Observation of natural convection and particle ejection from stainless steel single pits

Single pits were visually and electrochemically followed during their growth on a S41500 martensitic SS microelectrode. Solid particles were observed ejecting from the growing pit and after extraction, EDX determined they were mostly iron-based hydroxides/oxides. These particles were carried by fluid flow originating from the pit at a velocity of 21 μm/s. Finite element modeling confirmed the observed flow speeds are achievable by natural convection due to metal ion density gradients. While the pit solution does not experience flow, the bulk solution pH was observed to decrease illustrating how natural convection affects the bulk solution chemistry. • Solid particles are ejected from growing single pits. • Particles illustrate substantial fluid flow originating from single growing pit. • Flow is natural convection from density change between local metal ion concentrations. • Natural convection changes bulk solution chemistry, illustrated with pH decrease. • FEM simulations indicate convection only occurs outside the pit.