Alloy 20 or C-276: What the Comparison Can Tell You
Alloy 20 is considered for selected sulfuric-acid and related chemical services. The C-276 alloy family is often considered when chloride-bearing or mixed corrosive conditions raise concern about localized corrosion. Neither description approves a material for a specific pump. The decision depends on the actual liquid composition, concentration range, temperature, contaminants, flow conditions and component specification.
For an ANSI process-pump wet end, keep three questions separate: is the alloy family suitable for the process, is the supplied casting or wrought part specified correctly, and does the replacement match the installed pump and duty? A “yes” to one does not answer the others.
Do not use a universal corrosion red line
Do not select or reject either alloy from acid name, one chloride value or a single temperature threshold. Use corrosion data that matches the actual chemistry and exposure conditions, then confirm the material form and purchase requirements with engineering.
Specify the Pump Casting Grade, Not Only the Alloy Name
Alloy names and UNS designations are commonly used for wrought products, while a pump casing or impeller is usually a casting. A purchase description should identify the product form, applicable material specification and grade. Similar service intent does not make two forms identical or automatically interchangeable.
| Material reference | Common product form | Pump casting reference | Purchasing check |
|---|---|---|---|
| Alloy 20 / UNS N08020 | Wrought alloy reference | CN7M is commonly used for Alloy 20-family cast components | State the governing casting standard and CN7M grade; verify the actual chemistry and heat number on the MTR. |
| HASTELLOY C-276 / UNS N10276 | Wrought alloy reference | ASTM A494 CW12MW is commonly associated with cast C-276-family components | Confirm the specified casting grade, standard edition, heat treatment, traceability and inspection requirements. |
| CW2M / UNS N26455 | Cast nickel-alloy grade | C-4-family casting reference | Do not use CW2M as a synonym for C-276 or substitute it for CW12MW without an approved engineering basis. |
| CX2MW / UNS N26022 | Cast nickel-alloy grade | C-22-family casting reference | Specify the actual grade and product form required by the project; do not infer interchangeability from a family name. |
ASTM A351 covers austenitic steel castings for pressure-containing parts; ASTM A494 covers nickel and nickel-alloy castings. The applicable specification depends on the grade, component and purchase requirements. See the ASTM A351 scope and ASTM A494/A494M scope. For more detail on C-276 product-form verification, see the C-276 pump-parts material guide.
Review the Actual Chemistry, Not a Generic Acid Limit
For sulfuric acid, concentration alone is not enough. Temperature, aeration, impurities, velocity and deposits can change corrosion behavior. Published Alloy 20 data also warn that commercial contaminants and heat-transfer conditions can change laboratory results. For C-276, manufacturer corrosion tables are tied to specific laboratory solutions and test conditions; they are useful for screening, not a guarantee for an operating pump.
Hydrochloric acid, mixed acids and chloride-bearing process streams likewise need condition-specific review. A material’s performance in one chloride test does not establish suitability for every acid mixture, oxidizing condition, temperature or component geometry. Avoid translating a PREN estimate or a critical-pitting result into a sulfuric-acid service limit.
| Process input | Why the reviewer needs it | Evidence to provide |
|---|---|---|
| Chemical identity and mixture | A trade name or primary acid can omit contaminants and secondary constituents that affect corrosion. | Process composition, SDS and known contaminants. |
| Normal, minimum and upset concentration | Start-up, dilution, cleaning and process upsets may differ from the nominal concentration. | Operating range and cleaning / flushing steps. |
| Normal and maximum temperature | Temperature changes corrosion behavior and may affect seals and clearances. | Continuous, transient and metal-temperature conditions if known. |
| Chlorides, fluorides, pH and redox condition | These may change the controlling corrosion mechanism and candidate alloy. | Analysis data, process specification or documented range. |
| Aeration, solids, velocity and deposits | Flow, entrained solids, stagnant zones and deposits can make service differ from static laboratory data. | Operating description, solids data and prior failure observations. |
Use the Chemical Resistance Guide only for conditions it explicitly lists; do not interpolate an unlisted concentration or temperature. For the narrower sulfuric-acid and caustic-soda impeller discussion, see the sulfuric-acid impeller material guide. Carpenter’s Alloy 20 guidance and Haynes’ C-276 corrosion data both explain why the test conditions and solution chemistry matter.
Use a Review Sequence Before Releasing the RFQ
This workflow is a screening and documentation sequence, not an automatic alloy selector. If the process data or material record is incomplete, hold the selection for review rather than infer a safe envelope.
Material and replacement review
Move through each gate with the same process case and component scope.
