Abstract
Recently, damage caused by liquid droplet impingement erosion (LDIE) in addition to flow-accelerated corrosion (FAC) has frequently occurred in the secondary side steam piping of nuclear power plants, and the damage-occurring frequency is expected to increase as their operating years’ increase. In order to scrutinize its causes, therefore, an experimental study was conducted to understand how the behavior of LDIE-FAC multiple degradation changes when the piping of nuclear power plants is operated for a long time. Experimental results show that more magnetite was formed on the surface of the carbon steel specimen than on the low-alloy steel specimen, and that the rate of magnetite formation and extinction reached equilibrium due to the complex action of liquid droplet impingement erosion and flow-accelerated corrosion after a certain period of time. Furthermore, it was confirmed at the beginning of the experiment that A106 Gr.B specimen has more mass loss than A335 P22 specimen. After a certain period of time, however, the mass loss tends to be the opposite. This is presumed to have resulted from the magnetite formed on the surface playing a role in suppressing liquid droplet impingement erosion. In addition, it was confirmed that the amount of erosion linearly increases under the conditions in which the formation and extinction of magnetite reach equilibrium.
Highlights
An experiment was performed under the same conditions of forming the magnetite oxide film as the inside surface of the secondary side piping of nuclear power plants, and it was confirmed that the LDIE degradation occurring on the pipe surface of the secondary side of nuclear power plants is caused by multiple degradation of flow-accelerated corrosion by the flow of water film and liquid droplet impingement erosion by droplets colliding on the water film
The reason why this phenomenon appeared in A106 Gr.B specimen later than A335 P22 specimen is estimated to be that the magnetite formed on the surface played a protecting role from being damaged by LDIE
It was confirmed that the surface shape and the degree of magnetite formation of A335 P22 specimen were similar at 400 hours and 500 hours
Summary
A lot of damage has occurred due to liquid droplet impingement erosion even in the secondary side steam piping of nuclear power plants. M. Hwang et al, an experiment was performed under the same conditions of forming the magnetite oxide film as the inside surface of the secondary side piping of nuclear power plants, and it was confirmed that the LDIE degradation occurring on the pipe surface of the secondary side of nuclear power plants is caused by multiple degradation of flow-accelerated corrosion by the flow of water film and liquid droplet impingement erosion by droplets colliding on the water film. It was identified that the material loss was large in carbon steel at the beginning of the experiment, but the material loss occurred more severely in low alloy steel after a certain period of time These findings were reflected in the LDIE prediction theory of the ToSPACE program developed by KEPCO E&C [6]. The experiment was conducted under the same conditions as the previous experiment except for the water chemistry control facility
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