Alloy 20 vs 316 Stainless: Which for Sulfuric Acid Service
Steelco Metals | Materials Engineering Series |
Introduction
Sulfuric acid is the most widely produced industrial chemical in the world, used in fertiliser production, petroleum refining, battery manufacturing, and chemical synthesis. It is also one of the most corrosive environments that process equipment must endure. When engineers specify materials for pumps, heat exchangers, vessels, and piping in sulfuric acid service, two alloys frequently appear on the shortlist: 316 stainless steel and Alloy 20 (UNS N08020). Understanding their differences in this specific service environment is critical.
316 Stainless Steel Overview
Capabilities and Limitations
316 stainless steel is the standard austenitic workhorse of the chemical process industries. Its addition of 2–3% molybdenum improves chloride pitting resistance significantly over 304, and it performs well in many dilute acid environments. However, sulfuric acid presents a specific corrosion challenge that molybdenum content alone cannot fully address.
In sulfuric acid service, 316 stainless performs acceptably only within a narrow concentration window — approximately 0–5% and above 95% concentration at or near ambient temperature. In the intermediate range (5–95% concentration), particularly at elevated temperatures, 316 stainless corrodes rapidly and is generally considered unsuitable without additional protection. Stress corrosion cracking (SCC) is also a concern for 316 in sulfuric acid environments with chloride contamination.
Alloy 20 Overview
Purpose-Built for Sulfuric Acid Resistance
Alloy 20 was specifically developed to resist sulfuric acid corrosion across a broad concentration range. Its composition — approximately 34% nickel, 20% chromium, 3.5% copper, and 2.5% molybdenum — provides a multi-mechanism defense against acid attack.
The copper content (3.0–4.0%) is particularly significant. Copper provides active cathodic protection that reduces the acid’s aggressive attack on the alloy matrix, extending the viable service range dramatically compared to standard stainless steels. Alloy 20 is resistant to sulfuric acid at concentrations from 5% to 98% and at temperatures that would rapidly destroy 316 stainless.
Corrosion Performance Comparison
Corrosion Rate and Reliability Differences
In 50% sulfuric acid at 66°C — a common process condition in chemical manufacturing — 316 stainless steel exhibits corrosion rates that render it impractical for equipment designed for years of service. Alloy 20 in the same conditions shows corrosion rates typically below 0.5 mm/year — acceptable for most pressure equipment design codes.
At 98% sulfuric acid and ambient temperature, the situation reverses partially — concentrated acid passivates many stainless grades. However, even here, Alloy 20 offers greater reliability against localized corrosion and temperature excursions that can occur during process upsets.
Fabrication and Weldability
Welding and Fabrication Considerations
Alloy 20 is fully weldable using matching filler metal (Alloy 20 filler per AWS A5.14). Its stabilization with niobium prevents sensitization and intergranular corrosion in the heat-affected zone — a critical advantage in fabricated equipment where weld quality directly affects service life.
316 stainless is easier to source and fabricate, but for genuine sulfuric acid service in the critical concentration range, fabrication ease does not compensate for inadequate corrosion resistance.
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The Cost vs. Consequence Equation
Alloy 20 carries a material premium over 316 stainless — typically 2.5 to 4 times higher per kilogram. However, when equipment failure in sulfuric acid service means unplanned shutdown, acid release, and potential safety incidents, the lifecycle cost argument overwhelmingly favors specifying the correct material from the outset. Alloy 20 is not an upgrade — it is the appropriate specification for sulfuric acid service in the intermediate and elevated temperature range.
Steelco Metals Supply Capability
Steelco Metals supplies Alloy 20 and 316 stainless in plate, bar, pipe, and fittings with full MTRs. Speak with our technical sales team for your process requirements.
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