2016 Volume 56 Issue 3 Pages 495-497
A small Devanathan-Stachurski cell was newly designed to ascertain the hydrogen absorption behavior into re-sulfurized carbon steel during pitting corrosion. In 0.1 M NaCl (pH 5.5), a pit was generated on a small area with MnS inclusions, the current of hydrogen permeation current increased sharply when the solution (capillary) was placed on the steel surface, and the current density returned to the background after the capillary was removed. On the other hand, no pitting was observed on the area without the inclusions, and no permeation current was also observed. The time variation of the permeation current density was measured in 0.1 M NaCl (pH 2.0) and boric-borate buffer with 1 mM NaCl (pH 5.5) solutions. The permeation current from a small area with MnS inclusions was large compared with that of an area without the inclusions. After the experiments, a deep pit was generated on the surface with the inclusions. MnS inclusions were likely to accelerate the steel dissolution and the hydrogen absorption into the steel.
Carbon steels are well known for their susceptibility to hydrogen embrittlement. The Devanathan-Stachurski cell1) has been successfully used for the study of the hydrogen absorption. The surface concentration and diffusion rate of hydrogen can be obtained quantitatively through the permeation current. The hydrogen absorption rate is affected by the existence of the hydrogen sulfide species (H2S, HS−, and S2−).2) These species are thought to be released from MnS inclusions in corrosion environments.3,4) MnS inclusion is also known as an initiation site for pitting corrosion of steels. A microelectrochemical cell is suitable for understanding the mechanisms of pitting at the inclusions. In this study, to ascertain the hydrogen absorption behavior of steel during pitting at MnS inclusions, a small Devanathan-Stachurski cell was designed, and its applicability was verified.
AISI 1145 re-sulfurized carbon steel was used (Table 1). The typical length of MnS inclusions was around 20 μm. The front and back surfaces of the specimen (1 mm in thickness) were mechanically polished down to 1 μm diamond paste. Pitting corrosion tests were performed in a 0.1 M NaCl solution and a borate buffer solution (pH 5.5, a mixture of 0.3 M H3BO3 and 0.075 M Na2B4O7) with 1 mM NaCl. The pHs of the 0.1 M NaCl solution was were adjusted to 5.5 and 2.0 with HCl. All measurements were conducted at 298 K. On the hydrogen detection side of the specimen, Pd-plating was employed. A thin Pd film, with ca. 100 nm thickness, was electrochemically deposited. After the electrochemical plating, an additional Pd film (ca. 100 nm) was plated by means of vapor deposition, since the inclusion surfaces were found not to be covered with the electroplating layer.
| C | Si | Mn | P | S | Cu | Ni | Cr | Al | Ti | N |
|---|---|---|---|---|---|---|---|---|---|---|
| 0.46 | 0.19 | 0.8 | 0.011 | 0.052 | 0.01 | 0.02 | 0.13 | 0.035 | 0.001 | 0.007 |
Figure 1 shows the schematic illustration of the small Devanathan-Stachurski cell newly designed in this study. The upper surface of the specimen was the hydrogen entry side, and the lower one was used for hydrogen detection. To generate a pit in the small area, a microelectrochemical cell composed of a single glass tube was used.4) The electrode area of the micro-capillary was determined to be 1.3×10−8 m2 (approximately 130 μm in diameter).
Schematic illustration of hydrogen permeation cell.
On the hydrogen detection side, an acrylic block with a small hole (3.0 mm in diameter) was adhered with an epoxy resin, and then a polyether ether ketone (PEEK) T-joint was assembled. A small Ag/AgCl electrode filled with a saturated KCl solution was connected to the T-joint. Potentials cited in this paper were given versus the standard hydrogen electrode (SHE). The electrolyte of the hydrogen detection side was a deaerated 0.1 M NaOH solution. The Pd-coated surface was polarized at 0.1 V. To maintain well-deaerated conditions, the deaerated NaOH solution flowed at a rate of 0.1 mL min−1 during the experiments. Once the background current reached a steady-state, the capillary (solution) was attached on the specimen to generate pitting at open-circuit potential (OCP).
