Indoor corrosion of Pb: Effect of formaldehyde concentration and relative humidity investigated by Raman microscopy

In this work the effect of relative humidity (RH) and formaldehyde (H₂CO) concentration on Pb corrosion was investigated; a possible synergism between the aldehyde and CO₂ effects was also considered. Triphasic aqueous salt solutions were used to produce 54% and 75% RH that, together with the 100% RH condition, were combined with 0, 0.62, 55 and 2.0 10² mg m⁻³ formaldehyde concentrations to compose the wanted environments.The results pointed to the conclusion that even at low RH (54%) formates are produced at the metal surface as a consequence of formaldehyde adsorption, indicating that the aldehyde has an active role in Pb corrosion; formates were also observed at relatively low H₂CO concentration (0.62 mg m⁻³). No synergism between formaldehyde and carbon dioxide were observed as demonstrated by the Raman images from a corroded Pb coupon, showing that formate and carbonate contributions to the corrosion products were not spatially related.When compared to other volatile organic compounds (VOCs), formaldehyde harmful effect towards metals is frequently underestimated and the results here reported clearly indicate that, even at low RH, its concentration in indoor environments, where it tends to be produced and accumulated, has to be carefully controlled.