Corrosion Control in the Desalination Industry

Desalination is a viable solution to the 21th century´s shortage of freshwater. The most widely used desalination processes are thermal and membrane. Other modern techniques apply solar and electrical energy for evaporation and electrodialysis. Many desalination plants (DP) are located in desertic/arid regions with a harsh climate and limited rainfall. About one-fifth of the DPs operate in the Middle East, with Saudi Arabia producing half of the world desalted water. The final selection of materials of construction for plant equipment must be a compromise between technological and economic factors. Corrosion problems and their solutions in several DPs in the Middle East, USA and Mexico are presented.


Introduction
The environment quality, worldwide water scarcity and clean energies have been established today as central disciplines in modern science, engineering and technology .They are already being linked to the crucial, actual problems of climate change and global warming. Innovative desalination technology of saline water (SW) contributes to alleviate these problems by producing fresh water DPs have a high level of corrosion risk since they handle and process aggressive SW under severe operating conditions which include filtration, heat exchange, distillation, evaporation, circulation and high flow velocities .These SW sea, brackish and brines cause localized corrosion such as pitting, crevice, galvanic and stress corrosion. In addition, biological fouling and mineral scaling are frequent nuisances that alter the equipment surface performance and induce corrosion [1].

The Desalination Industry
Due to an increased population growth and the expectation of high living standards, demand for water and electricity in the desertic and arid regions of the world is soaring. Placing DPs combined with power generating units allows the heat extracted from the process to evaporate seawater. Desalination is the most viable solution to the 21th century's shortage of fresh water to be obtained from sources of saline waters (SW), mainly seawater and brackish water. Actual innovative desalination technology is less energy-consuming and more environmentally friendly. The economic and social relevance of the desalination industry is evident by the activities of the diverse international and national professional associations and R & D institutions involved in all the aspects of desalination science, engine ring and technology. It includes authorities from government, industry, and academia to address progress of vital importance for national and global prosperity [2]. This is a modern, dynamic industry based on the principles and practices of water chemistry and chemical engineering; it requires varied engineering materials, structures, installations, equipment and machinery that should function with industrial efficiency and labor safety to assure its economic performance. Use of suitable corrosion resistant alloys (CRAs): titanium, stainless steels, Ni-base alloys, Cu -Ni alloys and naval aluminum is the most direct means of preventing corrosion. Corrosion resistance is the main property to be considered in the choice of materials for plant equipment. Today about 15,000 DPs operate worldwide with an estimated total production capacity of 32 million m 3 /day, in the Mediterranean coast countries, the Middle East, South America deserts, the Canary and Caribbean islands, all places with limited water supplies. The world largest plant in Saudi Arabia produces 1 Mm 3 /day.An advanced seawater DP was installed in 2005 in Ashkelon,Israel with a capacity of 100 Mm 3 /year. It is operated by IDE Technologies, uses Seawater Reverse Osmosis (SWRO) technology and employs state-of-the-art means for recovery of energy from an independent, combined cycle electricity station, with a capacity of 80 MW [3,4].

Desalination Processes and Plants
There is no universal desalination process; every tipe of SW requires a process adapted to its characteristic and performance. The DPs are fed with seawater, containing 35 g/l of total dissolved solids (TDS) or brackish water with TDS in the range 2 to 5 g/l, taken from briny well or wells infiltrated by seawater. Two main desalination technologies are implemented worldwide: Membrane separations process e.g. Reverse Osmosis (RO). Under high pressure the water molecules contained in seawater pass through a selective membrane while the dissolved salt ions do not pass through the membrane. The membranes are made from polymers specially developed and manufactured to serve in DPs. The thermal processes are based on improved distillation, evaporation and condensation technologies with the aim to save energy and to obtain fresh water with a low level of TDS and at a low cost operation. In general, the thermal processes are more expensive than RO but distillation produces pure water independent of the quality and salinity of the feed water. The cost of desalting brackish ground water is generally less than the cost of desalting seawater due to a lower TDS content. The average expense for desalting brackish water is 0.50 USD/m 3 and for seawater 1.5 USD/m 3 .

Desalination Equipment and Materials
To maintain this continuous and effective operation, a diversified assembly of equipment is employed in DPs including significant quantities of pipes of varying sizes for both operational processes and transmission lines. Table 1  The construction of DPs and the fabrication of their equipment require a wide spectrum of engineering materials, metallic and nonmetallic, that should display a reasonable endurance to the fluids handled and processed and to the plant installation environment. The prime consideration during the selection of materials of construction is their corrosion characteristics. CRAs used in the desalination industry are classified into two large groups: • Ni-containing alloys e.g. Ni base alloys, Cu-Ni alloys and stainless steels (SS).

