Titanium and super stainless steel welded tubing solutions for sea water cooled heat exchangers

Owing to its outstanding resistance to general and localised corrosion, titanium has become established as the preferred material for sea water cooled heat exchanger tubing, be it for power plant surface condensers or heat exchangers in thermal desalination plant or in the chemical and petrochemical processing industry. Periodic increases in the price of titanium have generated interest in cost effective alternatives using so called 'super alloys': highly alloyed stainless steels showing improved corrosion resistance over conventional compositions. Taking sea water cooled power plant condenser tubes as an example, the mechanical and corrosion/erosion properties of titanium are reviewed together with those of three frequently considered alternatives, UNS S31254 super austenitic alloy, and UNS S44735 and S44660 super ferritic alloys. The results of ASTM G48 pitting and crevice corrosion tests and of electrochemical studies in model sea waters are reported. The effect of heat treatment on the corrosion resistance of S44735 has also been examined. It is concluded that while titanium has a superior balance of corrosion and other properties, super alloys can provide excellent performance in service provided certain precautions are observed.