Series editors' preface - Wood Tools

Table 8.5 Galvanic series in seawater
DIRECTION OF ELECTRON FLOW
Source: LaQue, 1948
Most anodic, active or base
Magnesium/magnesium alloys
Zinc
Aluminium
Mild steel
Wrought iron
Cast iron
Chromium stainless steel (most stainless steel
fittings)
Lead–tin solder (50:50)
Lead
Tin
Brasses
Nickel (active)
Copper
Bronze
Silver solder
Nickel (passive)
Silver
Gold
Platinum
Most cathodic, passive, or noble
phases in metals of mixed composition
(alloys). This may be a result of the presence
of salts or the interaction of alloyed metals.
Salts function as electrolyte formers, or by
creating local areas of aggressive corrosion.
Two arrangements of metals are used to
predict the potential for corrosion between
metals – the electromotive force (EMF) series
and the galvanic series. The EMF series ranks
metals according to their standard oxidation or
reduction potentials but is of limited use for
predicting interactions between metals
because this varies with environment (Uhlig
and Revie, 1985). A galvanic series ranks the
measured oxidation or reduction potential of
metals (their corrosion tendencies) in a
specific environment (Table 8.5).
The galvanic series for seawater is often
used as a general approximation of how
metals will inter-react. Metals may occupy two
positions in the galvanic series – in the passive
state a layer has formed on the metal and it
is therefore less prone to corrosion. Galvanic
corrosion is a type of electrochemical corrosion
that occurs when two dissimilar metals
form a galvanic couple in the presence of an
electrolyte such as water or moisture from
high RH. Electrons flow from the anodic metal
to the cathodic metal, causing corrosion of the
Deterioration of other materials and structures 321
anodic metal. The intensity of the electron
exchange and hence corrosion, is determined
by the metals’ ranking in the galvanic series,
although factors, such as pH and the nature
of the electrolyte, can complicate this picture.
The metal that is ranked lower on the galvanic
scale will corrode before the metal which is
higher on the scale. The further apart on this
scale the alloyed metals are, the greater the
corrosion potential will be. The electron
exchange protects the more cathodic metal
(traditionally called more noble) whilst causing
the more anodic metal (traditionally called
more base) to corrode faster.
Galvanic corrosion has important implications
for the conservation of historical metal
alloys such as pewter and brass. Chemical
removal of corrosion products of metal, for
example, may cause the preferential attack of
one component of an alloy. The presence of
impurities in a metal that was not intentionally
alloyed can cause galvanic corrosion,
particularly if the impurity metal is lower on
the electromotive (EMF) scale. Heath and
Martin (1988) found the corrosion of lead inlay
on urushi objects exposed to acetic acid
vapour in storage was inhibited by the
presence of tin and copper in lead inlay but
accelerated by the presence of ferrous
impurities.
Role of moisture
It is easy to see that when a metal surface is
exposed to rain or mist it will become wet and
therefore it is possible for a corrosion cell to
be set up on its surface. The atmosphere
under these conditions is at 100% RH, that is
the air is saturated with water vapour. Metal
surfaces can suffer corrosion even when
exposed to RH of less then 100%. Even though
a metal surface may not be visibly wet, a thin
film of moisture can still be present that can
support a corrosion cell. At very low RH this
film becomes very thin and corrosion effects
diminish and become negligible. Whilst there
are various contaminants that occur in polluted
air which greatly accelerate corrosion and
while water on its own does not usually give
rise to very serious damage, it is essential for
corrosion that enough moisture should be
present. The tarnishing of silver is accelerated
by high relative humidity, whilst that of lead,
tin and pewter is not much affected.