Concrete Reinforcement
Stainless steel reinforcing bar, welded wire mesh and cable for pre- and post-tensioning of concrete should be considered for new concrete installations, replacement, and repair in chloride (coastal or deicing salt) exposure applications. Stainless steel can be used in combination with carbon steel in concrete repair projects, when both metals are fully embedded, without concern about galvanic corrosion.
Stainless steel is not a new reinforcement material. The first known use of stainless steel reinforcing bar was the Progreso Pier in Mexico, which was completed in 1941. NACE paper 07240 documents its performance relative to a nearby carbon steel reinforced pier. Today, in corrosive coastal applications like bridges, piers and seawalls, 2304 and the more corrosion resistant 2205 duplex stainless steels are the most common choices. Types 316L and 316LN are also used.
The Hassan II Mosque in Morocco began to show structural deterioration of the carbon steel rebar only ten years after its inauguration: the concrete walls, which were built into the Atlantic Ocean, started to crumble under the steady attack of the saltwater waves. After a spectacular reconstruction, replacing large parts of the original structure and reinforcing it with 1,300 metric tonnes of moly grade 2205 duplex stainless steel, the Mosque seawall restoration should last 100 years. Read the article Duplex rebar restores resistance to waves for more information.
Larger Structural Sections - Statue of Liberty Restoration
Since it’s installation in New York in 1886, the Statue of Liberty has become one of the most well known sculptures in the world. However, many people are not aware that galvanic corrosion caused structural deterioration, making a significant restoration necessary in 1986.
The original structure of the 46-meter (151-foot) statue had a framework of puddled iron consisting of three distinct components – a central pylon, secondary framing and armature bars. The central support pylon has four legs connected by cross bracing. A double-helix staircase rises through its center. The secondary iron framework connects 1850 puddled iron bars (51 x 16 mm or 2 x 0.625 inches) to the pylon. These iron bars create the armature support for the copper sheets. About 1,500 U-shaped copper saddles connected the armature to the outer copper envelope sheets (2.4 mm or 0.094 inch thick and weighing 73 metric tons or 80 short tons).
Galvanic corrosion occurs when dissimilar metals such as iron and copper are placed in direct contact and there is regular exposure to an electrolyte (moisture containing salt and pollutants). The relative position of metals in the galvanic series and their surface area ratio can be used to predict the likelihood of galvanic corrosion. See the Nickel Institute publication Guidelines for Corrosion Prevention 11 024 for more information. Copper is much more noble than iron. The design details that led to corrosion problems on the Statue of Liberty are similar to those of some of the curtain wall and sunscreen galvanic corrosion failures that have occurred on buildings.
Designers Auguste Bartholdi and Gustave Eiffel anticipated the problem and tried to electrically isolate the materials. Unfortunately the envelope design and subsequent modifications to it allowed rain containing coastal salt and acid from pollution to seep behind the copper. Condensation from temperature and humidity changes and humidity also added the moisture necessary for corrosion.
The designers originally used shellac-impregnated asbestos pads and paint on the iron to separate the metals. However, both failed and galvanic corrosion began to occur. At least 10 paint coatings were applied to the copper over the 100-year period and then layers of paint were applied to both the copper and iron to try to extend the structure’s life.
Severe corrosion of the iron armature had occurred by 1981 and it’s expansion due to corrosion also caused copper damage. Molybdenum-containing Type 316L stainless steel was chosen to replace the entire iron armature of the statue except for a few bars in the right sole of the foot. It was selected because no galvanic corrosion problem would be expected between copper and stainless steel in this type of environment and their coefficients of thermal expansion are the same. Although it was not needed based on the environmental conditions, skived Teflon was used between the metals to electrically isolate them in case the environment became more severe.
Originally the armature bars were connected to the secondary iron framework. The new Type 316 armature is now supported by a high-strength, highly corrosion resistant molybdenum containing duplex stainless steel (UNS S32550) secondary framework. The original central angle iron secondary framework was retained after cleaning and application of both a zinc primer and epoxy coating. These layers protect it from further corrosion and help to electrically separate the iron from the stainless steel.
Type 316 stainless steel was also used to replace the circular stair treads, for emergency elevator components, and for a redesigned pedestal stairway.
