Localized Electrochemical Methods Applied to Cut Edge Corrosion

ECS Transactions, 2008

Electrochemical processes occurring on the cut-edge of a galvanized steel immersed in NaCl solutions were studied using numerical simulations, and in situ current and pH profiles measured over the cut-edge. These results clearly demonstrate that only the steel surface remote from the zinc coating is cathodically active, oxygen reduction being strongly inhibited in the vicinity of zinc. This trend was confirmed by local polarization curves recorded on these distinct areas. Ex-situ AES and SEM analysis and cathodic polarization curves in solutions containing Zn 2+ ions led to conclude that this cathodic inhibition was related to the fast nucleation of a dense Zn(OH) 2 film on the steel surface. After a long term exposure, a new galvanic coupling takes place between the Zn(OH) 2 covered area, showing an anodic activity, and the remaining steel surface covered by bulky white zinc corrosion products.

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.

Transactions of the IMF, 2002

Corrosion and co"osion inhibition of coil-coated galvanised steel has been studied using an electrochemical sandwich cell which models the geometric situation at sheet steel cut edges. The galvanic currents flowing during immersion in a solution simulating typical acid rainwater were measured. Also, potentia-dynamic polarisation was carried out on the galvanic cell where the individual currents on the steel and =inc, as well as the total current, were determined. Where no inhibitor is present in solution. the corrosion rate is controlled by oxygen diffusion to the steel cathode. In the presence of strontium chromate the cathodic reduction of oxygen on the steel cathode is substantially reduced in addition to the expected anodic passivation of =inc. This is presumably due to reduction of chromate at the cathode forming a protective chromium oxide film. Some alternatives to chromate were also tested. Molybdate/phosphate pigments and calcium ion-exchange pigments alone gave poor inhibition. However, they were found to act synergistically in combination giving an inhibitive efficiency closely approaching that of chromate.

Corrosion Science, 2016

A model electrode consisting of a narrow zinc anode and a split iron cathode was used for assessing galvanic corrosion at cut edges. This setup aimed at a better understanding of the mechanisms of galvanic corrosion at cut edges and of the effective resolution of microprobe electrochemical techniques, namely the scanning electrochemical microscope (SECM), the localized electrochemical impedance spectroscopy (LEIS) and the scanning vibrating electrode technique coupled with the scanning ion-selective electrode technique (SVET/SIET). Hydroxyl ions diffuse across the cathode, in the direction of the anode, until they reach the critical area of precipitation above the cathode, at a distance from the edge of the cathode. The zinc corrosion products ultimately precipitate on the iron cathode, once the critical pH and solubility limits for precipitation of zinc corrosion is reached, at a distance from the edge that is determined by the diffusion of Zn 2+ and counter-diffusion of OH-. These precipitates revealed no inhibition on the cathodic reaction. The local activity on the zinc was easily detected by the LEIS and the SVET, whereas the zinc corrosion products were detected by the SECM. The local a.c. admittance measured on the cathode closer to the zinc anode at the early stages was insensitive to the local activity despite the d.c. ionic current measured in solution, whereas for longer immersion times an increase of admittance is explained by an increase in the solution conductivity.

Materials Sciences and Applications, 2012

From industrialized baths, free of cyanide, the corrosion behavior of electrodeposits of zinc and zinc alloys was studied by means of electrochemical tests in aerated solution of 3.5% (0.6 M) NaCl at pH 8.2. In literature, several studies are found about zinc coatings and zinc alloys, for example, Zn-Ni and Zn-Co, nevertheless there is little about the ternary alloy Zn-Fe-Co. The Fe presence in the alloy results in a good adhesion to the substrate and allows application of these materials at higher temperatures. The electrochemical tests were carried out by obtaining open circuit potential curves with immersion time, potentiodynamic polarization curves and cyclic voltammetry. From the obtained results, the large potential differences observed between the steel and the electrodeposits showed that the last protect the substrate, acting as a sacrifice metal. The tests disclosed similar behaviors in both the current densities and the corrosion potential for electrodeposits of Zn and Zn-Fe-Co. After chromate passivation process, a significant decrease in corrosion density was noted for Zn and when the system was de-aerated there was change in the cathodic process mechanism.

