Passive film conductivity

Fig. 1a illustrates the CSAFM map with 0.8 V applied tip bias of passive film formed before hydrogen charging. CSAFM measurements were conducted in the air with 328 nN tip normal load to obtain a clear current map image, prevent film fracture and limit electron tunneling effects.17,18 The ferrite and austenite phases can be identified in the CSAFM current maps based on our previous work.17,18,19 In Fig. 1a, the passive film on grain boundaries exhibits much higher conductivity (brighter color) than passive film on either the ferrite or austenite grains. This is because the grain boundaries act as hydrogen trap sites, thus affecting interfacial processes occurring at the material’s surface8 and creating unfavorable conditions for the development of stable, insulating passive film. Moreover, the passive film on austenite grains shows higher conductivity (brighter color) than on ferrite grains (darker color), which means that passive film on ferrite grains is more insulating (more stable) than on austenite grains. The current measured during CSAFM is negatively correlated with the corrosion resistance of the scanned area. It can be said that areas of the passive film with high CSAFM-measured current also have high pitting susceptibility. The low conductivity on the ferrite grains is attributed to enriched chromium oxides in the passive film formed in these areas.20 Current spikes in Fig. 1a are related to nanoscale defects present in the passive film. Figure 1b shows an optical image of pit initiation and propagation after hydrogen charging and FeCl 3 exposure. More hydrogen is adsorbed in the austenite grains than in the ferrite grains due to hydrogen having a higher solubility and a lower diffusivity in the face-centered cubic austenite phase compared with the body-centered cubic ferrite phase.21,22 It can be concluded that hydrogen facilitates pitting corrosion initiation at grain boundaries and austenite grains, i.e. hydrogen makes the passive film less stable and more conductive.

Fig. 1 a Current-sensing atomic force microscopy 3D current map of the air-formed passive film with high current sites at grain boundaries and the austenite phase; b Optical image showing pitting corrosion after hydrogen charging and FeCl 3 exposure Full size image

It is generally accepted that pitting is a result of the localized breakdown of the passive film, which usually occurs at specific sites of low film stability.23 In our previous research,24 pitting of a hydrogen-charged specimen exposed to a 6% FeCl 3 solution initially occurred at the grain boundaries or at the austenite grains, and then appeared at the ferrite grains. Therefore, areas with poorly insulating passive film, such as grain boundaries and austenite grains, can be considered more susceptible to pitting corrosion. Pitting nucleation occurs at grain boundaries because these areas gather hydrogen preferentially, which causes the passive film to easily break down. Meanwhile, the vast majority of hydrogen gathers in the austenite grains, which can accelerate the breakdown of the passive film and cause pitting nucleation in this area. However, under the same hydrogen charging conditions, less hydrogen gathers in the ferrite grains, so pitting nucleation appears later here compared to other areas.

Passive film current density

Combining the contact area calculated by the Hertzian elastic contact formula with the current output of the CSAFM, yields the average current density of passive film at the grain boundaries and austenite grains of 0.074 nA/nm2 and 0.003 nA/nm2, respectively. The maximum current density at the grain boundaries and austenite grains is 0.109 nA/nm2 and 0.077 nA/nm2, respectively. Current flow is negligible on the ferrite grains, corresponding to black regions in Fig. 1a. Thus, it can also be deduced that pitting corrosion will preferentially begin at the grain boundaries or in the austenite phase of 2507 duplex stainless steel, which matches our previous results.21,24,25