Views: 0 Author: Monica Publish Time: 2026-04-13 Origin: Site
Short answer: No — not safely, and not for long.
If you are specifying pipe for a 98% sulfuric acid system and you are considering 316H/UNS S31609, this article will explain why that is a dangerous choice, what actually happens to stainless steel in 98% concentrated sulfuric acid, and which alloys genuinely work in this service.
Stainless steel relies on a chromium oxide passive film to protect itself from corrosion. In most environments, this film is stable and self-repairing.
But, at concentrations above 95% and at or near ambient temperature, austenitic stainless steels show moderate to acceptable corrosion rates in static conditions. The moment temperature rises above approximately 40–50°C, corrosion rates increase dramatically—often by an order of magnitude per 10°C increase.
The NACE and DECHEMA corrosion data tables show the following general corrosion rates for austenitic stainless steels in 98% H₂SO₄:
Temperature | Corrosion Rate for 316L/316H (approx.) |
Ambient (20°C) | 0.1 – 0.5 mm/year (borderline acceptable) |
40°C | 1 – 3 mm/year (unacceptable) |
60°C | 5 – 15 mm/year (severe attack) |
80°C | >20 mm/year (rapid destruction) |
Conclusion: 316H may survive at ambient temperature in static or very slow-flow 98% H₂SO₄ for a limited period, but any temperature rise or flow velocity increase will accelerate attack rapidly.
This is the central question, and the honest answer is that none of the three is a good choice for 98% sulfuric acid service, particularly at elevated temperatures. Here is how and why.
Property | 316H | 304H | 321 |
UNS Designation | S31609 | S30409 | S32100 |
Carbon (%) | 0.04 – 0.10 | 0.04 – 0.10 | ≤ 0.08 |
Chromium (%) | 16 – 18 | 18 – 20 | 17 – 19 |
Nickel (%) | 10 – 14 | 8 – 10.5 | 9 – 12 |
Molybdenum (%) | 2 – 3 | None | None |
Titanium (%) | None | None | 5×C min |
Stabilization | None | None | Ti-stabilized |
Tensile Strength | 515 MPa min | 515 MPa min | 515 MPa min |
Yield Strength | 205 MPa min | 205 MPa min | 205 MPa min |
High-Temp Creep Strength | Excellent | Good | Good |
Sensitization Resistance | Low (high C) | Low (high C) | High (Ti-stabilized) |
Relative Corrosion Resistance in H₂SO₄ | Best of the three | Moderate | Poorest of the three |
316H in 98% H₂SO₄
316H's 2–3% molybdenum content gives it a measurable advantage over 304H and 321 in sulfuric acid environments. Molybdenum strengthens the passive film and reduces pitting susceptibility. This is why 316-family alloys are generally the first choice among standard austenitic grades when sulfuric acid is involved.
However, the H variant's higher carbon content introduces a risk: if the pipe has seen temperatures in the 425–850°C sensitization range during manufacture or service (such as in weld heat-affected zones), chromium carbides precipitate at grain boundaries, depleting the surrounding metal of corrosion-resistant chromium. This sensitized microstructure is highly vulnerable to intergranular corrosion in acidic media.
In concentrated sulfuric acid at elevated temperatures, a sensitized 316H Steel weld zone can corrode at catastrophic rates.
Conclusion on 316H: The best performer of the three in H₂SO₄ due to molybdenum, but the higher carbon content creates sensitization risks, and the overall corrosion resistance is still inadequate for anything above ambient temperature in 98% acid.
304H in 98% H₂SO₄
304H Steel has higher chromium than 316H (18–20% vs. 16–18%), which generally improves oxidizing acid resistance. However, it contains no molybdenum. In sulfuric acid service, molybdenum is the more important element — it directly contributes to passive film stability in reducing acid conditions.
304H also carries the same sensitization risk from its elevated carbon content. Without molybdenum and with the same sensitization vulnerability, 304H consistently shows higher corrosion rates than 316H in sulfuric acid test data.
Conclusion on 304H: Inferior to 316H for sulfuric acid service. The higher chromium does not compensate for the absence of molybdenum.
321 in 98% H₂SO₄
Stainless Steel 321 is titanium-stabilized, meaning the titanium preferentially combines with carbon to form titanium carbides rather than chromium carbides. This makes 321 essentially immune to sensitization.
However, 321 contains no molybdenum and has chromium levels similar to 316H. In sulfuric acid service, the absence of molybdenum is a serious limitation. Testing consistently shows 321 performing similarly to or worse than 304H in H₂SO₄ environments.
The titanium stabilization is valuable for high-temperature service and weld integrity, but it does not provide any benefit for sulfuric acid corrosion resistance.
Conclusion on 321: The least suitable of the three for sulfuric acid service. Good sensitization resistance cannot compensate for the complete absence of molybdenum.
316H > 304H > 321
But to be direct with you: this ranking is like asking which of three wrong tools is the least wrong. In 98% sulfuric acid at elevated temperatures, none of these three grades provides acceptable engineering performance.
