Abstract
Atomic force microscopes (AFMs) are capable of high-resolution mapping of structures and the measurement of mechanical properties on nanometre scales within gaseous, liquid and vacuum environments. The contact mode high-speed AFM (HS-AFM) developed at Bristol Nano Dynamics Ltd. operates at speeds that are orders of magnitude faster than conventional AFMs, and is capable of capturing multiple frames per second. This allows for direct observation of dynamic events in real-time, with nanometre lateral resolution and subatomic height resolution. HS-AFM is a valuable tool for the imaging of nanoscale corrosion initiation events, such as metastable pitting, grain boundary (GB) dissolution and short crack formation during stress corrosion cracking (SCC). Within this study HS-AFM was combined with SEM and FIB milling to produce a multifaceted picture of localised corrosion events occurring on thermally sensitised AISI 304 stainless steel in an aqueous solution of 1% sodium chloride (NaCl). HS-AFM measurements were performed in situ by imaging within a custom built liquid cell with parallel electrochemical control. The high resolution of the HS-AFM allowed for measurements to be performed at individual reaction sites, i.e. at specific GB carbide surfaces. Topographic maps of the sample surface allowed for accurate measurements of the dimensions of pits formed. Using these measurements it was possible to calculate, and subsequently model, the volumes of metal reacting with respect to time, and so the current densities and ionic fluxes at work. In this manner, the local electrochemistry at nanoscale reaction sites may be reconstructed.
Highlights
Within this study, localised corrosion events were investigated occurring on thermally sensitised AISI 304 stainless steel in an aqueous solution of 1% NaCl
Post-corrosion analysis of the surface using scanning electron microscopy (SEM) revealed that these pits were a result of severe intergranular attack (IGA) working in combination with intragranular pitting events
Intragranular pits present across the sample surface were round in appearance rather than the elongated shape observed for intergranular pits
Summary
PaperStainless steels are important for structural applications and are widely used in the nuclear power industry due to their good mechanical properties and tolerance of elevated temperatures.[1,2,3] They are, susceptible to corrosion, which is the cause of a signi cant number of failure events in the nuclear sector.[2,4,5] Methods to detect and model the effects of corrosion are important for the prevention of these failure events.Corrosion is an irreversible process that causes material deterioration as a result of dissolution.[6,7] It is an electrochemical process in which a potential difference across the material/environment interface results in the ow of ions and electrons through an electrolyte.[6,7] The process of metallic corrosion is a conglomerate of anodic dissolution of the metal:.
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