Managing corrosion in desalination plants

1 Introduction

Nowadays, humankind is afflicted by three crucial problems: the worldwide water scarcity, the generation of clean energy without greenhouse emissions and the production of food for a population of about eight billion people.

Corrosion and pollution are pernicious processes that impair the quality of the environment and the durability of structure engineering materials. They are intertwined phenomena as many pollutants accelerate corrosion and corrosion products: rust, oxides, and salts contaminate water bodies, e.g. lakes, rivers, and aquifers, that supply water for human consumption. The viable solution for this grievous situation is the desalination of saline water (SW) to obtain fresh water (Schorr, 2011).

Fresh water is used in three vital sectors of human life and economy: domestic for body and home sanitation; agricultural for irrigation of crops to grow grains and fruits; and industrial production of food for human and animal nutrition. Figure 1 shows the global water distribution.

Figure 1:

Distribution of world water.

A desalination plant (DP) is a complex, huge organized structure managing physicochemical processes. DP involve operations comprising filtration, distillation evaporation, agitation, circulation, and heat exchange employing diverse equipment such as pumps, pipelines valves, turbines, and compressors and using corrosion-resistant materials, metallic and non-metallic, to manufacture this equipment. This whole structure exhibits a high level of corrosion, handling corrosive SW, under continuous turbulent and regular motion of fluids, sometimes at elevated temperatures. Corrosion control methods and techniques are implemented to prevent, mitigate and avoid corrosion.

Numerous national and international professional associations of chemists, engineers and economists; research and development centers, institutes and universities and global industrial enterprises dealing with the diverse aspects of the desalination industry (DI) demonstrate the economic and social importance and relevance of this worldwide industry. The European Desalinization Society organizes congress, conferences and courses on all the scientific, technical and economic aspects, dealing with the central subject of application of engineering materials. The cooperation of government, academy and industry authorities will assure the supply of fresh water demanded by the inhabitants of our planet (Table 1).

To prevent the dangers of bioterrorism by poisoning water, private safety and security organizations are looking after the DPs. Moreover, the science, engineering and technology of desalination is treated in a great number of books and journals, including the management and control of corrosion in DPs (Table 2).

Because of the great abundance of scientific and technical information, editorials, publishers and educational authorities use reviews as a practical tool for dissemination of knowledge. These reviews devote a section to the analysis of the corrosion issues and their solutions, in the relevant industries. The authors of the present work have already published reviews in this journal on corrosion management in industries such as SW (Charrach et al., 1990), geothermal wells (Valdez et al., 1999), petrochemical corrosion (Schorr, 1996), phosphoric acid corrosion control (Schorr & Valdez, 2016), corrosion in electronics devices, corrosion in natural gas facilities (Valdez et al., 2015) and corrosion inhibitor applications (Cheng et al., 2018), in boiler corrosion (Charrach et al., 1990).

2 Water: fresh and saline

As the world’s population grows, fresh water resources are getting increasingly scarce. Nevertheless, there is plenty of water around, in oceans and seas, but they are salty, not suitable for human consumption, therefore driving a global boom for the desalination industry (Habib et al., 2001).

The significant difference in chemical composition between fresh and SW determines the diverse corrosion performance of the engineering materials utilized in the sectors of a DP, which treats and manages these waters (Table 3; IDE Technologies, 2004, 2014; IDE Newsletter, 2014).

The US Geological Survey groups SW in three salinity categories: low 1000–3000 ppm; moderate 3000–10,000 ppm and high 10,000–35,000 ppm. Some SW are slightly alkaline because of the presence of carbonate, bicarbonate and hydroxide compounds.

ASTM Standard D 1129 deals with the “Definition of Terms Relating to Water” (Annual Book of ASTM International Standards). It classifies SW in three categories: seawater, brackish water (BW) and brines; it presents the characteristics and properties of water, the phenomena typical of water behavior such as corrosion, erosion, scaling and fouling, the water soluble salts, suspensions and sediments and more.

The oxidation-reduction potentials of water measures the ratio of the activities of the oxidized to the reduced species present in water. This value (in volts) is determined applying the ASTM standard D1498 (ASTM D 1498).

BW have oxidation-reduction potential (also called redox potential) values in the range of 0.10–0.60 V, but when they are contaminated with H₂S, the values decrease to electronegative values: −0.28 to −0.50 V (Charrach, 1990, p. 335).

2.3 Brine

Brines, natural and industrial, are aqueous, concentrated solutions of salts, mainly NaCl, with a salt content of 3.5% up to 26%, a saturated solution such as waters of the Great Salt Lake, Utah, US and the Dead Sea, Jordan-Israel (Table 3; ASTM D 3875).

