ENGINEERING MATERIAL GUIDE
Quick answer: Select impeller metallurgy from the actual chemical, concentration and temperature first. Then verify impurities, solids, velocity, cavitation and erosion risk, as well as the rest of the wetted system. There is no universal best alloy for every sulfuric acid or caustic soda duty.
An impeller purchase can look simple on a parts list: confirm the pump model, choose a metal and place the order. Corrosive chemical service makes that shortcut risky. A material that screens well for one sulfuric acid concentration can require caution or engineering review at another concentration. The same is true for sodium hydroxide. Temperature changes the condition again, and the rotating impeller adds velocity, solids, cavitation and geometry-sensitive wear to the decision.
For a chemical-process buyer, the practical objective is not to name the most expensive alloy. It is to identify a defensible candidate material, document the service condition and confirm that the selected casting, impeller geometry, sealing system and other wetted components belong in the same verified configuration. This guide explains that reasoning without duplicating the site’s interactive material database.
Why Impeller Material Selection Is Different From a Generic Wetted-Part Check
The impeller is both a wetted part and a rotating hydraulic component. Its vanes accelerate the liquid, create a pressure field and operate with clearances that affect pump performance. Chemical compatibility is therefore necessary, but it does not by itself predict impeller service life.
Local velocity can be much higher around vane leading edges, discharge tips and close-clearance regions than a simplified line-velocity value suggests. Suspended solids can repeatedly strike those surfaces. If the liquid is chemically aggressive, mechanical removal of a protective surface film may expose fresh metal and accelerate erosion-corrosion. A table rating based on chemical environment alone cannot describe that interaction.
Cavitation is a separate mechanism. Vapor bubbles form when local pressure falls sufficiently and then collapse as pressure recovers. The resulting surface damage can resemble corrosion or abrasive wear, but changing to a more corrosion-resistant alloy does not correct inadequate NPSH available, suction losses or operation outside an acceptable range. The hydraulic cause still has to be removed.
Surface degradation also matters because an impeller is shaped to deliver a defined duty. Rounded vane edges, pitting, loss of diameter or widening clearances may change head, efficiency, balance and vibration. A casing, cover, shaft sleeve, mechanical seal and elastomer can have different failure risks even when they see the same bulk liquid. Final material selection must therefore review the complete wet end, not just the impeller in isolation.
Start With Concentration and Temperature, Not the Alloy Name
“Sulfuric acid” is not a complete material-selection condition. Neither is “caustic soda.” The purchasing specification must attach concentration and temperature to the chemical name because those variables can change the preliminary rating. The relevant temperature is the credible service range, including normal operation, upset or cleaning conditions where applicable, rather than a convenient room-temperature value.
Impurities can also change behavior. Chlorides, oxidizing contaminants, dissolved gases, aeration, solids and another chemical in a mixture may make a nominally similar duty materially different. A batch process can pass through intermediate concentrations during filling, dilution or cleaning. Those transient conditions deserve review when they contact the impeller.
The ANSI Pumps Pro chemical resistance guide for ANSI pump materials returns exact listed conditions. It does not interpolate between an unlisted concentration and temperature. Its A/B/C/D results are preliminary screening information, not a corrosion-rate model or a blanket approval for every casting grade.
Use the exact condition. Search the listed chemical, concentration and temperature. If no exact row exists, do not infer a rating from the nearest value.
Sulfuric Acid Impeller Material Selection
Sulfuric acid service must be evaluated condition by condition. Concentration affects the chemical environment, while temperature and contamination can change the expected behavior again. A phrase such as “sulfuric acid pump material” is useful for search, but it is not enough for a technical quotation.
What the current material guide shows
The following compact table reproduces only three selected rows from the current guide. The condition remains attached to every rating. Use the interactive guide for other exact listed conditions and read the full limitations before selecting a candidate.
| H2SO4 condition | 316 SS | CD4MCu | Alloy 20 | C-276 | Titanium | Nickel 200 | Alloy 400 |
|---|---|---|---|---|---|---|---|
| 50% at 120°F | B | B | A | A | B | C | D |
| 80% at 75°F | C | C | A | B | C | D | D |
| 98% at 75°F | B | C | C | B | D | D | D |
Screening note: Screening ratings apply only to the listed concentration and temperature. Do not interpolate an unlisted condition from the nearest row.
The table is useful precisely because it prevents a universal ranking. At 50% and 120°F, Alloy 20 and C-276 are rated A in the current guide, while 316 SS and CD4MCu are B. At 80% and 75°F, Alloy 20 remains A, but C-276 is B and 316 SS is C. At 98% and 75°F, 316 SS and C-276 are B, while Alloy 20 is C. These are screening outcomes for the listed rows, not corrosion-rate predictions or approvals for an unlisted process.
