It is well known that the susceptibility of hydrogen embrittlement of steels increases with increasing their strength. Because hydrogen induces the delayed fracture of high-strength steels, the usage of the steels is restricted. Hydrogen entry into the steel occurs due to the cathodic reaction of atmospheric corrosions, and it is strongly affected by non-uniformity of steel surfaces. Thus, it is important to clarify the distribution of hydrogen entry into steels. However, it is difficult to observe the distribution of hydrogen entry into steels under atmospheric corrosion conditions.

We studied the detection of hydrogen absorbed into steels by using a TiO₂ thin film. The TiO₂ thin film is formed in one side of the specimen surface, which is the hydrogen detection side. Hydrogen atoms are absorbed from the other side of the specimen surface, which is the hydrogen entry side, and diffused to the hydrogen detection side. If TiO₂ reacts with hydrogen atoms to form H_xTiO₂ in the hydrogen detection side, the distribution of hydrogen entry can be observed by the color change of TiO₂. The objective of this study is to detect absorbed hydrogen into the pure iron by TiO₂.

The specimens used in this study were pure irons (99.99 mass %), sheet with the thickness of 1 mm. The specimens were heat-treated under vacuum for 5 hour at 1173 K. After the heat-treatment, the specimens were polished by a diamond paste down to 1 μm and cleaned with acetone. Pd was electroplated on the hydrogen detection side of the specimens, and the TiO₂ thin film was formed on the Pd-electroplated layer by reactive magnetron sputtering. The hydrogen detection using TiO₂ was evaluated by the hydrogen permeation tests. The hydrogen entry side of the specimen was attached to the electrochemical cell in the hydrogen entry side, and polarized cathodically during the permeation test. The cathodic polarization was conducted for 10 hour at -0.5 V vs. SHE in a naturally aerated 0.1 M H₂SO₄solution at 298 K. The hydrogen detection side was observed by an inverted microscope throughout the test period.

Surface appearances of the specimen in the hydrogen detection side before and after the hydrogen permeation test are shown in Fig. 1(a) and (b). The color of TiO₂ changed to dark yellow ocher after the hydrogen permeation test. Surface appearance of the specimen in the hydrogen entry side after the hydrogen permeation test is shown in Fig. 1(c). The corrosion area in Fig. 1(c) corresponded to the polarized area. The polarized area was equivalent to the region colored dark yellow in Fig. 1(b). This shows that the change of the color was caused by the hydrogen entry. The color change of the TiO₂ thin film is thought to be due to the formation of H_xTiO₂.

RGB-values in the region A shown in Fig. 1(a) and (b) are measured to evaluate the time dependence of the color change of TiO₂ quantitatively. The time dependence of RGB-value changes of TiO₂ in the region A is shown in Figure. 1(d). The RGB-values of the TiO₂ thin film drastically decreased in the period from 0.5 to 3 hours after the start of the hydrogen permeation test. After that, the RGB-values approached to the constant values. The B-value showed the largest change among the RGB-values, and the B-value is suitable for the evaluation of the absorbed hydrogen detection by TiO₂.