Tech Briefs

Liquid Coatings for Reducing Corrosion of Steel in Concrete

Structures could be protected more easily and less expensively.

Inorganic coating materials are being developed to slow or stop corrosion of reinforcing steel members inside concrete structures. It is much simpler and easier to use these coating materials than it is to use conventional corrosion-inhibiting systems based on impressed electric currents. Unlike impressed electrical corrosion-inhibiting systems, these coatings do not require continuous consumption of electrical power and maintenance of power-supply equipment. Whereas some conventional systems involve the use of expensive arc-spray equipment to apply the metallic zinc used as the sacrificial anode material, the developmental coatings can be applied by use of ordinary paint sprayers.

A coating material of the type under development is formulated as a liquid containing blended metallic particles and/or moisture-attracting compounds. The liquid mixture is sprayed onto a concrete structure. Experiments have shown that even though such a coat resides on the exterior surface, it generates a protective galvanic current that flows to the interior reinforcing steel members. By effectively transferring the corrosion process from the steel reinforcement to the exterior coating, the protective current slows or stops corrosion of the embedded steel. Specific formulations have been found to meet depolarization criteria of the National Association of Corrosion Engineers (NACE) for complete protection of steel reinforcing bars ("rebar") embedded in concrete.

A coating of this type can be applied thick enough to afford protection for ten years or longer. The coating can easily be maintained or replaced to ensure continued protection of the reinforcing steel for an indefinite time.

The costs of protecting structures by use of these coating materials are expected to be less than (or in some cases, comparable to) the costs of protection by most conventional methods:

  • Typical costs of installing impressed-electric-current systems range from 10 to 30 dollars per square foot (about 110 to 330 dollars per square meter) [prices as of year 2000]. After installation, these systems incur additional costs of electrical power, inspection, and maintenance.
  • The costs of installing sacrificial systems based on thermally sprayed zinc typically range between 10 and 20 dollars per square foot (about 107 to 215 dollars per square meter). Like the present developmental systems, sacrificial-zinc systems require very little maintenance once they are installed.
  • Another type of sacrificial system involves the use of zinc sheet and electrically conductive glue. The costs of installing these systems typically range from 8 to 18 dollars per square foot (about 86 to 194 dollars per square meter). These systems also require very little maintenance after installation. Both this and the preceding sacrificial-zinc system have been said to offer 10-year life expectancy. However, according to NACE, pure zinc coats on concrete structures provide only partial protection because of their low driving voltages. Upon exposure, the zinc can become passivated, such that during dry weather, it does not supply protective current to steel rebar.
  • The costs of protecting structures by use of the developmental coating materials have been estimated to range from 5 to 9 dollars per square foot (about 54 to 97 dollars per square meter).

This work was done by Louis G. MacDowell of Kennedy Space Center and Joseph Curran of Dynacs, Inc.

This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Technology Programs and Commercialization Office, Kennedy Space Center, (321) 867-8130. Refer to KSC-12049.