The natural durability of lead under normal
conditions is due to the formation of protective insoluble salts which are
formed on the surface. Moisture and carbon dioxide in the atmosphere combine to
form weak carbonic acid and this reacts with bright metallic lead during periods
of rain or overnight dews to form the normal grey patina typical of lead roofs.
This thin continuous surface layer protects the metallic lead from further
attack. On the underside of lead sheets these protective layers may not
form.
Most lead roofs are unaffected by underside
corrosion or contain only a small amount of corrosion products, possibly as a
result of damp conditions when the roof was laid. However, some roofs have been subject to
severe underside corrosion as a result of on-going damp conditions, the presence
of carboxylic (acetic and formic) acids given out by acid-containing substrates.
The major corrosion product is the white hydrocerussite or white lead
(2PbCO3.Pb(OH)2). However, in the presence of
carboxylic acids, the red and yellow lead oxides of Massicot and Litharge may be
formed.
Underside corrosion of lead requires the presence
of moisture and an absence of carbon dioxide. The accumulation of condensate
which is depleted of carbon dioxide creates a risk of underside
corrosion.
When a thin film of moisture condenses on the
underside surface, this becomes rich in oxygen and carbon dioxide. This forms
the protective grey hydrocerussite patina, similar to that formed on the topside
of lead by normal weathering.
When thick films or droplets of condensation form
on the underside surface, these are unable to absorb sufficient carbon dioxide
to form a stable passive coating. Instead, the intermediate lead hydroxide forms
on the metal surface and this migrates to the outside of the droplet where it
eventually reacts with carbon dioxide to form the hydrocerussite. Because this has formed on the outside of the
droplet (as compared to the metal surface) it is loose and non-protective and
this allows further corrosion to occur during the next condensation period.