The Oregon Department of Transportation (ODOT), using highly alloyed stainless steel reinforcing bar in its concrete structures, is building a bridge here that is expected to provide maintenance-free service for an amazing 120 years, nearly 2.5 times the service life of the bridge it is replacing. The new bridge, carrying U.S. 101 over the Haynes Inlet Slough near the coastal town of Coos Bay, is using what is believed to be more stainless steel rebar than any bridge in North America – 362,878 kg or nearly 400 tons. Yet this is not an ordinary stainless steel because it had to meet some very challenging requirements for corrosion resistance, strength and site seismicity. Along the Oregon coast, the marine environment is very hostile to bridges. Salt-laden air and fog from the Pacific Ocean condense under the deck and T-beams of this bridge. Wind blows the chloride-containing moisture underneath the structures, initiating corrosion. Rain washes the chlorides off the road surface, but flushes away nothing below. Farwest Steel Corp., Eugene, Ore.,a steel distributor and rebar manufacturer, suggested trying Alloy 2205, a duplex stainless steel provided by Talley Metals Technology, Inc., Hartsville, S.C., a subsidiary of specialty materials expert Carpenter Technology Corp. ODOT was counting on Farwest Steel to fabricate the rebar it needed from rolled mill stock. Talley’s 2205 stainless has a duplex microstructure, mixing austenite and ferrite phases, that gives the alloy the required 75 ksi yield strength. This is 25% more yield strength (per ASTM-A-955) than that of austenitic Types 316LN and 304 stainless steels (60 ksi) which are more common candidates for bridge rebar. With a careful balance of chromium, molybdenum and nitrogen content, 2205 stainless steel offers superior resistance to chloride pitting, crevice corrosion, stress corrosion cracking and general corrosion in many environments. Its resistance to corrosion is substantially better than that of the Type 316LN stainless ODOT used in previous bridge reconstruction.
