THE ROLE OF CARBON-DIOXIDE IN THE ATMOSPHERIC CORROSION OF ZINC - A LABORATORY STUDY

We report on a laboratory study of the atmospheric corrosion of zinc i n air containing different concentrations of carbon dioxide (CO2) (<1, 350, 1000, and 40,000 ppm CO2). The samples were exposed to synthetic atmospheres with careful control of CO2 concentration, sulfur dioxide (SO2) concentration, relative humidity, and flow conditions. The rela tive humidity was 95%. Mass gain and metal loss results are reported. The corrosion products were analyzed quantitatively and qualitatively by a combination of grazing-angle x-ray diffraction, scanning electron microscopy, gravimetry, and quantitative analysis for carbonate. The corrosion rate of zinc increased with increasing CO2 concentration. In the presence of carbon dioxide Zn4CO3(OH)(6).H2O formed. Hydrozincite , Zn-5(CO3)(2)(OH)(6) was only identified after exposure to high CO2 c oncentration. Zinc hydroxycarbonate was converted into hydroxysulfate exposed to air containing 225 ppb SO2.Zn4SO4(OH)(6).4H(2)O was produce d in all exposures including SO2. The zinc hydroxycarbonate surface fi lm formed in the presence of CO2 was not protective in humid SO2 pollu ted air.