Seawater resistance and PREN

In seawater and brine, stainless steels fail mainly by pitting and crevice corrosion. Resistance is ranked by the pitting resistance equivalent number, PREN = %Cr + 3.3 × %Mo + 16 × %N. As a guide, a PREN of 40 or more is generally needed for reliable service in natural seawater at ambient temperature, especially at crevices such as flange gaskets.

Reverse-osmosis plants

In seawater reverse-osmosis (SWRO) plants, high-pressure pumps feed seawater to the membranes at pressures of around 60–80 bar, and concentrated brine leaves the membranes at similar pressure. The high-pressure feed and brine piping is typically super duplex pipes and tubes with super duplex flanges and fittings, chosen for their strength and resistance to pitting and crevice corrosion. Low-pressure pretreatment and intake lines often use GRP or lined pipe, with duplex, SMO 254 or 316L for metallic parts, and product-water lines use 316L.

Thermal desalination

Multi-stage flash (MSF) and multi-effect distillation (MED) plants use large evaporator shells, water boxes and tube bundles. Titanium pipes and tubes are widely used for heat-transfer tubes in seawater, together with copper-nickel alloys, and evaporator shells and internals are made from duplex steel sheets and plates, SMO 254 sheets and plates and 316L. Duplex steels have largely replaced clad carbon steel in modern evaporator shells.

Water treatment and seawater cooling

Municipal and industrial water treatment plants use 304L and 316L stainless steel for piping, tanks and filters, with 316L or duplex where chlorides or chlorination levels are higher. Power stations and process plants on the coast use SMO 254 pipes and tubes, super duplex and titanium in seawater cooling systems and condensers.

Flanged joints and bolting

Flanged joints are the most common site of crevice corrosion in seawater systems, because the gasket face is a tight crevice. Super duplex flanges with non-absorbent gaskets, and super duplex or other compatible bolting, are used so that the joint is as resistant as the pipe. Carbon steel bolts on stainless or duplex flanges corrode quickly in seawater spray and should be avoided or protected.

Fabrication quality

Much of the corrosion seen in desalination plants starts at welds rather than in the base metal. Duplex and super duplex welds must be made with qualified procedures, controlled heat input and correct filler so that the ferrite balance and corrosion resistance match the pipe; heat tint must be removed and welds pickled. Supplying material with full mill certification, and specifying ferrite and G48 testing of weld procedures, helps ensure long life.

Products for desalination

We supply seamless pipes and tubes, welded pipes and tubes, flanges, pipe fittings, plates, sheets, round bars and fasteners in these materials, with super duplex bolting to avoid galvanic corrosion at flanged joints. Supplementary tests – ferrite measurement, impact testing, ASTM A923 and ASTM G48 pitting tests – and third-party inspection are available on request. All industries we serve are on the industries page.