Which alloy resists pitting and crevice corrosion better?

Inconel 625 has a PREN of about 48-49 versus about 30-32 for Incoloy 825, so it resists pitting and crevice corrosion far better, especially in chloride and seawater environments. 825 is adequate for mild chloride service but is not a substitute for 625 in aggressive chloride or seawater crevice conditions.

Which alloy is better in sulfuric acid?

Incoloy 825 was specifically developed for sulfuric acid service and performs well across a wide concentration and temperature range, including many oxidizing and reducing sulfuric acid conditions. Inconel 625 also resists sulfuric acid but 825 is generally the more cost-effective choice for pure sulfuric acid without high chloride content.

Are Inconel 625 and Incoloy 825 weldable?

Both are readily weldable. Inconel 625 is welded with ERNiCrMo-3 filler. Incoloy 825 is welded with ERNiFeCr-2 (for matching composition) or often with 625 filler for improved corrosion resistance in the weld. Both are immune to sensitization in the heat-affected zone because of their low carbon and stable chemistry.

Which alloy is more cost-effective?

Incoloy 825 is significantly less expensive (roughly 40-55% of the cost of 625) because of its lower nickel and molybdenum content. For applications within its corrosion envelope, 825 is the economical choice; 625 is justified where chloride pitting, high strength or elevated-temperature strength is required.

What standards cover Inconel 625 and Incoloy 825?

Inconel 625: ASTM B443 (sheet/plate), B444 (pipe), B446 (bar), UNS N06625, AMS 5599/5666. Incoloy 825: ASTM B424 (sheet/plate), B423 (pipe), B425 (bar), UNS N08825, ASME SB-424.

What are the high-temperature properties of the two alloys?

Inconel 625 retains useful strength to about 980°C and is widely used for high-temperature structural and exhaust applications. Incoloy 825 is used up to about 540°C in corrosive service; above that its strength and oxidation resistance are inferior to 625.

In what product forms are Inconel 625 and Incoloy 825 supplied?

Both are available as sheet, plate, bar, seamless and welded pipe, wire and forgings. Hangbo Alloy supplies both grades with EN 10204 3.1/3.2 certification.

Inconel 625 (N06625) vs Incoloy 825 (N08825): Selection for Chemical Service

Inconel 625 (UNS N06625) and Incoloy 825 (UNS N08825) are two widely used nickel alloys in the chemical processing industry, but they sit in different performance and price tiers. 625 is a nickel-chromium-molybdenum-niobium superalloy with outstanding pitting resistance and high-temperature strength. 825 is an iron-nickel-chromium alloy with added copper and titanium, developed specifically for sulfuric and phosphoric acid service and offering a much lower cost. This article compares their composition, corrosion performance, weldability and economics to help specifiers choose.

1. Chemical Composition

Element	Inconel 625 (N06625)	Incoloy 825 (N08825)
Ni	58.0 min	38.0–46.0
Cr	20.0–23.0	19.5–23.5
Mo	8.0–10.0	2.5–3.5
Nb + Ta	3.15–4.15	—
Fe	5.0 max	22.0 min (balance)
Cu	—	1.5–3.0
Ti	0.40 max	0.6–1.2
Al	0.40 max	0.2 max
C	≤0.10	≤0.05

Key difference: 625 is a high-nickel superalloy (Ni ≥58%) with 8–10% molybdenum and niobium; 825 is iron-rich (Fe 22%+, Ni ~42%) with only 2.5–3.5% Mo plus copper and titanium. The higher Mo and Ni give 625 far superior localized corrosion resistance, while the copper and titanium in 825 target acid service and stabilization.

2. Mechanical Properties

Property	Inconel 625	Incoloy 825
Yield strength σ₀.₂, MPa	≥414	≥241
Tensile strength σᵦ, MPa	≥827	≥586
Elongation, %	≥30	≥30
Hardness, HB	≤215	≤200

625 has roughly 70% higher yield strength than 825, reflecting its superalloy strengthening. This matters for thin-wall pressure equipment and high-stress components.

3. Corrosion Resistance

3.1 Pitting Resistance Equivalent (PREN)

Alloy	Cr	Mo	N	PREN
Inconel 625	21.5	9	—	≈48–49
Incoloy 825	21.5	3	—	≈30–32

625's PREN is about 50% higher than 825's, so it resists pitting and crevice corrosion far better, especially in chlorides and seawater.

