904L Stainless Steel for Sulfuric Acid Tanks & Piping

904L stainless steel was developed for demanding acid-service environments
and remains an important material for selected sulfuric-acid systems in the
chemical, petrochemical and refinery industries.

However, specifying only:
“904L for sulfuric acid”
does not define whether the buyer needs plate for a storage tank,
pipe for a transfer line, tubing for a process circuit, bar for machined
components or forged material for flanges and valves.

More importantly, sulfuric acid corrosion cannot be evaluated from the alloy
name alone.
Acid concentration, temperature, aeration, chlorides and process contaminants
can substantially change material performance.

This guide connects:
sulfuric-acid equipment → raw-material form → current ASTM standard →
service conditions → inspection → RFQ.

Quick Answer: Where Does 904L Fit in Sulfuric Acid Service?

Type 904L / UNS N08904 is a high-alloy austenitic stainless steel containing
significant nickel, molybdenum and copper.

It was originally developed for sulfuric-acid service and is widely considered
for selected chemical and petrochemical applications involving
medium-concentration sulfuric acid.

Depending on the actual process conditions and engineering specification,
904L may be evaluated for:

• sulfuric-acid storage tanks;
• acid transfer piping;
• circulation lines;
• mixing and dilution equipment;
• selected heat-exchanger components;
• reactor-vessel components;
• flanges;
• valve components;
• selected refinery acid-handling systems.

These are application categories, not universal approvals.

The final material should be selected from the actual acid concentration,
temperature, contaminants, flow conditions, aeration and equipment design.

Why Do Copper, Nickel and Molybdenum Matter in 904L?

904L differs significantly from conventional 304L and 316L stainless steels.

Its alloy design combines:

• high nickel;
• approximately 4–5% molybdenum under the ASTM composition range;
• intentional copper addition;
• low carbon.

The copper addition is particularly important to resistance in selected
reducing-acid environments such as sulfuric acid.

Molybdenum contributes to improved resistance to localized corrosion in many
environments, while the high nickel level helps maintain the austenitic
structure and contributes to corrosion resistance.

Alleima states that its N08904-grade material was originally developed for
sulfuric-acid service and attributes its performance to high molybdenum
content together with copper alloying.

Technical reference:

Alleima N08904 / 904L Technical Data
.

904L Grade Designations and Current Product Standards

The current product standard should follow the actual material form.

A 904L tank shell and a 904L transfer pipe should not be purchased under
the same ASTM standard merely because both use UNS N08904.

For JISCO JUDE’s general grade information, see:

Super Austenitic Stainless Steels — 904L & 254SMO
.

ASTM A240 UNS N08904 Chemical Composition Limits

The following values represent ASTM A240 composition limits for N08904
flat products.
They should not be described as producer-specific typical heat chemistry.

Actual delivered material should be verified against the governing standard,
contract edition and Mill Test Certificate.

Which Sulfuric Acid Equipment Uses 904L?

1. Sulfuric Acid Storage Tanks

904L plate can be considered for sulfuric-acid storage tanks where the
process chemistry and design justify the alloy.

Possible starting products include:

• hot-rolled plate;
• cold-rolled sheet for thinner fabricated components;
• cut tank-shell blanks;
• head or cover material.

Tank-material selection should consider:

• acid concentration;
• normal operating temperature;
• maximum temperature;
• water ingress;
• chloride contamination;
• ventilation and aeration;
• cleaning and shutdown conditions.

Do not approve 904L from the words “sulfuric-acid tank” alone.

JISCO JUDE publishes:

Hot-Rolled Stainless Steel Sheet

and

Cold-Rolled Stainless Steel Sheet & Coil
.

Specific 904L availability, thickness and mill route should be confirmed
for each project.

2. Acid Transfer Piping

Transfer lines require a piping specification rather than a plate specification.

For N08904 process piping, ASTM A312/A312M may be applicable when specified
by the project.

The RFQ should define:

• NPS or outside diameter;
• wall thickness or schedule;
• seamless or welded route;
• length;
• end preparation;
• pressure;
• temperature;
• required NDE;
• certificate requirements.

Review JISCO JUDE’s:

Stainless Steel Tube & Pipe
.

3. Sulfuric Acid Circulation Lines

Circulation systems can experience different corrosion conditions from a
static storage tank because velocity, aeration, temperature and contamination
can change the corrosion mechanism.

The same 904L grade should not automatically be assumed to have identical
performance in:

• tank walls;
• high-velocity return lines;
• pump suction lines;
• heated circulation loops;
• dead legs.

Where process conditions differ substantially, each zone should be reviewed
separately.

4. Heat-Exchanger Components

904L can be used in selected heat-exchanger applications where conventional
316L does not provide sufficient corrosion resistance.

Possible components can include:

• tubes;
• tube plates;
• channels;
• shell components;
• acid-side piping.

The process side and cooling side must both be reviewed.