When Alloy 20 enters the discussion
Alloy 20 is often discussed for sulfuric acid pump components, but the phrase “Alloy 20 for sulfuric acid” still omits the decision variables. The current guide rates the Alloy 20 family A for the listed 50% at 120°F and 80% at 75°F conditions, B for 93% at 75°F and C for 98% at 75°F. That variation is why every material statement and purchase specification must retain concentration and temperature.
The commercial specification should also distinguish nominal alloy family from the actual material form. Alloy 20 commonly refers to UNS N08020 in wrought-product contexts, while CN7M is a cast grade used for pump components. A quotation should identify the required cast grade, applicable material specification, heat or melt traceability and any project-specific testing. A family name alone does not prove that a casting is correctly specified or supplied.
Why 316 SS, CD4MCu and C-276 cannot be ranked once for every duty
316 stainless steel, represented by CF8M in many cast pump-component applications, is not simply “good” or “bad” for all sulfuric acid. The selected guide rows move between caution and engineering review. CD4MCu also changes by condition. Wrought C-276 is associated with UNS N10276. For cast pump components, ASTM A494 CW12MW is the cast C-276 grade reference; CW2M belongs to the C-4 family and should not be used as a shorthand for C-276. Specify the exact alloy, material form and governing standard on the purchase document.
Pump Casting Grade Reference
| Alloy family | Cast pump-component reference |
|---|---|
| 316 family | CF8M |
| Alloy 20 / UNS N08020 | CN7M |
| Nickel 200 | ASTM A494 CZ100 |
| C-276 / UNS N10276 | ASTM A494 CW12MW |
Procurement note: Do not substitute a wrought designation for a cast grade. Put the exact alloy, UNS designation, material form and applicable ASTM standard on the RFQ and order.
A best-to-worst list would hide the information a buyer needs most: the actual chemical environment and the component form. It would also ignore solids, velocity, aeration, mixing transients and a plant’s previous failure evidence. Use the table to identify candidates, then review the duty and manufacturing specification.
Caustic Soda / Sodium Hydroxide Impeller Material Selection
Caustic soda pump material selection has the same condition-dependent requirement. Sodium hydroxide concentration and temperature must be stated, and a metallic impeller rating does not cover every seal face, gasket, O-ring, coating or non-metallic wetted component.
| NaOH condition | 316 SS | CD4MCu | Alloy 20 | C-276 | Titanium | Nickel 200 | Alloy 400 |
|---|---|---|---|---|---|---|---|
| 10% at 200°F | A | A | A | A | B | A | A |
| 50% at 200°F | A | A | B | B | D | A | B |
| 73% at 120°F | B | B | C | C | D | A | B |
Nickel 200 is rated A in these three selected rows, but that does not authorize extrapolation to every caustic mixture, contaminant or temperature. The selected titanium results move from B at 10% and 200°F to D at both 50% and 200°F and 73% at 120°F. Alloy 20 and C-276 move from A at the dilute listed condition to B or C as the selected condition changes.
The practical conclusion is not that one metal always replaces another. It is that concentration controls the screening discussion and that the full wetted system must be checked. A metallic impeller can still be paired with an unsuitable mechanical-seal elastomer, gasket or coating. The quote must identify those components separately.
Corrosion, Erosion-Corrosion and Cavitation Are Different Failure Modes
Corrosion
Corrosion is chemical or electrochemical deterioration of the metal in its environment. It may appear as relatively general loss, pitting, crevice attack or another localized pattern. The appearance, location and operating history help determine whether the selected material, an impurity, a stagnant region or a concentration and temperature change contributed to the failure.
Erosion-corrosion
Erosion-corrosion combines chemical attack with mechanical removal. High local velocity, suspended solids or repeated impingement can strip protective films and expose fresh metal. Damage may concentrate at vane entrances, tips or flow-turning regions. The corrective action can require both a metallurgy review and a hydraulic or system change, not merely a harder or more expensive alloy.
Cavitation damage
Cavitation damage follows bubble formation and collapse in low-pressure regions. A more resistant material may change damage tolerance, but it does not restore NPSH available, remove excessive suction loss or move the pump to a suitable operating point. Use the site’s NPSHa and pump calculators for a preliminary suction check, then review the installed system and pump curve.
| Observed pattern | Primary question | What to verify before selecting material |
|---|---|---|
| General or localized chemical attack | Is the metal compatible with the exact liquid condition? | Concentration, temperature, impurities, aeration and crevices |
| Directional wear or impingement | Are chemistry and mechanical removal interacting? | Solids, local velocity, geometry and flow distribution |
| Pitted, hammered surface near low-pressure regions | Is cavitation present? | NPSHa, suction losses, vapor pressure and operating point |
A Practical Impeller Metallurgy Selection Workflow
A defensible workflow preserves the condition and the evidence from initial screening through quotation. It also makes clear where a tool result ends and an engineering decision begins.
