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Corrosion inhibition by chromate at defects of organic coatings
RISE - Research Institutes of Sweden, Materials and Production, KIMAB. (Institut de la Corrosion)
RISE - Research Institutes of Sweden, Materials and Production, KIMAB. (Institut de la Corrosion)
2004 (English)In: EUROCORR 2004 - European Corrosion Conference: Long Term Prediction and Modelling of Corrosion, 2004Conference paper, Published paper (Refereed)
Abstract [en]

Chromate is known as one of the most effective inhibitors for a wide range of technically important metals and alloys, particularly aluminum alloys and galvanized zinc surfaces protecting steel against atmospheric corrosion. The very good protective ability of inhibiting systems with chromate became the subject of intensive research in recent years. The major reason of the research activity is the environmentally driven effort to limit or even diminish the usage of chromate because of its toxicity and possible human carcinogenity. The understanding of the mechanism of the chromate inhibition is believed to be essential in formulating the alternative inhibiting systems, which should provide at least comparable efficiency. The study was focused on the corrosion inhibition by chromate in primer coatings applied on hot-dip galvanized steel (HDG) under atmospheric weathering conditions, particularly at defected areas of the coatings such as scribes or cut-edges. The kinetics of chromate release from organic coatings to solution, chromate mobility ("throwing power") at defected areas and the composition and some properties of the forming protective layer on the zinc surface have been studied. To characterize the composition of the surface layers on zinc, Fourier Transformed Infra-Red (FTIR), Energy Dispersive X-ray (EDX) and X-ray Absorption Near Edge Structure (XANES) techniques were applied. The composition of solutions was analyzed by Ion Chromatography (IC). Based on the experiments and published results of other authors, a mechanism of the inhibition by chromate in the system pigmented coating / water / zinc in atmospheric conditions was proposed. After establishing contact between the coating and water, hexavalent chromium starts to dissolve rapidly and releases from the coating in the form of chromate, CrO 4 2-, dichromate, Cr 2O 7 2-, or bichromate, HCrO 4 -, according to actual pH and Cr(VI) concentration. The released ions are in neutral solutions stable with a minimal bias to be reduced to Cr(III). The ions of hexavalent chromium are transported toward unprotected bare zinc and adsorbed on the surface. Moreover, Cr(VI) is able to seek active, or formerly active, corrosion sites, e.g. corrosion pits, where it is preferentially adsorbed. On the zinc surface, chromate is partly reduced to Cr(III). The film formed on zinc in the presence of chromate in solution is composed mainly of Cr(III), with a smaller extent of Cr(VI). Further consumption of chromate available in the solution is slow and controlled by the aggressiveness of the solution rather than by the chromate concentration. The film formation and/or presence lead to a highly efficient inhibition of zinc corrosion, probably due to decrease in the oxygen reduction rate.

Place, publisher, year, edition, pages
2004.
Keywords [en]
Chromate, Inhibition, Organic coatings, Zinc, Atmospheric conditions, Chromate concentration, Corrosion inhibition, Corrosion pits, Corrosion sites, Energy dispersive x-ray, Film formations, Fourier, FTIR, Hexavalent chromium, Hot-dip galvanized steel, Intensive research, Metals and alloys, Neutral solution, Oxygen reduction rate, Pigmented coatings, Primer coatings, Protective layers, Research activities, Surface layers, Throwing power, Weathering conditions, X-ray absorption near-edge structure, Zinc corrosion, Atmospheric corrosion, Chromates, Chromium, Electrolytic reduction, Enzyme inhibition, Galvanizing, Metallurgy, X ray absorption near edge structure spectroscopy, Zinc coatings, Chromate coatings
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:ri:diva-40443Scopus ID: 2-s2.0-84867241261OAI: oai:DiVA.org:ri-40443DiVA, id: diva2:1361024
Conference
European Corrosion Conference: Long Term Prediction and Modelling of Corrosion, EUROCORR 2004; Nice; France; 12 September 2004 through 16 September 2004
Available from: 2019-10-15 Created: 2019-10-15 Last updated: 2019-10-15Bibliographically approved

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