Scanning electrochemical microscopy studies for the characterization of localized corrosion reactions at cut edges of coil-coated stee

The local distribution of anodic and cathodic sites in a corroding cut edge using the scanning electrochemical microscope is presented. Platinum and antimony microdisks were used as tips for combined amperometric/potentiometric operation. Local changes in electrochemical activity, oxygen concentration and pH distribution associated to the onset of localized anodes and cathodes were imaged in situ, as well as their evolution with time. Local alkalization and oxygen consumption related to cathodic activity were detected above the zinc layer covered with the thinner polymeric coating, providing a direct evidence of a differential aeration process effectively operating in an asymmetrical cut edge, in addition to the galvanic coupling of aluzinc and steel. Anodic activity leading to local acidification occurred at the other aluzinc layer at all exposures

Electrochemical impedance spectroscopy investigation of the corrosion resistance of a waterborne acrylic coating containing active electrochemical pigments for the protection of carbon steel

The aim of this work was to investigate the corrosion resistance of a water-based commercial paint system applied on carbon steel. Electrochemical impedance spectroscopy (EIS) in the conventional three-electrode configuration was employed to test the various components of the paint system during their exposure to 3 wt.% NaCl aqueous solution. An efficient protection against corrosion was provided by the acrylic polymer containing zinc phosphate pigments when it was applied as a multilayer system. In this way, the high density of pores produced in the polymer matrix during the curing process could be avoided to become direct ionic pathways for the transport of aggressive species from the environment towards the surface of the underlying metal substrate

Investigating metal-inhibitor interaction with EQCM and SVET: 3-amino-1,2,4-triazole on Au, Cu and Au-Cu galvanic coupling

In this work, the scanning vibrating electrode technique (SVET) and the quartz crystal microbalance technique (EQCM) were used to characterize the electrochemical interaction of two corrosion inhibitors, 2-Mercaptobenzothiazole and 3-Amino-1,2,4-triazole (ATA), in Au and Cu samples individually and with both metals connected in order to form a galvanic pair Au-C

SVET study of the interaction of 2-Mercaptobenzothiazole corrosion inhibitor with Au, Cu and Au-Cu galvanic coupling

This study reports an electrochemical investigation of the interaction between 2-mercaptobenzothiazole (2MBT), a corrosion inhibitor, and two noble metals (Cu and Au), either isolated or under galvanical coupling. The surface reactivity of the inhibitor-modified metals in chloride-containing aqueous solution of 0.1 mM NaCl was studied using the Scanning Vibrating Electrode Technique (SVET). The potential gradients in the electrolyte were detected by the SVET and converted to local ionic currents distributed at different anodic and cathodic sites, where electrochemical redox reactions are taking place. No external polarization was applied during SVET measurements, leaving the samples at the spontaneous free corrosion potential (FCP). The values of FCP showed that the metals are not protected by 2MBT. SVET revealed a moderate electrochemical activity of Au and very low in the case of Cu

Scanning electrochemical microscopy studies for the characterization of localized corrosion reactions at cut edges of painted galvanized steel as a function of solution pH

Coil coated steel sheets immersed in 1 mM NaCl solutions adjusted at both acidic and alkaline pH conditions were used to investigate the cut edge corrosion behaviour by scanning electrochemical microscopy. Combined amperometric/potentiometric operation revealed asymmetries in the distribution of localized anodic and cathodic activities along the cut edge related to the onset of a differential aeration mechanism. The anodic activity was initially located at the aluzinc layer coated with the thinner organic coating, whereas alkalization of the steel foil related to cathodic activity was limited by the buffering ability of the soluble metal ions. In this way, precipitation of corrosion products might block the cathodic sites, a process responsible for the eventual complete cessation of corrosion in alkaline solution for sufficiently long exposures

Imaging local surface reactivity on stainless steels 304 and 316 in acid chloride solution using scanning electrochemical microscopy and the scanning vibrating electrode technique

