August 12, 2016
As much as 40% of the global population is affected by potable water shortages. Not only does it have to be safe to drink, all countries need fresh water to support agriculture. Yet a very high proportion of our world's water reserves are salty or brackish and not fit for human consumption.
The solution? Desalination of seawater that is readily available from oceans has been an established technology for many decades. Today, development of that technology is growing rapidly in response to a steadily increasing global population.
Nickel alloys and nickel-containing high alloy stainless steels are playing a key role. These alloys have the right combination of properties to resist the extremely corrosive conditions that prevail in desalination plants.
The drive to desalinate
Modern desalination plants operate by utilising either distillation or reverse osmosis of seawater.
In thermal distillation plants, seawater is heated and evaporated to produce distilled water which is clean and safe to drink. The technologies most commonly employed are multi-stage flash (MSF) or multiple-effect distillation (MED).
In reverse osmosis (RO) plants, high pressure pumps are used to drive sea water through a membrane that permits clean desalinated water to pass through, but blocks the passage of dissolved salts found in seawater.
For both of these desalination technologies, the plant construction materials employed require high strength and excellent corrosion resistance.
Alloy answers under pressure
The older seawater distillation plants widely used copper-nickel alloys such as 90Cu-10Ni (UNS C70600) and 70Cu-30Ni (C71500) for many of the desalination plant elements.
The current interest around the world in larger RO plants requires construction materials that can perform well at high pressures and in corrosive seawater for up to 40 years. RO plants operate in pressurised seawater up to 90 bar and, temperatures up to 40°C, with biological contaminants and chlorine additions up to 30ppm.
Under these conditions, the standard austenitic and duplex stainless steels have experienced some localised corrosion attack, particularly crevice corrosion at pressurised seawater pipe flanges.
To overcome this, modern RO plants are constructed from nickel-containing super-austenitic and super-duplex stainless steels. Alloys in the 6Mo family (e.g. N08367 and S31254) and the super-duplex family (S32750 and S32760) are particularly well suited to deliver the long life for piping and fabricated components in a wide range of desalination plant environments.
The superduplex alloys have similar corrosion resistance to 6Mo super-austenitic stainless steel, but the duplex alloy has the added benefit of higher strength and improved fatigue resistance making it a more optimal material for high-pressure seawater feed piping and brine pumps.
These high alloy stainless steels are now being extensively used in RO desalination plants around the world, in efforts to reduce potable water shortages in the future.