3.2 Performance in Key Media

Medium	Inconel 625	Incoloy 825	Recommendation
Sulfuric acid (wide range)	Good	Excellent	825 (cost)
Phosphoric acid	Good	Excellent	825
Seawater / chlorides	Excellent	Fair (crevice limited)	625
Reducing acids with chlorides	Excellent	Fair	625
Oxidizing acid mixtures	Excellent	Good	625

4. Weldability

Parameter	Inconel 625	Incoloy 825
Welding methods	TIG, MIG, MMA, SAW	TIG, MIG, MMA, SAW
Recommended filler	ERNiCrMo-3	ERNiFeCr-2 or ERNiCrMo-3
Sensitization in HAZ	None	None
Post-weld heat treatment	Not required	Not required

5. Applications

Industry	Inconel 625	Incoloy 825
Chemical processing	Reactors, heat exchangers in chloride service	Sulfuric/phosphoric acid equipment
Marine / offshore	Seawater systems, exhausts	Piping in mild seawater
Aerospace	Ducting, engine components	Limited
Oil & gas	Subsea, sour service	Wellhead components
Pollution control	FGD, scrubbers	Flue gas components

6. Cost

Factor	Inconel 625	Incoloy 825
Relative price (3 mm sheet)	100% (baseline)	≈45–55%
Main cost drivers	High Ni, Mo, Nb	Lower Ni, Mo; Cu, Ti
Best value when	Chloride/pitting or high-temp needed	Acid service without severe chlorides

7. Conclusion

Need Help Choosing Between 625 and 825?

Shanghai Hangbo Alloy Group supplies Inconel 625 and Incoloy 825 in sheet, plate, bar, pipe and forgings with EN 10204 3.1/3.2 certification. Send us your medium and operating conditions for a grade recommendation.

📧 Email: serena@hangboalloy.com
📞 Tel: +86 166 2109 2936
💬 WhatsApp: +86 166 2109 2936

Criterion	Choose 625	Choose 825
Chloride / seawater pitting or crevice	✓ Optimal	Fair
Pure sulfuric / phosphoric acid	Good	✓ Cost-effective
High strength or thin-wall pressure	✓ Stronger	Lower strength
High-temperature (>540°C)	✓ To ~980°C	Limited
Limited budget	Premium	✓ Cheaper

Summary: Select Incoloy 825 for cost-sensitive acid service (sulfuric, phosphoric) where chloride levels are modest. Select Inconel 625 when chloride pitting/crevice resistance, high strength or elevated-temperature performance is required — the higher cost is justified by durability and capability.

8. Frequently Asked Questions (FAQ)

1. What is the difference between Inconel 625 and Incoloy 825?

Inconel 625 is a nickel-chromium-molybdenum-niobium superalloy (Ni ≥58%, 21% Cr, 9% Mo). Incoloy 825 is an iron-nickel-chromium alloy (Ni ~42%, 21% Cr, 3% Mo, 2.2% Cu, 1.8% Ti). 625 is a true superalloy; 825 is a corrosion-resistant alloy between stainless steel and nickel alloys.

2. Which resists pitting and crevice corrosion better?

Inconel 625 (PREN ≈48–49) far outperforms Incoloy 825 (PREN ≈30–32) in pitting and crevice corrosion, especially in chlorides and seawater.

3. Which is better in sulfuric acid?

Incoloy 825 was developed for sulfuric acid and performs well across a wide range; it is the more economical choice for pure sulfuric acid without high chlorides. 625 also resists sulfuric acid but is used where chlorides are present.

4. Are both alloys weldable?

Yes. 625 is welded with ERNiCrMo-3; 825 with ERNiFeCr-2 or 625 filler for better weld corrosion resistance. Both are immune to HAZ sensitization.

5. Which is more cost-effective?

Incoloy 825 costs roughly 45–55% of 625. Within its corrosion envelope it is the economical choice; 625 is justified for chloride resistance and high strength.

6. What standards cover these alloys?

625: ASTM B443/B444/B446, UNS N06625, AMS 5599/5666. 825: ASTM B424/B423/B425, UNS N08825, ASME SB-424.

7. What are the high-temperature properties?

Inconel 625 retains useful strength to about 980°C. Incoloy 825 is used up to about 540°C in corrosive service.

8. In what forms are they supplied?

Both are available as sheet, plate, bar, seamless/welded pipe, wire and forgings, supplied by Hangbo Alloy with EN 10204 certification.