The Influence of Salt Deposits on the Atmospheric Corrosion of Zinc. The Important Role of the Sodium Ion

The influence of salt deposits on the atmospheric corrosion of zinc was studied in the laboratory. Four chloride-containing salts, NaCl, and and four sulfate-containing salts, and were investigated. The salts were applied by spraying a saturated ethanol/water solution before exposure. The samples were exposed to purified humid air with careful control of relative humidity (95%), temperature (22.0°C), and air flow. The concentration of was 350 ppm and the exposure time was four weeks. The salts formed aqueous solutions on the metal surface. Mass gain and metal loss results are reported. The corrosion products were analyzed by gravimetry, ion chromatography, and X-ray diffraction. It was concluded that zinc corrodes by an electrochemical mechanism. Of the four cations studied, sodium was by far the most corrosive toward zinc. The corrosion of zinc was directly correlated with the amount of sodium ion and did not depend on whether or NaCl was added. The comparatively rapid corrosion caused by the two sodium salts is suggested to be due to the absence of insoluble sodium compounds. In contrast, the divalent cations precipitate in the cathodic areas due to the high pH values produced, resulting in the blocking of the cathodic sites. © 2002 The Electrochemical Society. All rights reserved.