Nanoscale Imaging of a Corrosion Reaction:  Sulfuric Acid Droplets on Aluminum Surfaces

Abstract

The morphological changes of the liquid and solid phases occurring during the corrosion of aluminum by sulfuric acid were imaged in situ with nanometer resolution using scanning polarization force microscopy and conventional atomic force microscopy. The rate of reaction between the acid and the metal is highly dependent on the relative humidity. At high relative humidities, a dramatic spreading of the liquid droplets is observed that is indicative of sulfuric acid-induced aluminum corrosion. The liquid layer consists of a mixture of aluminum sulfate product and sulfuric acid. At low relative humidity, an aluminum sulfate precipitate is formed which segregates from the unreacted liquid acid. This phase separation inhibits further corrosion, in agreement with the results of previous experiments using infrared reflection absorption spectroscopy. The results of these experiments offer new insights into the mechanisms of chemical corrosion at gas-liquid-solid interfaces.Atmospheric chemical corrosion constitutes a severe threat to the structural integrity of many metals and, as such, is responsible for considerable economical loss. 1-4 For example, sulfuric acid droplets from acid rain or dew deposition is the principal cause of corrosion of aluminum. Since atmospheric corrosion reactions occur at the solid-liquid interface and involve submicrometer thick liquid films and droplets, their study constitutes a difficult challenge. Optical imaging and spectroscopy techniques are excellent and nonintrusive but lack the necessary resolution. Atomic force microscopy (AFM) has very high resolution, but it is unable to image liquids, since the contact of the tip causes capillary wetting and strong perturbation of the liquid surface. In this work, we use a new technique, scanning polarization force microscopy (SPFM), 5,6 to image liquids with nanometer resolution. In combination with infrared reflection absorption spectroscopy (IRAS), we were able to follow in unprecedented detail the physical and chemical changes occurring during the corrosion of aluminum by sulfuric acid droplets in humid environments.In the SPFM technique, the electrically charged tip of an AFM scans the surface at a distance of a few hundred angstroms. Maintaining this tip-surface distance is critical in order not to perturb the liquid surface. The SPFM can be operated also as a standard contact mode AFM, and in this case, only the solid substrate is imaged while any liquid deposit is locally displaced by the tip.The aluminum samples were prepared by vapor depositing 2000 Å thick Al films on polished Si wafers. X-ray photoelectron spectroscopy indicated that the native oxide layer of the film was 20-30 Å thick, in agreement with published results. 7 Prior to use, the samples were rinsed with acetone and deionized water. The experiments were always carried out in humidity-controlled chambers.In-situ IRAS studies revealed that hydrated aluminum sulfate is the principal product of the corrosion reaction. 8 The rate of corrosion, measured by the appearance of h...

Wetting in the nanoscale: A continuum mechanics approach

“…Salmeron and coworkers adopted as solid substrates muscovite mica, cut and prepared as they report in their papers [40][41][42][43][44][45][46]. Our tests, on the other hand, were performed on silicon wafers for electronic use, cut as square 10 × 10 × 0.7 mm plates and used either as delivered or coated with 300 nm thick films of alumina or molybdenum by the sputtering technique.…”

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