Abstract
Nanometer-thick passive films, which impart superior corrosion resistance to metals, are degraded in long-term service; they are also susceptible to chloride-induced localized attack. Here we show, by engineering crystallographic configurations upon metal matrices adjacent to their passive films, we obtain great enhancement of corrosion resistance of FeCr15Ni15 single crystal in sulphuric acid, with activation time up to two orders of magnitude longer than that of the non-engineered counterparts. Meanwhile, engineering crystallography decreases the passive current density and shifts the pitting potential to noble values. Applying anodic polarizations under a transpassivation potential, we make the metal matrices underneath the transpassive films highly uneven with {111}-terminated configurations, which is responsible for the enhancement of corrosion resistance. The transpassivation strategy also works in the commercial stainless steels where both grain interior and grain boundaries are rebuilt into the low-energy configurations. Our results demonstrate a technological implication in the pretreatment process of anti-corrosion engineering.
Highlights
Nanometer-thick passive films, which impart superior corrosion resistance to metals, are degraded in long-term service; they are susceptible to chloride-induced localized attack
In order to clarify the corrosion events occurring during passive film degradation in acid media, we conducted experiments with the passivated single-crystal specimens immersed in sulfuric acid (Supplementary Fig. 2) and closely monitored the structural evolution of the film, paying special attention to the Me/F interface
The high angle annular dark-field scanning transmission electron microscopic (HAADF-STEM) images of the passive films prior to and after the immersion in sulfuric acid, clearly reveal that the sharp, well-defined, and straight interface in the pristine passive film becomes undulating after the immersion in acid solution (Fig. 1 and Supplementary Fig. 3)
Summary
Nanometer-thick passive films, which impart superior corrosion resistance to metals, are degraded in long-term service; they are susceptible to chloride-induced localized attack. As early as the 1930s, it was found that an anodically-formed oxide film on unalloyed iron could be thinned or even completely eliminated in sulfuric acid; whereas, the same film could survive for considerably longer periods without dissolution when it was transferred from the metal to plastic support with no electrical contact with the metal[9,10]. The component of chromium oxide highly retards the reductive dissolution of the passive film and the Cr content in the passive film is well known to be correlated with the corrosion resistance. The iron oxide component of the film would suffer reductive dissolution, which is considered to be the most important process for the stability of stainless steel[13]. Developing novel strategies to mitigate the reductive dissolution of the iron oxide component in the passive film of stainless steel would impart improved corrosion resistance
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