Natural brines are located at salt lakes, around salty lands (salar in Spanish) and in arid regions formed by strong evaporation. Industrial brines are used in the food industry for conservation and in power plant cooling fluids, for deicing of frozen roads.

Brine is a byproduct of DPs, which is disposed by conducting it back into the sea, near the coast, or into solar evaporation ponds and probably for industrial application of their salt. Brine disposal constitute an ecological issue as it may affect vegetable and animal life; therefore, these ponds are lined with flexible, impermeable rubber or plastic sheets to avoid infiltration into the aquifer. Brines are disposed in accordance with the environmental regulations of the countries involved (Ahmed et al., 2000).

An interesting aspect is the corrosion behavior of metals in salt concentrated and saturated brines. Figure 2 depicts the relation between salt concentration and steel corrosion rate (CR). It increases to a maximum at seawater content (3.5%) and then decreases nearing cero at the saturation content (25%) e.g. Great Salt Lake and Dead Sea, because DO reaches values near zero: 0.1 mg/l. (Figure 2)

Figure 2:

Effect of sodium chloride concentration on corrosion of iron in aerated solutions at room temperature.

On the basis of this knowledge, the carbon steel barges harvesting the salts in the evaporation pounds and the pipelines conducting the salt slurry to the salt treatment plants, are non-protected with industrial paints and other corrosion protection systems.

3 Desalination industry

Desalination, transforming SW into potable water, did not start yesterday; it is know from biblical times. The people of Israel, wandering in the Sinai desert was desperately searching for water, then their leader Moses found bitter water. In an act of magic he threw a piece of wood into the water that become fresh water (Exodus 15:22–25). Is this an ancient practice for water treatment, whose procedure has been lost in the eons of history?

The ancient nations of the Middle East and the Mediterranean Sea: Hebrews, Egyptians, Mesopotamians, Phoenicians, Greeks and Romans built public water works: flood control facilities, aqueducts, canals, reservoirs, pipelines made from corrosion-resistant stone and wood for the supply of fresh water for their population.

The US created the Office of Water Research and Technology and in 1952 the Congress passed “The Saline Water Act” to promote the DI, particularly in the southwest arid/desertic regions.

The DI is spread out in five continents: America, Europe, Asia, Africa and Australia, in many countries but for the shake of brevity the DI is presented only in Mexico, USA and Israel. These nations possess a strong economy, a modern agriculture; a high-tech industry and a dynamic society. These countries comprehend vast regions of dry arid/desertic climate, high temperatures, low level of rains, and ground BW, therefore they are erecting DPs. Their Universities and R&D institutes manage materials and corrosion departments and laboratories that implement methods and techniques for corrosion control in natural environments and industrial facilities (Balaban, 2004).

The DI is in the middle of an expansion and modernization program designed to construct more efficient and larger DPs that will reduce production costs. The maintenance of its infrastructure assets requires a robust understanding of the integration between global climate change and the materials engineering-structure-climate-interaction, induced by variations in humidity, temperature, solar radiation, drought and pluvial precipitation mainly during extreme events (Roberge, 2010). The Institute of Materials, Minerals and Mining (IOM3) London has published a special issue of its journal: Corrosion Engineering, Science and Technology, which brings together papers examining climate change induced corrosion (Valdez et al., 2010, 2011; Schorr, 2011).

Mexico is a vast, maritime country, surrounded by seas and oceans, having a complicated distribution of water resources, with plenty of water in the southeast region whereas in the northwest there is water scarcity, e.g. the State of Baja California. Extraction of underground water in areas near to the sea coast causes saline contamination of the underground aquifer. The cities of Rosarito, Tijuana, Ensenada and Hermosillo had installed some DPs and other huge desalination projects are in the planning stage.

The national Autonomous University of Mexico (UNAM) has created a Project for Multidisciplinary Investigation for Leadership and Academic Improvement (its Spanish acronym IMPULSA), with SW desalination as its central activity, applying renewable sources of energy. Impulsa maintains cooperation links with desalination institutes in Spain, Israel and USA (Hirart et al., 2013).

The southwest US states constitute a dry/desertic region, affected by frequent droughts; then supplies of clean water becomes scarce. To overcome this crucial situation, the largest DP in the Western Hemisphere, applying the Reverse Osmosis (SWRO) process, was built at Carlsbad, CA, that will provide 200 K m³/day of fresh water to the whole area (Figure 3). This was the result of an international cooperation, with American, Israeli and Spanish desalination enterprises covering the financial, technical, equipment and engineering materials aspects of this special DP. Because of the continuing drought in California, the plant completion was advanced to late 2015. The US DI continues to expand, technologies are being innovated and high quality water is supplied to the nation (The Carlsbad Desalination Project, 2014).

