Saltwater is one of the most aggressive fluids encountered in pumping service. Chloride ions, dissolved oxygen, biological growth, and abrasive sediment combine to attack most ferrous materials within months. Specifying the right materials, coatings, and certifications is essential for marine, offshore, desalination, aquaculture, and coastal industrial applications.
Why saltwater is corrosive
Seawater contains approximately 3.5% dissolved salts, dominated by sodium chloride. Chloride ions attack the passive oxide layer that normally protects stainless steel, leading to pitting and crevice corrosion. Dissolved oxygen drives general corrosion of carbon steel and cast iron. Biological growth — barnacles, mussels, microbial slime — creates oxygen-depleted zones that accelerate localised corrosion. Sediment in coastal waters adds erosion to the chemical attack.
The result: standard cast iron pumps fail in months, austenitic stainless (304/316) suffers pitting and crevice corrosion in warm or stagnant seawater, and even bronze degrades if galvanic conditions are unfavourable.
Common materials for saltwater pumps
| Material | Properties | Typical Use |
|---|---|---|
| Marine bronze (nickel-aluminium bronze, NAB) | Excellent seawater resistance, good erosion resistance, traditional choice | Impellers, casings, propellers, marine cooling pumps |
| Duplex stainless steel (e.g. 2205, 1.4462) | Higher strength than austenitic stainless, good chloride resistance | Process pumps, ballast pumps, moderate-temperature seawater |
| Super-duplex stainless steel (e.g. 2507, 1.4410) | Excellent pitting and crevice corrosion resistance (PREN >40) | Offshore seawater lift, fire main, deep-sea applications |
| 6Mo / super-austenitic (e.g. 254 SMO, AL-6XN) | Better corrosion resistance than duplex in some conditions, higher cost | Aggressive seawater, desalination, FGD scrubbers |
| Nickel alloys (Inconel 625, Hastelloy C-276) | Extreme corrosion resistance, very high cost | Severe service, contaminated seawater with H₂S or chemicals |
| Titanium (Grade 2, Grade 5) | Effectively immune to seawater corrosion at low velocities, expensive | Specialised desalination, deep-sea, defence |
Pitting Resistance Equivalent Number (PREN)
For stainless steels, the standard measure of chloride pitting resistance is PREN — a weighted sum of chromium, molybdenum, and nitrogen content:
PREN = %Cr + 3.3 × %Mo + 16 × %N
Higher PREN means better pitting resistance:
- 304 (1.4301): PREN ≈ 18 — not suitable for seawater
- 316 (1.4401): PREN ≈ 24 — limited to fresh seawater service, short-term
- 2205 duplex (1.4462): PREN ≈ 35 — suitable for most seawater service
- 2507 super-duplex (1.4410): PREN ≈ 42 — proven for aggressive seawater
- 6Mo (e.g. 254 SMO): PREN ≈ 44 — high-end seawater applications
Avoiding galvanic corrosion
When two different metals are connected in seawater (an electrolyte), the more noble metal accelerates corrosion of the less noble one. A bronze pump fitted with carbon steel piping will cause the steel to corrode rapidly at the connection.
Practical rules:
- Pair similar materials (bronze with bronze, duplex with duplex).
- Use isolating gaskets and sleeves when mixing materials.
- Apply sacrificial anodes (zinc or aluminium) at strategic points.
- Avoid carbon steel fasteners on bronze or stainless components.
Certifications and class society approvals
For marine and offshore service, pumps frequently require approval from a classification society — DNV, Lloyd's Register, Bureau Veritas, ABS, RINA, ClassNK. Approval covers material certificates (EN 10204 3.1 or 3.2), hydrostatic testing, performance testing, and documentation. Materials in sour service (H₂S) must comply with NACE MR0175 / ISO 15156. For potable water service in desalination, materials must meet WRAS, NSF 61, or KTW certifications. Specify these requirements early — material change after fabrication is usually impossible.