Corrosion Management in Desalination Plants

Corrosion in Desalination Plants

?A desalination plant is exposed to different and diversified kinds of environments such as seawater, seawater-air and salt-air aerosols, corrosive gases, very fast or extremely slow moving liquids, particulates contained in high velocity fluids or deposit-forming liquids. All of them create a number of corrosion related problems.

?Corrosion in desalination plants can cause a variety of undesirable consequences, including loss of equipment, unplanned shutdowns, expensive repairs, leaks and contamination of products as well as serious personal hazards.

?The construction of a desalination plant requires a wide spectrum of materials (metals, non-metals and composites) which would not only satisfy the design and functional requirements of the particular component but should also have reasonable endurance to the environment surrounding the plant installations. One of the prime considerations during the selection of construction materials for a particular unit or component is its corrosion characteristics.

?In process industries like desalination plants corrosion consideration be more important than the other factors while carrying out the material selection for plant constructions. Thus the selection and corrosion control are synonymous as far as safe operation and maximum output from the plant facilities are concerned.

Factors Affecting on Corrosion in Desalination Plants

• Environment

A desalination plant is exposed to different kinds of environments such as seawater, seawater-air and salt-air aerosols, corrosive gases, very fast or extremely slow moving liquids, particulates contained in high velocity fluids or deposit-forming liquids all of them create a number of corrosion related problems.

Atmospheric corrosion in distillation plants installed in coastal zones is driven by the effect of marine aerosols containing humid particles of NaCl. It is promoted by the formation of moisture layers on metal surfaces and by the chemistry of these layers. Climate change could influence the macro- and micro-surface effects controlling either the chemistry or the duration of these wet surfaces. Furthermore, hygroscopic salts absorb moisture and form deliquescent salt particles which can greatly accelerate the corrosion of metals.

• Operating Conditions

The desalination plants offer a huge number of corrosion problems due to their operation in relatively aggressive environments, very fast or slow moving liquids, particulates contained in high velocity fluids or deposit forming liquids. All of them create a number of corrosion related problems.

Corrosion Prevention Strategies in Desalination Plants

• Material Selection
• Application of Coatings, Paints and Linings
• Cathodic Protection
• Chemical Treatment

Corrosion monitoring is the important practice of assessment of corrosion prevention program. Modern electrochemical, electronic, mechanical, non-destructive and computational devices are applied for corrosion monitoring. Weight loss coupons, electrical resistance (ER) probe, linear polarization resistance (LPR) technique are more common techniques for determining corrosion rate of the liquid.

 ?Scaling

Highly concentrated saline water tends to form thick scales by deposition of dissolved and suspended solid such as carbonates, silicates and hydroxides of calcium and magnesium. To prevent CaCO3 scaling the feed water is treated with sulfuric acid converting the carbonate to carbon dioxide CO2 that escapes from the water. When phosphates and phosphonates are applied to provide scale protection, careful control is implemented to avoid scaling by calcium phosphate [Ca3(PO4)2] adding organic polymers containing carboxylic acids for control of calcium phosphate deposition.

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 sulfur, iron, manganese and other water-soluble heavy metals compounds, bacteria, odor and colour.

Some alkaline chemicals e.g. soda ash neutralize the acidity found in some brackish waters, helps reduce corrosion and extends the life of 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 acid, hydrous iron oxide, aluminum oxide and organic substances in the feed water by special pretreatments.

?Biofouling

All natural water systems contain a wide range of microorganisms that can cause operational problems, if not controlled. These organisms grow predominantly in slime-enclosed biofilms attached to surfaces.

Biofilms may form very rapidly, restricting the flow of water through a membrane. The formation of a biofilm of microorganisms on the surface of the desalination membranes of SWRO and BWRO plants and on the contact surfaces of thermal desalination plants such as to cause a measurable reduction of the production capacity of the desalination system is typically referred to as biofouling.

Although most aquatic organisms typically causing biofouling are not pathogens, their excessive growth could have a negative effect on desalination plant’s overall performance and efficiency. Once a biofilm establishes itself and fouls a membrane, it may be extremely difficult, or even impossible to remove.

Desalination plants with open intakes typically incorporate facilities for biofouling control which include the use of chlorine or other oxidants or biocides to control excessive bio-growth. Thermal desalination plants usually practice continuous chlorination, while most membrane system practice intermittent or shock chlorination.

Aquatic organisms, including mussels, barnacles, clams and mollusks may grow in intake channels, pipes and equipment, causing operational problems. Open intake systems are usually equipped with provisions for hindering of bio-growth and periodic removal of aquatic organisms from the plant intake facilities in order to maintain reliable and consistent plant performance.

