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INTRODUCTION TO CELLS: TERMINOLOGY AND BACKGROUND 287

frequently withstand force-10 gales. Having built the rig, we appreciate how impor-
tant is the need of maximizing its lifetime. And one of the major limits to its life
span is corrosion.
Oil rigs are made of steel. The sea in which they stand contains vast quantities of
dissolved salts such as sodium chloride, which is particularly ‘aggressive’ to ferrous
metals. The corrosion reaction generally involves oxidative dissolution of the iron, to
yield ferric salts, which dissolve in the sea:

Fe (s) −−→ Fe 3+ (aq) + 3e − (7.10)

If left unchecked, dissolution would cause thinning and hence weakening of the legs
on which the rig stands.
One of the most ingenious ways in which corrosion is inhibited
is to strap a power pack to each leg (just above the level of the In reality, several of
sea) and apply a continuous reductive current. An electrode couple the iron compounds
would form when a small portion of the iron oxidizes. The couple are solid, such as rust.
would itself set up a small voltage, itself promoting further disso- This clever method of
lution. The reductive current coming from the power pack reduces averting corrosion can
any ferric ions back to iron metal, which significantly decreases also arrest the corro-
sion of rails and the
the rate at which the rig leg corrodes.
undersides of boats.
Clearly, we want the net current at the iron to be zero (hence no
overall reaction). The rate of corrosion would be enhanced if the
power pack supplied an oxidative current, and wasteful side reac- The simplest definition
tions involving the seawater itself would occur if the power pack of equilibrium in an
produced a large reductive current. The net current through the electrochemistry cell is
iron can be positive, negative or zero, depending on the potential that no concentrations
applied to the rig’s leg. The conserver of the rig wants equilibrium, change.
implying no change.
All the discussions of electrochemistry so far in this chapter concern current – the
flow of charged electrons. We call this branch of electrochemistry dynamic, implying
that compositions change in response to the flow of electrons. Much of the time,
however, we wish to perform electrochemical experiments at equilibrium.
One of our best definitions of ‘equilibrium electrochemistry’ says
the net current is zero; and from Faraday’s laws (Table 7.1), a zero Electrochemical mea-
current means that no material is consumed and no products are surements at equilib-
formed at the electrode. rium are made at zero
But this equilibrium at the oil rig is dynamic: the phrase current.
‘dynamic equilibrium’ implies that currents do pass, but the cur-
rent of the forward reaction is equal and opposite to the current of the back reaction,
according to
(7.11)
I (forward, eq) =−I (backward, eq)
and the overall (net) current is the sum of these two:

I (net) = I (forward) + I (backward) (7.12)