Investigating Iron Corrosion with In Situ Liquid TEM

Corrosion is the environmental degradation of metallic structures over time due to an electrochemical reaction on the surface. Whilst it starts at the surface, it can often get deep into the structure locally, leading to catastrophic structural failures. Understanding corrosion process at the most fundamental level is of great scientific and commercial interest, as it underpins novel and efficient corrosion mitigation approaches and reliable prediction models. Despite the importance of early stages of the phenomena, earlier works have been largely limited to speculation based on ex-situ studies of iron and steel pieces, which are unable to capture many subtle sub-stages of the process. Moreover, removal of samples from the native state necessarily results in quenching/transformation before characterisation. It is important to understand the nucleation stage of the dynamic process under 'real conditions', through in-situ techniques. &lt;div&gt; Here, we demonstrate the use of liquid TEM to study iron corrosion in sodium chloride solution, using a commercially available liquid holder. Two silicon nitride chips is assembled with a spacer in-between, and clamped in a metal holder. Microfluidic connections allow liquid flow, and the spacer sets the liquid layer thickness in the beam path. The sample is a thin film of iron (30-50 nm) deposited, via electron beam evaporation, on one of the silicon nitride membrane chips, before assembly. The sample is imaged in a FEI Philips Tecnai 20 LaB6 TEM. A combination of bright and dark field imaging, and selected area electron diffraction is employed to capture the early reaction stages.<br/>&lt;/div&gt; &lt;div&gt; We address the two key processes – iron dissolution in solution and iron oxide nucleation process. The nature of the nucleation process (homogenous or heterogeneous), corrosion precursor (amorphous or crystallinity) and its evolution is investigated. We find that nucleation initiates on the metal surface, with preferential growth along the grain-boundaries of the iron film (quantified). We also identify the early-stage nuclei to be crystalline, characterised by its diffraction pattern. On the basis of our findings, we present a primary mechanism for nucleation of iron oxides on a reactive iron surface.&lt;/div&gt; &lt;div&gt; &lt;/div&gt;