In recent years, high-strength steels have been widely used in many engineering fields such as automobiles and construction. However, hydrogen embrittlement (HE) of high-strength steels has been a great concern because HE susceptibility of steels strongly relates to the strength of them. Especially, high-strength steels are often used in atmospheric corrosion environments and hydrogen atoms are generated by corrosion reaction. Some of the generated hydrogen atoms are absorbed into steel and they relate to occurrence of HE.

In atmospheric corrosion environments, steel corrosion proceeds with forming iron rust on the surface. The rust layer may affect hydrogen entry behavior into the steel. Therefore, it is needed to investigate the effect on rust layer on hydrogen entry behavior into steel. In this study, hydrogen entry behavior into steel exposed in an outdoor environment is discussed based on the results of hydrogen permeation current and corrosion potential.

Experimental

Hydrogen permeation current for steel exposed in an outdoor environment was measured by Devanathan-Stachurski method¹. A sheet of plain carbon steel (0.020%C, 0.01%Si, 0.18%Mn, 0.015%P, 0.007%S, bal. Fe) was used as a material. The steel sheet was exposed at Choshi exposure test and research center (Chiba, Japan). The exposure test started at August, 2016 and the exposure period was changed from 1 to 6 months. After the exposure test, the steel sheet was cut into a small coupon of 25 mm^w x 25 mm^l x 0.8 mm^t. The exposed steel surface was washed with Milli-Q water (18 MΩ•cm). One side of the steel surface facing to the air was used as a hydrogen-entry side and the opposite surface of the steel was used as a hydrogen-withdrawal side for hydrogen permeation tests. The hydrogen-withdrawal side was polished and electroplated with Pd ca. 400 nm in thickness before hydrogen permeation test.

A 30 μL of Milli-Q water was placed on steel to measure hydrogen permeation current. During drying of the water droplet, corrosion potential for the steel was measured with a Kelvin probe simultaneously with hydrogen permeation current.

Results and Discussion

In this study, hydrogen entry into steel exposed in an outdoor environment was investigated with hydrogen permeation current and corrosion potential measurements. It was found from the results of hydrogen permeation current and corrosion potential that hydrogen entry into steel after the exposure test was observed during drying of a water droplet. Therefore, hydrogen absorption reaction into the exposed steels occurs with corrosion.

Figure shows change of the amount of hydrogen entry for the exposed steels in the test environment. The amount of absorbed hydrogen into steel was calculated by time-integral of hydrogen permeation current during drying of the water droplet. The plots at 0 month are obtained from the hydrogen permeation data for a bare steel. As shown in the figure, the amount of hydrogen entry into the exposed steel decreases significantly after one-month exposure and then it keeps almost constant independent of exposure period. After one-month exposure, the steel surface was wholly covered with iron rust. These results indicate that the formation of iron rust affected the amount of hydrogen entry into steel.

Reference

1. M. A.V. Devanathan, A. Stachurski, Proc. Roy. Soc., A270, 910 (1962).