United States Department of Transportation
i
Development of a New Sacrificial Cathodic Protection System for Steel Embedded in Concrete
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1997-05-01
[PDF-3.26 MB]
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Abstract:This is the interim report for a study to evaluate the use of sacrificial cathodic protection for reinforced and prestressed concrete bridge members. Cathodic protection (CP) using impressed current is an accepted and common method used to provide corrosion protection to steel in concrete. Sacrificial cathodic protection, however, is relatively easy to apply, does not require that the anode be electrically isolated from the steel, and does not require an external power supply. Attempts to apply sacrificial anode CP to bridge structures have met with limited success because conventional sacrificial anode alloys operate at a low fixed potential and the conductivity of concrete often does not permit satisfactory current output. The Florida Department of Transportation has applied arc-sprayed zinc to several bridges in the marine environment. This study examined the performance of existing sacrificial alloys in different environments for sacrificial CP, and developed new sacrificial alloys for protecting steel in concrete. The study consisted of both laboratory and field tasks. The first task consisted of tests to identify suitable environments for sacrificial alloys. In this task, test blocks with and without a sprayed zinc coating were exposed to northern marine, northern semi-rural, and southern marine environments; a laboratory environment; and seawater splash zone exposure. The results indicate that a sprayed zinc anode can reduce the corrosion of embedded steel in concrete. In the second task, two bridges with a zinc sacrificial anode in the Florida Keys were inspected and tested. Data on anode current output, anode and cathode potential, and anode and cathode depolarization were recorded. The results indicate that pure zinc does not provide long-term protection because of eventual passivation in non-wet areas. The third task tested existing sacrificial alloys for their performance when applied to concrete. The laboratory studies into existing commercial sacrificial alloys did not produce favorable results and so further laboratory work to develop new alloys was conducted. The above studies indicated the need to identify a sacrificial alloy or alloys that could provide adequate CP current for the long term. The fourth task consisted of this developmental study. The result was a series of aluminum-zinc-indium alloys that are capable of maintaining reasonably high potentials and current output under a range of concrete conditions for longer than pure zinc or aluminum. Field testing on a bridge structure is scheduled.
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