Formaldehyde oxidation and lead corrosion

Carbonyl compounds are of concern in museums because of their potential to damage artifacts. The mode of damage for aldehydes is far from clear and seems to require oxidation. Lead coupons were exposed to gaseous formaldehyde (methanal) and formic acid(methanoic acid) in various humid oxidizing atmospheres. The resulting corrosion products were weighed and identified using X-ray diffraction analysis after as much as 56 days exposure. When formaldehyde was in more oxidizing environments (atmospheres containing hydrogen peroxide) it caused heavy corrosion but in the dark, with no oxidant, there was only very slight tarnish, even at high humidities. This suggests that oxidation is an important control on corrosion by formaldehyde. Thermodynamic calculations indicate that at low formic acid concentrations (below about 1 ppb) the corrosion products should be carbonates (plumbonacrite and hydrocerussite) while at higher concentrations lead formate would be expected. The mineralogical complexity and the importance of oxidants make it very difficult to set standards for formaldehyde in the museum environment.