Atmospheric corrosion of lead - The influence of formic acid and acetic acid vapors

The present laboratory study investigates the influence of low concentrations of formic acid vapor and the combination of acetic and formic acid vapors on the atmospheric corrosion of lead. The samples were exposed to synthetic air with careful control of relative humidity (95%), temperature (22.00 degrees C), flow conditions and the concentration of formic acid (160 ppb), acetic acid (170 ppb) and CO2 (350 ppm). Exposure time was one, two and four weeks. Corrosion products were analyzed by gravimetry, ion chromatography, quantitative carbonate analysis, X-ray diffraction and environmental scanning electron microscopy. Cross sections of the corroded surface were prepared by focused ion beam milling. Formic acid vapor is very corrosive toward lead, although somewhat less so than acetic acid. The corrosion products, consisting of plumbonacrite (Pb10O(OH)(6)(CO3)(6)) and lead formate hydroxide [Pb(HCOO)(OH)], are evenly distributed. The combination of acetic and formic acid has a synergistic effect on lead corrosion. The corrosion products found were plumbonacrite and massicot (beta-PbO) together with an unidentified phase. Corrosion attack in the mixed pollutant exposure is more localized compared to the acetic and formic acid exposures, clearly indicating the electrochemical nature of corrosion