Process envelope
Record composition, concentration range, temperature, contaminants and operating changes.
Candidate alloy family
Compare only against corrosion data that covers the stated chemistry and exposure.
Product form and grade
Name the actual casting or wrought specification, grade and required records.
Pump and duty
Confirm interfaces, component scope, seal arrangement and duty with drawings and curve data.
RFQ evidence
Agree MTR, traceability, inspection, test and deviation requirements in the order.
Separate B73.1 Dimensions, Internal Fit and Hydraulic Duty
ASME B73.1 includes dimensional interchangeability requirements for pumps with the same standard dimension designation and specified installation interfaces. That scope does not establish that every internal component, seal arrangement, material or hydraulic curve is interchangeable. Review those items separately against the installed configuration and the approved drawings. See the guide to dimensional interchangeability versus component compatibility.
| Review area | Examples to confirm | Useful evidence |
|---|---|---|
| Dimensional | Pump size / frame or group, casing registers, nozzles, mounting, shaft extension and critical interfaces. | Approved drawing, sample measurements and dimensional inspection report. |
| Mechanical | Shaft, key, coupling, bearing arrangement, seal chamber, retained parts and running clearances. | Assembly drawing and configuration-specific fit-up data. |
| Hydraulic | Flow, head, speed, impeller trim, NPSH, absorbed power and operating range. | Applicable pump performance curve and documented duty point. |
| Material | Casing, impeller, cover, sleeves, wear parts, seal faces, elastomers and gaskets. | Specified product form, grade, MTR / heat traceability and required PMI. |
For performance review, use an applicable pump performance curve from the ANSI Pump Technical Resources and confirm that its pump size, speed, impeller and material configuration match the offer. For installed equipment, use the dedicated Goulds 3196 replacement or Durco Mark III replacement route to verify the actual supply scope; neither a brand name nor a B73.1 designation alone proves component fit.
Agree the Inspection and Material Records Before Ordering
Specify the records required for each wetted component and pressure-containing part. The exact inspection and acceptance criteria belong in the project specification and purchase order; a supplier should not imply a test was performed unless its record identifies the supplied heat or component.
Portable X-ray fluorescence can screen many alloying elements, but it does not provide a dependable carbon measurement. If a carbon limit is part of the acceptance requirement, name an appropriate laboratory method in the inspection plan. A PMI report supplements the material certificate; it does not by itself establish corrosion suitability.
Prepare a Complete Alloy Wet-End RFQ
Include the following information where available. If you only have a nameplate, OEM part number or drawing, identify what is known and what still needs confirmation.
Use the OEM part-number cross-reference when you have a part number or position code. Send the installed pump configuration and process data with the request so material, fitment and performance checks can be reviewed together.
Frequently Asked Questions
Is Alloy 20 the same material as CN7M?
No. Alloy 20 / UNS N08020 is commonly used as a wrought-alloy reference; CN7M is a casting grade used for related pump-component service. Specify the actual casting standard and grade rather than treating the designations as identical.
Is CW2M the cast equivalent of Hastelloy C-276?
No. CW2M is a different cast nickel-alloy grade associated with the C-4 family. ASTM A494 CW12MW is commonly associated with cast C-276-family components. Confirm the required specification and product form on the drawing and purchase order.
Can I select Alloy 20 or C-276 from sulfuric-acid concentration alone?
No. Concentration must be reviewed with temperature, aeration, impurities, velocity, solids, deposits and process transients. Use condition-specific corrosion data and request review when the actual service falls outside the published data.
Does ASME B73.1 guarantee that every replacement part will fit and perform the same?
No. B73.1 defines a dimensional framework for the applicable pump designation and interfaces. Internal parts, material, seal arrangement and hydraulic performance still require configuration-specific confirmation.
What should I send when requesting an alloy replacement wet end?
Send the installed pump model and size, nameplate, OEM part number or drawing, required component scope, process chemistry and temperature, duty point, material requirement and required inspection records.
Sources Used for Material and Pump-Standard Notes
- ASTM A351/A351M scope — austenitic castings for pressure-containing parts.
- ASTM A494/A494M — nickel and nickel-alloy castings, including composition and heat-treatment requirements.
- ASME B73.1 — chemical-process pump design and dimensional interchangeability scope.
- Haynes International C-276 technical data — wrought-alloy properties and condition-specific laboratory corrosion data.
- Carpenter Technology corrosion guidance — service variables affecting Alloy 20-family and other alloy performance.
- MetalTek cast-alloy profiles — CW2M, CW12MW and CX2MW family references.
