CHEMISTRY OF CORROSION 5
W
F (it) M
n
corroded
=
(Eq. 2.8)
where:
W
corroded
= mass (weight) of corroded/electrodeposited
metal
F = Faraday ’ s constant
i = current in amps
t = time of current passage
m = molar mass of the element in question
n = ionic charge of the metal in question
The amount of a substance consumed or produced at
one of the electrodes in an electrolytic cell is directly
proportional to the amount of electricity that passes
through the cell. Methods of measuring the corrosion
current are diffi cult and are discussed in Chapter 7 ,
Inspection, Monitoring, and Testing.
Electrode Potentials and Current
The Electromotive Force (EMF) Series is an orderly
arrangement of the relative standard potentials for pure
metals in standard, unit activity (1 Normal, 1 N), solu-
tions of their own ions (Table 2.1 ). The more active
metals on this list tend to be corrosion susceptible and
the less active, or noble metals, will resist corrosion in
many environments.
It should be noted that two sign conventions are fol-
lowed in publishing the EMF series. This can cause
confusion, which can be avoided if the reader under-
stands that active metals like magnesium and aluminum
will always be anodic to carbon steel, and corrosion -
resistant metals like silver and palladium will be
cathodic.
The EMF series shows equilibrium potentials for
pure metals in 1 N (one normal or unit activity of ions)
solutions of their own ions. While this is the basis for
much theoretical work in corrosion and other areas of
electrochemistry, pure metals are seldom used in indus-
try, and oilfi eld corrosive environments never have 1 N
metal ion concentrations. The more practical galvanic
series (Figure 2.1 ), which shows the relative corrosion
potentials of many practical metals, is often used in cor-
rosion control. This is based on experimental work in
seawater and serves as the basis for many corrosion -
related designs.
The galvanic series in seawater shown in Figure 2.1
is widely used for engineering designs. Some authorities
claim that the relationships between various alloys must
be determined for each environment, but this is seldom
done. The reason for this precaution is that zinc and
be very close, for example different metallurgical phases
on a metal surface, or they can have wide separations,
for example, in electrochemical cells caused by differ-
ences in environment or galvanic cells between anodes
and cathodes made of different materials.
Electrolyte Conductivity
The electrical conductivity of an environment is deter-
mined by the concentration of ions in the environment,
and the resulting changes in corrosivity can be under-
stood by considering Ohm ’ s Law:
EIR=
(Eq. 2.7)
where:
E = the potential difference between anode and
cathode, measured in volts
I = the electrical current, measured in amperes
R = the resistance of the electrical circuit, determined
by the distances between anode and cathode and
by ρ , the resistivity of the elec trolyte, which is
usually expressed in ohm - centimeters ( Ω - cm). In
most cases, the distance between anode and
cathode is not known, but the changes in the cor-
rosion rate can be monitored and correlated in
changes in resistivity, for example, the changes in
resistivity of soils caused by changes in moisture
content which alter the ionic content of the soil
electrolyte.
The resistivity of liquids and solids is determined by
the ions dissolved in the bulk solution. Hydrocarbons
such as crude oil, natural gas, and natural gas conden-
sates are covalent in nature and are very poor electro-
lytes because they have very high resistivities. Oilfi eld
corrosion is usually caused by chemicals in the water
phase that, among other things, lower the natural resis-
tivity of water, which is also mostly covalent. Water is a
very effi cient solvent for many chemicals, and most oil-
fi eld corrosion occurs when metal surfaces become
wetted by continuous water phases having dissolved
chemicals which lower the natural high resistivity (low
conductivity) of water.
Faraday ’ s Law of Electrolysis
The mass of metal lost due to anodic corrosion currents
can be determined from Faraday ’ s law for electrolysis,
Equation 2.8 , which is also used by the electroplating
industry:
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