Hydrogen Entry Behavior into Pre-Rusted Steel in an Atmospheric Corrosion Environment

Abstract

Introduction In recent years, high-strength steels have been widely used in many engineering fields such as automobiles and construction. However hydrogen embrittlement has been a great concern because the susceptibility of the steels to hydrogen embrittlement is highly relevant with the strength of the steels. Especially, since high-strength steels are often used in atmospheric corrosion environments, hydrogen atoms generated by corrosion reaction can cause of hydrogen embrittlement. In addition, hydrogen absorption behavior would depend on the formation of rusts on steel. Therefore, it is needed to clarify hydrogen absorption mechanism into rusted steels in atmospheric corrosion environments. In this study, effect of rusts on hydrogen entry behavior of steels in an atmospheric corrosion environment is discussed based on the results of hydrogen permeation current and corrosion potential. Experimental In this study, in order to investigate the effect of rusts on the hydrogen entry behavior into steel, hydrogen permeation current for pre-rusted steel was measured by Devanathan-Stachurski method1. A sheet of plain carbon steel (0.018%C, 0.01%Si, 0.18%Mn, 0.017%P, 0.007%S, bal. Fe) was used as a material. The sheet was cut into a small coupon of 25 mmw x 35 mml x 0.7 mmt. The steel surface for a hydrogen-withdrawal side was electroplated with Pd ca. 400 nm in thickness. The other surface was rusted by applying a 30 μL of 100 mM NaCl droplet on its surface and exposed to wet-dry corrosion cycles. During the wet-dry corrosion cycles, relative humidity was changed repeatedly from 40 % to 95 % at a constant rate of 22.5 %/h and 25 oC. A 30 μL of Milli-Q water (18 Mω•cm) was placed on the pre-rusted steels to measure hydrogen permeation current. During drying of the water droplet, corrosion potential for the pre-rusted steel was measured with a Kelvin probe simultaneously with hydrogen permeation current. Results and Discussion Fig. 1 shows changes of corrosion potential and hydrogen permeation current for pre-rusted steels during drying of a water droplet. The water droplet was placed on the pre-rusted steel at 0 min in Fig. 1. As shown in Fig. 1, corrosion potential for the pre-rusted steel exposed to 3 wet-dry cycles keeps almost constant of ca. -0.7 V vs. SSE during the drying of the droplet and the evolution of hydrogen permeation current can be slightly observed. By contrast, in the case of 30 wet-dry cycles, corrosion potential is less noble and hydrogen permeation current becomes larger compared with the case of 3 wet-dry cycles. These results suggest that hydrogen entry behavior of the pre-rusted steels is related to the characteristics of the rusts after the exposure to the wet-dry corrosion cycles. Reference M. A.V. Devanathan, A. Stachurski, Proc. Roy. Soc., A270, 910(1962). Figure 1