Long-Term Barrier Effects of CR (III)- and CR (VI)- Treated Zinc Surfaces on Metal Release

Long-term barrier effects of Cr(III) and Cr(VI)
treated zinc surfaces
D. Lindström, I. Odnevall Wallinder, C. Leygraf
– Div. Corrosion Science, School of Chemical Sciences and Engineering, KTH,
Dr. Kristinas v. SE-100 44 Stockholm, Sweden
www.corrosionscience.se, phone: +46 8 7906739, fax: +46 8 20 82 84
davidb01@kth.se, ingero@kth.se, chrisl@kth.se

The recently implemented legislation regarding handling of chemicals in the European Union, and a generally increased environmental awareness in the society have generated a need to quantify diffuse metal flows from metallic product surfaces. The use of Cr(VI) in surface treatments on external constructions (e.g. on galvanized steel) is questioned and powerfully restrained by this legislation. Therefore, environmentally friendly alternatives based on e.g. Cr(III) are now available on the market.

In this study, corrosion-induced changes in barrier effects in terms of release of zinc and chromium and changes in surface properties with time were investigated of bare zinc sheet and continuously galvanized steel with surface treatments based on Cr(III) or Cr(VI) in long-term field exposures at an urban (Stockholm, Sweden) and a marine site (Brest, France). Essential key information is generated to assess potential adverse environmental effects caused by the use of these surface treatments.

Preliminary results from the urban site show the Cr(VI)-based surface treatments, at least initially, to have a higher barrier effect for zinc release compared to the Cr(III)-based surface treatment. Both surfaces resulted in significantly lower release rates of zinc compared to bare zinc sheet. Chromium was released from both surface treatments (Cr(VI)>>Cr(III)) at significantly lower rates compared to zinc, whereby the rates decreased with time.