Page 11 - OpenAccess-April-2021

TECHNICAL PAPER

Table 1: Cathodic Charge required to passivate steel at the chloride levels and current densities shown

Current Density (mA/m ) 2 30 50
-
% Cl in Mortar 1 2 3 2 4
Cathodic Charge (kC/m ) 15 120 190 74 108
2
reduced as some anode systems fail, monitoring equipment multiplied by time) delivered to the steel until passivation of the
become antiquated and lack of adequate maintenance makes steel was achieved increased with the level of chloride but was
the systems inoperable with the average service life of any CP lower at the higher current density of 50 mA/m , as summarised
2
system falling to 15 years [19] . Inevitable additional costs are in Table 1.
involved in maintaining and prolonging correct operation of
the system. It appears that there is a requirement by structure Passivity of the steel was assumed when the 24-hour depolarised
managers and owners for simpler CP systems which will involve potential had reached -150mV vs Ag/AgCl, 0.5M KCl (-141mV vs
less maintenance and monitoring requirements. SCE) as indicated in EN 12696: 2016 [10] .
ISO EN 12696:2016 [10] has clear performance criteria that need As revealed in the introduction, work by Pedefferri [25] and
to be continuously satisfied to ensure that the system is working. Presuel-Monreno et al. [26] had demonstrated that passivity of
A much-used criterion is that a depolarisation potential of 100 the steel can be maintained in a corrosive environment for
mV should be achieved when the system is temporarily turned considerable periods, by a process they termed Cathodic
off for a period of 24 hours. The standard also defines that a Prevention, by applying a current density of 0.4-2 mA/m . Even
2
successful CP system either passivates the steel or reduces the constant exposure to highly corrosive environments could
corrosion rate of the steel reinforcement which implies, correctly, not initiate corrosion at a current density of 1 mA/m 2 [27] . Once
as shown earlier, that achieving 100 mV of polarisation does corrosion is arrested, it would appear reasonable to suggest
not necessarily mean that corrosion has been arrested short- that a second stage of a process, based on Cathodic Prevention,
term. Nonetheless, it has been shown in several cases that if is likely to protect the steel from further corrosion. Thus, a CP
a CP system is running for an extended period, e.g. 5 years, system based on a Two-Stage process appears to be a viable
and is then turned off, corrosion of the steel does not reinitiate corrosion mitigation method.
over a significant time period [20] . As briefly mentioned earlier,
this phenomenon is believed to be caused by some secondary This understanding formed part of a process that has enabled
effects, primarily, the increase in alkalinity and reduction in the recent development of a simple to install and operate
chloride concentration at the steel/concrete interface [21] which in Two-Stage Corrosion Mitigation system [28,29] as illustrated in
-
effect reduces the [Cl ]/[OH ] ratio considerably below the critical Figure 16.
-
ratio for initiation or maintenance of corrosion. It has also been
suggested that realkalisation of the acidified pits occurs which A field trial on a real structural element, a column underneath a
allows steel repassivation within them [22] . Once repassivation of bridge in the UK (Figure 17), was performed which demonstrated
the steel is achieved, the application of cathodic prevention can the effectiveness of the system.
then maintain the passive conditions long-term [23] .
All cracked and spalled concrete that had occurred on two
It was soon realised that a system that can arrest steel corrosion edges of the column had been removed and repaired with an
relatively early and can then switch to cathodic prevention appropriate repair mortar. A total of six anodes were installed
mode over the longer term is realistically possible. It was in a grid formation at a spacing of 600 mm in such a way so
important, however, to identify the desired current density as to surround the repaired areas. Two silver/silver chloride
and overall charge delivery to the steel reinforcement for standard reference electrodes were embedded within the test
successful corrosion arrest to occur before the current density is areas in-between the anodes to enable monitoring of the steel
reduced to the lower cathodic prevention current density levels potentials both during application of the current and following
(0.4-2 mA/m ) which have been shown to be easily achieved by disconnection. A junction box connected the anodes to the
2
galvanic anodes [14,23] . steel externally, so disconnection of the anodes, either the ICCP
To develop a viable Stage-1 procedure in which corrosion arrest component or the galvanic component,was possible at any
of corroding steel can be achieved, a series of experiments were stage. The main results are detailed in Figure 18. The range of
conducted, the results of which are published elsewhere [24] . current densities applied either during Stage-1 (ICCP anodes) or
What was discovered was very interesting. At two constant Stage-2 (galvanic anodes) is indicated at the top of the graph.
2
2
current levels, 30 mA/m and 50 mA/m , the corrosion of pre- All potentials shown are 24-hour depolarised potentials, i.e. the
corroded steel plates in mortars containing increasing doses current was interrupted for 24 hours before the steel potential
of chloride by weight of cement as NaCl, was arrested after a was recorded. The anodes were disconnected for two longer
length of time under polarisation. The required charge (current periods starting at 130 days and 260 days to allow extended

68 THE INDIAN CONCRETE JOURNAL | APRIL 2021