Water pumps are critical for industrial and agricultural business processes. Most of these pumps are subjected to extreme environmental conditions such as variable temperatures and flow velocities. The water being pumped might contain corrosive contaminants, abrasive solids and vapor bubbles. (For background reading, see Understanding Corrosion in Pumps and How to Deal With It.) Hence we must devise suitable corrosion prevention and protection strategies for reliable and economical operation of the water pumps.
Materials for Pumps and Valves
The choice of water pump materials depends upon parameters such as:
- Water’s pH level
- Water impurities
- Air and vapor entrapment
- Flow turbulence
- Liquid pressure
- Liquid temperature and range
- Pump duty type (intermittent or continuous)
- End use of water
Normally cast iron is chosen for valve and pump bodies in normal applications. For impellers, the material is selected on the basis of its suitability for casting, machinability, strength, cavitative wear resistance, abrasion resistance and resistance to corrosion. Cast iron impellers are occasionally used mainly on account of their lower initial cost. They are inferior for cavitation, corrosion and abrasion resistance. Brass is sometimes used for steam and water, but not for seawater applications, as corrosion resistance is inadequate. Comparatively, bronze has better machinability and casting properties. Bronze impellers also have moderate cavitation resistance. For a higher resistance to cavitation damage and corrosion damage (for freshwater and seawater), phosphor bronze or nickel aluminum bronze can be used.
Glass-reinforced plastic (GRP) can be used for pump impellers that pump seawater. Stainless steel can be chosen for severe duty high-speed pumps on account of their improved cavitation resistance. This metal has moderate corrosion resistance and abrasive wear resistance. From a cavitative corrosion resistance point of view, the life of a stainless steel impeller can be almost 200% higher than a comparable mild steel impellor. Ceramic coatings can minimize cavitation corrosion.
Seawater pumps and those pumping contaminated water face a severe corrosion and abrasion damage risk. High-speed pumps with high fluid velocity may face cavitation corrosion risk as well. Higher temperatures and restrictions in fluid flow paths also add to the risk of cavitation corrosion.
Types of Corrosion Damage
Water pumps are exposed to electrochemical and mechanical degradation phenomenon that are closely interrelated. Cavitation corrosion, erosion corrosion and fatigue are initially studied as physical mechanisms, though electrochemical and chemical reactions are necessarily occurring simultaneously. Conversely, galvanic corrosion, uniform corrosion, crevice corrosion and pitting are primarily studied as electrochemical reactions.
Generalized Corrosion, Pitting and Crevice Corrosion
Uniform or generalized corrosion in the pump may increase the clearances and cause the pump to lose its operating efficiency. Wear debris may simply add to the severity. Generalized corrosion may occur all along the length and breadth of the wetted surface of the pump. This may be minimized by providing a protective coating.
Localized pitting corrosion may occur at locations where a protective coating has been removed. The presence of a high concentration of chlorides in the water can cause pitting of metals. An example is stainless steel, where the chrome oxide protective layer is destroyed and an electrochemical reaction results in the formation of localized pits.
Local crevice corrosion may result at tightly held surfaces, such as in coupling flanges.
When two dissimilar metals establish an electrical connectivity (through a moist medium in most cases) then galvanic corrosion can begin. The corrosion rate depends on their position in the electrochemical series. For example, if a weldment is different from the parent metal, there is a risk of galvanic corrosion. (For more information, see An Introduction to the Galvanic Series: Galvanic Compatibility and Corrosion.) The material used for valves, piping and the pump components must be studied to evaluate the risk of galvanic corrosion in the entire system.
The combination of corrosion and cavitation can have a disastrous effect on pump efficiency and longevity. Cavitation is the phenomenon of the formation of liquid vapor bubbles in a low-pressure zone and their collapsing on the surface of the impeller, the casing and other parts of the pump in a high-pressure zone.
Cavitation creates shock waves that speed up other forms of corrosion locally. It causes the removal of protective films of oxides on the metallic substrate. On the other hand, if the surface in the low-pressure zone inside the pump is already corroded due to pitting or other forms of corrosion, then local turbulence is created with even less severe operation, and cavitation can begin. Metals with moderately hard surfaces are more prone to cavitation damage. Softer materials such as polyurethanes and soft metals disperse the energy of collapsing bubbles, and result in a lower rate of damage.
