Local electrochemical characteristics of pure iron under a saline droplet I: Effect of droplet size on electrochemical distribution

Abstract

Despite historically being the focus of much research and measurement development, atmospheric corrosion under saline droplets and its underlying mechanisms continues to require more accurate investigations. Toward this goal, the electrochemical distributions of pure iron under different volumes of 3.5% NaCl droplets at different elapsed times were investigated using newly developed concentric three-electrode array measurements. The results showed that the electrochemical distribution under a droplet was directly affected by the droplet size. Distributions of corrosion potential and galvanic current on the CTEA under a 110 μL droplet were circumferentially symmetric; whereas, the electrochemical distribution under a 25 μL droplet was characterized by a neighboring peak-valley pair, and that of a 340 μL droplet exhibited multiple adjacent peaks and valleys. The asymmetric electrochemical distributions under droplets with volumes of 25 μL and 340 μL can be attributed to the difference between the droplet sizes and the effective influencing distance of the corrosion couple, which is determined by the corrosion potential difference between the anode and cathode, the galvanic current, and the anodic and cathodic polarizabilities. The standard deviations of galvanic current trend indicated a greater tendency towards localized corrosion under the 110 μL droplet after a coupling period of 60 min. The results suggested that a droplet with the proper size and electrochemical distribution symmetry is more likely to initiate and develop local atmospheric corrosion.