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Corrosion processes at cut edges of galvanized steels proceed as highly localized electrochemical reactions between the exposed bulk steel matrix and the protective thin metallic coating of a more electrochemically active material. Scanning microelectrochemical techniques can thus provide the spatially resolved information needed to assess the corrosion initiation and propagation phenomena, yet most methods scan cut edge sections as embedded in insulating resin to achieve a flat surface for scanning purposes. In this work, the galvanized coatings on both sides of the material were concomitantly exposed to simulated acid rain while characterizing the cut edge response using SECM and SVET techniques, thereby maintaining the coupled effects through the exposure of the whole system as rather realistic operation conditions. The cut edges were shown to strongly promote oxygen consumption and subsequent alkalization to pH 10–11 over the iron, while diffusion phenomena eventually yielded the complete depletion of oxygen and pH neutralization of the nearby electrolyte. In addition, the cathodic activation of the exposed iron was intensified with a thinner coating despite the lower presence of sacrificial anode, and preferential sites of the attack in the corners revealed highly localized acidification below pH 4, which sustained hydrogen evolution at spots of the steel-coating interface. [ABSTRACT FROM AUTHOR]

The study of corrosion of magnesium and its alloys has emerged a hot topic in the applications of lightweight structural materials. The inherently high electrochemical activity of bare magnesium surfaces still lacks a convincing mechanism to describe the observed experimental characteristics, and it has prompted the development of various types of protective coatings with the aim of slowing metal dissolution. In recent years, new instruments and techniques have been developed to study with spatial resolution the local corrosion processes that occur in metallic materials in general, and for magnesium and its alloys in particular, both for bare surfaces and coated. Scanning microelectrochemical techniques, such as local electrochemical impedance spectroscopy (LEIS), scanning electrochemical microscopy (SECM), scanning vibrating electrode technique (SVET), scanning ion-selective electrode technique (SIET) and scanning Kelvin probe (SKP) can provide information about the local electrochemical activity of metallic surfaces. In the present work, the applications of these techniques in corrosion studies of magnesium and its alloys are reviewed. Assessment of corrosion mechanisms, barrier properties of conventional coatings and active corrosion behavior of self-healing coatings are examined. Limitations and future developments in this area are discussed. [ABSTRACT FROM AUTHOR]

Scanning Electrochemical Microscopy (SECM) is presented as an essential tool for the local characterization of the still uncertain mechanism for magnesium corrosion. The reaction leading to magnesium release and hydrogen evolution from separated magnesium cathodes and anodes has been imaged using an adequate combination of the operation modes available in SECM. Magnesium ion selective microelectrodes (Mg-ISME's) were used for the visualization of the heterogeneously distributed release of magnesium (II) species. Antimony microelectrodes detected the pH gradients in the adjacent electrolyte resulting from either water or magnesium electrolysis, whereas platinum microdiscs were used to monitor the concomitant local evolution of hydrogen. Alkalization and H2 generation were observed over the magnesium strip polarized as cathode, whereas a small local acidification was observed above the strip polarized anodically, at which extensive heterogeneous magnesium release was also imaged. [ABSTRACT FROM AUTHOR]

• Characterization of the near-surface deformed layer (NSDL) in FSWelded 2098-T351. • Spatially resolved electrochemical activity in NSDL obtained by potentiometric SECM. • Mg-enriched bands affect the electrochemical behavior of welded areas. • The NSDL has an anodic electrochemical behavior with respect to the bulk material. • Highest activity for Mg2+ dissolution observed in the HAZ of the retreating side. The development of heterogeneous electrochemical activity in the welded zones of aluminum alloy 2098-T351 by friction stir welding (FSW) associated with the formation of a near-surface deformed layer (NSDL) upon exposure to an aqueous chloride-containing solution was characterized using scanning electrochemical microscopy (SECM) in potentiometric operation. A solid-contact Mg2+ ion-selective microelectrode allowed in situ monitoring of the corrosion reactions sites for magnesium dissolution from different zones of the FSW weld upon exposure to a chloride-containing aqueous environment. In this way, localized corrosion reactions developing in the galvanically coupled joint/heat affected zones (WJ/HAZ) of the weld were detected and imaged with spatial resolution. The most active domains for local Mg2+ concentrations were associated with the HAZ of the retreating side (RS), and these corresponded to Mg oxidation from the Mg-enriched oxide bands in NSDL. [Display omitted] [ABSTRACT FROM AUTHOR]

