A scanning Kelvin probe investigation of the factors influencing secondary cathodic spreading of chloride-containing water droplets on metallic surfaces

摘要

It is well known that during the atmospheric corrosion of certain metals such as iron and zinc, adifferential aeration cell becomes established within a water droplet, giving rise to the evolution of asecondary spreading phenomenon beyond its original physical boundaries. The secondary zone isproduced by cathodic oxygen reduction originating at the perimeter of the droplet, which in turndraws sodium ions and associated water from the droplet bulk. Thus, a thin electrolyte layercomposed of dilute aqueous sodium hydroxide surrounds the primary droplet and grows radiallywith time. Here a combination of in-situ, height-regulated Scanning Kelvin probe (SKP)potentiometry and time-lapse photography is used to characterise localised corrosion occurring ona range of pure metal and alloy surfaces in the presence of sodium chloride (NaCl) droplets. Thesurfaces under investigation include iron, carbon steel, Fe-Cr alloys of differing Cr content, nickel,Ni-electroplated steel, aluminium, copper, zinc, brass, and magnesium. The principal aim is toidentify surfaces which sustain secondary cathodic spreading and, in instances where thephenomenon is observed, to characterise the associated localised corrosion cell potentials alongwith the kinetics of propagation. Following initiation of atmospheric using water droplets of knownvolume and concentration, secondary cathodic spreading around the main body of corrosivesolution is studied at fixed relative humidity levels maintained by means of electrolyte reservoirs ofthe same composition as the initiating droplet. For passive surfaces, including stainless steel,nickel and pure aluminium there is no evidence of secondary spreading under circumstances wherepassivity in maintained within the droplet. However, under conditions where passivity breakdown isinduced within the droplet, leading to pit initiation and propagation, then secondary spreading isobserved, where expansion of the secondary zone radius is shown to be proportional to the squareroot of holding time. This is in line with previously published observations on kinetics, confirmingthat the process is migrationally controlled. The phenomenon is not observed for magnesiumchloride containing droplets of similar concentration, confirming that the cation concerned must becapable of transporting charge into a zone of increased alkalinity. For reactive metals such aszinc, brass, copper and iron it is shown that the physical shape changes considerably over timeand loses a significant proportion of its volume within a typical 48h holding time at fixed RH. It isproposed that this change in droplet volume observed stems from a combination of Na~+ removalfrom the droplet into the secondary spreading zone coupled with Cl~- entrapment within insolublecorrosion product at the droplet centre. The implications of these observations will be discussed interms of secondary spreading limiting, rather than accelerating atmospheric corrosion.