Corrosion resistance of novel coatings on packaging steels

ECCS (Electrolytically Chromium Coated Steel) is historically made from a Cr(VI) based bath, but REACH legislation makes market expansions following this route restricted. A novel chromium based coating alternative has been developed by Tata Steel using a Cr(III) containing electrolyte. Initial studies were undertaken to understand the relationship between deformation of the packaging steels and the resultant impact on corrosion performance. An organic overcoat (PVB) was applied to strained samples and an in-situ scanning Kelvin probe technique (SKP) was used to determine rates of PVB cathodic disbondment. The Cr(VI) derived coatings were fully resistant to cathodic disbondment post-deformation. The Cr(III) derived coatings exhibited increased rates of cathodic disbondment with increasing uniaxial strain. Enhancements of the Cr(III) based coating were explored using a citric acid passivation treatment. SVET (scanning vibrating electrode technique) was used to study aqueous corrosion mechanisms and time lapse photography was utilized to measure PVB cathodic disbondment on the Cr(III) coated substrate. The best performance for localised corrosion was identified at a passivation time of 5 minutes. However, 15 minutes of treatment produced improvements in suppressing cathodic disbondment rates of the PVB overcoat. The primary mechanisms for the passivation treatment with citric acid was identified as chromium oxidation and the dissolution of surface iron. Prolonged exposure to citric acid incurred a detrimental effect in aqueous corrosion circumstances, however an enhanced performance during cathodic disbondment studies. The influence of changes in total chromium coating weight and chromium oxide levels on corrosion resistance were studied, revealing a correlation between higher coating weights and resistance to filiform corrosion. SVET experiments revealed the resistance to localised corrosion activity was not dependent on total coating weight, but the levels of chromium oxide on the Cr(III) coating.