Corrosion inhibitors - Concrete Repair - page 182

P ENETRATING VAPOUR PHASE INHIBITORS

These products, typified by Sika Ferroguard 903 and a number of other
competitive materials, have been promoted for use on concrete suffering
from reinforcement corrosion and often for concrete containing significant
levels of chloride contamination.
Migrating corrosion inhibitors are typified by the amino alcohol based
corrosion inhibitors, which coat the embedded steel with a monomolecular
layer that keeps the chlorides ions away from the embedded steel. They
also inhibit the reaction of oxygen and water at the cathodic sites on the
steel which are an essential part of the corrosion process. As a result amino
alcohols can be regarded as both anodic and cathodic inhibitors.
Thangavel and co-workers at the central electrochemical research institute
in India (Thangavel et al., 2009) concluded that

• Alkalinity of the concretes was not affected by adding either MCI or
admixed inhibitors. Among three concretes, Portland Slag Cements
performed well under macro cell condition.
• Migrating inhibiting systems enhanced the passive condition of
embedded steel in concrete by three times and admixed inhibiting
systems enhanced the passive condition by two times when compared
with the respective control concretes.

• Migrating inhibiting systems showed a 60% reduction in macrocell

current, whereas admixed inhibiting systems showed only a 40%
reduction in macrocell current in the case of PSC concretes.

• The prolonged passive condition of embedded steel, lowest macrocell

and integrated current, lower free chloride contents, lowest corrosion
rate, and visual observation data confirmed the better corrosion
resistance of steel embedded in migrating corrosion inhibiting systems.

Figure 10.3 shows some of their results, though it should be noted that
after 12 months the potential values for the treated system, while lower,
were approaching -350 mV (equivalent to -450 mV approx Cu/CuSO4).
There are a number of reasons why measurement of potential may not be a
good guide to corrosion behaviour in concrete, including possible poisoning
of the reference electrode, if an embedded system is used. For this reason
corrosion current measurements are usually more reliable. Figure 10.4
shows the effect on macrocell current, which is a better guide to behaviour,
but again does not show anything near the tenfold reduction in corrosion
current that would indicate a significantly effective inhibitor. Such values,
while confirming some effect, cannot be taken as proof of effectiveness in
the field.
In 2003/4 Grantham and co-workers carried out an evaluation of two
corrosion inhibitors, Flexcrete Cemprotec MCI 2020 and Sika FerroGard
903, for possible remediation of chloride contamination to a car park