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Tuesday, March 16, 2010: 3:40 PM
214 D (Henry B. Gonzales Convention Center)
Conventional hot-dip galvanized zinc coated (HDG) and novel hot-dip galvanized Zn-Al-Mg alloy coated (ZM) steel sheet samples, both comprising a coating thickness of 7 μm, were exposed to standardized salt spray test. Cross-sections of the corrosion samples were analyzed by using SEM and EDS. On HDG corrosion proceeds very fast and the steel substrate is attacked even after 100 h of exposure. ZM samples showed a completely different behavior. Cross-section analysis showed that the entire metallic ZM coating is converted into a stable, adherent aluminum-rich passive layer, which protects the steel substrate against corrosive attacks. This layer is the main reason for the enhanced corrosion resistance of the ZM coating in sodium chloride-containing environment. The chemical composition of this protective layer was analyzed by XRD and FT-IR and identified as zinc aluminum carbonate hydroxide.
Above this steel substrate near passive layer different types of corrosion products are formed on top. The visual inspection of the corroded ZM samples showed, that there exist noticeably two different areas on the sample surface: On the one hand white regions with thick layers of white corrosion products and on the other hand dark-grey regions, where the corrosion product layer seemed to be much thinner than in the white ones. SEM analysis of the sample surface showed that this difference originates from different types of corrosion products formed in the distinctive areas: in the white regions very porous and voluminous corrosion products are formed. EDS analysis indicated the presence of zinc, oxygen, and small amounts of magnesium. In contrary to that the structure of the corrosion products in the grey area is completely different. In these regions crystals can be observed which contain also chlorine beside zinc and oxygen. Raman analysis of the crystals indentified them as zinc chloride hydroxide, Zn5(OH)8Cl2.
Additional XPS analyses were carried out to identify the corrosion products. Concerning the elemental composition XPS analysis confirmed the findings from EDS analysis: Magnesium is enriched in the white regions, whereas chlorine is only found in the dark regions. In the white region zinc is existent in form of zinc hydroxide, Zn(OH)2, and zinc carbonate hydroxide, Zn5(OH)6(CO3)2. The existence of zinc oxide, ZnO, could be clearly excluded. Magnesium was found to be existent in form of MgCO3 only. XPS analysis also showed that in the white areas more inorganic carbonate – bound in zinc carbonate hydroxide and magnesium carbonate – can be found in comparison to the dark areas.
In the dark areas, where no magnesium, but decisive amounts of chlorine were found, zinc chloride hydroxide was identified by using XPS and therefore confirms the findings from Raman analysis. In addition zinc chloride, ZnCl2, zinc hydroxide and zinc carbonate hydroxide were detected in the dark areas.
Above this steel substrate near passive layer different types of corrosion products are formed on top. The visual inspection of the corroded ZM samples showed, that there exist noticeably two different areas on the sample surface: On the one hand white regions with thick layers of white corrosion products and on the other hand dark-grey regions, where the corrosion product layer seemed to be much thinner than in the white ones. SEM analysis of the sample surface showed that this difference originates from different types of corrosion products formed in the distinctive areas: in the white regions very porous and voluminous corrosion products are formed. EDS analysis indicated the presence of zinc, oxygen, and small amounts of magnesium. In contrary to that the structure of the corrosion products in the grey area is completely different. In these regions crystals can be observed which contain also chlorine beside zinc and oxygen. Raman analysis of the crystals indentified them as zinc chloride hydroxide, Zn5(OH)8Cl2.
Additional XPS analyses were carried out to identify the corrosion products. Concerning the elemental composition XPS analysis confirmed the findings from EDS analysis: Magnesium is enriched in the white regions, whereas chlorine is only found in the dark regions. In the white region zinc is existent in form of zinc hydroxide, Zn(OH)2, and zinc carbonate hydroxide, Zn5(OH)6(CO3)2. The existence of zinc oxide, ZnO, could be clearly excluded. Magnesium was found to be existent in form of MgCO3 only. XPS analysis also showed that in the white areas more inorganic carbonate – bound in zinc carbonate hydroxide and magnesium carbonate – can be found in comparison to the dark areas.
In the dark areas, where no magnesium, but decisive amounts of chlorine were found, zinc chloride hydroxide was identified by using XPS and therefore confirms the findings from Raman analysis. In addition zinc chloride, ZnCl2, zinc hydroxide and zinc carbonate hydroxide were detected in the dark areas.