• New potentiometric Al3+ion-selective microelectrode for corrosion monitoring on aluminum-based materials. • SECM imaging of local ion concentration distributions over 2098-T351 al-cu-li alloy welded by FSW. • Spatial resolution of Al3+ local concentrations arising from the base material and from distinct weld zones. • High active domains for Al3+ associated to microgalvanic coupling were resolved and characterized. • Electrochemical data correlate well with high-resolution XPS spectra of the tested surfaces. A novel potentiometric Al3+−ion selective microelectrode (ISME), with internal solid contact, based on the use of a neutral carrier morin as ionophore is reported. The ability of the ISME to image local ion concentration distributions was tested on aluminum alloy surfaces freely corroding in an aqueous solution containing chloride ions. The microelectrode was then used as the sensing tip for scanning electrochemical microscopy (SECM) in potentiometric operation to monitor the reactive sites associated with the dissolution of aluminum that developed in the 2098−T351 Al−Cu−Li alloy as a result of welding by the Friction Stir Welding (FSW) process. The ISME detected differences in the local concentrations of Al3+ species arising from the 2098−T351 Al−Cu−Li alloy (base material) and from the coupled weld joint/heat affected zones (WJ/HAZ) of the alloy produced by the FSW process. More active domains for Al3+ dissolution were found in the HAZ regions coupled to WJ, more specially in the HAZ of the advancing side (AS). These results demonstrate that the Al3+−ISME presented in this work can be used to monitor corrosion sites on aluminum alloys surfaces with combined chemical and spatial resolution. [Display omitted] [ABSTRACT FROM AUTHOR]

• Microelectrochemical characterization of the electrochemical activity on welded zones in FSW 2098-T351 Al-Cu-Li alloy. • Imaging of galvanic coupling effects and local electrochemical activity on welded zones using amperometric and potentiometric probes. • Galvanic interactions within and between the coupled welded zones were visualized by SVET and SECM analyses. • Pt and Sb/Sb 2 O 3 probes used to map reactive sites developed on the coupled welding joint/heat affected zones (WJ/HAZ). • Spatially-resolved images of anodic and cathodic sites associated with oxygen consumption and local pH changes were monitored and studied. Localized electrochemical methods supported by surface analytical characterizations were employed to investigate galvanic coupling effects and local electrochemical activity developed along the welded zones in Friction Stir Welded 2098-T351 Al-Cu-Li alloy. The investigation was carried out in the coupled weld joint/heat affected zones (WJ/HAZ) for both, the retreating (RS) and the advancing (AS) sides. The correlation between the surface chemistry, the microstructural characteristics and the electrochemical activity of these welded areas was studied. The results showed the development of galvanic interactions within and between the WJ and HAZ regions, which were imaged using the scanning vibrating electrode technique (SVET), and scanning electrochemical microscopy (SECM). SVET analyses showed that HAZ was more susceptible to the development of anodic sites than WJ. SECM in amperometric operation mode showed that WJ coupled to HAZ exhibited higher oxygen consumption and higher cathodic activity compared to HAZ. Furthermore, SECM in potentiometric operation showed alkalinization around WJ and increased acidity in HAZ, mainly at sites of severe localized corrosion (SLC). Based on the SVET and SECM results in combination with the surface analyses, it is proposed that the microgalvanic cells formed within these welded zones are due to the presence of secondary phases in the 2098-T351 alloy and their interactions with the adjacent matrix. Image, graphical abstract [ABSTRACT FROM AUTHOR]

• The detrimental effect of cathodic polarization on the passive state of titanium was determined using SECM. • Cathodic polarization produces electrical conductivity changes in the surface film facilitating the occurrence of electron transfer reactions. • The modification of the passive state was quantitatively described in terms of electron transfer rates at the titanium surface. • Occurrence of hydrogen intercalation into the titanium dioxide film was proved operating SECM in the redox-competition mode. The effect of cathodic polarization on the electrochemical behavior of the thin titanium dioxide film formed by anodic pretreatment over pure commercial titanium metal for biomaterial application was investigated in situ using scanning electrochemical microscopy (SECM). Quantitative information on the electron transfer rates (k eff) at the titanium surface was obtained using the feedback operation of SECM with ferrocene-methanol (FcMeOH) as electrochemical mediator. An increase of k eff values with the increase of the negative polarization was detected, a feature that correlates well with the decrease of titanium oxide resistance with increasing cathodic polarization observed using electrochemical impedance spectroscopy (EIS). In addition, SECM operation in the redox competition mode proved that hydrogen was absorbed in the surface oxide film leading to changes in conductivity and electrochemical reactivity. [ABSTRACT FROM AUTHOR]

• Triple-barrel microelectrode for real simultaneous pMg and pH monitoring. • Multi-electrode microprobe arrangement containing an internal reference. • Electric field effects elimination in galvanic corrosion processes. • Improved chemical resolution than with a conventional reference electrode. Simultaneous monitoring of pH and Mg2+ distributions above AZ63 magnesium alloy, either spontaneously corroding or galvanically coupled with iron, was achieved using SECM in the potentiometric operation. By introducing an internal micro-reference electrode in a multi-barrel arrangement of the ion-selective microelectrode used as scanning probe, superior performance was achieved compared to conventional single-barrel and double-barrel assemblies. In this way, the impact of the overestimated acidification accompanying metal dissolution using conventional tips was established from model experiments using non ion-sensitive open micropipettes and pH antimony microelectrodes. It is shown that the simultaneously acquired pH and pMg maps provide complementary information on the spontaneous and galvanic corrosion of AZ63 magnesium alloy. [ABSTRACT FROM AUTHOR]