2205 vs 904L sulfuric acid iso-corrosion curve

If you are selecting stainless steel for sulfuric acid (H₂SO₄) service, neither 2205 nor 904L should be selected simply because one grade has a higher PREN or more alloying elements.

The 2205 vs 904L sulfuric acid isocorrosion curve shows that corrosion resistance changes significantly with sulfuric acid concentration and temperature. In general, 904L provides a broader corrosion-resistance range in sulfuric acid, particularly in medium-concentration sulfuric acid, while 2205 can provide a cost-effective solution in selected dilute or higher-concentration conditions.

 

What Is a Sulfuric Acid Isocorrosion Curve?

An isocorrosion curve is a material-selection chart that plots sulfuric acid concentration against temperature at a specified corrosion-rate limit.

For the commonly published stainless-steel curves, the boundary represents approximately:

Corrosion rate = 0.1 mm/year (4 mpy)

Conditions below the curve generally indicate that the material is expected to remain within the selected corrosion-rate criterion, while conditions above the curve indicate increasing corrosion risk. Outokumpu describes its isocorrosion diagrams in this way and uses them to establish material limits over different acid concentrations and temperatures.

epresentative safe-concentration limits (≈0.1 mm/yr)

Temperature	2205 max % H₂SO₄	904L max % H₂SO₄	Read
20°C (room)	~40%	Full range	904L dominant
40°C	~20%	Full range	904L dominant
60°C	~12%	Broad (intermediate narrows)	904L dominant
80°C	~8%	Dilute + concentrated regions	904L dominant
100°C	~5% (very dilute only)	Dilute + >90% concentrated	904L dominant

 

Why Does 904L Usually Perform Better in Medium-Concentration Sulfuric Acid?

Chemical composition is one of the primary reasons. Compared to 2205, 904L contains significantly more nickel and molybdenum, as well as an addition of copper. The high nickel content in this highly alloyed grade reduces active corrosion rates compared to duplex steel grades with lower nickel content.

Element	2205	904L	Corrosion relevance
Cr	21–23%	19–21%	Passive-film formation
Ni	4.5–6.5%	24–26%	Improves resistance to active corrosion
Mo	2.5–3.5%	4–5%	Improves resistance in reducing/acidic media
Cu	Low	1–2%	Particularly useful in sulfuric acid environments
N	Higher	Low	Important for localized corrosion resistance

 

2205 vs 904L: The Sulfuric Acid Concentration Matters

One of the most common misconceptions when selecting stainless steel is the belief that a higher H₂SO₄ concentration inevitably leads to a higher corrosion rate. However, the corrosive nature of sulfuric acid is not that straightforward; its corrosivity changes significantly with concentration. Sulfuric acid can be most corrosive at intermediate concentrations, meaning that a higher acid concentration does not necessarily imply more severe corrosion.
This highlights a crucial factor in material selection: a 20% H₂SO₄ solution is not necessarily safer than an 80% solution. Therefore, the suitability of a material must be determined based on the specific combination of concentration and temperature.

 

2205 vs. 904L: Which is more cost-effective?

For many projects, 2205 offers the potential for lower total material costs, whereas 904L provides a greater margin of safety regarding corrosion resistance in many acidic applications.
However, comparing unit prices (USD/kg) alone is insufficient.
Purchasers should consider:
Material price + thickness + fabrication + welding + corrosion allowance + maintenance + replacement risk
2205 possesses significant mechanical advantages, with a yield strength far exceeding that of standard austenitic stainless steel grades. This means that, with proper design of pressure-bearing components, thinner material can often be used.
However, 904L may prove more economical if the use of 2205 results in excessive corrosion allowance requirements, reduced equipment service life, frequent maintenance, unplanned downtime, or premature equipment replacement.

 

FAQ

Is 2205 stainless steel resistant to sulfuric acid?
Yes, 2205 has useful resistance to sulfuric acid in selected concentration and temperature ranges. Its performance is strongly dependent on the exact H₂SO₄ concentration, temperature and contaminants. Published SAF 2205 data includes an isocorrosion curve based on a 0.1 mm/year corrosion-rate criterion.

 

Is 904L better than 2205 for sulfuric acid?

Generally, 904L provides a broader corrosion-resistance range in many sulfuric-acid conditions, especially medium concentrations. However, 2205 can outperform 904L in certain high-concentration conditions, so the exact operating point must be checked.

What is the corrosion rate represented by an isocorrosion curve?

The commonly published stainless-steel sulfuric-acid curves discussed here represent approximately 0.1 mm/year (4 mpy) corrosion rate.

 

Does higher sulfuric acid concentration always mean higher corrosion?

No. Sulfuric acid can be particularly aggressive at intermediate concentrations, so corrosion behavior is not simply proportional to acid concentration.

 

Does chloride affect 2205 vs 904L sulfuric-acid resistance?

Yes. Chloride contamination can significantly change the corrosion boundary. Published data for sulfuric acid containing 2,000 ppm chloride shows a different corrosion-resistance envelope from pure sulfuric acid.

 

Is 2205 cheaper than 904L?

Typically, 2205 can be more economical because it contains far less nickel than 904L. However, the correct comparison should include material thickness, fabrication, service life, maintenance and replacement costs.

 

Can 904L be used for concentrated sulfuric acid?

It depends on the concentration and temperature. Do not assume that 904L is suitable for all concentrated H₂SO₄ conditions. Isocorrosion data should be checked against the actual operating point, and severe service may require a higher-alloy material or actual-process testing.

 

Is PREN enough to select 2205 or 904L?

No. PREN is particularly useful for comparing localized corrosion resistance, especially in chloride environments. Sulfuric-acid uniform corrosion also depends on active/passive behavior, nickel, molybdenum, copper, temperature, concentration and contaminants.

 

Which is better for a sulfuric acid heat exchanger, 2205 or 904L?

There is no universal answer. If sulfuric acid is the primary process medium, 904L is often the stronger starting candidate, while 2205 may offer a more economical solution when the concentration-temperature point is within its proven resistance range. Heat-transfer-side water chemistry and chloride levels must also be evaluated.