93-98% Sulfuric Acid: What Actually Holds Up in the Tank vs. the Piping

For 93–98% sulfuric acid, I'd separate the answer into bulk storage and transfer/feed piping, because I would not automatically use the same material for both.

Carbon Steel Is Still a Legitimate, Conventional Choice

For a large bulk installation, carbon steel is still a legitimate, conventional choice for 93–98% sulfuric at roughly ambient temperature. Concentrated sulfuric forms an iron-sulfate film on carbon steel that can substantially reduce corrosion. Industry guidance specifically covers 93% and 98% acid stored in carbon-steel equipment. (Per Accuris Tech Store's guidance on sulfuric acid storage.)

But there are conditions attached to that answer: keep water out of it. If moisture enters and starts diluting the acid, that protective behavior changes dramatically. That's why I like a properly designed vent with a desiccant dryer, adequate corrosion allowance, good tank-bottom design, secondary containment, and an actual inspection program — not just "we bought a steel tank 15 years ago." (Per General Industries, Inc.'s sulfuric acid storage guidance.)

For many municipal/water-treatment installations, particularly smaller systems, I also like a purpose-built XLPE sulfuric-acid tank with an oxidation-resistant inner system. One established system specifies XLPE with an antioxidant liner and a 2.2 specific-gravity rating for 93–98% acid. That's important because concentrated sulfuric is roughly twice the density of water — this isn't a standard water tank with an "acid compatible" sticker slapped on it. (Source: Poly Processing Blog, sulfuric acid tank systems.)

XLPE at 98%: One Tank Manufacturer Flags a Failure, Another Lists It as Compatible

A second tank manufacturer, ProTank, names three commercial-strength concentrations directly — 93 wt%, 96 wt%, and 98.5 wt% — and publishes a specific-gravity/freezing-point table for them: 93% runs about 1.83 SG with a freezing point around -21°F, while 98% runs about 1.84 SG with a freezing point around 30°F. That freezing-point swing matters for outdoor tank siting and secondary containment sizing in a way a single "93–98%" label doesn't tell you. A chemical producer's own data sheet (Nouryon) publishes slightly different numbers for the same two concentrations — -20°F at 93% and 40°F at 98% — a 10°F spread from ProTank's figure at the top end. I'm not picking a side; both are published, and the spread itself is the lesson: near 98%, a small change in concentration moves the freezing point a lot, and the number that matters is on the certificate of analysis for the load you actually received, not a rounded catalog figure. They also flag that any tank over 4,000 gallons at 94%+ H2SO4 should go through engineering review before fabrication — not just ordered off a size chart. Their guidance independently confirms the same carbon-steel-passivation point I cited above from General Industries, Inc. — a second vendor landing on the same answer for 93%+ service. (Source: ProTank.)

Now the part I'm not going to smooth over: ProTank's own published guidance states that XLPE fails within six months of service at 98% sulfuric acid. That's a specific, direct claim from a tank manufacturer. Poly Processing's own catalog — the same source I cited above for the 2.2-SG oxidation-resistant XLPE system — lists 98% sulfuric as XLPE-compatible, with no such caveat.

I'm not going to pretend those two positions agree, and I'm not going to pick a side for you without your actual service conditions. If you're looking at XLPE at or near 98% concentration, that's exactly the kind of conflict I'd want resolved with the specific tank manufacturer's engineering group before fabrication — ask them directly whether their antioxidant/liner system has been tested and warrantied at your guaranteed concentration and operating temperature, in writing, not just "the catalog says compatible." A generic compatibility listing and a documented six-month field failure can both be accurate descriptions of different formulations, wall constructions, or temperature conditions — but I'm not going to guess which one applies to your tank from a desk.

Piping Is Where I'd Be More Conservative

If somebody tells me "It's sulfuric acid. Just use PVC," I'm stopping the submittal.

For 93% acid, good-quality PVDF has established applications. Asahi specifically lists its PVDF system for sulfuric acid up to 93%. (Source: Asahi America.)

Once we're talking 94–98.3%, I prefer looking at ECTFE or another fluoropolymer system specifically engineered for concentrated sulfuric. Asahi's current water/wastewater guidance separates the ranges this way:

Their published water-treatment guidance specifically recommends ECTFE piping for approximately 94–98.3% sulfuric. (Source: Asahi America water/wastewater guidance.)

