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5.5.7.2 Formation of galvanic cells, corrosion Such a concentration cell can be formed, for example, by two
Corrosion processes can be clearly described with the help of iron electrodes, one of which is fixed in iron-reinforced con-
a galvanic cell. If, for example, a metal rod is immersed into crete while the other lies in the ground (Figure 5.5.7.2.3).
an electrolyte, positively charged ions pass into the electrolyte Connecting these electrodes, the iron in the concrete becomes
and conversely, positive ions are absorbed from the electrolyte the cathode of the concentration cell and the iron in the
by the metal band. This is called “solution pressure” of the ground becomes the anode. The latter is therefore destroyed
by ion emission.
metal and “osmotic pressure” of the solution. Depending on For electrochemical corrosion it is generally the case that the
the magnitude of these two pressures, either the metal ions larger the ions and the lower their charge, the greater the
from the rod pass into the solution (the rod becomes nega- transport of metal associated with the current flow i (this
tive compared to the solution) or the ions of the electrolyte means that i is proportional to the atomic mass of the metal).
deposit on the rod (the rod becomes positive compared to the
electrolyte). Voltage is thus applied between two metal rods in In practice, the calculations are carried out with currents
the same electrolyte. flowing over a certain period of time, e.g. one year. Table
In practice, the potentials of the metals in the ground are 5.5.7.2.1 specifies values which define the effect of the corro-
measured with the help of a copper sulphate electrode which sion current (current density) in terms of the quantity of metal
consists of a copper rod immersed into a saturated copper sul- dissolved. Corrosion current measurements thus make it pos-
phate solution (Figure 5.5.7.1.1) (the reference potential of sible to calculate in advance how many grammes of a metal
this reference electrode remains constant). will be eroded over a specific period.
In the following, it will be described how two rods made of Of more practical interest, however, is the prediction if, and
different metals are immersed into the same electrolyte. A volt- over which period of time, corrosion will cause holes or re-
age of a certain magnitude is now created on each rod in the cesses in earth electrodes, steel containers, pipes etc. Thus,
electrolyte. A voltmeter can be used to measure the voltage it is important whether the current attack will be diffuse or
between the rods (electrodes); this is the difference between punctiform.
the potentials of the individual electrodes compared to the As far as the corrosive attack is concerned, it is not solely the
electrolyte. magnitude of the corrosion current which is decisive, but also,
How does it come that current flows in the electrolyte and that in particular, its density, namely the current per unit of area of
material is transported, that is corrosion occurs? the discharge area.
If, as shown here, the copper and the iron electrode are con- It is often not possible to directly determine this current densi-
nected via an ammeter outside the electrolyte, for example, the ty. In such cases, potential measurements are carried out from
following (Figure 5.5.7.2.1) is observed: In the outer circuit, which the extent of the available "polarisation" can be read
the current i flows from + to –, namely from the “more pre- off. The polarisation behaviour of electrodes is discussed only
cious” copper electrode according to Table 5.5.7.2.1 to the briefly here.
iron electrode. Let us consider the case of a galvanised steel strip situated in
In the electrolyte, in contrast, the current i must therefore flow the ground and connected to the (black) steel reinforcement
from the “more negative” iron electrode to the copper elec- of a concrete foundation (Figure 5.5.7.2.4). According to our
trode to close the circuit. In general, this means that the more measurements, the following potential differences occur here
negative pole passes positive ions to the electrolyte and hence with respect to the copper sulphate electrode:
becomes the anode of the galvanic cell, in other words it is ¨ Steel (black) in concrete: –200 mV to –400 mV
dissolved. The metal is dissolved at those points where the cur- ¨ Steel, galvanised, in sand: –800 mV to –900 mV
rent enters the electrolyte. A corrosion current can also arise ¨ Steel, galvanised, as good as new: about –1000 mV
due to the concentration cell (Figure 5.5.7.2.2). In this case,
two electrodes made of the same metal immerse into differ- Thus, there is a potential difference of 600 mV between these
ent electrolytes. The electrode in electrolyte II with the higher two metals. If they are now connected above ground, a current
concentration of metal ions becomes electrically more positive i flows in the outer circuit from concrete steel to the steel in the
than the other. sand, and in the ground from the steel in the sand to the steel
in the reinforcement.
This process is also referred to as polarisation. Connecting the The magnitude of the current i is now a function of the voltage
two electrodes enables the current i to flow and the electrode difference, the conductance of the ground and the polarisation
which is electrochemically more negative dissolves. of the two metals.

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