Passive film breakdown and pit nucleation on 304 and 316 stainless steels in chloride-containing media were investigated using scanning microelectrochemical microscopy (SECM) and the scanning vibrating electrode technique (SVET). Experiments were performed for the alloys either at their corresponding open circuit potential, or under applied polarization, as to image domains of similar topography but different resistance against breakdown of the passive layers formed on these steels. Identification of the iron released species was accomplished, showing that pitting occurs with the formation of iron (II) species only. Detection of iron (III) species occurred when the steel sample was polarized at high positive overpotentials because it served as the reaction site to oxidize the iron (II) ions released from a propagating pit. The obtained results have revealed some difference of the reactivity of both specimens. Moreover the effect of the galvanic coupling has been investigate

New developments in scanning microelectrochemical techniques: a highly sensitive route to evaluate degradation reactions and protection methods with chemical selectivity

The scanning electrochemical microscope (SECM) offers a highly sensitive route to evaluate degradation reactions and protection methods with chemical selectivity by using ion-selective microelectrodes as tips, thus operating SECM potentiometrically. Spatially resolved imaging of electrochemical reactivity related to each component of the investigated material can thus be effectively monitored selectively both in situ and in real time. The applicability of this method has been illustrated using two practical examples of galvanic corrosion processes, namely a model Zn-Cu pair, and a metal-coating system consisting in the exposure of cut edges of coil-coated galvanized steel to aqueous saline environment. In this contribution, localized pH and zinc(II) ion distributions originated around the corroding systems immersed in 1 mM NaCl solution are show

Novel dual microelectrode probe for the simultaneous visualization of local Zn2+ and pH distributions in galvanic corrosion processes

Novel dual potentiometric microsensor probe has been developed for the simultaneous detection of Zn2+ concentration and pH distributions in the Scanning Electrochemical Microscopy investigation of corroding galvanized steel. The individual sensors show nearly theoretical behavior over a wide concentration range. The applicability of this probe is first demonstrated on a Fe-Zn galvanic couple, as it shows excellent performance in these simultaneous model experiments. In addition, linear scans recorded over a cut edge of coated galvanized steel evidences the complementary information gathered on the electrochemical behavior of the corroding sample, and adumbrates promising and feasible applications of multi-barrel microelectrodes in corrosion research

A novel scanning electrochemical microscopy strategy for the investigation of anomalous hydrogen evolution from AZ63 magnesium alloy

The evolution of hydrogen gas from corroding magnesium arises not exclusively from the cathodic half-cell reaction due to the consumption of the electrons released by the dissolving metal, but anodized magnesium generates significant amounts of H2 gas as well. In addition, the increase of the anodic overpotential enhances the rate of hydrogen gas generation. Therefore, spatially-resolved detection of the actual sites for hydrogen evolution related to anodically-activated sites is attempted using scanning electrochemical microscopy (SECM). This work describes a new experimental procedure for SECM based on a three-step (off-onoff) anodization operation sequence that was designed to obtain new insights into the behavior of magnesium and magnesium alloys when they are subjected to anodic polarization. Results obtained with scanning electrochemical microscopy (SECM) and scanning vibrating electrode technique (SVET) experiments presented here demonstrate the catalytic properties of the magnesium-oxide film for the hydrogen-evolution reactio

Combined amperometric/potentiometric probes for improved chemical imaging of corroding surfaces using Scanning Electrochemical Microscopy

Chemical visualization of corrosion processes using scanning electrochemical microscopy (SECM) in combined amperometric/potentiometric operation has been achieved by developing novel multi-barrel probes as tips. A Pt-based amperometric disc probe is employed for the detection and characterization of reactive sites on a corroding system, whereas a Sb-based disc microelectrode is employed to visualize local solution pH changes. Quasi-simultaneous imaging of localized corrosion micro-cells on the surface and the associated pH variations in the electrolyte, resulting from both the electrolysis of dissolved metal ions from the local anodes and the consumption of an oxidizing agent at the local cathodes, can be obtained in the same solution without changing the probe. Galvanic corrosion of a model Cu-Fe pair in chloride-containing solution was visualized with high spatial resolution by recording either line scans or 2D-images using the novel Pt/Sb multi-barrel tip