A sulfuric-acid process side combined with chloride-containing cooling water
can create a different material-selection problem from sulfuric acid alone.

5. Flanges and Valves

Outokumpu lists flanges and valves among typical 904L applications.

For forged flange or valve components, the purchase order should state the
applicable forging specification rather than use ASTM A240.

Possible procurement information includes:

• F904L where required;
• flange class;
• size;
• facing;
• forging standard;
• inspection;
• traceability.

A cast valve body, forged flange and machined bar component may require
different product specifications even when the selected alloy family is 904L.

6. Reactor and Mixing-Vessel Components

Selected chemical and petrochemical reactors or mixing vessels can use 904L
where reducing-acid corrosion is an important part of material selection.

Agitation can introduce:

• higher local velocity;
• erosion-corrosion effects;
• air entrainment;
• temperature gradients;
• crevice conditions around attachments.

These effects should be included in the corrosion assessment.

Component-to-Raw-Material Procurement Matrix

Why Acid Concentration Alone Is Not Enough

Sulfuric-acid corrosion does not change linearly with concentration.

Two systems with different concentrations can show very different corrosion
behavior, and the result can change again when temperature, oxygen or
contaminants change.

For material selection, provide:

• normal acid concentration;
• minimum concentration;
• maximum concentration;
• normal temperature;
• maximum temperature;
• water dilution conditions;
• process contaminants.

Do not use a single corrosion chart point as proof that the alloy is suitable
for every operating or upset condition.

Alleima publishes isocorrosion data for N08904 in both deaerated and
naturally aerated sulfuric acid, illustrating why environmental condition
matters.

Technical reference:

Alleima Sulfuric Acid Corrosion Data
.

Temperature, Aeration, Chlorides and Contamination Matter

A corrosion assessment for sulfuric-acid service should not stop at acid
percentage.

Important variables can include:

• temperature;
• oxygen / aeration;
• chloride contamination;
• fluorides;
• metal ions;
• solids;
• flow velocity;
• shutdown deposits.

For example, Alleima publishes separate corrosion information for sulfuric
acid containing chlorides.

This demonstrates an important purchasing rule:
contaminated sulfuric acid is a different material-selection problem
from pure laboratory acid.

Reference:

Alleima — Sulfuric Acid with Chlorides
.

904L vs 316L in Sulfuric Acid Service

316L is widely used in chemical equipment, but it does not provide the same
alloy design as 904L for reducing-acid service.

904L contains:

• higher nickel;
• higher molybdenum;
• intentional copper.

This is one reason 904L can provide a larger corrosion margin than 316L in
many sulfuric-acid environments.

However:

904L should not replace 316L automatically.

If 316L already satisfies the project’s corrosion, fabrication and life-cycle
requirements, moving to 904L may increase material cost without creating
equivalent project value.

Likewise, if conditions exceed the practical corrosion margin of 904L,
simply moving from 316L to 904L may still be insufficient.

When Should a Higher Alloy Be Evaluated?

904L is not the upper limit of corrosion-resistant stainless and nickel alloys.

More severe service may require evaluation of:

• 254SMO;
• Alloy 28;
• Alloy 825;
• nickel-molybdenum or nickel-chromium-molybdenum alloys;
• other project-specific corrosion-resistant materials.

Possible triggers include:

• higher temperature;
• high chloride contamination;
• mixed acids;
• fluorides;
• erosion-corrosion;
• unfavorable crevices;
• prior 904L service failure.

Outokumpu’s chemical-process guidance positions 904L among highly alloyed
materials for severe process environments while also identifying higher-alloy
options for more demanding service.

Current ASTM Standards: Check Old Purchase Descriptions

904L has an unusually important standards-history issue.

Older datasheets, ERP descriptions and certificates can still refer to ASTM
B-series standards that historically covered UNS N08904.

Current ASTM scopes have reassigned N08904 to stainless-steel A-series
specifications.

This does not mean that every old certificate is invalid.

It means that new purchase orders should use the current applicable product
standard unless the project specifically requires an older edition.

Do not copy an obsolete material description from an old drawing or MTC
without checking the current standard scope.

Welding and Post-Fabrication Surface Condition

904L is a low-carbon austenitic stainless steel and can be fabricated by
conventional stainless-steel welding processes when an approved procedure is used.

Welding still requires control of:

• joint preparation;
• filler selection;
• heat input;
• interpass temperature;
• surface oxidation;
• contamination;
• post-weld cleaning.

For acid-service equipment, heavy heat tint, embedded carbon-steel contamination
or poorly cleaned weld areas can reduce corrosion performance even when the
base material is correctly certified as N08904.

The equipment specification should define required:

• pickling;
• passivation;
• surface cleaning;
• weld inspection;
• final contamination control.

What Should a 904L Sulfuric Acid RFQ Include?