At first, to confirm pitting causes hydrogen absorption, the hydrogen permeation behaviors were compared between non-corroded and corroded specimens. Figure 2 shows the time variation of the permeation current density for the steel exposed to the naturally aerated 0.1 M NaCl (pH 5.5) solution at OCP, and Fig. 3 exhibits the optical micrographs of the solution contact areas on the hydrogen entry sides. It is known that MnS inclusion acts as an initiation site for pitting; therefore, a pit was generated on the solution contact area with the MnS inclusion, however no pitting was observed on the area without the inclusion. As shown in Fig. 2(a) (the non-corroded specimen), no increase in the hydrogen permeation current was observed. For the corroded specimen (Fig. 2(b)), the permeation current increased sharply when the capillary (solution) was attached to the surface, and then the current gradually decreased with time gradually. Finally, the current density returned to the background after the capillary was removed. It was confirmed that pitting corrosion causes the hydrogen absorption.
Time variation of permeation current density for carbon steel exposed to to 0.1 M NaCl (pH 5.5). Solution contact areas without MnS (a) and with MnS (b).
Optical micrographs of steel surfaces. (a) Solution contact area without MnS after exposed to 0.1 M NaCl (pH 5.5), (b, c) solution contact areas with MnS before and after exposed to 0.1 M NaCl (pH 5.5).
The same experiments were performed in the naturally aerated 0.1 M NaCl (pH 2.0). Figure 4 shows the comparison of the permeation current density and the optical micrographs of the solution contact areas on the hydrogen entry side. Corrosion occurred at OCP even in the area without the inclusion since the pH value of 2.0 was lower than the de-passivation pH of the steel. The permeation current increased sharply when the capillary was attached to the steel, however the current density decreased within 100 s, though the solution was still placed on the surface. It was suggested that the steel dissolution of the entry side was stopped immediately due to repassivation. On the other hand, for the area with the inclusions, the permeation current density of around 40 nA cm−2 was detected measured at the steady state. The hydrogen permeation current of the area with the inclusions was larger than that of area without the inclusions. In the acidic solution, the deep pit was also observed after the experiment (Fig. 4(b)). The inclusions were likely to accelerate the steel dissolution and hydrogen permeation.
Time variation of permeation current density for carbon steel and optical micrographs of steel surfaces exposed to 0.1 M NaCl (pH 2.0). Solution contact areas without MnS (a) and with MnS (b).
The naturally aerated boric-borate buffer solution at pH 5.5 was also used to ascertain the effect of the inclusions on hydrogen permeation. Figure 5 presents the time variation of the permeation current and the corrosion potential of the hydrogen entry side. The decrease in corrosion potential was observed for both the areas with and without MnS inclusion; however, the hydrogen permeation current was detected measured only for the area with the inclusion. Those results suggests that the MnS inclusions accelerate the hydrogen absorption into the steel.4) As seen in Fig. 2(a), no corrosion occurred on the area without the MnS inclusion in 0.1 M NaCl at pH 5.5. However, pitting was generated in the boric-borate buffer with 1 mM NaCl at pH 5.5. The cause of this difference is unclear. As shown in this study, the microelectrochemical approach appears to be a promising and useful technique to understand the hydrogen absorption into steel during corrosion
Time variation of permeation current density and corrosion potential for carbon steel exposed to 0.3 M H3BO3 −0.075 M Na2B4O7 solution (pH 5.5) with 1 mM NaCl. Solution contact areas without MnS (a) and with MnS (b). Inserted are SEM images of solution contact areas after exposure.
(1) A small Devanathan-Stachurski cell was newly designed to ascertain the hydrogen absorption behavior of steel during corrosion. It was confirmed that pitting corrosion causes the hydrogen absorption into the steel.
(2) The MnS inclusions were likely to accelerate the steel dissolution, and also the inclusion accelerated the hydrogen absorption.
This work was supported by JSPS KAKENHI Grant Number 25630319. This research was supported by Japan Society and Technology Agency (JST) under Collaborative Research Based on Industrial Demand “Heterogeneous Structure Control: Towards Innovation Development of Metallic Structural Materials.”