Here is what the industry actually specifies for concentrated sulfuric acid service:
For Ambient Temperature Storage and Low-Velocity Transfer
Carbon steel ASTM A106 Grade B/C is the traditional choice for 93–98% H₂SO₄ at ambient temperature. The iron sulfate passive layer that forms in concentrated acid provides reasonable protection at low temperatures and in static or slow-flow conditions. This is why tanker trucks, storage tanks, and transfer lines for concentrated sulfuric acid are almost always carbon steel.
Limitation: Carbon steel corrodes rapidly if acid concentration drops below about 85%, if temperature exceeds 40°C, or if flow velocity is high.
For Process Piping at Elevated Temperatures
Hastelloy C276 (UNS N10276) is one of the most widely used alloys for hot concentrated sulfuric acid. Its high molybdenum (15–17%), chromium (14.5–16.5%), and tungsten content provides excellent resistance across a wide range of H₂SO₄ concentrations and temperatures.
Hastelloy B3 (UNS N10675) and Hastelloy B2 (UNS N10665) are specifically designed for reducing acid environments, including hot, concentrated hydrochloric and sulfuric acids. They show very low corrosion rates in 98% H₂SO₄ at temperatures up to 80°C and beyond.
Alloy 20 (UNS N08020 / Carpenter 20) was actually developed specifically for sulfuric acid service. Its composition (33–37.5% Ni, 19–21% Cr, 2–3% Mo, 3–4% Cu) makes it exceptionally resistant to both concentrated and dilute sulfuric acid across a wide temperature range. It is often the first recommendation for sulfuric acid piping systems where temperature and concentration vary.
Service Condition | Recommended Material | Notes |
98% H₂SO₄, ambient temp, static | Carbon steel A106 | Standard industry practice; monitor for dilution |
98% H₂SO₄, ambient temp, flowing | Carbon steel or Alloy 20 | Flow velocity increases corrosion in CS |
98% H₂SO₄, 40–80°C | Alloy 20 or Hastelloy B3 | Stainless steels not suitable |
98% H₂SO₄, >80°C | Hastelloy B3 or C276 | Specialist alloys required |
Variable concentration (50–98%) | Alloy 20 or Hastelloy C276 | Must cover the dangerous 40–80% range |
Oleum / fuming sulfuric acid | PTFE-lined pipe or specialty alloys | Consult specialist for each case |
Q: Can 316H pipe be used for 98% sulfuric acid at room temperature?
Short-term contact at ambient temperature may not cause immediate failure, but 316H is not a material for this service. Corrosion rates at 20°C in 98% H₂SO₄ for 316-family stainless steels are in the range of 0.1–0.5 mm/year under ideal conditions. There are better materials for this service at minimal additional cost.
Q: Why does carbon steel work for 98% sulfuric acid but 316H does not perform better?
In concentrated sulfuric acid above about 85%, iron forms a dense iron sulfate (FeSO₄) layer that acts as a physical barrier between the steel and the acid. This mechanism is specific to iron in concentrated, near-anhydrous acid.
Stainless steel relies on a different mechanism — a chromium oxide passive film — which is not as stable in this specific environment. The two passivation mechanisms are fundamentally different, which is why more expensive stainless steel does not automatically mean better performance in every corrosive environment.
Q: What pipe standard applies to Alloy 20 piping?
Alloy 20 seamless and welded pipe is produced under ASTM B464 (welded) and ASTM B729 (seamless). Fittings are covered under ASTM B366. Flanges are produced per ASTM B462 (ASME Class 150–2500). Alloy 20 is also covered under ASME Section VIII for pressure vessel fabrication under the SB-designated equivalent standards.
Q: Is 317L a better choice than 316H for sulfuric acid?
317L (UNS S31703) contains 3–4% molybdenum compared to 316L's 2–3%, giving it meaningfully better pitting and crevice corrosion resistance. In moderate concentrations of sulfuric acid at low temperatures, 317L does outperform 316L and 316H.
However, in 98% concentrated acid at elevated temperatures, 317L still falls short of what is required. Alloy 20 or Hastelloy B3 remain the correct choices for demanding concentrated acid service.
Q: How do I know if my stainless steel pipe has been sensitized?
The ASTM A262 test series (Practices A through F) is the standard method for detecting sensitization in austenitic stainless steels.
Practice A uses an oxalic acid etch to reveal grain boundary structure under a microscope. Practice C (Huey test in boiling 65% nitric acid) and Practice E (Strauss test in copper sulfate-sulfuric acid solution) detect corrosion-susceptible microstructures in welded or heat-affected zones.
These tests should be performed on weld procedure qualification coupons for any stainless steel pipe intended for corrosive service.
316H is not suitable for 98% sulfuric acid service at elevated temperatures. At ambient temperature it may survive short-term, but it is not a reliable choice for this application.
Among 316H, 304H, and 321, the ranking is 316H > 304H > 321 in concentrated H₂SO₄—molybdenum content is the key differentiator. However, none of the three is genuinely suitable.
For hot or flowing concentrated sulfuric acid, Alloy 20 (UNS N08020) is the most widely recommended material. Hastelloy B3 and C276 are the choices for the most demanding conditions.
If you are specifying pipe for a sulfuric acid system, do not anchor on familiar grades like 316H simply because they are commonly available. The right alloy for this service is specific and you can ask our company.