Figure 3:

Carlsbad, CA, the largest desalination plant in the Western Hemisphere.

The DI started in Israel with the construction of DPs around 1960, establishing a cooperation between Mekorot-the National water company and IDE Technologies, the national desalination enterprise, installing thermal DP in several parts of the country, in particular in the southern arid region; reaching the Red Sea (Hofman, 2011; Tenne, 2011).

At its beginning IDE erected a desalination pilot plant at the premises of Israel Mining Industries (IMI), in Haifa. The IMI corrosion Laboratory assisted IDE in the selection of corrosion resistant alloys (CRA), carried out corrosion tests and evaluated corrosion problems in the pilot plant.

Today, IDE Technologies is considered as one of the prominent desalination companies developing, designing and erecting DPs worldwide following the BOT-Built-Operate-Transfer system. Table 4 lists the DPs operating now in Israel, supplying potable water to the whole country.

From 1998 the Israeli Desalination Society (IDS), helds yearly conferences on desalination science, technology and engineering, including corrosion management of DPs. Dr. Miriam Balaban, director of the European Desalinization Society and editor of desalination journals, organizes conferences and courses on desalination over the world, including prevention and avoidance of corrosion (Israel Desalination Society).

IDE technologies focuses on the development, engineering construction and operation of membrane and thermal desalination facilities delivering 3 million m³/day of high quality water, worldwide.

4 Desalination processes, plants and equipment

Two main technologies are applied to desalinate SW: thermal evaporation, which converts SW into steam and separates the salt; and membrane separation, e.g. RO, a filtering process utilizing special plastic membranes (Asociacion Española de Desalacion y Reutilizacion; Ophir & Lokiec, 2005).

All these DPs use corrosion-resistant materials (CRM), metallic and non-metallic for the fabrication of the varied equipment employed in the production of pure water (Tables 5 and 6). DPs present a high level of corrosion risk as they handle and process aggressive SW under reverse conditions operating dynamic and static parameters:

Membrane separation processes apply semipermeable and ion-selective membranes to desalt seawater, using relatively high hydraulic pressure and the driving force for the separation between pure water and salts. Some RO membranes are manufactured from high-grade polymeric PVDF, which are very durable and less prone to damage. Such membranes impede the adhesion of foulants such as bacteria, proteins and mineral crystals (Malik et al., 2006; Figure 4).

Figure 4:

Diagram of desalination process by reverse osmosis.

Thermal desalting involves operations of boiling and evaporation. They are based on improved stages of distillation, compression and condensation with the purpose of saving energy and to produce water with a low content of total dissolved salt and a low cost for the potable water. Such processes are made more efficient by the application of multiple steps and the employment of vacuum, e.g. multiple effects distillation and multiple flash processes (Table 5; Malik & Al-Fozan, 2011).

It should be emphasized that these elevated temperatures and circulation of hot water and vapor require the use of metallic CRA for the prevention, mitigation and avoidance of corrosion damage (see Section 8). Furthermore, there is no universal desalination process; every type of SW requires a process adapted to its properties, characteristics and performance.

A DP converts SW into two products: a desirable potable water and disposable salty brine. The great variety of equipment, being handled in a DP, might be classified in two groups: mechanical and thermal equipment, in accordance with their particular operation. All the motion of the plant fluids is managed mainly by pipes, tubes, ducts, and valves to regulate their flow and velocity, vertical and horizontal pumps and all kind of storage tanks to contain different liquids. The thermal equipment, e.g. steam generators, condensers, flash chambers, heat exchangers, and distillation towers, handle hot and boiling water and vapor at different temperatures.

These varied equipment request CRM that display a reasonable endurance to the DP fluids, which are described in Section 6.

5 Corrosion, scaling and fouling

The three central problems that occupy maintenance engineers of DPs are corrosion, scaling and fouling of the equipment. Sometimes they occur simultaneously, are interrelated and influence each other. These phenomena, their extent and intensity depend on the chemical and biological properties of the feed SW and the DP streams (stagnant or circulating) and the thermal and mechanical conditions of operation (Roberge, 2000; Valdez & Schorr, 2010a,b,c; Valdez et al., 2012).

DPs suffer from diverse types of corrosion, e.g. uniform, general or localized, that appears in a specific part of the equipment, such as water boxes, heat exchangers of tubes or plates, demisters, pump impellers and more. Combined mechanical erosion and electrochemical corrosion are encountered in pump hubs and bends in pipelines conveying liquids and slurries at high velocities (Roberge, 2008; Schorr et al., 2011).

