2.3. Coating
The coating is applied to overcome the barrier effect of the natural Al2O3 layer. Hence the zincate treatment is the most suitable coating process as the procedure includes the dissolution of the surface oxides while the deposition of the zinc layer takes place simultaneously. The coating mechanism is based on a simple redox reaction and is stated in the equations (1) and (2) [10].
Oxidation: Al2 l O3 + 2NaOH + 3H2 H NaAl(OH)4
Reduction: 3Na2Zn(OH) ( ( 4 NaAl(OH)4 + 4NaOH + 3Zn (deposited) (2)
Previous to the coating step the surface is cleaned from processing agents and organic materials using a neutral soap dispersion (brand name: Curatech TN 249/400). Sandblasting destructs the oxide layer, thus increases the effective surface for the following pickling steps. Furthermore it is common practice to disperse oxide layers prior to the zincate step with Sodium hydroxide and etch the pickling deposit by dipping the substrate into nitric acid [11]. The zincate step is performed using the commercial product CNF 11 (Schlötter Galvanotechnik). Adhesive forces of the zinc layer on the substrate are measured according to DIN EN ISO 4624 Peel-of tests. Therefore a steel pin is glued on the zinc layer and pulled of perpendicular to the surface. Maximal forces at the point of fraction are measured. Recent investigations by Rübner et al. [12] indicate an important influence by the thickness of the applied zinc layer. To increase the amount of zinc on the substrate surface a supplemental galvanizing process is added to the process chain. Galvanizing parameters are given in Tab. 1.