| Step | Decision | Required output |
|---|---|---|
| 1 | Identify the exact fluid or mixture | Chemical names, composition and process contaminants |
| 2 | Record concentration | Normal range plus credible transient or cleaning conditions |
| 3 | Record temperature | Normal, maximum and relevant startup or upset values |
| 4 | Screen listed compatibility | Exact-row A/B/C/D result or a documented data gap |
| 5 | Review service modifiers | Impurities, solids, aeration, velocity and mixing sequence |
| 6 | Review damage mechanism | Corrosion, erosion-corrosion, cavitation or another verified cause |
| 7 | Verify the component configuration | Material form, pump model, impeller geometry, interfaces and sealing system |
At Step 4, A means generally resistant under the listed reference condition, B means use with caution, C means engineering review is required and D means not recommended. None of these letters confirms a corrosion rate, component life or the suitability of an unlisted mixture. When there is no exact row, record the gap and escalate it rather than interpolating.
At Step 7, dimensional and material confirmation come together. Identify the pump model, size, frame or group, shaft arrangement, impeller design and trim, seal chamber and retained components. For replacement work, provide a part number, drawing, sample or measured evidence. The ANSI pump selection worksheet and broader ANSI pump technical resources can help organize the duty before quotation.
What to Send With an RFQ for a Corrosive-Service Impeller
A useful RFQ lets the supplier confirm both service and fitment. Send the data together so that a material family is not separated from the condition it is expected to handle.
- Chemical or mixture: include relevant contaminants, cleaning fluids and batch transitions.
- Concentration: normal range and any credible maximum or intermediate condition.
- Temperature: normal, maximum, startup and cleaning values where applicable.
- Solids and impurities: type, approximate loading and particle information if known.
- Hydraulic duty: flow and total dynamic head, plus operating range when available.
- Pump identity: manufacturer, model, size, frame or group and speed.
- Current impeller: material, trim or diameter, design and part number if available.
- Failure evidence: photos, locations, service hours, inspection notes and previous analyses.
- Fitment evidence: drawing, sample, part number, dimensions or nameplate photo.
For a replacement impeller or wet-end package, the material review should be combined with the component interface review. See ANSI pump replacement parts for the supported aftermarket pathway. If density and motor load are also part of the duty review, continue with how specific gravity affects centrifugal pump power.
Need confirmation beyond the screening table?
Send the exact fluid, concentration, temperature, pump model and any failure or fitment evidence. We will review impeller material, damage mechanism and component configuration.
Frequently Asked Questions
What is the best impeller material for sulfuric acid?
There is no universal best material. Select a candidate from the exact sulfuric acid concentration and temperature, then verify impurities, solids, velocity, cavitation or erosion risk, material form and the complete wetted system.
Is Alloy 20 suitable for sulfuric acid pumps?
It can be a candidate for some listed conditions, but its rating changes with concentration and temperature. The current guide rates Alloy 20 A at 50% H2SO4 and 120°F and at 80% and 75°F, B at 93% and 75°F, and C at 98% and 75°F. These are preliminary screening ratings, not universal approval.
Can 316 stainless steel be used for sulfuric acid service?
Only after checking the exact condition and the complete service. In the selected guide rows, 316 SS is B at 50% and 120°F, C at 80% and 75°F, and B at 98% and 75°F. Do not extrapolate those results to another concentration, temperature or mixture.
What pump material is used for caustic soda?
Several metallic materials screen differently by condition. In the selected NaOH rows, Nickel 200 is A, while titanium, Alloy 20 and C-276 change ratings as concentration and temperature change. Final selection must also review contaminants, solids, seals and the complete wetted system.
Why does caustic concentration matter for material selection?
Concentration changes the chemical environment and can change a material’s screening result. For example, the selected current-guide rows show different outcomes at 10%, 50% and 73% NaOH. Keep concentration and temperature attached to every recommendation.
Does a corrosion-resistance rating also cover mechanical seals and elastomers?
No. The listed table compares metallic material families. Mechanical-seal faces, elastomers, gaskets, coatings and other non-metallic wetted parts require their own compatibility review.
Can I select an impeller material from a compatibility table alone?
No. Use the table for preliminary screening, then review concentration, temperature, impurities, solids, aeration, velocity, cavitation and erosion history, material form, pump configuration and the full sealing and wetted system.
Technical Sources
- ANSI Pumps Pro: Chemical Resistance Guide for ANSI Pump Materials
- ANSI Pumps Pro: Technical Resources
- KSB Centrifugal Pump Lexicon: Cavitation
Editorial scope: ANSI Pumps Pro manufactures and supports ANSI process pumps and replacement components. This article provides a condition-based screening workflow. It does not certify a material for an unlisted duty or replace project-specific corrosion, hydraulic, seal and fitment review.
