The formation of a natural passive layer on hot-dip galvanized coatings increases their corrosion resistance. Although the basic mechanisms of passive layer formation are known, the quantitative determination of the evolution of its passive character, in particular its surface resistivity, and its development under atmospheric weathering remains largely unexplored for zinc-5% aluminum (Zn5Al) coatings.
Zn5Al-galvanized test sheets were exposed to atmospheric weathering for four weeks during summer and winter, and the development of polarization resistance measured using a gel-type electrolyte over these periods. Further investigation included stripping away the formed passive layer midway through the exposure period and electrochemical characterization of the passive layer that re-formed after under continue weathering.
The results showed a steadily increasing resistivity of the corrosion product (passive) layer over the course of the weathering period; this was confirmed by supplementary macroscopic and microscopic analyses. Exposure conditions affected the speed of development. Polarization resistance values were higher by approximately 50 kΩcm2 during the winter compared to summer. Upon interruption of exposure and removal of the passive layer, it rebuilt up much more rapidly after exposure resumed in comparison with the initial layer.
The use of gel electrolyte for determining the polarization resistance of a metal surface is a novel and useful method. To the best of the authors’ knowledge, this study has, for the first time, established the suitability of this electrochemical test method for quantifying the coating resistance of Zn5Al galvanized steel after calibrating the measurement parameters.
