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Molybdenum (Mo) is used in several types of stainless steel to improve corrosion resistance, particularly pitting and crevice corrosion resistance in chloride-containing solutions.

When it comes to building or repairing reinforced concrete bridges so that life-cycle costs are minimized, many authorities are specifying the use of stainless steel reinforcing bars (rebars) instead of the traditional carbon steel rebars.

Stainless steels containing molybdenum (Mo) are the alloys of choice when "stainless rebar" is specified. The most commonly specified alloys are shown below (nominal compositions in weight %, balance iron):

These alloys can provide long-term corrosion resistance when concrete is exposed to chloride-containing environments, e.g., road salts and sea water. They have been used as rebar in highway bridges, ramps & barrier walls, parking garages, sea walls & marine facilities, building foundations & restorations, and tunnels.

The US Federal Highway Administration (FHWA) organized extensive corrosion testing to find rebar materials that could extend the lifetime of reinforced concrete bridges to 75-100 years, when the concrete was contaminated with chlorides. Their 1998 report concluded that Type 316 stainless steel rebars would be capable of providing that required lifetime.

Image courtesy of Dunkirk Specialty Steel

In Europe, Mo-containing stainless steel rebars have been widely used over the last 20+ years. In North America, their use in highway bridges has been steadily growing over the last 10 years, with many large bridges being constructed or extensively repaired. Two highway bridge projects that are currently underway help to illustrate the use of stainless rebar in the USA.

The Driscoll Bridge.

This bridge will span the Raritan River in New Jersey. When completed, the new bridge will carry 8 lanes of vehicles. Type 316LN and Alloy 2205 stainless steel rebar was specified for use in the bridge deck and other elements. Thus far, a total of 1300 tons of stainless rebar has been specified.

The Woodrow Wilson Bridge

This bridge system will span the Potomac River to link Virginia and Maryland. The first 12-lane bridge (of the twin bridges) is scheduled for completion in late 2005/early 2006. The second bridge should be completed in late 2007/early 2008. Thus far, 1100 tons of stainless steel rebar (Type 316LN and/or Alloy 2205) has been specified. An aerial view of the project can be viewed by linking to the following web site: www.roadstothefuture.com/Woodrow_Wilson_Bridge.html

The use of Mo-containing stainless steel rebar is expected to continue to increase as Federal, State, Provincial and municipal authorities demand much lower maintenance costs and longer lifetimes for their road and highway bridges. Along with the stainless rebar, other stainless steel products such as tie-wire, couplers and dowels are used to complete these corrosion resistant structures

For additional information on Mo-containing stainless steels, please visit the IMOA web site at www.imoa.info or contact Dr. Nicole Kinsman, IMOA Technical Director at nkinsman@imoa.info