Cavitation intensity is influenced by various pump parameters such as:
- Low pressure or vacuum at suction nozzle (pressure going below the vapor pressure of the liquid can cause a buildup of vapor bubbles)
- Higher water temperature
- Higher water velocity at suction with pressure drop
- Pressure reduction in the suction pipe due to fittings such as elbows or valves
- Blockages due to the buildup of contaminants
Solutions for Cavitation
Solving cavitation may be attempted by:
- Increasing the suction pipe’s inside diameter
- Solving bottleneck restriction in the suction area
- Cooling the water
- Removing unnecessary restrictive valves and fittings
- Adding a pressure booster pump to add system pressure at suction
- Operating pump at lower rpm (by operating the pump motor with a lower voltage and frequency in the case of an induction motor)
- Using a ceramic or polymeric (epoxy) coating and ensuring a smooth impellor surface, thus reducing cavitation corrosion
Erosive corrosion generally occurs due to solid contaminants present in the water being pumped. Metal loss due to erosion is dependent upon the impact angle of the solid matter, nature of the solid contaminant, the hardness of the surface and the fluid velocity. This damage can be minimized by proper material selection and applying ceramic and other protective coatings on the surfaces. (More ideas can be found in Combating Cavitative Corrosion and Erosive Corrosion.)
Microbiologically Influenced Corrosion
Microbiologically influenced corrosion (MIC) occasionally occurs in pumps that handle borewell water and in water pumps used by the oil and gas industry. While some microorganisms induce corrosion directly, others actively assist preexisting corrosion reactions.
Different types of organisms that influence corrosion include:
- Sulfate-reducing bacteria
- Sulfur oxidizers
- Iron oxidizers
Iron ions present in the water enable microbes to produce iron hydroxide, oxidize manganese and create metal precipitate on the surface. Steel components, for example, are also susceptible to acidic corrosion, which are caused by certain organisms assisted by sulfur-reducing bacteria.
If the water being pumped carries solid contaminants, the flow rate and velocity become critical. At lower flow rates, solids may settle down, thus resulting in crevice corrosion at some locations. If the flow velocity of water is designed for a higher value, then erosion and cavitation may occur for which suitable protective coatings must be specified.
Changes in the chemistry of the water being pumped may influence the corrosion rate. A temperature rise can increase the rate of corrosion. When a pump is out of service for a long period, there is the risk of increased corrosion due to trapped water and air. In such cases, adding a corrosion inhibitor to the water inside the pump during a prolonged outage may be considered. Draining the entrapped water can also be attempted.
The outer surfaces of pump flanges and valve and hand-wheels will require protective coatings, particularly in corrosive environments such as seawater, submerged conditions or splash prone conditions. Ceramic-lined valves and pipeline fittings are used in corrosive environments and higher temperatures. For higher erosion wear resistance of the metal surfaces, a mix of epoxy with ceramic particles can be used when the cavitation risk is low.
Non-reinforced polyurethane coatings can be used for higher cavitation resistance, due to its higher flexibility, adhesion strength and protection against erosion corrosion. Preparation of a perfect surface is the main secret behind good adhesion strength for the polymer coating. A non-reinforced elastomer is preferred because the coating works like a spring pad when bombarded by cavitative gas bubbles at jet speed. Certain polyurethane formulations meet the requirements of pumps used for drinking water. Polyurethane is flexible, so that it can expand and contract during the thermal expansion and contraction of the substrate.
Teflon and Other Coatings
Teflon polytetrafluoroethylene (PTFE) coatings are used in water pumps in the pharmaceutical and food industry as a non-toxic coating. It has high corrosion and heat resistance. The coating is easily removed. As a solid lubricant it also provides a low friction surface.
Polyester coatings with glass-flake reinforcement are also used for valves and pipefittings.
Neoprene coatings and claddings are used for seawater systems of pumping and piping, where wave action and splashing water can corrode the system components. Neoprene has the advantages of mechanical strength combined with a stronger adhesion to steel.
Thermal sprays of stainless steel, zinc and aluminum are occasionally used for valves and pipefittings, splash areas in seawater applications and high temperature applications.
When planning to coat the impeller surface, precautions should be taken to retain the designed shape of the vanes, because any shape change can impact the pumping efficiency and the life of the impeller. An imbalance induced by a coating can also result in vibration at higher speeds.
The corrosion risk of water pumps can be minimized by adopting suitable design parameters during the design stage and selecting suitable materials for components like impellers, valves and pipefittings. Anti-corrosion coatings reduce corrosion damage and improve pump efficiency and the economic life of the pumping system. Condition monitoring and preventive maintenance will contribute to the economical operation of the water pumps and the overall pumping system.