Combined chemical and EIS study of the reaction of zinc coatings under alkaline conditions

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, impedancespectroscopy in dependency of different pH-values. The setup allows holding the pH-value with a pre-cision of at least 0.05 by dosing the required amount of titrating agent to the system. If the alkaline pHregion 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 thick-ness. Cross section SEM shows an excellent agreement of 13 m after 7 h for ZnMg2Al2. At a constant pHvalue, 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 theAl 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 inthe behaviour of the two coatings are discussed.