Korrosionsverhalten von Zn-Al-Mg-beschichtetem Stahlband

Translated title of the contribution: Corrosion behaviour of Zn-Al-Mg alloy coated steel

Susanne Schürz-Peschka

Research output: ThesisDoctoral Thesis

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Abstract

This work focuses on the corrosion resistance of different hot-dip galvanising coatings in chloride-containing/chloride-free environment and under atmospheric conditions. For this purpose the following coating systems were investigated: the conventional hot-dip galvanising coating (ZnAl 0.2), two Zn-Al alloy coatings (Galvalume: ZnAlSi 55-1.6, Galfan: ZnAl 5) and five Zn-Al-Mg alloy coatings (ZnAlMg 0.3-0.5, ZnAlMg 2-1, ZnAlMg 2-2, ZnAlMg 3-3, ZnAlMg 4.5-3). The ZnAlMg 2-2 alloy coating system consisting of 2 wt.-% Al and 2 wt.-% Mg is currently produced on a large scale hot-dip galvanising line at voestalpine Stahl GmbH in Linz and already commercially available. Therefore this coating system is studied in more detail. All Zn-Al-Mg coated systems show a considerably enhanced corrosion resistance in neutral salt spray test compared to conventional hot-dip galvanised steel and ZnAl 5. The corrosion resistance of ZnAlSi 55-1.6 is comparable to those of the Zn-Al-Mg coating systems. A detailed analysis of ZnAlMg 2-2 coated steel showed that during exposure to neutral salt spray test the metallic coating is converted into a well adherent, compact and dense Al-rich protecting layer of zinc aluminium carbonate hydroxide, Zn6Al2(OH)16CO3 • 4 H2O, which covers and protects the subjacent steel substrate. Magnesium added to the metallic coating forms soluble magnesium chloride during exposure and is depleted from the sample surface. If ZnAlMg 2-2 coated steel is plastically deformed, the coating shows more or less decisive cracks. When these samples were exposed to salt spray test the dense zinc aluminium carbonate hydroxide layer can be formed. Due to the fact that this layer forms in a compact and crack-less way a sufficient corrosion protection is still given. When the plastic deformation is carried out on ZnAlMg 2-2 coated steel which was prior exposed to salt spray test, cracks in the zinc aluminium carbonate hydroxide layer can be observed. When these samples were re-exposed to salt spray test also the cracks in the protecting can be closed and a satisfactory corrosion resistance can still be found. When exposed to salt spray test at pH 3 ZnAlMg 2-2 coated steel again shows an enhanced corrosion resistance in comparison to conventional hot-dip galvanised steel due to the formation of a zinc aluminium carbonate hydroxide layer on the steel substrate. A depletion of magnesium can be found, too. When exposed to salt spray test at pH 12 both conventional hot-dip galvanised and ZnAlMg 2-2 coated steel show a worsening in corrosion resistance in comparison to tests under neutral and acid conditions. Though the formation of a zinc aluminium carbonate hydroxide layer can be observed, the layer cannot unfold its usual protecting properties under alkaline conditions. When a mixture of sodium chloride and ammonium chloride is used for the salt spray test ZnAlMg 2-2 shows a decline in corrosion resistance, whereas conventional hot-dip galvanised steel has a slightly increased corrosion resistance in comparison to salt spray test under neutral conditions. Besides magnesium zinc, too, is depleted from the ZnAlMg 2-2 coating during exposure to this testing medium. The formation of zinc aluminium carbonate hydroxide can not be observed under these conditions. In both examined chloride-free corrosion test ZnAlMg 2-2 coated steel shows a decisively enhanced corrosion resistance compared to conventional hot dip galvanised steel. In both test a protective layer of zinc aluminium carbonate hydroxide is formed. When painted ZnAlMg 2-2 coated steel is tested under atmospheric condition a corrosion creep of 1.1 mm can be observed at the cut edge after one year of exposure. In the corroded cut-edge area the Al-rich protecting layer forms.
Translated title of the contributionCorrosion behaviour of Zn-Al-Mg alloy coated steel
Original languageGerman
QualificationDr.mont.
Supervisors/Advisors
  • Mori, Gregor Karl, Assessor A (internal)
  • Kneißl, Albert, Assessor B (internal)
Publication statusPublished - 2010

Bibliographical note

embargoed until null

Keywords

  • hot-dip galvanising
  • corrosion protection
  • Zn-Al-Mg alloy coating

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