Series editors' preface - Wood Tools

320 Conservation of Furniture
Table 8.4 Lead corrosion products
Name Formula Colour
Lead formate Pb(CHO 2) 2 ‘White fuzzies’
Cerussite (lead carbonate) PbCO 3
Hydrocerussite (basic lead carbonate) Pb 3(OH) 2(CO 3) 2 Lead white
Anglesite (lead sulphate) PbSO 4
Cotunnite (lead chloride) PbCl 2
Phosgenite (lead carbonate chloride) Pb 2CO 3Cl 2
Leadhillite (lead sulphate carbonate hydroxide) PbSO 4(CO 3) 2(OH) 2
Penfieldite (basic lead chloride) Pb 2(OH)Cl 3
Lead monoxide (litharge, massicot) PbO
Plattnerite (lead dioxide) PbO 2
Galena (lead sulphide) PbS
Corrosion in many metals takes place on
both sides of the original surface, with ions
moving out into the environment in more or
less straight lines. This means that a lump of
corrosion on the surface is likely to be
matched by an area of depleted spongy metal
mixed with corrosion below the surface that
is similar in shape and extent (mirror imaging).
Corrosion is an electrolytic form of decomposition
which results when different electric
potentials are established either between
metals or between different points on a metal
surface. For this to occur one part of an object
must behave as an anode and another part as
a cathode and these must be connected
together so that electrons flow into the
cathode out of the anode. In addition an
(a)
(b)
(c)
electrolyte, forming a continuous link between
the substances which react at each electrode,
is required (Figure 8.2a). There are several
ways in which corrosion cells may be set up
in practice (Figure 8.2b). The presence of
particles of an impurity or contact with
another metal allows a difference of voltage to
be set up. In the presence of moisture containing
air, or some dissolved chemical substance
that will conduct electricity, an electric current
will flow and corrosive attack will begin.
Electrochemical corrosion occurs wherever
different electric potentials are established
either between metals or between different
points on a metal surface. Such different
electric potentials are normally due to local
areas of differing composition known as
Figure 8.2 Corrosion cells
(a) A simple corrosion cell consisting of iron in contact
with copper in the presence of an electrolyte (e.g.
acid). Positively charged ferrous ions form around the
iron electrode, whilst negatively charged hydroxyl ions
are produced around the copper electrode. (This is the
principle behind the construction of batteries)
(b) Galvanic corrosion occurs where two dissimilar
metals are in contact in the presence of an electrolyte.
In this example, the iron adjacent to the copper will be
most severely corroded
(c) Pitting corrosion describes very localized corrosion at
points where there are cracks in a protective coating or
oxide film or where the protective layer is thin. The
area where the protective later has been disrupted acts
as an anode, whilst the adjacent (still protected) metal
acts as a cathode causing corrosion pits to develop