Environmental factors that affect the entry of hydrogen into materials are briefly summarized with respect to electrochemical reactions on electrode. Effects of promoters, particularly of H2S, are reviewed concerned with their mechanism. The effects of localized corrosion, such as crevice corrosion and pitting corrosion, and atmospheric corrosion under wet/dry cycles on hydrogen entry are reviewed. Further, interactions between electrochemical reactions on the surface of electrode and plastic deformation, which appear on polarization during stressing, and vice versa, are noticed as phenomena that suggest some surface effects on internal dislocation dynamics.
Carbon steels are considered to be the most promising material for an overpack container used for the geological disposal of radioactive wastes. In order to know effective alloying elements that increase the corrosion resistance in the disposal environment, corrosion tests of a commercial carbon steel and several kinds of low alloy steels were performed in compressed bentonite containing a simulated solution of bentonite contact water with chloride at pH 8 to 13. The most effective alloying element in the solutions from pH 8 to 12 was Ni. The second effective one was Al. The addition of Ti, Cr, Si, Cu, or Nb slightly increased the corrosion resistance. The most harmful element that decreased the corrosion resistance was Mo. In the solution with pH 13, all the steels except Mo- and Si-containing steels were passivated and the difference in the effect of alloying elements was very small. An FeCO3 layer was formed on all the steels as a corrosion product in the solutions with pH 8 to 12.
The results of exposure test by using the test piece which simulates cathodically protected marine steel structure are reported. The exposure examination used the test piece which applied seawater resistant stainless steel from splash zone to the upper part of submerged zone. The value of cathodic current to submerged zone after five months was almost equal to carbon steel. But, cathodic current to seawater resistant stainless steel near high water level was smaller than the value of carbon steel. And, the cathodic current to the high water level of spring tide was larger than other parts.
Carbon steel was immersed in an aqueous solution of NaNO3 in a closed system for observing both the chemical interaction between metal and NO3-, and the effect of nitrate on the generation rate of H2 gas. The experimental pH range of the solution was 10.0-13.5 which corresponds to that of pore fluid in cementitious material. The cathodic current density showed a “Tafel equation type” potential dependency in aqueous solution containing NO3- or NO2-. In spite of the accelerated cathodic reaction due to the existence of nitrate, the corrosion rate of carbon steel was not accelerated in the nitrate solutions. This fact suggests that the system is controlled by the anodic reaction. The nitrate reduction accompanied by the corrosion of carbon steel is considered to be a series reaction such as NO3-→NO2-→NH3. The nitrate reduction reaction competes with the water reduction reaction (hydrogen evolution reaction) within the anodic controlled condition, therefore nitrate strongly reduces the hydrogen generation rate (1/100-1/500 of the cases without nitrate in 1.0 mol dm-3 NaNO3 cases). The generation rates of NH3 were independent of the concentration of NO3- over the range of 1.0×10-3∼1.0 mol dm-3.