Without a doubt, water pumps are critical for industrial and agricultural business applications and processes. Most water pumps are subjected to extreme environmental conditions. Sometimes the water being pumped may contain abrasive solids, corrosive contaminants, and vapour bubbles. For this reason, you need to actively prevent corrosion from occurring if you want to extend the life of your water pump. In this article, we explore corrosion prevention for water pumps.
Materials for Pumps and Valves
The choice of materials used for a water pump depends on several factors, including:
- 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
Cast iron is often chosen for valve and pump bodies in regular applications. For a water pump’s impellers, the material is chosen based on its suitability for casting, strength, cavitation wear resistance, abrasion resistance machinability, and corrosion resistance. Sometimes cast iron impellers are used, but that is mainly because of their lower initial costs. The material is, unfortunately, inferior for abrasion resistance, cavitation, and corrosion.
Brass is another option sometimes used for steam and water applications, but not seawater applications, as corrosion resistance is not adequate for this material. For a higher resistance to cavitation and corrosion damage (for freshwater and seawater), phosphor bronze or nickel aluminum bronze can be used.
Glass-reinforced plastic or GRP can be used for pump impellers that pump saltwater, while stainless steel is often chosen for heavy-duty high-speed pumps because of its improved cavitation resistance. In fact, stainless steel impellers are almost 200% higher than a comparable mild steel impellor when noting corrosion resistance.
Lastly, ceramic coatings can minimise cavitation corrosion.
Corrosion Risks
Seawater pumps and those pumps that handle contaminated water face severe corrosion and abrasion damage risk.
High-speed water pumps with high fluid velocity may face cavitation and corrosion.
Higher temperatures and restrictions in fluid flow paths also add to the risk of cavitation and corrosion.
Types of Corrosion Damage
Water pumps are exposed to electrochemical and mechanical degradation phenomena that are closely interrelated. Below are the different types of corrosion damage:
- Cavitation corrosion
- Erosion corrosion
- Fatigue
The above are studied as physical mechanisms. Conversely, the below are primarily studied as electrochemical reactions:
- Galvanic corrosion
- Uniform corrosion
- Crevice corrosion
- Pitting
Generalised Corrosion, Pitting, and Crevice Corrosion
Uniform or generalised corrosion in a water pump may increase the clearances. This will cause a pump to operate inefficiently. Generalised corrosion may occur along the entire length and width of the wet surface of the pump. You can minimise this damage by providing a protective coating on the exposed elements.
Localised pitting corrosion occurs at locations where a protective coating no longer exists or has been removed. If there is a high concentration of chlorides in the water, it can cause the pitting of the metals. Let’s use stainless steel as an example, where the chrome oxide protective layer is destroyed, and an electrochemical reaction results in the formation of localised pits.
Galvanic Corrosion
Galvanic corrosion occurs when two dissimilar metals establish electrical connectivity – through a moist medium in most cases. The corrosion rate depends on their position in the electrochemical series. To avoid galvanic corrosion, carefully inspect the material used for your water pump’s valves, piping, and other pump components, and evaluate the risk of galvanic corrosion in the entire system.
Cavitation Corrosion
The combination of cavitation and corrosion can cause severe damage and affect your pump’s efficacy and longevity. When rapid changes in a liquid create small, vapor-filled cavities where the pressure is low, then the process of cavitation occurs. These bubbles or cavities can collapse when placed into high-pressure water, which causes a shock wave that can damage the impeller. Basically, the changes in pressure cause bubbles, and when the bubbles increase in pressure and burst, they cause shockwaves within the pump. It is the shockwaves that wear down the pump’s impeller and body (volute) over time.
Cavitation creates shock waves that speed up other forms of local corrosion. It can also remove the protective films of oxides on the metallic substrate. If the surface in the low-pressure zone inside the pump has pre-existing corrosion, then local turbulence is created with less severe operation, and cavitation begins.
Metals with hard surfaces are more prone to damage from cavitation, while softer materials, such as polyurethanes and soft metals disperse the energy of collapsing bubbles. This results in a lower damage rate.
Various pump parameters influence the intensity of cavitation, such as:
- Low pressure or vacuum at the suction nozzle (pressure going below the vapour pressure of the liquid can cause a buildup of vapour bubbles)
- Higher water temperature
- Higher water velocity at suction with a drop in pressure.
- Pressure reduction in the suction pipe due to fittings.
- Blockages due to the buildup of contaminants.
Solutions for Cavitation
Below are a few ways you can solve the issue of cavitation:
- 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 the pump at a lower rpm (by running the pump motor with a lower voltage and frequency in the case of an induction motor).
- Using a ceramic or polymeric (epoxy) coating ensures a smooth impellor surface, thus reducing cavitation corrosion.
Erosive Corrosion
When there are solid contaminants in the pumped water, then erosive corrosion may take place. 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
- The fluid velocity.
To minimise this damage, ensure the proper material selection and apply ceramic or other protective coatings on the surfaces.
Microbiologically Influenced Corrosion
MIC or microbiologically influenced corrosion occasionally occurs in water pumps that handle borehole water and in pumps used the gas and oil industry. Some micro-organisms directly induce corrosion, while others actively assist pre-existing corrosion reactions.
The different types of micro-organisms that influence water pump corrosion include:
- Sulfate-reducing bacteria
- Sulfur oxidizers
- Iron oxidizers
The Bottom Line
Water pump corrosion can be minimised by adopting suitable design parameters during the design stage of your system and by choosing resistant materials for components like the impellers, pipe fittings, and valves.
Anti-corrosion coatings can significantly reduce corrosion damage and improve your water pump’s efficiency and longevity. Regularly maintaining your water pump system will help you identify early symptoms of corrosion to eradicate the problem before it causes severe damage.
The best advice is to use a professional to help you choose the right water pump system for your needs. Contact Jordan Pumps for professional advice, installation, and maintenance of your water pump.
