Desalination is the process of removing minerals, predominantly salt, from seawater for human consumption or the irrigation of crops. Although it is a relatively energy intensive process, and therefore more costly than freshwater extracted from rivers or groundwater, it is required in many regions of the world where freshwater is scarce.

There are currently around 19,000 desalination plants in operation around the world, supplying more than 300 million people with their needs. However, this number is expected to increase significantly in response to population growth and development.

Desalination processes

Thermal desalination

Here, the water is purified by evaporating it from the seawater. The vapour given off is condensed on cooling, leaving the impurities behind. The two most common forms of thermal desalination are:

Multistage flash distillation - the product of the heated water is reheated multiple times, each time at a lower pressure than the last. By using waste process heat from conventional power plants, which is high-volume, low temperature and otherwise difficult to exploit, it is possible to operate such plants with far less energy than alternative processes such as reverse osmosis.

Multiple-effect distillation - the salt water solution is heated and the pure water produced flows into the next chamber. The heat energy it carries is used to boil it again, producing more vapour.

Filtration desalination

Reverse osmosis dominates in terms of the number of plants in operation. Here, pressure is used to push the water solution through a membrane, with the membrane preventing the larger solutes (the salt) from passing through. It is generally considered to be the least energy consuming of all the large-scale processes.

Desalination processes handle waters of varying compositions, either as a feature of their location (seawater, brackish water) or the process employed (concentrated seawater, distillate). The input pH is usually between pH 6.0-8.0, and the level of oxygenation and chloride ions will vary by location.

Corrosion challenges

Erosion corrosion

Erosion corrosion is an acceleration in the rate of corrosion due to the relative motion of a corrosive fluid and a metal surface, due to the repeated removal and repair of the protective passive layer. This can occur, for instance, when sand is entrained into elements of the desalination system handling seawater.

Erosion-corrosion is most prevalent in soft alloys. Corrosion resistant alloys that form a protective passive film commonly show a limiting velocity above which corrosion rapidly accelerates. Generally speaking:

  • Carbon steels (including cast irons) will corrode appreciably at all velocities and are unsuitable, unless protected, for long-term application in pumps, valves and pipework exposed to a corrosive environment.
  • Copper-base alloys and austenitic cast irons perform reasonably well at low and moderate flows, but at high velocities they will suffer erosion-corrosion at about 1 mm/year.
  • Stainless steels and nickel-copper alloys show practically no corrosion at low velocities, but will have a general tendency to pit. At velocities > 1 m/s further pitting will stop and there is negligible corrosion up to 40 m/s.

Corrosion fatigue

This kind of fatigue is only really a concern in components that are subject to high levels of repeated cycling i.e. shafts found in pumps. Stainless steels are widely used shaft materials for seawater and brine pumps. In general, alloys with high strength and corrosion resistance have good corrosion fatigue strength.

Duplex and super duplex stainless steels are attractive - particularly as they have much higher strength than standard austenitic grades allowing their use at higher design stresses (reducing diameter and cost). Also, these alloys often have improved pitting and crevice corrosion resistance.

Ferralium® 255 - SD50 is widely used in all levels of pumps, with yield strengths of > 85 ksi (586 N/mm2) and a Pitting Resistance Equivalent number (PREN) > 40. Our high-performance Fermonic® 50 austenitic stainless steel is also used for this application, supplied in high-strength form with a yield strength of > 105 ksi (725 N/mm2) and a PREN of 34, while retaining excellent toughness at cryogenic temperatures.

Pitting and crevice corrosion

In chloride-containing near-neutral solutions, pitting and crevice corrosion will mostly influence the choice of materials. Pitting corrosion is a form of extremely localised corrosion leading to the creation of small holes. It occurs at weak points in the protective passive surface layer, and requires both the presence of an aggressive species in the environment (chloride ions) and an oxidising potential (aerated water).

Crevice corrosion often occurs under deposits and at joints of fabricated components. In such regions the water may have a different level of aeration than elsewhere, resulting in an increase in metallic ions that attracts chloride ions to the crevice to maintain charge neutrality. The environment within the crevice becomes increasingly concentrated, forming hydrochloric acid and giving accelerated pitting corrosion.

Pitting resistance equivalent number (PREN)

Note: PREN = %Cr + 3.3× %Mo + 16× %N

Super duplex stainless steel grades achieve excellent resistance to pitting corrosion through their higher chromium content combined with molybdenum and nitrogen. Given the high cost (and volatility) of nickel prices, their relatively low nickel content can be an important cost advantage for their specification.

Another simple way to review the suitability of metals is the Critical Pitting temperature (CPT) which is a standardised laboratory test. Samples are exposed to an aggressive corrosive solution, with the CPT representing the temperature at which pitting corrosion is initiated. Sometimes clients will use the basic rule-of-thumb that metals should have a CPT > 40°C - something that is met by most metals that Langley Alloys presents to the market.

Practical materials selection

Alloy 316L

For a number of years Alloy 316L has been used. It is a common and widely available grade with reasonable corrosion resistance to freshwater, but in aerated seawater its level of performance is no longer accepted. It is therefore limited to use in the 'processed-side' of the plant i.e. in fresh water.

Alloy 317L

A step-up in performance can be achieved by using Alloy 317L, which benefits from additions of molybdenum and higher chromium content to increase its resistance to pitting corrosion. Performance against seawater is acceptable if there is little oxygen present.

Alloy 2205

Alloy 2205, a duplex stainless steel, has been increasingly used in a number of process industries - particularly in pulp and paper mills. Duplex alloys have improved corrosion resistance, and as their strengths are typically twice that of standard austenitic stainless steels, they also have much superior resistance to stress corrosion cracking. Therefore, it has been widely used for shells of large thermal desalination (multistage flash) plants. Despite this, it will not resist corrosion if exposed to highly aerated seawater.

The next step in performance

The next step-up in performance is achieved from super duplex stainless steels such as Ferralium®, with PREN values of > 40 providing excellent resistance to pitting, crevice and stress corrosion cracking. As such, they are widely used in applications where the seawater is heavily aerated, up to operating temperatures of approximately 35°C.

Ferralium® has been widely used in pumps and valves, in pipework and selected sections of the desalination plant. A general rule has been to use these higher corrosion resistant alloys on the 'seawater-side' of the plant.

Other considerations

In specific applications, greater corrosion resistance may be required. An example of this is in reverse osmosis desalination plants, where the backwash solution for cleaning membranes can be more acidic in nature.

Issues may also occur during plant stops and maintenance periods, as standing water may become significantly more aerated than during typical operation. An extension of this is the formation of bio-films and slime during shutdown periods, which then support crevice corrosion which persists long after re-start.

Common materials for pump and valve bodies include cast austenitic, duplex and super duplex grades of stainless steels. Duplex and super duplex grades such as Ferralium® are selected because of their improved corrosion-erosion and wear resistance in handling slurries and brines.

Copper alloys are widely used in applications such as heat exchangers, where their thermal conductivity is required. Although nickel alloys will perform well in seawater applications, costs are prohibitive for widespread application.