3654 Journal of The Electrochemical Society, 147 (10) 3654-3660 (2000) S0013-4651(00)02-083-8 CCC: $7.00 © The Electrochemical Society, Inc. When a painted steel sheet is cut at the factory, the bare metal is exposed in cross section on the cut edge, where it becomes a corro- sion risk regardless of the barrier properties of the paint layer on the face. Such “cut edge corrosion” is a well known problem particular to coil coated steel products 1,2 and may represent a major, but large- ly ignored, obstacle to the development of advanced on-line coating technologies such as plasma polymerization. Therefore, there is an immediate interest in understanding the corrosion protection mech- anisms which are provided by the coating layers against this form of corrosion. In a more general way, corrosion on the cut edge of galvanized steel products can be considered as a worst case scenario for sacrifi- cial protection by zinc coatings, because of the very unfavorable anode to cathode surface area ratio. The zinc layer is on the order of 5 to 20 mm thick as compared to around 800 to 1000 mm for the steel substrate. Therefore secondary mechanisms of protection involving the formation of protective films from zinc corrosion products and from water soluble inhibitors in the paint 2,3 play an important role on the cut edge of coil coated steel. The efficiency of chemical inhibition is highly dependent on the chemical and electrochemical environment which is found in the vicinity of the cut edge. The strong galvanic couple set up between the steel and the zinc leads to large electrical field variations, and the close proximity of the anodic and cathodic reactions leads to signif- icant changes in the chemical environment including increases in [OH 2 ] and [Zn 12 ]. The distribution of current is further altered by the precipitation of zinc-based corrosion products and the release of water soluble inhibitors present in the paint. Although extensive ana- lytical studies on the nature of the corrosion products have been un- dertaken in recent years, 4-7 their role in inhibiting the electrochemi- cal reactions has been given much less attention. In this work, we have used in situ electrochemical techniques with local resolution, the scanning vibrating electrode technique (SVET) and scanning pH microscopy, to investigate the interrela- tionship between electrochemical activity, pH changes, and corro- sion-product precipitation. In particular, localized polarization curves were obtained for the anodic and cathodic reactions in order to determine the rate-limiting processes on the cut edge and to ob- serve the inhibiting effect of corrosion products. Preliminary results have been previously presented concerning the polarization behav- ior 8,9 and localized pH measurements 10 on the cut edge. Experimental Sample preparation and model sample design.—Cut edge phe- nomena were simulated in the laboratory by embedding, in an epoxy resin, a commercial electrogalvanized low alloy carbon steel sample typical of that used in the automotive industry. The cut edge was exposed in cross section to form an electroactive area of 10 3 0.81 mm. The steel thickness was approximately 800 mm, and the zinc coating ranged from 5 to 10 mm. The cut edge surface was pol- ished to a 1 mm finish and particular care was taken during polish- ing to avoid spreading the zinc onto the steel surface. Single and double sided electrogalvanized steel were used as indicated in the text. Electrogalvanized steel was chosen for this work because of its high level of purity and reproducibility as an industrial product. For galvanic coupling experiments, a model cut edge sample was prepared. This consisted of separate steel and zinc samples mounted in the epoxy resin with approximately 2 mm of separation between them. A 250 mm zinc foil (Goodfellow, 99.95 1 %) and a sample of low alloy carbon steel of 800 mm thickness, were used. The edges of the sample were exposed to the electrolyte and the two metals were short-circuited by an external circuit. The large-area Zn/steel electrode was prepared by vapor deposit- ing a 5 mm thick Zn coating over one-third of a 5 3 10 mm low alloy carbon steel electrode. The deposition was done by masking the sur- face not to be coated by an adhesive tape (3M PTFE 5490, based on a silicone adhesive with upper temperature limit of 2048C), which was removed after the zinc deposition. The sample was sputtered with Ar ions before the deposition of the zinc in order to increase the metal/metal adherence. This sample was stored in a dissicator prior to use. The electrodes thus produced were exposed to NaCl solutions of variable concentration as described in the text. Because of a poor Zn to steel adherence, the electrodes were not submitted to any addi- tional cleaning regime. All solutions used deionized water (>5 MV- cm) and reagent grade NaCl. The electrolytes were aerated, and all experiments were conducted at ambient temperature. Current density measurements.—A commercial SVET system from Applicable Electronics was used in this work. 11,12 The probe consisted of an insulated Pt-Ir wire with a Pt black deposit at the tip on the order of 20 mm diam. The probe is vibrated at different fre- quencies in the parallel and perpendicular direction to the surface, with an amplitude on the order of 20 to 40 mm. The two-dimension- al (2D) vibration is accomplished by use of piezoelectric wafers dri- ven by sinewave oscillators. The parallel and perpendicular compo- nents of the local electric field are considered proportional to the parellel and perpendicular components of the current vector, J x and J z , and are measured by independent lock-in amplifiers. The second horizontal component of current, J y , is not measured with this sys- tem. The calibration parameters necessary to convert the electric field data into current were determined from a one-point calibration, performed by taking a measurement at a fixed distance from a point Localized Electrochemical Methods Applied to Cut Edge Corrosion K. Ogle,* ,z V. Baudu, L. Garrigues, and X. Philippe Irsid, Usinor Research, 57283 Maizières-lès-Metz, France Current density and pH mapping techniques have been used to characterize the chemical and electrochemical phenomena which occur on the cut edge of galvanized steel. pH variations between 7 and 11 were observed, primarily due to the formation of hydrox- yl ions by the cathodic reaction. Zinc-based corrosion products precipitated in zones of intermediate pH were identified as ZnO and 3Zn(OH) 2 ?2ZnCO 3 for model samples. The efficiency of these corrosion products as cathodic inhibitors was demonstrated by the absence of cathodic activity at open circuit and a 2300 mV negative shift of the onset potential for hydrogen formation in the affected zones. The cathodic current, localized over the steel, was independent of potential, consistent with a diffusion-limited reduction of oxygen. The anodic current, localized over the zinc, varied with potential, with a Tafel slope of 44 mV/decade for an order of magnitude decrease of potential below open circuit. The addition of SrCrO 4 to the electrolyte increased the Tafel slope to 63 mV/decade, consistent with a passivating inhibitor on the anode surface. © 2000 The Electrochemical Society. S0013-4651(00)02-083-8. All rights reserved. Manuscript submitted February 22, 2000; revised manuscript received May 19, 2000. This was in part Paper 582 presented at the Honolulu, Hawaii, Meeting of the Society, October 17-22, 1999. * Electrochemical Society Active Member. z E-mail: kevin.ogle@irsid.usinor.com