Hydrogen could form in the corrosion reaction and sometimes causes hydrogen-related deterioration of steel. Spatial distribution of hydrogen penetration in steels has been observed using a photocurrent mapping method. In this method, hydrogen penetrated from the backside of the steel sheet formed by the cathodic polarization or corrosion reaction causes an enhancement of photocurrent intensity under the photo-irradiation to the passive film in the anodic biased condition. This photocurrent enhancement may be provided by the photo-generated holes that oxidize hydrogen atoms efficiently on the passive film surface. This work conducted hydrogen penetration mapping using a photo-charge mapping method newly developed for rapid mapping measurement. In this method, a polarization current during single UV-light pulse irradiation was integrated to obtain the charge Q_a, and then a charge Q_b during the irradiation was subtracted to obtain a photo-charge Q_PC = Q_a-Q_b for a single pulse. This process was performed by the electronics circuit controlled by the computer. Combination of pulse system and X-Y galvano-mirror-scanning device for a light beam emitted from pulse UV (405 nm, 0.2 W) pulse laser, rapid photocharge mapping was achieved, e.g., 24 s for 40 x 40 = 1600 pixel map.

In the experiments, a Devanathan-type cell composed of a top cell of a humidity control room and a bottom electrochemical cell was constructed. A UV laser beam was induced to the bottom cell via a quartz window to scan the bottom iron surface. Fig. 1 shows an example of the time-transition of photocharge maps measured for an iron sheet. In the experiment, a droplet of NaCl solution was put on the surface, and the humidity of the top cell was changed to wet/dry/wet conditions. In the figure, a droplet grew with absorbing moisture at 48 ks, rust expanded and precipitated in the droplet at 131 ks, dried to form rust at 668 ks, and soaked again at 534 ks. In this process, penetration of hydrogen was observed at the bottom face. Initially, hydrogen appeared only at the center of the droplet and then expanded to the lateral direction by increasing the photocharge.

Fig. 1. Time-transition of top images of an iron sheet (0.1 mmt) with a single NaCl solution droplet and photocharge maps measured for the bottom side of the iron sheet polarized at 0.7 V _{vs. Ag/AgCl} in pH8.4 borate solution.