A key component of selecting a pump is identifying the proper materials of construction. This is critical for applications prone to corrosion, as improper material selection can lead to process interruptions and potential pump failures. Costs that arise from these issues can far exceed the cost of the pump if materials are improperly selected. Understanding the principles of corrosion and how it affects pump material can have a significant impact on maximizing the lifespan of a pump.
Types of Corrosion
While both chemical and galvanic corrosion break down metals, the root cause of corrosion varies. Chemical corrosion is a reaction that occurs when a chemical comes in contact with metal. For example, cast iron is not compatible with sulfuric acid. When the two come in contact, the molecules will react, causing the metal to break down or corrode.
Proper material selection is key to preventing chemical corrosion. For a pump that is constructed wholly of corrosion-resistant materials, one cost-effective alternative is for coatings and linings to be used to protect incompatible components. Unfortunately, abrasive components will eventually wear away this protective layer, ultimately leading to corrosion. If electing to use a coating or lining, it is best to establish a routine check of these components to determine if the coating is intact or a recoat is needed.
In comparison, galvanic corrosion is an electrochemical reaction between two dissimilar metals within a conductive electrolyte solution such as saltwater. Each metal component has a property called electrochemical potential (or nobility). When two dissimilar metals are applied in a conductive solution, the metal with the lower potential experiences an electrochemical reaction leading to corrosion, while the metal with the higher potential remains intact. In applications prone to galvanic corrosion, an anode—a sacrificial component made of a metal with low potential—is often installed externally to the pump. The anode assumes corrosion while key components and sealing surfaces remain intact.
Pump Wet End
A pump’s wet end consists of the impeller and volute, and it is a key area of consideration during the material selection process. Not only will this area be in contact with the fluid (pumpage), but it will also be in contact with any solids present in the pumpage. Additionally, if the pump has solids-handling capabilities (such as chopper, cutter or grinder), then the strength of the material and its abrasive resistance will also need to be considered. A popular material of construction for pump wet ends is cast iron, which is a strong, cost-effective option but has poor corrosion resistance. In the presence of saltwater, protective anodes would be necessary to prevent galvanic corrosion.
To prevent chemical corrosion, it is recommended to explore alternate materials such as stainless steel. While stainless steel comes at a higher price point, it offers similar strength properties as cast iron along with greater corrosion resistance. Stainless steel’s corrosion resistance is drawn from its chromium content within its alloy makeup. This chromium, when in contact with oxygen, forms a thin, transparent protective layer that prevents corrosion. Even in the presence of abrasive particles, the layer quickly reforms to fix any exposure caused by wear or scratches.
Another pump wet end material option typically found on smaller pumps is thermoplastics. While these plastics do not have the same strength as cast iron or stainless steel, they offer excellent chemical and wear resistance. Pumps with resin-molded components are suitable alternatives to traditional small cast iron pumps in sumps or prefabricated lift stations prone to both chemical and galvanic corrosion.
Smaller Components
Another key component that is often overlooked is the pump’s elastomers. While small and relatively inexpensive compared to the pump’s wet end, these components are critical to the pump, as they seal the various chambers from one another. Ignoring the material compatibility of the elastomers could lead to leakage and eventual pump failure.
A common pump elastomer option is nitrile, or Buna-N. This cost-effective option has strong mechanical properties and is resistant to oils, making it ideal for sealing a pump’s oil chamber. Buna-N has limited chemical resistance, particularly against strong acids or fluids with low pH levels. Another cost-effective elastomer option is ethylene propylene diene monomer rubber (EPDM), which works well in applications with water, steam and numerous chemicals. Fluoroelastomer (FKM), on the other hand, is an alternative option with poor resistance to oils, which can lead to sealing failure from swelling or cracking. In applications with more aggressive chemicals, FKM elastomers may be a better choice. While FKM is more expensive, it has high chemical resistance as well as a higher temperature range.
Another minor yet critical component to consider is the pump’s hardware. Whether it is bolts, washers, nuts, locking rings, etc., these components are important, as they fasten and provide structure to the pump assembly. A common material used is carbon steel, as it is durable and cost effective; however, it has poor corrosion resistance. In applications prone to chemical corrosion, it is recommended to select a pump with stainless steel hardware to provide similar strength and corrosion resistance. For applications prone to galvanic corrosion, users can elect to use stainless steel or titanium as an alternative. Titanium provides an even higher level of chemical resistance compared to stainless steel and is exceptional in saltwater applications or environments.
Material Compatibility
A large emphasis of pump selection is dependent on hydraulics and operating point. While erroneously selecting a pump based on specs can lead to premature failure, it can be argued that selecting the wrong material can be even more detrimental. Corrosion damage will persist whether or not the pump is in operation and is often undetectable outside of routine inspection. Therefore, it is important to consider all components when reviewing material compatibility.
Pump manufacturers should provide information regarding the materials of construction of each component via a parts list or sectional drawing. Many manufacturers also offer a material compatibility list showing how their components fare in various fluids. It is best practice to only use the guide for products from that specific manufacturer, as they use a variety of alloys and using another manufacturer’s compatibility list could lead to improper material selections. It is also recommended to refer to the specific manufacturer directly with questions on material compatibility.
For more on corrosion, visit pumpsandsystems.com/tags/corrosion.