Treatment of Water in Desalination Plants

• Pre-Treatment
• Feedwater is treated to protect the membranes by removing some contaminants and controlling microbial growth on the membrane and to facilitate membrane operation.
• Suspended solids are removed by filtration, pH adjustments (lowering) are made to protect the membrane and control precipitation of salts; antiscaling inhibitors are added to control calcium carbonates and sulfates.
• Iron, manganese and some organics cause fouling of membranes. A disinfectant is added to control biofouling of the membrane. Disinfection can involve chlorine species, ozone or UV light and other agents.
• Marine organisms, algae and bacteria must be eliminated, and if chlorine is used it should be neutralized prior to contact with the membrane.
• Post Treatment
• Product water must be treated to stabilize it and make it compatible with the distribution system.
• Adjustment of pH to approximately 8 is required.
• Carbonation or use of other chemicals such as lime may be applied, and blending with some source water may be done to increase alkalinity and TDS and stabilize the water.
• Addition of corrosion inhibitors like polyphosphates may be necessary.
• Post disinfection is also necessary to control microorganisms during distribution, as well as to eliminate pathogens from the blending process.
• Many systems blend back a portion of the source water with the desalinated water for mineralization. With seawater, this is usually limited to about 1% due to taste contributed by sodium salts.

Pre Treatment of Desalination Plants

• Thermal Desalination

For thermal desalination facilities the pretreatment process must address:

• Scaling of the heat exchanger surfaces primarily from calcium and magnesium salts (acid treated plants);
• Corrosion of the plant components primarily from dissolved gases;
• Physical erosion by suspended solids;
• Effects of other constituents such as oil, growth of aquatic organisms and heavy metals.
• Thermal desalination systems are quite robust and normally do not include any physical treatment other than what is provided by the intake (i.e. no additional filters or screens).

Chemical conditioning is utilized in thermal desalination in two treatment streams: the cooling water (which is the larger flow and generally returned to the feed source), and make up water (used within the desalination process).

• Membrane Desalination

For membrane desalination facilities the pre-treatment processes must address:

• Membrane fouling and scaling from metal oxides, colloids, and inorganic salts;
• Fouling or plugging by inorganic particles;
• Biofouling by organic materials;
• Chemical oxidation and halogenation by residual chlorine;
• Chemical reduction of chlorine
• Effects of other constituents such as oil, aquatic organisms and heavy metals.

Membrane desalination requires a higher degree of pre-treatment than thermal desalination processes. Membrane separation technologies were developed for the removal of dissolved salts but they also block the passage of filterable materials.

Performance Measurement    

In choosing between possible inhibitors, the simplest tests should be done first to screen out unsuitable candidates. The philosophy of initial screening tests should be that poorly performing candidates are not carried forward. An inhibitor that does poorly in early screening tests might actually do well in the actual system, but the user rarely has the resources to test all possible inhibitors. The inhibitor user must employ test procedures that strictly exclude poorer inhibitors even though some good inhibitors are excluded.

Inhibitors Performance Measurements

1- Lab Testing

• Physical Measurements
• Electrochemical Testing
• Scale Rig
• Well Test Rig
• Bug Bottle Test
• Compatibility test

2- Side Stream testing

• Corrosion side stream
• Biocide Side Stream
• Scale side stream

3. Trail Testing

• Test only part of the facility for at least one month

The Need for Corrosion Monitoring

• The rate of corrosion dictates how long any process plant can be usefully and safely operated. The measurement of corrosion and the action to remedy high corrosion rates permits the most cost effective plant operation to be achieved while reducing the life-cycle costs associated with the operation.
• Corrosion monitoring techniques can help in several ways:

1.     by providing an early warning that damaging process conditions exist which may result in a corrosion-induced failure.

2.     by studying the correlation of changes in process parameters and their effect on system corrosivity.

3.     by diagnosing a particular corrosion problem, identifying its cause and the rate controlling parameters, such as pressure, temperature, pH, flow rate, etc.

4.     by evaluating the effectiveness of a corrosion control/prevention technique such as chemical inhibition and the determination of optimal applications.

5.     by providing management information relating to the maintenance requirements and ongoing condition of plant.

Corrosion Monitoring Techniques

A large number of corrosion monitoring techniques exist. The following list details the most common techniques which are used in industrial applications:

• Corrosion Coupons (weight loss measurements)
• Electrical Resistance (ER)
• Linear Polarization Resistance (LPR)
• Galvanic (ZRA) / Potential
• Hydrogen Penetration
• Microbial
• Sand/Erosion

Other techniques do exist, but almost all require some expert operation, or otherwise are not sufficiently rugged or adaptable to plant applications.

Of the techniques listed above, corrosion coupons, ER, and LPR form the core of industrial corrosion monitoring systems.

  Inspection Techniques

• Ultrasonic Testing
• Radiography
• Thermography
• Eddy current/ magnetic flux

Comments

? Practical methods that minimize or eliminate corrosion include selection of suitable corrosion resistant alloys (CRA), application of coatings, paints and linings to carbon steel and galvanized steel equipment and cathodic protection.

? Corrosion resistance is the main property to be considered in the choice of materials for desalination plants equipment 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.

? Corrosion monitoring (CM) is the practice of measuring the corrosion by continuously exposing probes in a body of water or a operating distillation plant. Modern electrochemical, electronic, mechanical, non-destructive and computational devices are applied in the field of corrosion monitoring such as weight loss corrosion coupons, electrical resistance probes, LPR (Linear Polarization Resistance) probes, remote CM, expert programs and artificial neural networks.

? CM techniques provide daily warning of costly corrosion damage and critical information, where the damaging event is occurring and about the rate of deterioration. This information is essential to take decisions about the type, urgency and cost of preventive and curative measures to be applied on site without delay.