Influence of ppb Levels of SO2 on the Atmospheric Corrosion of Aluminum in the Presence of NaCl

The effect of part per billion (ppb) levels of SO2 on the atmospheric corrosion on Al (99.999%) was investigated in the laboratory at 22 degrees C. Exposure time was 20 and 672 h. The effect of SO2 concentration (46-1300 ppb), relative humidity (70- 99% RH), and the amount of NaCl applied prior to exposure (0- 250 mu g/cm(2)) was studied. In addition, the effect of O-3 and NO2 on the SO2 deposition rate was addressed. The samples were investigated by gravimetry, grazing incidence X-ray powder diffraction, ion chromatography, optical microscopy, and environmental scanning electron microscopy equipped with energy-dispersive X-ray fluorescence. SO2 accelerates the atmospheric corrosion of Al, forming corrosion product "islands" containing aluminum hydroxy sulfate. Combining NaCl and SO2 has a synergistic effect on the average corrosion rate. In contrast, the chloride-induced pitting of aluminum is significantly reduced by SO2. The corrosivity of SO2 is attributed to the acidification of the surface electrolyte that destabilizes the alumina passive film. The suppression of pitting corrosion by SO2 is attributed to the formation of aluminum hydroxy sulfates that are less soluble than the corresponding chlorides and slows down pit propagation. At relative humidities above 70% RH, there is a measurable steady-state deposition of SO2 that corresponds to the rate of formation of sulfate. The SO2 deposition rate is not limited by the oxidation of (IV)-valent sulfur on the surface. SO2 deposition is enhanced by sodium chloride because the corrosion reactions in the NaCl(aq) electrolyte result in high pH in the cathodic regions and the formation of aluminum hydroxide. Ozone in the ppb range has only transient effects on the SO2 deposition rate, whereas ppb levels of NO2 had no effect at (c) 2005 The Electrochemical Society.