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TECHNICAL PAPER

1000 10000
750 mm
600 mm 1000
2
300 mm
100 100
A)
Current (μ 10 Current Density (mA/M ) 10 1

0.1

1 0.01
0 2 4 6 8 10 12 14 16 18 20 22 0 10 20 30
Time (years) Time (years)

Figure 13: Reduction of current output of galvanic anodes embedded in Figure 14: Anode current density vs time for two types of anodes with
structural elements of a bridge in the UK different Aging Factors such that both anodes provide a current density
of 2 mA/m2 at 20 years
which achieve an Aging Factor of less than 3 years. The reduced
performance of halide activated anodes is thought to be caused that the level of current output is related to the reciprocal of
by the relative insolubility of the zinc corrosion products which the absolute temperature according to the Arrhenius equation
tend to block the pore structure of the encasing mortar close (Equation 3) as depicted in Figure 15.
to the zinc/mortar interface and interfere with current flow (17). Ea
The importance of the Aging Factor in the design of galvanic CP k = Ae – RT (3)
systems is considerable as the current density at a future time Where,
can be accurately estimated in order to design for long-term
protection. Figure 14 demonstrates this elegantly [15] . An alkali- k = Rate constant
activated anode set is, in this case, required to achieve a specific A = Frequency factor
current density of 2 mA/m at 20 years. For an Aging Factor Ea = Activation energy
2
of 13 years, the current density would need to be designed to R = Universal gas constant
deliver an early current density of around 7 mA/m . Furthermore, T = Absolute temperature (K)
2
the design will maintain a current density above 1 mA/m even
2
beyond 30 years. A typical halide anode set would require an A form of the equation [18] relates the corrosion rate, i.e. the
initial current density of over 2000 mA/m in order to achieve the current output from the zinc, to the reciprocal of temperature
2
required 2 mA/m at 20 years, clearly an improbability. (Equation 4). ∆Ea
2
Logi corr = Log A – ( 2.303RT ) (4)
The current density was also found to vary with concrete
temperature. Analysis of data sets of current output of galvanic Where,
anodes in relation to the concrete temperature had shown i corr = Corrosion current of zinc metal
∆Ea = Apparent activation energy of the corrosion process
100
The relationship shown represents a doubling of current density
every 10-15°C increase in temperature over ambient conditions.
Current density (mA/M ) 10 in all monitored galvanic CP installations (see, for example,
2 The same variation of current output with temperature was seen

Figure 13). This knowledge allows a better calculation of the
required current density for particular climatic conditions, for
example, a higher current output will be expected in tropical
conditions compared to cooler climates.

4. THE NEED FOR SIMPLER ICCP SYSTEMS
1
0.003 0.0032 0.0034 0.0036 0.0038 0.004
There is no doubt that ICCP of steel reinforced concrete has,
Reciprical of temperature (1/°K) over the last decades, become a well-established technique for
Figure 15: Relationship between the logarithm of current density of a controlling reinforcement corrosion of structural elements. The
galvanic anode set and the reciprocal of concrete temperature in Kelvin expectation of long life protection has, however, been somewhat

THE INDIAN CONCRETE JOURNAL | APRIL 2021 67