Revealing How Alkali Cations Affect the Surface Reactivity of Stainless Steel in Alkaline Aqueous Environments

Abstract

Stainless steel is a ubiquitous structural material and one that finds extensive use in core-internal components in nuclear power plants. Stainless steel features superior corrosion resistance (e.g., as compared to ordinary steel) due to the formation of passivating iron and/or chromium oxides on its surfaces. However, the breakdown of such passivating oxide films, e.g., due to localized deformation and slip line formation following exposure to radiation, or aggressive ions renders stainless steel susceptible to corrosion-related degradation. Herein, the effects of alkali cations (i.e., K⁺, Li⁺) and the interactions between the passivated steel surface and the solution are examined using 304L stainless steel. Scanning electrochemical microscopy and atomic force microscopy are used to examine the inert-to-reactive transition of the steel surface both in the native state and in the presence of applied potentials. Careful analysis of interaction forces, in solution, within ≤10 nm of the steel surface, reveals that the interaction between the hydrated alkali cations and the substrate affects the structure of the electrical double layer (EDL). As a result, a higher surface reactivity is indicated in the presence of Li⁺ relative to K⁺ due to the effects of the former species in disrupting the EDL. These findings provide new insights into the role of the water chemistry not only on affecting metallic corrosion but also in other applications, such as batteries and electrochemical devices.

Figure 1

Representative force–distance (approach) curves of a 304L stainless steel substrate interacting with a silicon nitride lever (SNL-C) probe (tip diameter ≈ 12 nm) at 23 ± 3 °C in DI water, 10 mM KOH, and 10 mM LiOH solutions. The calculated Debye length (λ_D) in the 10 mM solutions is around 3 nm. In water, attraction between the negatively charged AFM tip and the positively charged steel surface occurs within separation distances of ≤8 nm. In KOH, both attraction and repulsion act on the AFM tip at separation distances of ≤5 nm. However, at surface separations >5 nm, repulsive forces are dominant due to both hydration forces and the abundance of ^–OH ions on steel surfaces. In LiOH, repulsive forces dominate both within and beyond 5 nm from the surface. This greater degree of repulsion of the AFM probe is consistent with the higher energy barrier to nanoscale structures presented by the layer of hydrated Li⁺ ions at the (steel–solution) interface.

Figure 2

Representative illustrations of structures of: (a) bulk water, (b) strongly hydrated 4-coordinate Li⁺ ions (kosmotrope), and (c) weakly hydrated 7-coordinate K⁺ ion (chaotrope). This illustration is representative only and not drawn to scale.

Figure 3

Illustration of the structure of the electrical double layer (EDL) that forms on an (oxidized) steel surface in LiOH solutions. The dehydration of Li⁺ ions results in the sorption of ^–OH species from the steel substrate and from adsorbed H₂O molecules. These actions lead to: surface dealkalization (i.e., due to enrichment of H⁺ ions at the steel–solution interface) and transport of the liberated (dissolved) Fe-species into the bulk solution. The resulting force–distance curves can be explained as follows: (1) the repulsion of the negatively charged silicon nitride tip within a few nanometers from the steel surface occurs due to the strong hydration forces surrounding the Li⁺ ions and an abundance of ^–OH ions, which shield the positively charged metal surface. (2) When the AFM probe touches the surface of the steel substrate, a repulsive contact force is observed due to the overlapping molecular orbitals of the tip and substrate. Therefore, (1) and (2) result in an overall (net) state of repulsive interaction.

Figure 4

Illustration of the structure of the electrical double layer (EDL) that forms on an (oxidized) steel surface in KOH solutions. The force–distance curves can be explained as follows: (1) repulsion of the silicon nitride probe is observed at tip–substrate separation distances >5 nm due to hydration forces presented by the hydrated K⁺ ions. These hydration forces are, however, weaker than those observed in hydrated Li⁺ ions. (2) Both attractive and repulsive forces act on the negatively charged SNL probe at distances of ≤5 nm from the steel surface because of interactions with the weakly hydrated, positively charged K⁺ ions, and van der Waals forces. (3) When the tip of the AFM probe touches the substrate, a repulsive force is observed.

Figure 5

Representative Tafel plots generated via potentiodynamic polarization of 304L stainless steel in 10 mM LiOH (red curve) and 10 mM KOH (dotted blue curve) solutions. The corrosion potential (E_corr) and corrosion current density (i_corr) were determined from the intersection of the extrapolated anodic and cathodic segments, which are shown by the dashed blue and red lines, respectively.

Figure 6

Evolution of the tip current measured using a 10 μm Pt ultramicroelectrode on the surface of 304L stainless steel immersed in 100 mM hydroxide solutions. The initial (and terminal, i.e., t = 75 min) currents measured in LiOH solutions are substantially higher than in KOH solutions, suggesting that a more inert surface persists in the latter case.

Figure 7

Representative plot of the tip current as a function of the applied potential. The open circuit potential (OCP) is represented by the dashed line. Herein, potentials were applied in the following order: (1) OCP (−0.4 V), (2) −0.3 V, (3) −0.5 V, (4) −0.2 V, (5) −0.6 V, (6) −0.1 V, (7) 0 V, (8) 0.1 V, (9) 0.2 V. Each applied potential was held for 5 min or until the rate of change in the tip current as a function of time (dI/dt) was less than 1.0 × 10^–11 A/min indicating near-equilibrium conditions. It is seen that in the presence of increasingly positive applied potentials, the tip current exponentially decayed to a limiting value in KOH, suggesting inhibited surface reaction kinetics.