Abstract
The corrosion of steel pilings in seawater was investigated during the past 30 years at two field locations in the United States, one in the Gulf of Mexico at La Costa Island, Florida (1971), and the other in Buzzard’s Bay, Massachusetts (1974). Three rows of 22 test steel H piles, 20.3 cm × 20.3 cm and 12.2 meters long, were installed at the Buzzards Bay, Massachusetts test site, where the test temperature varies from –1° C to +19° C. The steel H-plies were coated with various protection systems, such as coal tar epoxy, polyurethane, flame sprayed zinc and aluminum. Several of the piles were left uncoated for baseline comparison. Sacrificial cathodic protection was provided by anodes to some of the bare and coated steel pilings. Periodic inspections were conducted and a row of pilings was pulled out after 5 years for detailed inspections. The results of the long term coating evaluation showed that flame sprayed aluminum coating with a topcoat sealer performed best in the cooler temperature at Buzzard’s Bay and the thick polyester glass flake coating was the best performer in Florida. Coal tar epoxy coatings with zinc rich primers also performed well at both locations.
The average corrosion rates at the two test sites were determined by measuring the thickness of the flange after the steel piles were pulled out for detailed evaluation. Measurements indicate that, in some cases, concentrated corrosion was encountered, where the corrosion rates were more than twice as much within 1 m below the mean low water line, compared to corrosion rates in other regions of the immersed zone. These higher corrosion rates could be attributed to “accelerated low water corrosion (ALWC),” a rapid form of microbial assisted corrosion. The solutions include thicker steel, protective coatings, cathodic protection and design considerations.