Corrosion, Processes and Advanced Materials in Industry
In addition, nonmetallic materials such as plastics: polyethylene (PE), polypropylene (PP), polyvinylchloride (PVC) and composites in particular fiber reinforced plastic (FRP) are employed mainly for piping and storage vessels [5]. Those CRAs have an outstanding corrosion resistance, mechanical strength and weldability. This corrosion resistance is due to the tenacious, durable and self-healing, protective film of metal oxides formed in the presence of air and moisture. According to ASTM standard D 4194, SS UNS S31600 should be used for all wetted parts of RO devices. Furthermore, it warns about the use of piping made of carbon steel, cast iron and galvanized or cadmium plated carbon steel to avoid contamination by corrosion products. The DPs have a high level of corrosion risk since they handle and process aggressive SW under severe operating conditions which include filtration for feed water pretreatment, heat exchange, cooling and condensing of water vapor, distillation and evaporation at different temperature, agitation and circulation and high flow velocities, often turbulent. CRAs producers provide engineering and development services to Original Equipment Manufacturers (OEMs) to ensure their suitability to the DPs processes and fluids.

Corrosion, scaling and fouling
Corrosion, scaling and fouling phenomena appear simultaneously in DPs. Scaling and fouling have a marked effect on corrosion, often associated with SW velocities. They originate in the SW, depending on their chemical and biological composition, their interaction with the equipment surface and the plant operational conditions such as pH, flow regime, temperature and pressure. The pH values of SW are in the range of 5 to 8 and the concentration of dissolved oxygen, the main corrodent, varies from 4 to 6 mg/l as a function of temperature and flow regime. SW contaminated with hydrogen sulphide H 2 S, a reductant, are slightly acidic and corrosive towards some CRAs, therefore H 2 S should be eliminated by mechanical or chemical methods [6].
Corrosion is an electrochemical process that takes place upon the metallic surface by reaction with the components of the SW. The dominant factors are its concentration of dissolved oxygen and its salinity. Most corrosion in industrial systems, such as DPs equipment, occurs at specific areas or parts of a piece of equipment indicating that corrosion is localized. Localized attack results from differences in aeration, concentration, temperature, velocity and pH. It occurs as pits, crevices, cracks, groove and eroded areas, generating particular types of corrosion such as pitting, erosioncorrosion, under deposits and stress corrosion cracking, in diverse sectors of the DPs which are vulnerable to corrosion failures [7]. Highly concentrated SW tends to form thick scales by deposition of dissolved and suspended solid such as carbonates, silicates and hydroxides. Chemical and physical pre-treatment of feed water is required to remove substances that would interfere with the desalting operation and will damage the equipment, in particular, the plastic membranes of the RO process. Pretreatment with ozone, a powerful oxidant and biocide will remove sulphur, iron, manganese and other water-soluble heavy metals compounds, bacteria, odor and color. Some alkaline chemicals e.g. soda ash neutralize the acidity found in some brackish waters, helps reduce corrosion and extends the life of the equipment. Citric acid removes iron and polyphosphates reduce iron staining but these pretreatments are rather expensive. Scaling is controlled by introducing additives to inhibit crystal growth, reducing temperatures and salt concentration. Inorganic, colloidal particles e.g. silica or silicic acids, hydrous iron oxide, aluminum oxide and organic humic substances (HS) in the feed water foul filtration and RO membranes, therefore this fouling matter are removed from feed water by special pretreatments. Furthermore, a particular type of corrosion, known as microbiologically influenced corrosion (MIC) develops under these complex organic and inorganic deposits. The CRA desalination equipment should be maintained clean and smooth to avoid calcareous scaling on heat transfer surfaces and to diminish the propensity to biological fouling on polished surfaces. Acidic and alkaline cleaning is a mechano-chemical operation easily implemented in CRA Advanced Materials Research Vol. 95 equipment, to remove biological fouling and mineral scale since they alter the equipment surface performance and induce corrosion.

Corrosion Protection and Control.
Corrosion engineering and technology develop and apply methods and techniques of prevention and protection to avoid the interaction of the equipment and its construction materials with the corrosive factors of the DP environment. Practical methods that minimize or eliminate corrosion include selection of suitable CRAs and application of coatings, paints and linings to carbon steel and galvanized steel equipment Corrosion resistance is the main property to be considered in the choice of materials for DPs but the final selection must be a compromise between technological and economic factors. It is sometimes more economical to use a high-priced CRA that will provide long and trouble-free service than to use a lower priced material that may require frequent maintenance or replacement. The selected CRA should be able to perform its function safely for a reasonable period of time and at a reasonable cost. Today, the main and fastest source of information on corrosion control of industrial equipment, plants and facilities is the Internet. Data bases and computers based expert systems dealing with selection of materials, their properties and corrosion control for many environments and industries are listed in Roberge´s Handbook [8] and ASM Handbook 13 C [9]. Corrosion tests made in the service of the desalination industry may consist of several phases: laboratory, desalination pilotplant and DP. The results are useful in the CRA selection process. Modern DPs that are constructed from CRAs and that apply recognized methods of corrosion prevention and control can expect prolonged equipment service life and relative freedom from corrosion.