Useful information on pump corrosion and chemical compatibility
What is chemical compatibility?
The wetted parts of pumps may be constructed from a number of materials:
- Metals (for example: cast iron, stainless steel, nickel alloys) for casings and other components where mechanical strength is important.
- Thermoplastics (for example: PTFE, PVDF, PP) for casings, linings, bearings, gaskets and seals when dealing with corrosive chemicals.
- Ceramics (for example: alumina, silicon carbide) for seal faces and bearings.
- Elastomers (for example: Buna N®, Viton®) for flexible components such as shaft seals and O-rings.
The resistance of exposed metals to corrosion and of thermoplastics and elastomers to chemical damage (such as softening or swelling) can vary widely with temperature, the presence of abrasives, impurities or entrained gases. In selecting a pump for a particular application, it is important to understand how all exposed materials (including seals and shaft components) interact with the pumped fluid. This is referred to as chemical compatibility.
What is corrosion?
Corrosion in pumps results from chemical reactions between the pumped fluid and exposed metal surfaces. It results in impaired performance and increased maintenance costs. In extreme cases, corrosion can cause the premature failure of equipment.
In terms of the chemistry, corrosion is the result of metals being oxidised. Iron, for example, will readily corrode in the presence of air and moisture to form iron oxide (rust). This does not form a strong bond with the underlying metal and flakes off to expose more bare metal to continuing corrosion. Stainless steel and other alloys offer greater corrosion resistance because they have a protective layer of chromic and other oxides that do bond strongly to the underlying metal.
How does corrosion occur?
Pumps experience several different types of corrosion:
Uniform corrosion is a consistent and constant deterioration over the entire surface of an exposed metal component. It is predictable, so the operational lifetime of the component can be estimated from test results or standard corrosion charts. Uniform corrosion can be minimised by coating the metal surfaces with a protective layer (metallic or non-metallic) or by installing a pump constructed from more resistant materials such as stainless steel or nickel-based superalloys.
Galvanic corrosion occurs when there are combinations of different bare metal surfaces (for example, stainless steel and copper) in the presence of an electrolytic medium such as an acid or alkali. This essentially sets up a battery with charge and material transferred between the different metal surfaces. To avoid or minimise galvanic corrosion, it is necessary to assess the materials used throughout a system: valves, piping and pump components.
Pitting corrosion is a localised effect, normally following damage to a protective surface layer. Once exposed, the underlying metal begins to corrode, leaving irregular pits with sharp edges. This can be a problem when a fluid contains abrasive particles and it is exacerbated at high flow rates. Cavitation can also promote pitting corrosion.
Stress corrosion cracking is a localised effect brought on by stresses in a component. The combination of mechanical stress and corrosion may lead to fracturing.
Intergranular corrosion is a degradation of metal around grain boundaries. It is more commonly found in stainless steel pump castings or around welds.
Crevice corrosion arises because of stagnant pools of fluid retained in confined areas, for example, around flanges. It is particularly a problem when pumps are only used intermittently.
Microbiologically influenced corrosion (MIC) can be a problem in pumps that handle untreated water. Depending on the conditions, some bacteria can induce or promote corrosion.
What is cast iron?
Cast iron is commonly used to manufacture pump housings because of its low cost and the ease with which parts can be cast and machined. When used in applications with water or aqueous solutions with pH values between 6 and 10, cast iron, which contains about 4% carbon, quickly acquires a protective graphite layer. In fact, poorer quality cast iron with a higher carbon content is more resistant to corrosion than steel.
What is stainless steel?
Stainless steel contains a significant proportion of chromium (at least 15%), which forms a thin, inert oxide layer on the surface. Unlike iron oxide, this binds strongly to the metal surface and therefore protects the underlying metal in environments that would be highly corrosive for cast iron or carbon steel. Other elements such as nickel, manganese and molybdenum may also be added to stainless steel to further enhance corrosion resistance. Under the right conditions, the protective oxide layer is self-repairing if it is scratched or damaged.
There are four main types of stainless steel based on their crystal structures: austenitic, ferritic, martensitic and duplex. The most common types of stainless steel used in pump manufacture are the 300 series in the austenitic family, particularly 304 and 316. The 304 grade contains 18% chromium and 8-10% nickel. The 316 grade also includes 2% molybdenum and has greater resistance to acids and to localized corrosion.
Cast iron, steel or stainless steel?
In some cases, the use of carbon steel or cast iron is preferred to stainless steel for cost reasons. Corrosion is accepted, modelled reasonably accurately, and factored into on-going maintenance and equipment replacement costs. However, if the expected corrosion rate is high and seriously limits service life, then stainless steel pumps, although more expensive, may be cost-effective in the long term. It is important to select an appropriate grade of stainless steel for the specific application by referring to compatibility charts and consulting pump manufacturers. If the pumped fluid contains solids, stainless steel may be subject to accelerated corrosion. Abrasive particles can damage stainless steel’s passive, protective layer exposing the underlying metal to corrosion.
