The pH of the fluid being pumped is a critical factor in selecting pump materials. Incorrect material selection leads to corrosion, premature failure, and potential contamination of the pumped fluid.
Why pH Matters
Both acidic (low pH) and alkaline (high pH) fluids can attack pump components. Temperature and chemical concentration amplify corrosion effects.
Understanding pH
The pH scale measures acidity or alkalinity from 0 to 14:
| pH Range | Classification | Examples | Recommended Materials |
|---|---|---|---|
| 0-2 | Extremely Acidic | Battery acid, concentrated acids | Hastelloy, Titanium, PTFE-lined |
| 2-4 | Strongly Acidic | Mine drainage, pickling solutions | Duplex SS, 316SS with caution |
| 4-6 | Mildly Acidic | Fruit juices, acid rain, wine | 316 Stainless Steel |
| 6-8 | Neutral | Pure water, milk, most potable water | Cast Iron, 304/316 SS |
| 8-10 | Mildly Alkaline | Seawater, baking soda, soap | Cast Iron, 316 SS |
| 10-12 | Strongly Alkaline | Ammonia solutions, lime slurry | Duplex SS, Nickel alloys |
| 12-14 | Extremely Alkaline | Caustic soda, bleach, lye | Hastelloy, Monel, special alloys |
High Chloride Environments
For seawater or high chloride applications, consider epoxy-coated internals or duplex stainless steel to prevent pitting and crevice corrosion. Standard 316SS has limits in chloride-rich environments.
Material Options
Cast Iron
pH Range: 6-10
Economical for neutral water. Not suitable for acidic or aggressive fluids.
316 Stainless Steel
pH Range: 4-10
Good corrosion resistance for mildly acidic/alkaline fluids. Chloride limits apply.
Duplex Stainless
pH Range: 2-12
Excellent resistance to pitting and stress corrosion. Higher chloride tolerance.
High-Alloy/Special
pH Range: 0-14
Hastelloy, titanium for extreme conditions. Highest cost but broadest compatibility.
Elastomer Selection
Seals, O-rings, and diaphragms require compatible elastomers. The wrong elastomer can swell, harden, or dissolve in aggressive fluids.
Viton (FKM)
Best For: Oils, fuels, hydrocarbons, many acids
Avoid: Ketones, esters, amines, hot water/steam
Temp Range: -20°C to +200°C
EPDM
Best For: Water, steam, dilute acids/bases, ozone
Avoid: Oils, fuels, petroleum-based fluids
Temp Range: -50°C to +150°C
PTFE (Teflon)
Best For: Almost all chemicals, extreme pH
Avoid: Molten alkali metals, fluorine gas
Temp Range: -200°C to +260°C
Nitrile (NBR)
Best For: Petroleum, mineral oils, water
Avoid: Ozone, strong acids, ketones
Temp Range: -40°C to +120°C
Elastomer Selection Tip
When in doubt, PTFE-based seals offer the widest chemical compatibility but may require higher seal pressures. Always verify with chemical resistance charts for your specific fluid.
Selection Guide
When selecting materials, consider:
- pH of fluid: Primary factor in material selection
- Temperature: Corrosion rates typically double for every 10°C increase
- Chemical concentration: Dilute acids can be more aggressive than concentrated
- Chloride content: Critical for stainless steel selection
- Abrasives: Solids content affects wear resistance requirements
Important
Always verify material compatibility with the pump manufacturer. Chemical resistance charts are guides only – actual conditions may vary.
Safety Considerations
Working with aggressive fluids requires proper safety protocols to protect personnel and equipment.
Personal Protection
- Chemical-resistant gloves appropriate for the fluid
- Face shield and safety glasses
- Protective clothing and apron
- Respiratory protection if vapours present
Maintenance Safety
- Flush system with neutralising agent before maintenance
- Depressurise and drain completely
- Allow cooling for hot fluid systems
- Have spill containment ready
Critical Safety Warning
Never mix incompatible chemicals. Some combinations can cause violent reactions, toxic gas release, or explosions. Always consult Safety Data Sheets (SDS) and follow site-specific procedures.
Summary
Key Takeaways
- Know your fluid’s pH before selecting pump materials
- Consider temperature and chemical concentration effects
- Cast iron is economical for neutral water only
- Consult chemical resistance charts and manufacturer recommendations