Berlin 2012 – scientific programme

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O: Fachverband Oberflächenphysik

O 87: Surface chemical reactions

O 87.7: Talk

Friday, March 30, 2012, 12:00–12:15, MA 042

Electrochemically triggered phosphating of Si-Al-coated steel — •Paul Schneider, Andreas Erbe, and Frank Uwe Renner — Max-Planck-Institut für Eisenforschung, Düsseldorf, Germany

Tri-cation phosphating has been investigated on Si-Al-coated steels. Phosphate crystal growth on these surfaces can be initiated by cathodic polarisation, leading to hydrogen evolution and subsequent generation of a pH gradient. The electrochemically generated pH gradient replaces the pH gradient which is obtained by pickling on Fe and Zn-rich surfaces whose oxides are not stable under the technologically relevant pH. The dependence of the crystal morphology and composition on the applied potential has been investigated by scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDX), X-ray diffraction and X-ray photoelectron spectroscopy (XPS). The obtained phosphate layers mainly consist of hopeite with varying Mn content. In a second step, the time dependence of the crystal nucleation and growth has been studied by switching the passive surface to a potential where crystal growth was observed. First crystals were observed after 2 s of polarisation. Subsequently, both number of crystals as well as crystal size increases. A complete surface coverage is obtained after 2 min of active phosphating.