(Invited) Effect of Rust on Hydrogen Uptake into Exposed Steels during Drying of NaCl Droplets
Authors: Tada Eiji , Ajito Saya , Ooi Azusa , Nishikata Atsushi 
Published: December 2020

In recent years, high-strength steels have been widely used in many industrial fields such as automobiles and construction. However, hydrogen embrittlement (HE) of high-strength steels has been a great concern because HE susceptibility of the steels strongly relates to the strength of them. Since the steels are often used in atmospheric environments, hydrogen atoms generated by corrosion reaction are absorbed into steels and can cause HE. Therefore, to use the high strength steel safety, it is needed to clarify hydrogen absorption mechanism into steels in atmospheric environments. However, in atmospheric environments, corrosion reaction is considered to occur under wet/dry cyclic conditions. Especially, since corrosion product layers consisting of iron rust is formed during steel corrosion, they can affect the rates of both corrosion reaction and hydrogen uptake. In this study, the effect of rust on hydrogen uptake into steels is discussed based on the results of hydrogen permeation current and corrosion potential.
A sheet of plain carbon steel (0.020 % C, 0.01 % Si, 0.018 % Mn, 0.015 % P, 0.007 % S, Bal. Fe) was used as a material. The size of steel sheet was 150 mmw x 75 mml x 0.8 mmt. Both surfaces of the sheet were ground with SiC papers down to P500 grit and then degreased. The steel sheet was exposed for 1 month at Choshi exposure test site of Japan weathering test center (Chiba, Japan). It was located about 4 km far from the coast.
The steel sheet after the exposure test was covered with iron rust completely. The sheet was cut into small coupons of 25 mmw x 25 mml x 0.8 mmt. One side of coupon surfaces working as hydrogen-detection side was ground with SiC papers down to P1200 grit to remove iron rust. Then this side was electroplated with Pd ca. 400 nm in thickness. For other side of the coupon surfaces for hydrogen-entry side, iron rust was removed partly. This side of the steel surfaces was subjected to a wet/dry corrosion test. A sample was finally prepared after both surfaces were cleaned with Milli-Q water (18 MW cm at 25 oC). The exposed surface area of the sample was fixed about 1 cm2 for both hydrogen-entry and hydrogen-detection sides by coating with rubber-type masking agent.
Hydrogen uptake behavior of rusted steel was investigated by Devanathan-Stachurski method [1]. A coupon of rusted steel was set in this cell. Test solution filled in the hydrogen-detection-cell was 0.1 M NaOH. In this cell, Pd-electroplated surface was polarized at a constant potential of +0.1 V vs. Ir wire. On the opposite side of steel surfaces for hydrogen-entry side, a 30-mL droplet of 10 mM NaCl was placed on the rusted steel to measure hydrogen permeation current. During the drying of the droplet, corrosion potential for the rusted steel coupon was measured with a home-made Kelvin probe (KP) simultaneously with hydrogen permeation current [2-4].
On the exposure test at Choshi, iron rust was formed on the whole steel surface. XRD and FT-IR analysis revealed that the iron rust consisted mainly of a- and g-FeOOH. In addition, amorphous rust may be involved in the rust.
Hydrogen uptake behavior of the rusted steel with various percentages of the rusted area was observed during the drying of NaCl droplet. The amount of hydrogen entry was calculated by integrating the transient of hydrogen permeation current during the drying. The amount of hydrogen entry, N
H depended on the percentage of rusted area were . For N
H of 0 % of rusted area a bare steel before exposure test was used. The N
H was increased with decreasing the percentage of rusted area. When the percentages of the rusted area were 90 and 98 %, corrosion potentials during drying of a NaCl droplet were more negative than that of 100 %. The change of corrosion potential may affect the increase of N
H.

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