Page 258 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

236 CORROSION CONTROL AND PREVENTION

4.9.1.10 Electrochemical Chloride Extraction This method has been reviewed
by Virmani and Clemena (16). When a direct current is passed through concrete, the
mobile ions such as chloride, hydroxide, sodium, potassium, calcium will migrate,
with each ion moving toward the electrode of opposite charge. The feasibility of
removal of chloride from concrete without the excavation of contaminated concrete
from a structure was explored in the 1970s by the Kansas DOT. Chloride ions could
be expelled from concrete by passing direct current between the steel bars and anode
as in CP at considerably greater current densities. However, the high levels of current
used resulted in adverse effects on the concrete such as decrease in concrete-to-steel
bond strength, increase in porosity, and increased cracking in concrete. These
observed effects resulted in decreased interest in this method of chloride removal
from concrete. Subsequent studies showed that in keeping the current level below
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2
5 Amp/m (0.5 A/ft ) the adverse effects could be avoided.
2
Even at densities of the order of 5 Amp/m the possible hydrogen-induced crack-
ing does not favor electrochemical method of chloride removal from prestressed con-
crete structures. Pilot scale treatments showed it to be feasible and simple to treat
full-sized reinforced-concrete bridge members, although difficult to conduct the treat-
ment on concrete piers. One of the main difficulties is to predict the duration of
treatment to reach the chloride levels to acceptable levels where corrosion is under
2
control. Preliminary studies suggested a total charge of 600–1,500 A-h/m with a total
treatment time of 10–50 days.
It is impossible to remove all the chloride from concrete by the electrochemical
method. But the level of chloride in contact with the steel is reduced by 45–95%. Field
data have shown that the removal of chloride by electrochemical technique results in
stopping corrosion for 8 years. It is predicted by FHWA that the electrochemical
method of removal of chloride will extend the life of bridges by as much as 20 years
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2
(31). About 372,000 m (4,000,000 ft ) concrete has been treated by this method.
The costs of electrochemical removal of chloride are:
1. Treatment of bridge decks: $53–129/m 2
2. Treatment of substructures: $107–215/m 2
3. Treatment of very small substructures: $269/m 2

4.9.1.11 Deicing Salts Calcium magnesium acetate (CMA) and potassium acetate
(PA) have been found to be the most promising deicing agents. These salts contain
76% and 61% acetic acid, respectively. Annually, about 15.4 billion kg (17 million)
tons of rock salt (sodium chloride) are used for deicing in the United States. A study
conducted in 1987 showed that 910 kg (1 ton) of road salt costing only $50 caused
more than $1450 in damages to vehicles, bridges, and the environment. CMA cost
is $1–10/kg versus $0.04/kg of NaCl. Because of the large disparity in cost, CMA
usage will be limited to critical structures sensitive to corrosion.
CMA is not only costly but also slower in action than rock salt. When applied
as a solid, CMA exhibits marginal performance in light traffic freezing rain and dry
and cold storm conditions. However, when CMA is applied as a solid or concentrated
solution, the rate of action is similar to that of rock salt. The New York City DOT has