In hostile atmosphere, near the seacoast or in tropical areas, with high temperature and humidity due to heavy rains and with airborne chlorides, the DP installations and equipment should be protected with industrial paints and coatings.

Dissolved and suspended salts, such as carbonates and silicates, tend to sediment on the equipment surfaces forming hard, thick scales. To prevent this nuisance, feed SWs are acidified to dissolve the carbonates, which converts into CO₂ emitted into the air. Some BWs that contain weak acid, e.g. H₂S, are neutralized by alkaline Na₂CO₃ (Semiat et al., 2000).

Under complex organic and calcareous, porous scales, microbiologically influenced corrosion (MIC) develops, producing severe corrosion damage. To avoid these phenomena, scale inhibitors that impede the formation of crystals and corrosion inhibitors that slow down either the corrosion anodic or cathodic reaction, or both, or form a protective film on the equipment surface are added to the relevant DP streams under corrosion risk.

SWs, with their rich content of gases, minerals and organics, such as live plankton, constitute an adequate medium for the growth of bacteria, fungi, mollusk and algae, forming colonies strongly adhered to the metal surface. Under these colonies, corrosion develops as these organisms secrete acidic chemicals (Al-Muhanna & Habib, 2016).

To combat this grievance, many biocides are applied: gaseous chlorine (Cl₂), sodium hypochlorite (NaOCl), chlorine dioxide (ClO₂) and bromine salts, e.g. NaBr as they avoid the formation and accumulation of biological fouling (Moncmanova, 2007).

Mechano-chemical cleaning of equipment surfaces is done with weak solutions of acids: citric, sulfuric and phosphoric or alkalis, depending on the chemical and physical nature of the fouling sediments. A special method, called cleaning in place, permits the removal of foulants without suspending the equipment operations, which is adequate for heat-exchanging metallic surfaces.

6 Corrosion-resistant materials

One of the main activities in the development of a DP, particularly during the design stage, is the selection of CRM, which assures a continuous efficient operation without shutdowns, a low expense in maintenance and in replacement of corroded equipment (Schorr et al., 2005, 2012a,b).

The central aim of this section is to assess the CRMs considered as suitable for DPs. Moreover, cases of corrosion occurring in the diverse sectors of a DP are presented as a warning and alert for DP designers and constructors, to prevent such occurrences (Malik et al., 2015; Neroufel et al. 2017).

The numerous engineering materials listed are classified into two large groups: metallic alloys and plastic, which include polymers, rubbers, elastomers and composites (Table 7; Malik, 2001a,b; Malik et al., 2010).

7 Corrosion protection and control

It is widely accepted by corrosion institutions and professionals that corrosion protection and control technologies for industrial facilities and natural environments are based on four general methods:

1. Proper selection of corrosion-resistant engineering materials: metallic, plastic, ceramic and composites for the fabrication of the DP structures, installations, machinery and equipment, to assure a long-live service.

2. Application of industrial paints, polymeric coatings and rubber linings, compatible with the DP fluids, to protect the basic materials of construction.

3. Corrosion protection with sacrificial anodic metals such as aluminum and magnesium or employing impressed direct electrical current.

4. Use of corrosion inhibitor (CI) that hinder the corrosion reactions or form an absorbed protective film on metal surfaces or annihilate the DO in water systems. A new family of CI called “green” CI has entered service in industrial systems (Sastry, 2011; Cheng et al., 2018).

8 Conclusion and recommendations

Desalination is the most viable solution to the 21st century’s shortage of fresh water, to be attained from sources of SW. Actual innovative desalination technology is less energy-consuming and more environmentally friendly.

Corrosion resistance is the main property to be considered in the choice of CRM for DPs, but the final selection must be comprised between technological and economical factors. The selected CRM should be able to perform its function safely for a reasonable period of time and at a reasonable cost.

The surface of desalination equipment should be maintained clean and smooth to avoid mineral scale and to diminish the propensity of biological fouling. Mechano-chemical cleaning is implemented to remove scale and fouling.

Corrosion testing at the service of DPs mainly consists of several phases: laboratory test, desalination pilot plant and exposing specimens in DPs. Performing ASTM and NACE International standard tests is recommended, so these numerical and experimental results might be compared with those of similar test in other DPs (ISO M845, 1995; Al-Muhanna & Habib, 2016; Annual Book of ASTM International Standards, Vol. 03, 02).

Modern DPs that are constructed from CRMs and follow recognized methods of corrosion protection and control can expect prolonged equipment service life and relative freedom from corrosion.