High alloyed stainless steels and nickel alloys have proven their resistance to seawater in many marine, offshore and oil and gas applications but have not been widely used in seawater plate heat exchangers in the past. Therefore, immersion tests in artificial seawater under crevice conditions described in ASTM G-48 D for multiple crevice washers and electrochemical studies have been performed to examine their suitability for seawater cooled plate-type heat exchangers. The materials tested were the 6%Mo stainless steels alloy 926 (UNS N08926) and alloy 31 (UNS N08031), as well as the nickel alloys 825 (UNS N08825), 625 (UNS N06625), C-276 (UNS N10276) and 59 (UNS N06059). The chlorine level was set at 3mg/L to simulate one intermittent chlorination a day and the temperature set at 50°C and 75°C. In nonchlorinated seawater only the alloys 825 and 625 showed slight crevice attack at 75°C. In chlorinated seawater alloy 31 was resistant at 50°C; only alloy 59 was fully resistant at both 50°C and 75°C.
From the test results it may be concluded that alloy 59 is a promising candidate material for seawater cooled plate-type heat exchangers whereas under the conditions studied alloy 31 might be limited to temperatures up to 50°C.
Many chemical plants and offshore facilities use heat exchangers cooled by seawater. Further seawater applications are water boxes, pumps, piping systems, fittings, valves, fasteners, filters, condensers, evaporators, overlay welding, etc. In recent years, a notable increase in the offshore oil and gas production has been observed. Also, requirements for large volumes of cooling water by modern industry have often resulted in plants being situated near the ocean, particularly in arid areas such as the Middle East. This has increased the need of suitable materials for handling seawater, particularly as the marine environment is accepted as the most corrosive natural environment /1/. In particular, seawater cooled plate type heat exchangers experience severe corrosive conditions. To avoid biofouling by microorganisms oxidizing biocides like sodium hypochlorite are often used in the industrial seawater systems. Hypochlorite and the associated free chlorine have a strong impact on corrosion, since it shifts the redox potential to more positive values /2/. These conditions are highly corrosive to many stainless steels and may cause severe crevice and pitting corrosion.
Titanium plays a special role in cooling systems, particularly where seawater or brackish water is used as a coolant. It is used in the chemical, petrochemical and refining industry, offshore industry, marine and ship building industry as well as power generation. Titanium is resistant to seawater at all temperatures even in the presence of hypochlorite and has been used in almost all seawater cooled heat exchanger applications in the past. In order to eliminate corrosion and heat transfer problems from production plants, cooling systems consisting of a primary and a secondary cooling circuit are frequently adopted. The primary circuit is a closed circuit and removes the heat from the production equipment using conditioned plant water.