| Article ID | Journal | Published Year | Pages | File Type |
|---|---|---|---|---|
| 185719 | Electrochimica Acta | 2014 | 7 Pages |
•An electrochemical unit for automatic EIS and pH modulation is used.•ZnMg2Al2 and ZnAl53 are studied in alkaline solutions.•Amount of consumed sodium hydroxide allows following hydroxide formation and film thickness.•Cross sections of 13 μm show excellent agreement with consumed hydroxide.•ZnAl53 consumes more hydroxide due to soluble aluminate formation.
Two different zinc coatings of composition ZnMg2Al2 (Zn + 2 wt.% Mg + 2 wt.% Al) and ZnAl53 (Zn + 53 wt.% Al) were investigated in aqueous solution at pH 12.0 with a novel setup, the so-called impedance titrator. This device is able to perform electrochemical measurements including, but not limited to, impedance spectroscopy in dependency of different pH-values. The setup allows holding the pH-value with a precision of at least 0.05 by dosing the required amount of titrating agent to the system. If the alkaline pH region is investigated, hydroxide ions are consumed in the course of passive layer formation. The amount of consumed hydroxide allows to quantitatively follow the formation of the hydroxide film and its thickness. Cross section SEM shows an excellent agreement of 13 μm after 7 h for ZnMg2Al2. At a constant pH value, the hydroxide concentration is constant and film formation is well defined and kinetically characterised. The consumption of hydroxide by the ZnAl53 coating is higher resulting from the solubility of the Al as aluminate under alkaline conditions. The composition of the precipitates contains less than 3 wt.% of Al. The observed processes and the formation of corrosion products are recorded and differences in the behaviour of the two coatings are discussed.
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