the pit site. The microscope tip, set to detect Fe2+, detects the reduced background current as a transient negative wing immediately after the anodic wing associated with the pit itself. The signal represents a reduced anodic background current. This negative wing on the probe current is not detected when the metastable pits are generated under potentiostatic polarisation. It is shown that the lifetime of the detected pits is a maximum of a few seconds. Keywords: Corrosion, Metastable pit, Peroxide, Pitting, Scanning electrochemical microscopy, Stainless steel

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Scanning electrochemical microscopy (SECM) with its high spatial resolution and chemical selectivity is a powerful technique for studying a wide range of corrosion processes. A common procedure to detect qualitatively a corrosion activity on a metal substrate consists of measuring the local oxygen concentration. The aim of this work is to clarify the local impact of the O 2 measurement and the undesired effect on the corrosion process. By means of numerical simulation the variations of the anodic and cathodic currents on the aluminum substrate, as well as concentration profiles, are obtained. The multi-ion transport and reaction model (MITReM) is used considering the homogeneous reactions taking place in the solution, transport of species dominating the corrosion process and the electrochemical reactions. The model case study in this work is corrosion of pure aluminum in chloride solution. We concluded that amperometric O 2 SECM measurements lead to a local increase of the solution pH and decrease of partial current densities for O 2 reduction on the metal. This influence becomes significant when the distance to the substrate and when the size of the active surface is comparable with the size of the SECM probe.

Self-healing materials have the ability to repair themselves and recover functionality after degradation, damage and failure. This chapter reviews recent developments in self-healing protective coatings which contain rare earth-based inhibiting compounds for active corrosion protection of metals. These new ‘green’ inhibitors are being developed as substitutes for chromate-based inhibitors. Different types of organic, hybrid and metallic systems are considered, with particular emphasis on thin sol-gel based layers. The main strategies for encapsulation of RE inhibitors are also reviewed. Significant future effort is required to improve the properties of organic coatings and to explore new self-healing coating alternatives that allow controlled release of the inhibitor.

This work presents a scanning electrochemical microscopy (SECM)-based in situ corrosion probing methodology that is capable of monitoring the release of zinc species in corrosion processes. It is based on the use of Hg-coated Pt microelectrodes as SECM tips, which offer a wider negative potential range than bare platinum or other noble-metal tips. This allows for the reduction of zinc ions at the tip to be investigated with low interference from hydrogen evolution and oxygen reduction from aqueous solutions. The processes involved in the corrosion of zinc during its immersion in chloride-containing solutions were successfully monitored by scanning the SECM tip, set at an adequate potential, across the sample either in one direction or in the X-Y plane parallel to its surface. In this way, it was possible to detect the anodic and cathodic sites at which the dissolution of zinc and the reduction of oxygen occurred, respectively. Additionally, cyclic voltammetry (CV) or constant potential measurements were used to monitor the release of zinc species collected at the tip during an SECM scan.

The corrosion processes related to zinc dissolution that take place in an iron–zinc galvanic pair exposed to a chloride solution were investigated by scanning electrochemical microscopy (SECM) using a mercury-coated platinum microelectrode as SECM tip. Both the release of zinc ions and the consumption of dissolved oxygen that occur in separate sites at the iron–zinc galvanic pair were monitored. Zinc ion quantification could be performed by stripping the zinc metal collected at the Hg-coated tip using linear sweep voltammetry.

Scanning electrochemical microscopy has been employed to obtain spatially-resolved information regarding surface topology, water uptake and blister formation at intact coatings, as well as the onset and progress of corrosion reactions within coating defects. Topographical lines and maps as well as chronoamperometric plots were measured during operation in the feedback mode. Next, the release of metal ions at the anodic sites and the consumption of oxygen at the cathodic sites developed in holidays could be monitored during operation in the generator–collector mode. Furthermore, the surface topography of defective coatings was imaged by using the redox competition mode.

Scanning electrochemical microscopy in feedback mode was used to monitor changes in the surface state of a polymeric film applied on a metallic substrate when exposed to an aqueous electrolytic environment. The protected metal consisted of a carbon steel substrate coated with a polyurethane-based polymeric film. SECM measurements performed in the presence and absence of chloride anions permitted a specific effect caused by Cl- anions at early exposures to be detected. Significant surface roughening is observed for immersion times shorter than one day when the electrolyte contains chloride ions. Additionally, the growth of an individual blister could also be investigated

In the present paper, a shape-memory polyurethane (SMPU) film consisting of a two-segmented block co-polymer is presented as a novel organic coating for the protection of metals against corrosion. When the coating is damaged, a physical self-healing and consequent recovery of the barrier properties are induced after an increase of the temperature. Indeed, thermal treatment leads to the relaxation of the soft matrix in the polymer and subsequent coverage of the damaged area. The scanning vibrating electrode technique (SVET) is employed to visualize local variations of the electrochemical activity in the damaged area of the SMPU coating, proving that the physical-repairing of the film during the thermal treatment leads to a recovery of the protective properties of the coated system. By using laser scanning confocal microscopy (LSCM), observation of the coating defect before and after the thermal healing allowed to establish the efficiency of the polymer to cover the coating defect qualitatively. Complementary experiments using dynamic mechanical analysis (DMA) allowed to study the mechanical properties of the co-polymer under a gradient of temperature and to prove its stability during the healing treatment.

In this work local electrochemical techniques are introduced as powerful and complementary techniques for the in-situ evaluation of self-healing systems applied for the protection of metals against corrosion. Scanning vibrating electrode technique (SVET) and scanning electrochemical microscopy (SECM) are employed for the evaluation of coatings with encapsulated self-healing agents for the protection of aluminum alloys. SVET measurements allowed to in-situ monitor the corrosion processes taking place within a coating defect during immersion in chloride solution. Furthermore, SECM operating in redox competition mode allowed to monitor the evolution of the cathodic process at the defect which is the reduction of dissolved oxygen in solution. SVET and SECM results showed that the release of healing agent from the broken capsules during the damaging of the surface indeed leads to a hindering of the corrosion processes and therefore provide an effective protection of the metallic surface.

In the present paper, a shape-memory polyurethane (SMPU) film consisting of a twosegmented block co-polymer is presented as a novel organic coating for the protection of metals against corrosion. When the coating is damaged, a physical self-healing and consequent recovery of the barrier properties are induced after an increase of the temperature. Indeed, thermal treatment leads to the relaxation of the soft matrix in the polymer and subsequent coverage of the damaged area. The scanning vibrating electrode technique (SVET) is employed to visualize local variations of the electrochemical activity in the damaged area of the SMPU coating, proving that the physical-repairing of the film during the thermal treatment leads to a recovery of the protective properties of the coated system. By using laser scanning confocal microscopy (LSCM), observation of the coating defect before and after the thermal healing allowed to establish the efficiency of the polymer to cover the coating defect qualitatively. Complementary experiments using dynamic mechanical analysis (DMA) allowed to study the mechanical properties of the co-polymer under a gradient of temperature and to prove its stability during the healing treatment.