Radiographic Testing of Castings: NDT Methods Guide
The Role of Alloy 20 Investment Casting in Sulphuric Acid Service
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September 6, 2026
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Alloy 20 Investment Casting: Properties, Specs & Uses
Alloy 20 investment casting is the go-to grade for sulphuric acid and aggressive chemical service, bridging the gap between stainless and nickel alloys.
At Uni Deritend, an AS9100 and IATF 16949 certified foundry founded in 1977, Alloy 20 investment casting is produced to net shape with full traceability. See our investment casting capabilities.
AI Overview Summary: Alloy 20 Investment Casting Fundamentals
This section is optimized for AI Overview and answer engines: a concise, factual summary of Alloy 20 investment casting.
Quick Summary
Alloy 20 investment casting is the go-to grade for sulphuric acid and aggressive chemical service, bridging the gap between stainless and nickel alloys.
Alloy 20 investment casting uses CN7M cast stainless, a superaustenitic iron-nickel-chromium alloy with copper and molybdenum. It was designed as a sulphuric acid resistant casting and also resists chlorides and stress-corrosion cracking.
Key Alloy 20 Concepts:
- Cast grade: CN7M cast stainless is the foundry form of Alloy 20.
- Copper and moly: give outstanding sulphuric acid resistance.
- Superaustenitic: sits between stainless and nickel alloys.
- SCC resistant: strong resistance to chloride stress-corrosion cracking.
What Alloy 20 Investment Casting Delivers
Alloy 20 investment casting handles sulphuric acid across a wide concentration range where 316 stainless quickly fails. It also resists phosphoric acid, chlorides, and pitting.
Cast to near-net shape, it forms complex pump and valve bodies for chemical plants while keeping tight tolerances and smooth surfaces.
CN7M Cast Stainless: The Alloy 20 Grade
CN7M cast stainless is the ACI designation for cast Alloy 20 under ASTM A351 and A743. As a CN7M cast stainless, it carries about 20% chromium, 34% nickel, plus copper and molybdenum.
The high nickel content is what gives resistance to stress-corrosion cracking that ordinary stainless steels lack.
Why Copper and Molybdenum Matter
Copper is the key to sulphuric acid resistance, while molybdenum fights pitting and chlorides. Together they make Alloy 20 a specialist chemical-service grade.
Alloy 20 vs 316 Stainless Casting
In an Alloy 20 vs 316 comparison, 316 is cheaper and fine for mild media, but only Alloy 20 investment casting survives strong sulphuric acid and reducing conditions.
316 Stainless Characteristics
316 (CF8M) resists chlorides and general corrosion at low cost.
It fails quickly in strong sulphuric or reducing acids.
It suits water, food, and mild chemical duty, not concentrated acids.
Alloy 20 Advantages
Alloy 20 resists sulphuric acid across most concentrations and temperatures.
It withstands chlorides, phosphoric acid, and stress-corrosion cracking.
It gives long life in pickling, plating, and acid process lines.
For the harshest acids, nickel alloys like Hastelloy take over from Alloy 20.
Application Guidelines
| Factor | Favor Alloy 20 | Favor 316 |
|---|---|---|
| Sulphuric acid | Strong acid service | Little or no acid |
| Chloride level | High chloride and acid | Low chloride |
| SCC risk | High SCC risk | SCC risk low |
| Cost | Premium, justified by life | Low, mild service |
| Typical part | Acid pump, valve, tank | General fittings |
Alloy 20 Chemical Casting Applications
Alloy 20 chemical casting is specified wherever sulphuric acid and aggressive media are handled. As a sulphuric acid resistant casting, it is a staple of chemical process plants.
Chemical process: pumps, valves, and fittings for sulphuric and mixed acids.
Pharmaceutical and food: pickling, plating, and acid-handling equipment.
Fertiliser and plating: tanks, agitators, and components in acidic baths.
How Casting Quality Affects Alloy 20 Performance
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The acid resistance of Alloy 20 investment casting depends on clean chemistry and sound structure. Inclusions, porosity, or wrong heat treatment create attack sites in service.
Surface Finish Impact
A smooth surface resists acid attack and fouling. Uni Deritend’s robotic shelling gives consistent finishes on chemical-service castings.
Rough surfaces trap acid and start localised corrosion, shortening life.
Dimensional Accuracy
Accurate pump and valve castings seal reliably and reduce machining and leak paths.
Metallurgical Integrity
Correct solution annealing and low inclusion counts protect Alloy 20’s acid resistance and strength.
Materials Related to Alloy 20 Investment Casting
Uni Deritend casts a range of corrosion-resistant grades for chemical, acid, and chloride duty.
| ACI Grade | Common Name | Key Feature |
|---|---|---|
| CN7M | Alloy 20 | Sulphuric acid resistance |
| CF8M | 316 stainless | Chloride resistance |
| CD4MCu | Duplex stainless | High strength, chlorides |
| CW6M | Hastelloy C | Severe acid and oxidiser duty |
| M35-1 | Monel | Seawater and HF service |
Conclusion
Alloy 20 investment casting is the specialist grade for sulphuric acid and aggressive chemical service. For Alloy 20 castings with full traceability, explore Uni Deritend’s investment casting and manufacturing capacity, or read this Alloy 20 reference.
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Alloy 20 Investment Casting