Stainless Steel vs Cast Iron Submersible Pumps
Both materials are used in deep well and irrigation pumps, and both are produced by FANPO across overlapping flow and head ranges. The right choice depends on water chemistry, sand content, expected service life and how the pump will be maintained — not on which one is generally better.
The short answer
| Stainless steel | Cast iron | |
|---|---|---|
| Body material | AISI 304 stainless steel body, impeller and diffuser | Cast iron body with a robust hydraulic design |
| Corrosion resistance | High. Suited to aggressive, chloride-bearing and slightly acidic water | Moderate. Best in conventional, neutral groundwater |
| Efficiency over time | Holds its hydraulic surface and efficiency longer | Surface roughening over years can reduce efficiency |
| Weight | Lighter for the same duty, easier to install and pull | Heavier, which matters on deep installations |
| Mechanical robustness | Thinner sections, excellent corrosion life | Thick walls, tolerant of rough handling |
| Initial cost | Higher | Lower |
| FANPO range | 6″, 8″ and 10″ series, up to 130 m³/h and 633 m | 5″, 6″, 7″ and 8″ series, up to 162 m³/h and 436 m |
| Typical use | Drinking water, aggressive or brackish groundwater, long unattended service | Agricultural irrigation, high-flow transfer, conventional wells |
Maximum flow and maximum head are series limits across the whole range and do not occur at the same operating point.
Choose stainless steel when…
The water is chemically aggressive
Chloride content, low pH or dissolved gases attack cast iron far faster than AISI 304 stainless steel.
In coastal, brackish or geothermally influenced aquifers, stainless steel is usually the only sensible option.
The pump supplies drinking water
Stainless steel does not shed iron oxide into the water, so it avoids the taste, colour and staining problems associated with corroding iron components.
The installation is deep or hard to reach
A lighter pump is easier and cheaper to install and to pull for service.
When a single extraction costs a crane and a day of work, the material that fails later is the cheaper material.
Long unattended service is expected
Municipal, industrial and export installations that are expected to run for years with minimal intervention benefit from the slower degradation of stainless steel.
High head is required
The 6030 series reaches 633 m and the 8095 reaches 551 m, the highest heads in the FANPO range.
Choose cast iron when…
The water is conventional groundwater
In neutral, low-chloride agricultural aquifers a cast iron pump delivers many years of service at a lower initial cost.
Flow matters more than head
The 8535 series reaches 162 m³/h, the highest flow in the FANPO range, and the 7021 delivers 120 m³/h in a 7″ body.
The borehole is 5″ or 7″
FANPO produces 5″ and 7″ pumps only in cast iron. In those casings there is no stainless alternative in the range.
Budget drives the decision
On large irrigation schemes with many wells, the difference in initial cost across a whole project can be substantial and may be the right trade-off in benign water.
Handling conditions are rough
Thick cast sections tolerate the knocks of field installation better than thin stainless pressings.
Deciding from the water analysis
Let the water report make the decision
A simple water analysis answers the material question faster than any general argument. The values below are indicative guidance for typical borehole water; have an unusual analysis reviewed before ordering.
| What the analysis shows | Practical implication | Suggested material |
|---|---|---|
| Neutral pH, low chloride, low sand | Standard agricultural groundwater | Cast iron is normally sufficient |
| Elevated chloride or brackish water | Accelerated corrosion of iron components | Stainless steel |
| Low pH / acidic water | Aggressive to iron surfaces | Stainless steel |
| High sand content | Abrasive wear dominates over corrosion | Either material will wear; address the sand first with a separator or a repositioned intake |
| Drinking water supply | Iron oxide affects taste, colour and staining | Stainless steel |
| High iron or manganese | Deposits form on hydraulic surfaces regardless of material | Plan for periodic cleaning; stainless is easier to clean |
Sand is the one problem no pump material solves. If sand content is high, the intake position and well development matter more than the choice between stainless steel and cast iron.
Where the two ranges overlap
Stainless steel submersible pump series
AISI 304 body, impeller and diffuser.
| Series | Size | Max stages | Power | Max flow | Max head |
|---|---|---|---|---|---|
| 6017 | 6″ | 40 | 0.75–30 HP | 20 m³/h | 460 m |
| 6024 | 6″ | 33 | 1.5–40 HP | 32 m³/h | 442 m |
| 6030 | 6″ | 40 | 1.5–70 HP | 43 m³/h | 633 m |
| 6046 | 6″ | 25 | 3–60 HP | 59 m³/h | 372 m |
| 6060 | 6″ | 25 | 3–60 HP | 72 m³/h | 370 m |
| 8075 | 8″ | 22 | 5.5–100 HP | 96 m³/h | 423 m |
| 8095 | 8″ | 22 | 7.5–175 HP | 110 m³/h | 551 m |
| 8096 | 8″ | 22 | 10–200 HP | 125 m³/h | 467 m |
| 10110 | 10″ | 20 | 10–200 HP | 130 m³/h | 549 m |
Cast iron submersible pump series
Cast iron body with a robust hydraulic design.
| Series | Size | Max stages | Power | Max flow | Max head |
|---|---|---|---|---|---|
| 5004 | 5″ | 6 | 2–10 HP | 40 m³/h | 85 m |
| 6012 | 6″ | 10 | 5.5–50 HP | 48 m³/h | 295 m |
| 6013 | 6″ | 25 | 5.5–75 HP | 60 m³/h | 420 m |
| 6019 | 6″ | 18 | 7.5–60 HP | 84 m³/h | 270 m |
| 7021 | 7″ | 25 | 12.5–150 HP | 120 m³/h | 435 m |
| 8420 | 8″ | 18 | 15–175 HP | 117 m³/h | 436 m |
| 8530 | 8″ | 14 | 25–175 HP | 144 m³/h | 414 m |
| 8535 | 8″ | 14 | 30–175 HP | 162 m³/h | 405 m |
In 6″ and 8″ the two ranges overlap almost completely, so the material can be chosen on water chemistry alone. In 5″ and 7″ only cast iron is available; in 10″ only stainless steel.
Frequently asked questions
Is stainless steel always the better pump?
No. In neutral, low-chloride groundwater a cast iron pump can run for many years, and the money saved may be better spent on a larger pipe or a proper pumping test.
Does stainless steel resist sand better?
Not meaningfully. Sand wears the hydraulic surfaces of both materials. The solution is to reduce the sand reaching the pump, not to change the body material.
Which material is more efficient?
The hydraulic design sets the efficiency, not the material. Over years of service, however, stainless steel tends to retain its original surface and therefore its efficiency better.
Can I put a stainless pump on a cast iron motor?
The motor is a separate component and is selected on power, diameter and cooling. See the FANPO submersible motor range.
What about 5″ and 7″ wells?
FANPO produces the 5″ and 7″ series in cast iron only. If your casing dictates those sizes, the material decision is already made.
How do I decide for my own well?
Send a water analysis together with the flow and head requirement through the contact page. It is also worth reading the pump selection guide first.