Pump Materials and Corrosion Resistance: Understanding Pump Liquid and Material Selection
Selecting the appropriate pump material is an important factor in ensuring reliable operation and long service life. Different liquids have different characteristics that can affect the corrosion resistance of pump materials.
Understanding how materials react to various operating environments, including fresh water and seawater, is essential when selecting suitable materials for pump applications.
1. Understanding Corrosion Resistance
The corrosion resistance of materials varies depending on their operating environment.
According to the Corrosion Data Survey by the National Association of Corrosion Engineers, material corrosion resistance is evaluated based on corrosion rates and classified into four categories:
- Excellent: 0.050 mm/year or below
- Good: 0.050–0.500 mm/year
- Usable: 0.500–1.27 mm/year
- Unusable: 1.27 mm/year or above
Although stainless steel is known for its corrosion resistance, it can experience localized corrosion, such as pitting and crevice corrosion, in environments containing chloride ions.
Therefore, understanding the characteristics of the liquid and its potential effects on pump materials is important during material selection.
2. Fresh Water and Its Effect on Pump Materials
Fresh water is a general term covering groundwater, lake water, river water, tap water, and industrial water.
The corrosiveness of fresh water varies depending on its characteristics, including the influence of bacteria and chemical composition.
Fresh water corrosiveness is generally evaluated based on calcium carbonate precipitation.
Methods such as those introduced by Larson & Buswell and Ryznar consider several water quality parameters, including:
- Water temperature
- Calcium ion concentration (Ca²⁺)
- M alkalinity
- Total dissolved matter
- pH level
These parameters are used to evaluate water quality conditions and the tendency for calcium carbonate precipitation.
When calcium carbonate precipitation occurs, corrosion of metal materials may decrease.
Water quality evaluation also has a correlation with the occurrence of red water under certain pH and dissolved matter concentration conditions.
Understanding these characteristics helps assess the potential effects of fresh water on pump materials.
3. Seawater and Corrosion Risks
Seawater is highly corrosive, and completely preventing corrosion of metal materials can be extremely expensive.
Therefore, it is important to understand the characteristics of corrosion and select materials economically while considering pump applications and expected service life.
Several types of corrosion may occur in seawater environments.
Pitting Corrosion
Pitting corrosion occurs due to localized breakdown of the protective layer on metal surfaces.
This type of corrosion affects specific areas of the material rather than the entire surface.
Crevice Corrosion
Crevice corrosion occurs due to localized breakdown of the protective layer in narrow spaces between metal components or between metal and non-metal components.
It occurs through a mechanism similar to pitting corrosion.
The likelihood of crevice corrosion can vary depending on the area ratio between surfaces exposed to seawater and surfaces forming crevices.
Galvanic Corrosion
Galvanic corrosion occurs when different metals come into contact while submerged in seawater.
Galvanic action can cause the metal with the lower electric potential to corrode.
Therefore, the combination of different metal materials should be carefully considered in seawater applications.
4. Effect of Chlorination on Corrosion
Chlorination is another factor that can influence the corrosion behavior of pump materials.
For carbon steel, chlorination can increase the tendency for localized corrosion.
The EBARA technical reference indicates that a residual chlorine concentration of 0.3 ppm is generally acceptable for preventing the adherence of biological organisms on carbon steel surfaces.
However, the potential effects of chlorination on corrosion should still be considered when evaluating material suitability.
5. Effect of Flow Velocity on Pump Materials
Flow velocity is another important consideration in pump material selection.
Copper-based materials can generally be used only at relatively low flow velocities.
In contrast, stainless steel materials can be used at higher flow velocities, although they may experience pitting corrosion under low-flow conditions.
The EBARA technical reference also indicates that flowing water through the system once a week may help minimize pitting corrosion in stainless steel materials.
Therefore, actual operating flow conditions should be considered when evaluating the corrosion resistance of pump materials.
6. Corrosion Prevention Methods
Several methods can be applied to reduce corrosion risks, particularly in seawater pump applications.
Cathodic Protection
Cathodic protection is a method of corrosion prevention using anodes or an external electrical power source.
Protective Lining
Applying a protective lining to surfaces in contact with the liquid is a relatively inexpensive and effective corrosion prevention method.
The service life of the lining mainly depends on the permeability of the lining material and its adhesion to the metal surface.
Use of Filling Materials
Crevice corrosion may be reduced by inserting filling materials into narrow spaces, such as flange mating surfaces.
Anti-Corrosion Alloy Metal Overlay
Applying corrosion-resistant alloy materials, such as CREVALLOY or TORIBAROI, can effectively help prevent crevice corrosion.
These methods can be considered based on the characteristics of the liquid, operating environment, and pump material.
The corrosion resistance of pump materials depends on various factors, including the characteristics of the liquid, material properties, chlorination, and operating flow velocity.
Fresh water and seawater present different corrosion conditions, making material selection an important consideration for pump applications.
By understanding corrosion characteristics and applying appropriate prevention methods, the risk of corrosion can be reduced while supporting reliable pump operation and service life.
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