Corrosion Science, 2018

The combined use of the microcapillary cell (MEC) and scanning vibrating electrode technique (SVET) and lowangle cross sections was employed to elucidate the role of each coating region on the protection of the cut-edge corrosion of galvanized steels. Different compounds are involved in the blocking action of the corrosion products: Zincite (ZnO) on the steel substrate, hydrozincite (Zn 5 (OH) 6 (CO 3) 2) at the coating/steel interface, and Simonkolleite (Zn 5 (OH) 8 Cl 2) and ZnO on the different coating regions in different proportions. The coating surface is also active at the initial stage and during long-term protection and thus, must be considered in experimental simulation of the cut-edge corrosion.

Electrochimica Acta, 2011

Electrochemical processes occurring on the cut-edge of a galvanized steel immersed in NaCl solutions were studied using numerical simulations, and in situ current and pH profiles measured over the cut-edge. These results clearly demonstrate that only the steel surface remote from the zinc coating is cathodically active, oxygen reduction being strongly inhibited in the vicinity of zinc. This trend was confirmed by local polarization curves recorded on these distinct areas. Ex-situ AES and SEM analysis and cathodic polarization curves in solutions containing Zn 2+ ions led to conclude that this cathodic inhibition was related to the fast nucleation of a dense Zn(OH) 2 film on the steel surface. After a long term exposure, a new galvanic coupling takes place between the Zn(OH) 2 covered area, showing an anodic activity, and the remaining steel surface covered by bulky white zinc corrosion products.

Electrochimica Acta, 2010

The effect of benzotriazole (BTAH) in high concentration on the cut edge corrosion of galvanized steel in sodium chloride solution was studied, using electrochemical polarization, EIS, SVET and XPS. It was observed that there is the formation of a BTAH surface film on both steel and zinc, which polarizes anodically both metals, greatly reducing their electrochemical activity and changing the rate control from diffusion at the cathode to mixed control. The protective film undergoes numerous events of local activation and repassivation. The XPS data revealed that the BTAH becomes differently bound to zinc depending on the metal being under self-corrosion or coupled to steel. Although the BTAH reduces the electrochemical activity of both materials, there is significant reduction of the cathodic protection effect of zinc.

Scanning electrochemical microscopy (SECM) and the scanning vibrating electrode technique (SVET) were employed to identify the local reactivity and the cathodes and anodes on a cut edge of galvanized steel. The SECM was used in the amperometric feedback mode, using a redox mediator, for sensing the conductivity of the surface and also in the redox competition mode for locating the depletion of the cathodic reactant. Good agreement was observed in acidic solution between the location of the conductive regions of the surface and the depletion of oxygen estimated from amperometric lines across the cut edge. In nearly neutral medium, non-uniform activity of the steel surface was observed and correlated well with the ionic current flows in solution and with the accumulation of zinc corrosion products on steel. In alkaline medium the entire steel surface operated as electron source for the regeneration cycle of the redox mediator. Irrespective of the pH of the solution, a maximum of the feedback observed over the cut edge is consentaneous with the thermodynamic stability of iron in the reduced form. SECM provided useful information regarding the steel conductivity, whereas the zinc anodes could only be resolved by the SVET.

ChemElectroChem, 2018

Zinc electrodes were polarized cathodically at moderate overpotentials in NaCl 0.6 M solutions under potentiostatic conditions for 7 to 17 hours at room temperature. Corrosion products were characterized by optical microscopy, XRD, Raman microscopy, XPS and FIB-SEM. Close to the open circuit potential, the corrosion products were formed by simonkolleite and the electrochemical response exhibits anodic features. At more negative potentials, the current density remains cathodic throughout the polarization and the deposits on the electrode surface consist almost solely of ZnO. The soluble zinc species necessary for ZnO deposition originate from localized dissolution of the substrate in the form of pits. This effect is assigned to the strong alkalinisation of the surface due to oxygen reduction. Despite developing greater surface area than bare zinc substrates, the nanostructured ZnO deposits reduced the cathodic activity.