Q. Why are galvanized substrates more prone to voltage rupture than other metallic substrates? What process variables can you change to eliminate voltage rupture? — J.T.
A. Galvanized substrates are characterized by a layer of zinc (Zn) deposited over a steel substrate. The Zn layer can be deposited by several mechanisms, including continuous or bath hot dip, electrogalvanizing, mechanical, etc.
When the layer of galvanizing is not 100-percent tight or closed to the substrate (Zn layer voids or micro-cracks, small pinholes or large porosity), there is the possibility of creating a galvanic cell between the iron base and the zinc coating, because the two metals have different oxidation-reduction potentials. These galvanic cells are typically the nucleation sites responsible for voltage rupture in electrocoatings. The interaction between the galvanic cells in the substrate and the applied voltage in the electrocoat bath can create points of excessive voltage. This excessive voltage can create burns or ruptures in the electrodeposited coating.
To minimize or eliminate voltage rupture, first try reducing the voltage. In order to minimize application voltage and still meet minimum film thickness requirements, however, you also will have to modify other process variables to compensate, such as increasing percent solids, bath temperature, solvent levels or dwell time.
You also can ask the galvanizer to improve the zinc coating to eliminate voids or porosity. In most cases, galvanized coatings are more resistant to rupture than galvanealed coatings because there is less iron in the coating.