Effect of Relative Humidity on Crack Growth Mechanism of Hot Dip Galvanized Steel in Atmospheric Environment
Authors: Hasegawa Kayo , Morita Motoaki , Motoda Shinichi 
Published: February 2020

A hot dip zinc galvanizing is currently used as an effective corrosion protection technique for steels. There are a lot of reports concerning to the corrosion of coated metals. However, due to its difficulties for evaluation, the corrosion fatigue strength of galvanized steel has not been investigated. For this reason, the effect of galvanizing on the fatigue strength has not been understood yet. In the previous work, we studied the fatigue of zinc galvanized S45C steel at room humidity. In order to investigate the atmospheric corrosion fatigue strength of galvanized steel, we conducted the fatigue test focusing on the fracture mechanism of hot-dip galvanized steel in the corrosive atmospheric environment.
The substrate of the test material was S45C steel rod. Prior to the galvanizing process, S45C steel rod was pickled to remove the flux. The specimens were immersed in the galvanizing bath at 450℃ for 3 min. The corrosive environment was prepared in a glass sealed container controlled by the humidity control system. Load controlled fatigue tests were carried out at a load ratio of 0.01 and a frequency of 10 Hz (sine wave). SEM observation was performed to study the morphology of fracture surface and to determine the crack initiation sites.
In the cross sectional observation of galvanized layer, the Zn-Fe alloy layer was constructed in delta-one (δ

1
) phase and zeta (ζ) phase, and the top of surface was eta (η) phase which was comprised of the pure zinc layer. When the maximum stress was 90% of yield stress, the fatigue strength at RH=80% was mostly equal to that at room humidity (approximately RH=30%). However, the great difference in the fatigue strength was observed between the specimen at room humidity and that at RH=80% when the maximum stress was less than 80% of yield stress. The number of cycles to failure at RH=80% were increased approximately two times compared with that at room humidity. Stage II was defined as the main crack stable propagation region and the shape of fracture surface at stage II at RH=80% was different from that at room humidity. Multiple crack initiation sites at room humidity were observed on the η phase or a Zn-Fe alloy layer/η phase interface. Those crack initiation sites can be distinguished individually. They were existed some fracture surfaces of stage II, and these shapes were all crescent. On the other hand, there were multiple crack initiation sites at RH=80%, and every crack initiation site was located on the top of the surface. The location of crack initiation sites existed close to each other. Although some crack initiation sites were observed, those were existed in a fracture surface of stage II. The shape was elliptical and was the same with that of S45C steel. This suggests that those cracks coalesced with each other in the process of multiple cracks growing, and finally a fracture surface of stage II was formed. The oxygen contents of oxide formed on the fracture surface when the maximum stress was less than 80% of yield stress showed a larger value at RH=80% than that at room humidity. We consider that the galvanized layer in high-humidity corrosive atmosphere had been hardened by the oxide layer and resulted in the increase of fatigue strength.