Characterization of corrosion products from atmospheric exposures for up to 5 years

Four metallic materials of economic significance were exposed to an urban atmosphere up to five years. These materials were low carbon steel, zinc (as galvanized steel), copper and aluminum all of commercial quality. The corrosion rate was measured after 1, 2, 3 and 5 years using a weight loss method. Sulfur dioxide deposition rate and climatological parameters were determined monthly. At the same time, different techniques were used to characterize the corrosion products developed on the above materials. Results obtained by polarization resistance technique (Rp) in a 0.1 M Na2SO4 solution revealed that, the longer the exposure time, the greater the corrosion products protection, as result of broadening and compactness of such products, this was verified by scanning electron microscopy (SEM). Through x-rays diffraction (XRD), the expected crystalline compounds on steel and copper were identified and also, the transformation of middling protective initial products into other more protective was followed. For aluminum and zinc it was not possible to identify the crystalline compounds perhaps due to the very thin corrosion products layers. So, these materials were analyzed by means of Raman spectroscopy. α-Al2O3 was identified on an aluminum sample whereas on zinc any corrosion product could not be identified due to excessive fluorescence. © 1998 by NACE International.

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