98% and 98+% Are Not the Same Animal

Commercial "98%" sulfuric can creep above about 98.3%, where free SO3 becomes an important consideration. Asahi documented cases where facilities switched from 93% to 98+% acid and experienced stress cracking in thermoplastic piping — including PVDF. (Source: Asahi America case documentation.)

So before specifying the piping, I'd ask the sulfuric supplier: "What is the guaranteed concentration range — not the nominal concentration?" If their COA can say 98.0–98.8%, that's materially different from designing around exactly 98.0%.

For roughly 98.3% service, Asahi's sample specification uses ECTFE carrier pipe, PTFE-related sealing materials, and offers double containment. (Source: Asahi America sample specification.)

Carbon Steel and Specialty Alloys in Metallic Piping

Carbon steel piping can work with concentrated sulfuric under controlled temperature and low velocity, because you're relying again on that protective corrosion film. But velocity, elbows, turbulence, weld geometry, shutdown conditions, and dilution can remove or disturb that film.

That's why something that looks perfectly acceptable in a 10,000-gallon static steel tank isn't necessarily what I want in a half-inch pump discharge line moving acid through six elbows and a control valve.

For demanding metallic service, materials such as Alloy 20 or specialty high-silicon sulfuric-acid stainless alloys come into the discussion. There are specialty alloys now proven in concentrated sulfuric under considerably higher temperatures and velocities. (Per Metso's alloy guidance.)

Alloy 20 isn't just a piping and tank-wall material — I've seen it confirmed on the pump side too. Carver Pump has a published case study where a power-generation customer's self-priming, non-metallic PVDF horizontal pump was failing every 4–6 weeks handling sulfuric acid runoff from a flue-gas-scrubber secondary-containment system. Carver's fix was a vertical sump pump — now the G2S series — built with Alloy 20 wetted parts, rated to 22 feet of submergence and up to 2,500 GPM. The first set of pumps had already run nearly a year in service at time of publication. That's a genuine third vendor independently confirming Alloy 20 for sulfuric-acid wetted-parts service. (Source: Carver Pump.)

One nuance I'd flag before anyone assumes that's an off-the-shelf pump option: Carver's own G2C/G2S ordering code lists only four standard material-of-construction options — A (all cast iron), B (316 stainless-fitted), C (all 316 stainless steel), and X (Special). Full Alloy 20 wetted-parts construction, like the case study above, falls under code X — not a standard catalog SKU. In Carver's own standard component table, Alloy 20 only appears as the material for the slinger. If your service calls for Alloy 20 wetted parts, I'd plan on special-order lead time, not catalog-pump lead time. (Per Carver's G2C/G2S technical spec sheet.)

Don't Casually Specify 316 Stainless

People see "stainless" and think corrosion problem solved. Sulfuric acid doesn't work that way. Its corrosion behavior with stainless alloys changes enormously with concentration and temperature, and dilution/upset conditions can completely change the answer. (Per Outokumpu's corrosion guidance.)

A Real Design Basis, If You Handed Me a Municipal-Water Project

If you handed me a municipal-water project using 93–98% sulfuric acid, I'd probably start the design basis around:

• Tank: sulfuric-specific XLPE with oxidation-resistant inner surface, 2.2 SG, or engineered carbon steel for larger storage.
• Bulk transfer piping at 93%: PVDF, subject to temperature/pressure verification.
• Bulk transfer at 94-98.3%: ECTFE would be high on my list.
• Potential 98.3+% / free-SO3 service: don't blindly extend the PVDF specification — I'd get the acid supplier's concentration limits and have the piping manufacturer approve the application. ECTFE/PFA/PTFE or a suitable specialty metallic system may be warranted.
• Gaskets/wetted valve components: typically PTFE or specifically qualified fluoropolymer/elastomer components, selected as part of the complete piping system rather than independently.
• Secondary containment: absolutely something I'd evaluate for both the tank and chemical piping.

Frequently Asked Questions

Freeze Protection Isn't Sized to the Chemical's Name — It's Sized to Concentration

The freezing-point spread above isn't trivia. Before the first cold night, I want three things on paper: the concentration from the certificate of analysis (not the purchase order), the coldest ambient temperature the equipment actually sees, and every run, valve, and vent that holds acid without flow — and how each one stays above its freezing point, whether that's heat tracing and insulation, circulation, or a drainable design. A frozen valve isn't just a delay; a valve you can't operate is a line you can't isolate.