Duplex 2205 stainless steel - corrosion resistance for nuclear power plants

Designers of cooling water systems for nuclear power plants now have an additional option when it comes to choosing corrosion resistant piping material.

Piping in 2205 duplex stainless steel, which contains around 3.2 per cent molybdenum, can reduce maintenance costs, shutdown times and improve plant safety compared with other types of stainless steel piping.  Most nuclear plants currently in operation were built using carbon steel piping, sometimes lined with either cement or rubber, in their cooling water systems.  The most common problems with these systems are fouling (blockage) and leakage, often caused by microbiologically influence corrosion (MIC), which is a very aggressive form of corrosion caused by microbes in the water.

Reliability issues with this type of piping mean constant surveillance and consequently high maintenance costs. As a result, some plant operators replaced their carbon steel piping with stainless steel, either Type 304L (UNS S30403), Type 316L (UNS S31603) or 6% Mo stainless steel (UNS N08367).  With aggressive untreated waters, where MIC is a concern, Types 304 L and 316L are not viable options which means plant operators either have to upgrade to a 6% Mo grade or accept the high maintenance costs of carbon steel.

Thanks to its molybdenum content, 2205 duplex stainless steel has improved corrosion resistance and is much more resistant to MIC than either 304L or 316L stainless steels. It has been used by the Duke Power Company in field trials at its Catawba Nuclear Station in South Carolina, USA.

In the trial, which began in 2002, 2205 was used to replace the above-ground portion of piping that supplies make-up water to the main condenser cooling towers. The original system was made from lined carbon steel piping.

The project used 60 metres of 76.2 cm and 91.4 cm diameter piping with a standard wall thickness of 0.95 cm. The system was constructed in accordance with Code Case 153 for ASME B31.1, the Power Piping Code, which is part of the specifications governing the safe use of piping in power plants throughout America and is widely used in other parts of the world.

A thorough inspection of the system took place after 500 days and performance was found to be very good, revealing no evidence of fouling or corrosion. The piping has continued to perform very well over the last years, with the result that the Duke Power Company now has plans to use 2205 piping in other sections of their system.