Corrosion of copper and lead by formaldehyde, formic and acetic acid vapours

The formation of corrosion products and the corrosion sensitivity of copper and lead were studied under experimental conditions in which formaldehyde (methanal),formic (methanoic) and acetic (ethanoic) acid concentration, relative humidity (RH) and duration of exposure were varied. Levels of formic acid above 0.4 parts per million based on volume (ppmv) affect the appearance of copper at 75% RH, and at levels above 4 ppmv the copper gains weight at both 54 and 75% RH. The main compound found on copper was cuprite, copper(I) oxide. Lead has a higher sensitivity to formic acid: at levels as low as 0.04 ppmv lead becomes darker, and at above 0.1 ppmv weight gains were measurable at both 54 and 75% RH. In the presence of different levels of mixed carbonyl vapours at 75% RH, copper reacted mainly with formic acid. On the other hand, the reaction of lead was more complex. Acetic acid tends to form a thick white layer (composed mainly of plumbonacrite and possibly lead acetate compounds) on the lead surface, while formic acid tends to form a rather thin and darker layer of lead formate hydroxide. In the presence of formic acid, the action of acetic acid on lead was inhibited. At levels of formaldehyde up to 3 ppmv, no significant contribution of formaldehyde to the corrosion process on lead and copper was observed.