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Electrochemical study and microstructural characterization of conversion layers free of hexavalent chromium and without rinsing applied on galvanized surfaces

Grant number: 11/13367-4
Support type:Scholarships in Brazil - Scientific Initiation
Effective date (Start): October 01, 2011
Effective date (End): September 30, 2012
Field of knowledge:Engineering - Materials and Metallurgical Engineering
Principal Investigator:Hercílio Gomes de Melo
Grantee:Milton José Pimenta Segundo
Home Institution: Escola Politécnica (EP). Universidade de São Paulo (USP). São Paulo , SP, Brazil

Abstract

Passivation processes are very important both as a way to afford corrosion protection and also prior to application of painting systems. Industrially many of these processes are based on the use of hexavalent Chromium ions (CrVI+). However, although very efficient, these treatments must be banished from industrial use due to their pollutant and carcinogenic characteristics. The literature survey shows that, up to now, no pre-treatment is available for complete replacement of the CrVI+ based ones, as they do show neither equal corrosion protection nor similar adhesion properties. Particularly concerning galvanized surfaces, to make viable this replacement, it is necessary to take into account the existent galvanizing facilities, and avoid extra costs associated with adaptations and inclusion of new treatment steps, which would slow down productivity. In the continuous lines used to produce electrogalvanized steel, a CrVI+ passivation bath is employed that does not need washing after application. The alternatives to this treatment existing in the market are based on salts (silicates and Cr(III)). However, to afford good corrosion resistance, the surfaces need to be washed after these treatments application. Therefore, the direct substitution of CrVI+ based passivation by these new alternative treatments in existing electrogalvanizing facilities is not straightforward. The aim of this project is to investigate the corrosion resistance of conversion layers obtained from CrVI+ free passivation baths, which do not need the washing step after their application. To be efficient these layers must exhibit similar anticorrosion properties to their CrVI+ based counterparts, thus producing a more environmentally compliant treatment, which, at the same time, could be used in existing electrogalvanizing facilities.