Pump Materials Compared: Cast Iron, Bronze, Stainless Steel and Engineered Plastic
The fluid decides the material, not the price list.
This guide compares the six materials you will meet on pump casings and impellers: what each survives, where each fails, and which WALRUS pump series in the Valvenok range uses it.
Quick answer
What are you pumping?
Pick your fluid to see the material to start with.
Clean, cold fresh water: cast iron or engineered plastic with stainless 304 internals. Lowest cost, adequate life.
Salt, brackish or chlorinated water: stainless 316 as a minimum, 904L or bronze for seawater. Never cast iron in permanent service.
Sand, grit or shock loads: cast iron. Wall mass absorbs abrasion and hydraulic shock that wears thin stainless or plastic.
Coolant, light oil, hot water: stainless wetted parts with a seal elastomer matched to the fluid (NBR, HNBR, Viton or EPDM).
Pump materials comparison table
Compare the wetted-part materials side by side. PREN (Pitting Resistance Equivalent Number = %Cr + 3.3×%Mo + 16×%N) applies to stainless grades only; higher means harder to pit in chloride water.
Compare:
| Material | ||||||
|---|---|---|---|---|---|---|
| Typical use in a pump | Casings, diffusers, valve housings | Impellers, wear rings, marine casings | Impellers, shafts, casings on light-duty pumps | All wetted parts | All wetted parts, aggressive duty | Casings, covers on compact pumps |
| Corrosion behaviour | No passive film; rusts and graphitises in soft, acidic or saline water | Good in seawater; low-zinc grades resist dezincification | Passive chromium-oxide film; pits in chloride water | Molybdenum lifts pitting resistance for moderate chloride | High Ni and Mo; resists chloride pitting and reducing acids | Does not rust or pit; chemically inert to most water |
| PREN | — | — | ≈ 19 | ≈ 26 | ≈ 35 | — |
| Abrasion / shock | Excellent | Good | Good | Good | Good | Low |
| Relative cost | ££££(lowest) | ££££ | ££££ | ££££ | ££££ | ££££ |
| Best for | Dewatering, irrigation, municipal fresh water, closed heating circuits | Marine cooling, seawater transfer, fire-fighting | Clean fresh water, potable supply, boosting | Food, pharma, treated water, moderate chlorides | Seawater intake, brackish dewatering, mine water | Domestic and light-commercial clean water boosting |
| Avoid in | Seawater, brackish water, potable lines needing clean water | Ammonia-bearing fluids, high-velocity sand slurry | Coastal, brackish or chlorinated pool water | Continuous seawater, high-chloride warm water | Duties where 316 already lasts — it rarely pays back | Abrasive solids, high pressure, impact-prone sites |
How the materials score
Indicative ratings out of 5, for comparison only. Your fluid analysis overrides any chart.
Cast iron
1/5
Bronze
4/5
Stainless 304
2/5
Stainless 316
3/5
904L
5/5
Plastic (PPS)
5/5
Stainless grades by PREN: the chloride number that matters
AISI 304
≈ 19
AISI 316
≈ 26
AISI 904L
≈ 35
Scale 0–40. Cast iron, bronze and plastics have no PREN: they do not rely on a passive chromium film.
What the WALRUS range uses, and why
Four series from the Valvenok pump range show the trade-offs above in real products. Figures are from each series' product page.
Plastic casing + 304 internals
TP8 Automatic Booster
- Casing
- PPS
- Impeller
- SUS304
- Shaft
- SUS410
- Seal
- Ceramic + carbon + NBR
- Liquid temp.
- +4 to +40 °C
Rust-proof and low cost for clean-water boosting; not the pump for grit or high pressure.
View TP8 series Corrosion-resistant wetted parts
HS / TS Multistage Centrifugal
- Wetted parts
- Corrosion-resistant materials
- Fluid
- TS: plain and salt water
- System pressure
- Max. 8.5 kg/cm²
- Liquid temp.
- +4 to +40 °C
Choose TS over HS when the supply carries salt.
View HS / TS series Stainless steel construction
TPRK_E / TPHK / TPK Immersible
- Main parts
- Stainless steel
- Fluid
- Water, coolant, light oil
- Seal options
- SiC, carbon; HNBR / Viton / EPDM
- Liquid temp.
- 0 to +90 °C
Machine-tool coolant and wash systems, up to 267 m head.
View immersible series Seawater variant
PW Submersible
- Construction
- Corrosion-resistant, IP68
- Saltwater
- R-coded models
- Flow
- Up to 440 L/min
- Liquid temp.
- +4 to +35 °C
Specify the R code for sea-fish breeding or saltwater drainage.
View PW series Choose in four questions
- What is in the fluid? Get chloride, pH and solids figures. Chloride rules out cast iron and pushes stainless up the PREN scale.
- How hot does it run? Many compact boosters are rated to 40 °C; hot-water and immersible series go to 90 °C.
- Is there grit or shock? Sand and water hammer favour cast iron's wall mass over thin stainless or plastic.
- What does a failure cost? Where retrieval or downtime is expensive, pay for the higher grade up front.
Going deeper on borehole pumps?
For submersible borehole duty (water analysis limits, 904L vs 316L and dry-run protection), read our engineering guide: Stainless Steel vs Cast Iron Submersible Pumps.Frequently asked questions
Is a stainless steel pump better than cast iron?
Only when the fluid is corrosive. In clean fresh water cast iron lasts well, costs less and handles grit better.
What is the best pump material for seawater?
904L stainless or marine bronze for continuous duty; 316 for intermittent or lower-chloride service. Avoid cast iron and 304.
Are plastic pump casings reliable?
Yes for clean water inside their pressure and temperature rating. They will not rust, but they are not built for abrasive solids or impact.
Does the seal material matter as much as the casing?
Often more. Match the elastomer to the fluid and temperature: NBR for water and oils, EPDM for hot water, Viton for chemicals and higher temperatures.
Match the pump to your fluid
Eight WALRUS pump families, 50/60 Hz, up to 267 m head and 650 L/min. Send us your fluid, temperature and duty point.
