Alkaline cleaning of Zn?Al?Mg hot-dip galvanized steels: Mechanisms and surface oxide chemistry
Sprache des Titels:
Englisch
Original Kurzfassung:
Alkaline cleaning of Zn?Al?Mg coated hot-dip galvanized steel is a central process in the industrial galvanized steel production.
This process removes carbonaceous contaminants from the surface and modifies the surface chemistry profoundly. We implement a
combined analytical and surface science approach to characterize the dissolution mechanism and surface chemistry of Zn-Al-Mg
coatings after treatment with industrial cleaners with pH 9.3 and 12.7, respectively. Our data indicate that weak alkaline cleaning
can significantly increase the surface concentration of Zn-oxide, while strong alkaline cleaning dissolves the native oxide and
generates a transient Zn/Mg-hydroxide on the surface. The observed dissolution mechanisms are largely consistent with the
expectations from the Pourbaix diagrams, i.e. at pH 12.7 aluminium dissolution is expected while Mg is stable and forms a transient
passive film. In contrast, mild alkaline cleaning at pH 9.3 is dominated by Mg and Zn dissolution, while the native Al passive film
remains stable. Hence the cleaning provides an effective direct modification of the surface chemistry for subsequent process steps
during the coating. Mild alkaline cleaning offers an increase of Zn at the surface, which has important implications for subsequent
conversion and adhesive applications, that have been traditionally optimized for pure Zn coatings.