Detection of Hydrogen Permeated Under Atmospheric Corrosive Environment By Surface Potential Measurement
Authors: Katsumura Toshiki , Katayama Hideki , Akashi Takaya 
Published: February 2020

Higher strength steels have been used as some of car parts to make vehicle weight lighter and improve fuel efficiency. However, the higher strength of steels than 1200 MPa shows remarkable susceptibility to hydrogen embrittlement. Hydrogen embrittlement is one of serious problems for a fracture of steel materials. Therefore, it is important to understand the behavior of the hydrogen penetration and permeation caused by corrosion reaction. It is reported that the hydrogen embrittlement occurs even by a little amount of penetrated hydrogen under atmospheric corrosive environment. Some methods such as thermal desorption methods and electrochemical methods are suggested in order to evaluate the behavior of the hydrogen penetration and permeation. These methods can give the amount of permeated hydrogen. However, it is difficult to get the information for distribution of permeated hydrogen. The purpose of this paper is to investigate the possibility of visualization by surface potential measurement.
The surface potential was measured by the surface reaction measurement device having an X-Y-Z stage. The polished pure iron plates degreased with ethanol were used as samples. The thickness of the sample is approximately 0.5mm. One side of sample is for hydrogen entry and the other side is for detecting hydrogen permeation. The detective probe and X-Y-Z stage are installed to an acrylic case to control the environment around them. Relative humidity is thought to affect the value of surface potential, so the relationship between relative humidity and surface potential was examined using a relative humidity control system. The relative humidity can be controlled by the system from nearly 0% to 40%. The surface potential was measured at the center of the sample while varying the relative humidity from nearly 0% to 40%. The effect of exposure time to the acrylic case was also examined.
100μl droplet of 0.5 M NaCl solution was added on the sample surface to promote hydrogen entry by corrosion reaction. The surface potential distribution was measured during wetting and after drying. The measurement range was 20mm x 20mm, and the relative humidity in the acrylic case was controlled at nearly 0%.
The sample at 10% gave a lower surface potential than that at nearly 0%, and the surface potential hardly changed between 10% and 40%. It was reported that the surface potential shifted negatively by adsorption of molecular water on the sample surface. Except for the result at nearly 0%, the surface potential changed to a negative value with an increase of exposure time to the acrylic case. This is also attributed to the adsorption of water. These results suggest that the surface potential measurement should be conducted at nearly RH 0% to obtain an accurate value. In the measurement of the surface potential distribution change by corrosion reaction, the surface potential of the backside of sample decreased, and the negative potential area was spread with time. It is reported that when molecular hydrogen adsorbed on the surface of iron, the surface potential changed negatively. The surface potential distribution finally looked like the shape of the formed rust. These results imply that permeation of hydrogen produced by corrosion reaction at the entry side effects the surface potential and the distribution.