Conservation Science

separate metallic phases present on a single metal surface. Consider a strip of
copper (copper electrode) joined to a strip of iron (iron electrode) by an insu-
lated wire and immersed in sea-water (approximately 3 wt% sodium chloride).
The iron will start to dissolve (corrode as Fe^2 ) and release electrons according
to Equation (9)

(9)

In corrosion science, the iron electrode is referred to as the anode and this
is always where corrosion takes place.
The electrons travel through the wire to the copper electrode surface where
they react with dissolved oxygen gas in the seawater to form hydroxyl ions
according to Equation (10)

(10)

The copper electrode is called the cathode and no corrosion occurs at this
electrode. The electrical circuit is completed by the seawater electrolyte, which
carries a charge by the movement of ions through the solution. This would be via
the movement of chloride and sodium ions present in the seawater.
Single metals and alloys such as wrought iron, bronze and lead will corrode
even though they are not joined to a different metal or alloy. In reality, anodes
and cathodes are set up on the surface of the metal. This will be due to the dif-
ferent phases present in the alloy, e.g.iron (ferrite) will be the anode and
graphite the cathode in a cast iron, impurities such as sulfides present in
wrought iron and variation in the copper/zinc ratio in adjacent grains in a brass
alloy. The reactions taking place on the anode and cathode will be the same
except the anode may be a different metal if the object was not iron. With a
copper artefact, for example, the anode reaction will be:

(11)

where Cudenotes that copper is now present in the solution. The cathode
reaction will be the same as Equation (10) for all metals and alloys in natural
environments such as seawater, freshwater or soils. Cathodes and anodes will
also form on metal surfaces if moisture films condense on a metal surface on a
damp day or if the humidity inside a museum rises above a critical value.
From the above, the following five factors are required for corrosion to occur:

1. Anode – where metal dissolution occurs, i.e.corrosion.


2. Cathode – where the electrons are consumed by an electrode reaction.
   The cathode is protected.

CuCu 2 e

OeHOOH 22  42  4 

FeFe 2 e

132 Chapter 6