THE INFLUENCE OF CO2 AND NACL ON THE ATMOSPHERIC CORROSION OF ZINC - A LABORATORY STUDY

We report on a laboratory study of the atmospheric corrosion of NaCl-t reated zinc in air containing different concentrations of carbon dioxi de (<1, 100, 350, 1000, and 40,000 ppm). Each sample was exposed indiv idually to synthetic atmospheres with careful control of CO2 concentra tion and flow conditions. The relative humidity was 95%. The protectiv e proper-ties of carbonate-containing corrosion products were investig ated in 225 ppb SO2. Mass gain and metal loss results are reported. Th e corrosion products were analyzed quantitatively and qualitatively by a combination of X-ray diffraction, scanning electron microscopy: gra vimetry, and quantitative analysis for carbonate. The corrosion of zin c in the presence of NaCl resulted in heavy pitting of the metal. Carb on dioxide slowed down the corrosion rate by a factor of three to six. This is explained by the neutralization of the surface electrolyte re sulting in the precipitation of chloride in the form of insoluble Zn-5 (OH)(8)Cl-2. H2O. In the presence of NaCl large amounts of hydroxy car bonates formed. There is no indication that hydroxy carbonates are pro tective in a SO2 environment.