Effect of Hot Dip Galvanizing on Metal Fatigue
Hot dip galvanizing is one of the surface treatments for corrosion control. Hot dip galvanizing is used in many fields because of its ease of application to large steel products. Galvanized steel is one of the most used materials in corrosive environments such as bridges and ships due to its low cost and excellent corrosion resistance. Although the production of galvanized steel decreased immediately after the Lehman Shock (2008), the Sasago Tunnel accident (2012) and the decision to host the Olympic Games have stimulated new construction and reconstruction of structures, and the production volume is on the rise. Galvanized steel, which is actively used in this way, is expected to be a material with high corrosion resistance. In the hot dip galvanizing process, the steel is immersed in a plating bath of 400 degrees Celsius or higher, and the strength of the metallic material changes significantly as a result of the plating process. In general, however, the tensile strength of the material tends to increase. However, in general, the tensile strength of the material tends to be higher, so the effect of the plating process on the metal fatigue strength of hot-dip galvanized steel has been little investigated, perhaps because it is expected that the fatigue strength will also be higher
The figure below shows the S-N curve, which is a plot of the maximum loading stress on the vertical axis and the number of fatigue deformations that break on the horizontal axis. The materials used are S45C steel wire rod with 100 µm thick hot-dip zinc plating (plated material) and wire rod as-is (as-steel material). Comparing the fatigue strength of each material, it can be seen that although there was no difference in fatigue strength between the plated and unplated materials in the low-cycle, high-stress fatigue test of 10^4 cycles or less, the fatigue strength of the plated material was significantly lower in the high-cycle, low-stress fatigue test. Therefore, it can be seen that the 100µm thick plated material causes a reduction in fatigue strength. In past studies, we have modeled the initial crack initiation mechanism of this zinc plating. Currently, we are investigating the change in fatigue strength of various steel materials after galvanizing and the factors that caused this change.
Fig. Effect of zinc coating layer on fatigue strength
As an advanced research, we are also conducting research on corrosion fatigue. Research on corrosion resistance of zinc coating has been conducted for many years due to its importance. On the other hand, there are only a few reports of research on corrosion fatigue. The mechanism of corrosion fatigue that has been pointed out in past studies is that the pitting caused by corrosion becomes a stress concentration area and becomes a main crack. The reason for the reduction in fatigue life is thought to be that the corrosion products formed on the small cracks in the corrosive environment inhibit the reattachment of the cracks, but this has not yet been clarified.
However, in order to understand the fracture formation mechanism of corrosion fatigue of zinc plating, it is necessary to clarify the phenomenon of fatigue first, and then to understand the corrosion fatigue phenomenon.