What is a superalloy?
For dealing with caustic media such as acids and alkalis, particularly at elevated temperatures, nickel-based “superalloys” have a higher resistance to corrosion than stainless steel. These alloys are generally known by their tradenames, for example: HASTELOY®, INCOLOY®, INCONEL®, and MONEL®.
What is a thermoplastic?
Thermoplastics are resistant to most liquids that corrode metals. They are characterised by an ability to be softened by heating and cast into complex shapes. It is therefore possible to mould individual components or even an entire pump case. Mechanical strength can be reinforced with fillers such as glass fibre. However, physical properties are generally inferior to those of equivalent metal pumps.
PTFE (Teflon® is a common tradename) is used for many pump components including gaskets, seals and diaphragms because of its excellent corrosion resistance and chemical inertness. Unlike most thermoplastics, PTFE also has a wide operating temperature range (up to 250°C).
How are pumps lined?
Various pump components (shafts, sleeves, plungers, piston rods, impellers, containment shells) can be coated with metallic, non-metallic and ceramic compounds to achieve increased corrosion and wear resistance. Coatings can also be used to repair damaged or worn components.
To apply a metallic coating, various techniques can be used:
- Thermal spray: a stream of molten particles, generated using an electric-arc, plasma or a combustion process, is projected onto the base material.
- Spray and fuse: the coating is sprayed in liquid form onto the component which is subsequently heated to 950°C or more to fuse the coating.
- Plasma transferred arc (PTA): a welding process with elements of both thermal spray and spray and fuse techniques.
- Laser cladding: similar to PTA but using a laser as the heat source.
Table 1. Metallic coatings applied to pump components for added corrosion resistance
|Major constituents (example tradenames)||Application and uses|
|Good corrosion and wear resistance|
|Nickel-based alloys (e.g. Colmoney)||Good resistance to corrosion and heat|
|Excellent resistance in oxidizing and reducing media, such as acids, even at elevated temperatures. High resistance to cracking, pitting and stress corrosion|
|Aluminium oxide/titanium dioxide (e.g. Amdry/Metco)||Range of ceramic powders applied by thermal spray, resistant to wear and good thermal and chemical resistance|
|Chromium oxide||Improves corrosion, abrasion and wear resistance|
|Tungsten carbide (UCAR LW-15)||High wear resistance (for applications below 500°C)|
Thermoplastics, such as PTFE, can also be applied as a protective layer over metal components. These are usually applied as a powder and then heated. During subsequent cooling, the coating hardens. Some thermoplastic coatings can be reheated to correct and seal any flaws.
The coatings can be applied by:
- Electrostatic spray: electrical charging is used to coat the powder on the metal surface and the component is then heated, causing the powder to melt and flow.
- Preheat spray application: the metal part is preheated, and the coating sprayed onto it. It immediately melts and flows over the heated surface.
- Dip coating/fluidized bed: Preheated parts are immersed in a fluidized thermoplastic powder coating which is attracted to and subsequently fuses to the heated surface.
What is a Chemical Compatibility Chart?
Chemical compatibility charts are normally available from pump manufacturers. These describe the resistance of materials commonly used in pump construction to various chemicals. It is important to refer to these when selecting a pump to ensure that all wetted parts are chemically compatible with, and resistant to, the pumped medium. This can only be a guide as corrosion and, in the case of thermoplastics and elastomers, chemical damage such as softening or swelling, can vary widely with concentration, temperature, the presence of abrasives, impurities or entrained gases. It may be necessary to carry out tests under field conditions and to consult with pump manufacturers before committing to a system.
Table 2. Section of a chemical compatibility chart for Micropump’s range of gear pumps
The wetted parts of pumps may be constructed from a number of materials: metals, alloys, ceramics, thermoplastics and elastomers. In selecting a pump for a particular application, it is important to understand how all exposed materials interact with the pumped fluid. This is termed chemical compatibility.
Corrosion occurs because of chemical reactions between the pumped fluid and one or more of the metals used to construct pump components. There are several different types of corrosion with uniform and galvanic corrosion being the two most common types. Stainless steel resists corrosion because it contains chromium and other elements such as nickel, manganese and molybdenum, which form a thin, inert oxide layer on the surface. Under the right conditions, this protective oxide layer is self-repairing if it is scratched or damaged.
For dealing with strong acids or alkalis, particularly at elevated temperatures, nickel-based superalloys have a higher resistance to corrosion than stainless steel. Pumps constructed from, or lined with, thermoplastics are also a viable and economical alternative to superalloy pumps for handling certain aggressive chemicals such as hydrochloric or hydrofluoric acids. Metal components can also be coated with resistant alloys for added protection.
Chemical compatibility charts show the resistance of pump construction materials to various chemicals. It is important to refer to these when selecting a pump to ensure that all wetted parts are resistant to corrosion or other chemical damage by the pumped medium.