Page 4 - OpenAccess-April-2021

TECHNICAL PAPER

CATHODIC PROTECTION OF

STEEL REINFORCEMENT:

PAST EXPERIENCE,

PERFORMANCE AND

FUTURE OPPORTUNITIES GEORGE SERGI*

Abstract if the concrete undergoes either carbonation or chloride
contamination in the vicinity of the steel. Such effects can lead to
Impressed current cathodic protection (ICCP), following on from cracking and spalling of the cover concrete. In cases of chloride-
its early success in buried pipelines and submerged structures, induced corrosion, pitting develops at localised sites, whilst
was trialled on atmospherically exposed steel reinforced the remainder of the surface remains passive providing a large
structures in the 1970’s, first in the USA on bridge decks and cathodic area for oxygen reduction which drives the corrosion.
then more extensively in the UK as new and better performing For alkaline concrete, corrosion of steel may be expected to
inert anodes were developed. Cathodic Protection (CP) of steel
reinforced concrete soon became a well-established technique vary with potential and chloride content of the concrete, as
illustrated by Bertolini et al. and summarised in Figure 1.
[1]
for controlling reinforcement corrosion of structural elements. Domain A represents conditions that can lead to the initiation
Long-term maintenance of ICCP systems, however, started to and propagation of pits on initially passive steel. Domain B
be seen as a burden to most structure owners and managers as signifies conditions that allow pre-existing pits to propagate
it involves additional and continual costs. A requirement arose, but do not favour the initiation of new pits on initially passive
therefore, for simpler CP systems to be made available which steel. Domain C indicates conditions that do not allow the
will involve less maintenance and monitoring requirements. As initiation and propagation of pits, so that pre-existing pits
a first stage to simpler systems, galvanic cathodic protection tend to repassivate. Finally, Domains D and E represent
anodes were developed in the 1990’s, first, only to protect conditions that lead to highly negative potentials and are
steel reinforcement immediately around patch repairs but sufficiently reducing to render the passive film unstable. In
subsequently to control reinforcement corrosion over wider such cases, hydrogen is formed cathodically, increasing the
areas where corrosion risk was found to be high. It was the risk of hydrogen embrittlement of pre-stressed steel tendons.
combination of ICCP and galvanic anode systems, however, Thermodynamically, therefore, forcing the potential of the steel
that set the benchmark for a simpler alternative long-term into Domain C by applying sufficient cathodic current (4 to 6)
method for corrosion control of steel reinforcement. This would ensure that no new pits will propagate, and existing pits
paper attempts to follow the development of all CP systems will gradually repassivate. Even if polarisation is only sufficient to
utilized for atmospherically exposed steel reinforced structural place the steel potential in Domain B (4 to 5), the development
elements and looks at recently developed simpler systems of new pits cannot occur and a reduction in the intensity of
and methodologies that would likely form the future of the CP corrosion in any pre-existing pits will diminish depending on the
industry. level of polarisation.

Keywords: Cathodic Protection, Cathodic Prevention, Galvanic What Figure 1 also demonstrates quite elegantly is that an
CP, Corrosion Arrest, Two-Stage CP. increasing level of chloride reduces the steel potential at which
pitting can occur (pitting potential) and at which repassivation is
1. INTRODUCTION possible (repassivation potential) so that an increasing cathodic
current is required to force the potential of the steel below these
Concrete normally contains an alkaline pore liquid phase which levels as the chloride contamination rises. At the other extreme,
protects embedded steel from corrosion by allowing a highly there is a minimum concentration of chloride below which
dense passive oxide film to develop on its surface when oxygen pitting is not possible. This, of course, assumes a fixed level
is available. This form of protection can be lost, however, of alkali in the concrete. Applying a cathodic current to steel,

THE INDIAN CONCRETE JOURNAL | APRIL 2021 61
*Corresponding author : George Sergi, Email: georges@Vector-Corrosion.com