1. Application: tank, transfer line, circulation system, heat exchanger or another component.
2. Material: 904L / UNS N08904.
3. Product standard: ASTM A240, A269, A312, A479, A182 or another approved specification.
4. Product form: plate, sheet, coil, tube, pipe, bar or forging.
5. Dimensions: thickness, width, length, OD, wall or bar diameter.
6. Sulfuric acid concentration: normal, minimum and maximum.
7. Temperature: normal and maximum.
8. Chloride content: where known.
9. Other contaminants: fluorides, metal ions or process impurities where relevant.
10. Flow condition: stagnant, transfer, circulation or high-velocity service.
11. Pressure: operating and design pressure where applicable.
12. Welding: fabrication and approved WPS requirements.
13. Surface: mill finish and post-fabrication cleaning requirements.
14. Inspection: PMI, PT, UT, hydrotest or project-specific NDE.
15. Documentation: MTC / EN 10204 / project quality records.
16. Quantity: pieces, metres, sheets or total tonnage.
17. Drawing: latest approved equipment or component drawing.
18. Destination: country, port and required delivery date.

Example 904L Sulfuric Acid Tank RFQ

Example 904L Sulfuric Acid Transfer-Pipe RFQ

Incoming Inspection and Traceability

Depending on the order, incoming material verification may include:

• UNS N08904 identification;
• product standard;
• heat number;
• Mill Test Certificate;
• Cr / Ni / Mo / Cu chemistry;
• mechanical properties;
• plate thickness;
• pipe OD and wall;
• surface condition;
• PMI where required;
• NDE reports;
• full material traceability.

PMI is useful for checking alloy identity but should not replace complete
material documentation or the required mechanical and dimensional tests.

How JISCO JUDE Supports 904L Material Inquiries

JISCO JUDE publishes general information for:

• 
  904L & 254SMO Super Austenitic Stainless Steels
  
• 
  Cold-Rolled Stainless Steel Sheet & Coil
  
• 
  Hot-Rolled Stainless Steel Sheet
  
• 
  Stainless Steel Tube & Pipe
  

Additional JUDE Steel resources include:

• 
  904L Stainless Steel Grade
  
• 
  Stainless Steel Sheet & Plate
  
• 
  Processing Services & Capabilities
  

Specific 904L product form, thickness, source, mill route, pipe dimensions
and certification must be confirmed for each inquiry.

A general 904L listing should not be interpreted as confirmation that every
N08904 size, ASTM standard or special inspection requirement is available
from stock.

Frequently Asked Questions

Is 904L suitable for every concentration of sulfuric acid?

No.
Corrosion resistance changes with concentration, temperature, aeration and
contaminants.
The actual process conditions should be reviewed before selecting the material.

Why is 904L often considered instead of 316L for sulfuric acid?

904L contains substantially more nickel and molybdenum together with an
intentional copper addition.
This alloy design provides a larger corrosion-resistance margin than 316L
in many sulfuric-acid environments.

Can 904L be used for refinery sulfuric-acid systems?

It can be evaluated for selected refinery and petrochemical acid-handling
systems.
The responsible corrosion or materials engineer must confirm suitability
for the actual acid concentration, temperature, contaminants and process design.

Is ASTM B677 still the correct standard for new N08904 pipe orders?

Current ASTM B677 scope states that N08904 products previously covered by
B677 are now covered by ASTM A269/A269M and A312/A312M.
New orders should use the current applicable standard unless the project
specifically requires an older specification.

Is ASTM B625 still used for new 904L plate orders?

Current ASTM B625 scope states that N08904 flat products previously covered
by B625 are now covered by ASTM A240/A240M and A480/A480M.
Buyers should verify old drawings before copying historical material descriptions.

Can 254SMO automatically replace 904L?

No.
254SMO is a different highly alloyed stainless steel with a different
composition and corrosion profile.
A substitution requires engineering and purchaser approval.

Is 904L a universal seawater grade?

No.
904L provides high corrosion resistance, but severe warm seawater,
chlorinated seawater or tight crevices may require another alloy such as
254SMO, super duplex or a higher-alloy material.

Can standard 904L welding procedures be copied from 316L?

Do not assume so.
Although both are austenitic stainless steels, filler selection, heat input,
joint design and final corrosion requirements should follow the approved
904L fabrication procedure.

What should I send JISCO JUDE for a 904L sulfuric-acid quotation?

Send the equipment or piping drawing, UNS N08904 grade requirement,
product standard, dimensions, acid concentration, temperature, impurities,
pressure, welding and inspection requirements, quantity and destination.

Request 904L Material for Sulfuric Acid Equipment

Technical note:
This article supports preliminary raw-material selection and purchasing.
Final 904L suitability depends on sulfuric-acid concentration, temperature,
aeration, chlorides, contaminants, velocity, equipment geometry, welding,
design code and project corrosion assessment.
A general 904L datasheet should not be treated as a universal sulfuric-acid
corrosion guarantee.