Views: 50 Author: Monica Publish Time: 2026-04-13 Origin: Site
Table of Contents
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.
Head-to-Head Comparison
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
To be direct: this ranking is like asking which of three wrong tools is the least wrong. In 98% sulfuric acid at elevated temperature, none of these three grades provides acceptable engineering performance. The real question is which non-300-series alloy to use instead - and that is where 904L, Alloy 20, and Hastelloy B2 enter the picture.
The performance gap is enormous. 904L, Alloy 20, and especially Hastelloy B2 all survive where 316H degrades - at higher temperatures and with corrosion rates one or two orders of magnitude lower. The table below summarizes the four alloys side by side; the sections that follow explain the "why."
The reason is compositional. 316H depends almost entirely on a thin chromium-oxide film. 904L raises nickel, molybdenum, and copper. Alloy 20 pushes nickel to roughly a third of the alloy and adds copper on top of molybdenum. Hastelloy B2 abandons the stainless approach entirely and is built as a nickel-molybdenum alloy whose whole purpose is resisting reducing acids such as hot sulfuric acid.
316H vs 904L vs Alloy 20 vs Hastelloy B2 in 98% H2SO4
Property | 316H (S31609) | 904L (N08904) | Alloy 20 (N08020) | Hastelloy B2 (N10665) |
|---|---|---|---|---|
Alloy family | Austenitic stainless | Super-austenitic SS | Ni-Fe-Cr-Mo-Cu | Nickel-molybdenum |
Nickel (%) | 10 - 14 | 23 - 28 | 32 - 38 | Balance (~62 - 69) |
Chromium (%) | 16 - 18 | 19 - 23 | 19 - 21 | ≤ 1.0 |
Molybdenum (%) | 2 - 3 | 4 - 5 | 2 - 3 | 26 - 30 |
Copper (%) | None | 1 - 2 | 3 - 4 | None |
Carbon (%) | 0.04 - 0.10 | ≤ 0.02 | ≤ 0.07 | ≤ 0.01 |
Corrosion mechanism | Cr-oxide passive film | Cr-oxide + high Ni/Mo/Cu | Cr-oxide + high Ni/Cu/Mo | Ni-Mo, built for reducing acids |
98% H2SO4 resistance | Borderline, ambient/static only | Good up to ~80 deg C | Excellent (classic acid alloy) | Excellent, even hot |
Approx. rate @20 deg C, 98% | 0.1 - 0.5 mm/yr | < 0.1 mm/yr | < 0.05 mm/yr | < 0.025 mm/yr |
Practical service temp | ≤ ~20 - 30 deg C | ≤ ~80 deg C | ≤ ~100 deg C | Up to boiling (per service limits) |
Relative cost (316H = 1) | 1.0 | ~2.5 - 3x | ~3 - 4x | ~6 - 8x |
904L in 98% Sulfuric Acid
904L (UNS N08904) is a strong, cost-effective upgrade over 316H for 98% H2SO4, suitable up to roughly 80 deg C in concentrated acid. It is the sensible "next step up" when 316H is clearly failing but a full nickel alloy is not justified.
904L is a super-austenitic stainless with about 23-28% nickel, 4-5% molybdenum, and 1-2% copper. That chemistry gives it markedly better resistance to sulfuric acid than 316L/316H across most concentrations, and in 98% acid its corrosion rate at ambient temperature is typically below 0.1 mm/year. Its main limitation is the intermediate concentration range (roughly 40-60% hot), where even 904L can struggle - which is why Alloy 20 or Hastelloy grades are preferred when the acid strength varies. See our 904L stainless steel guide for composition and product forms.
Alloy 20 in 98% Sulfuric Acid
Alloy 20 (UNS N08020) is the classic, purpose-built sulfuric acid alloy and one of the best all-round choices for 98% H2SO4 service, performing well from ambient up to about 100 deg C and across a wide concentration band.
Alloy 20 was developed specifically for sulfuric acid. Its composition - about 32-38% nickel, 19-21% chromium, 2-3% molybdenum, and 3-4% copper - is tuned so that the copper and high nickel content directly attack the corrosion mechanism of sulfuric acid. In 98% concentrated acid it typically shows corrosion rates well under 0.05 mm/year, and it remains the go-to grade for sulfuric acid plant equipment, acid transfer lines, and tanks. Pipe is produced to ASTM B464 (welded) and ASTM B729 (seamless); more detail is in our Alloy 20 pipe guide and the Alloy 20 overview.
Hastelloy B2 in 98% Sulfuric Acid
Conclusion: Hastelloy B2 (UNS N10665) is the benchmark material for hot, concentrated sulfuric acid. Where 316H is destroyed above 40 deg C, Hastelloy B2 stays essentially inert even at elevated temperature, with corrosion rates often below 0.025 mm/year in 98% H2SO4.
Hastelloy B2 is a nickel-molybdenum alloy with roughly 26-30% molybdenum and essentially no chromium. That makes it superb in reducing acid environments - not only 98% sulfuric acid but also hot hydrochloric acid. It is one of the few alloys that remains reliable in continuously hot concentrated H2SO4, which is exactly the duty that defeats every 300-series stainless. Our Hastelloy B2 product page and the B2 vs B3 comparison cover grades, forms, and limitations.
Why Hastelloy B2 Beats 316H - and When B2 Itself Needs Replacing
The contrast is stark: 316H's chromium-oxide film is chemically unstable in concentrated H2SO4 above ambient, while Hastelloy B2's nickel-molybdenum matrix is fundamentally compatible with the reducing nature of the acid. That is why B2 costs several times more yet is specified where downtime or leakage is unacceptable.
Two caveats matter for specifiers. First, B2 is vulnerable to oxidizing contaminants - ferric or cupric ions, nitric acid traces, or air ingress in dilute acid can sharply raise its corrosion rate, so it is best in clean, reducing 98% acid. Second, B2 has historically suffered weld-zone sensitization from Ni-Mo phase precipitation; the modern successor Hastelloy B3 (N10675) largely resolves this and is often the preferred choice today. For very aggressive multi-acid or high-temperature duties, Hastelloy C276 or the C276 ultimate guide may be the safer call.
For cold static storage, carbon steel is the industry standard; for anything flowing, warm, or continuously in service, step up to Alloy 20, 904L, or a Hastelloy grade. Here is what is actually specified in the field.
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.
Limitation: carbon steel corrodes rapidly if acid concentration drops below about 85%, if temperature exceeds 40 deg C, or if flow velocity is high. 904L pipe is a common upgrade when those limits are approached.
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 their service limits.
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 attack, which is why it appears repeatedly in the selection table below. For background, see the Alloy 20 overview and Hastelloy B2 applications.
Material Selection Summary for H2SO4 Service
Material Selection Summary for H2SO4 Service
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 deg C | Alloy 20 or Hastelloy B3 | Stainless steels not suitable |
98% H2SO4, >80 deg 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 temperature, and only borderline at ambient. Among 316H, 304H, and 321, the ranking is 316H > 304H > 321 - but that ordering is academic because all three fall short.
When you compare 316H against the alloys that actually work, the picture is decisive: 904L is a competent, moderately priced upgrade; Alloy 20 is the purpose-built sulfuric acid grade; and Hastelloy B2 (or its successor B3) is the benchmark for hot, concentrated acid. Specify carbon steel A106 only for cold, static storage, and choose Alloy 20 or a Hastelloy grade for any flowing or warm duty. For a broader view of alloy selection, our chemical equipment and 316 vs 304 guide pages are useful next reads.
