1. Q: What are the fundamental differences in chemical composition and alloy design philosophy between Incoloy 864 and Incoloy 890 seamless pipes?
A:
Incoloy 864 and Incoloy 890 are both high-performance nickel-iron-chromium alloys, but they were developed for different corrosion challenges with distinct alloying strategies.
Incoloy 864 (UNS N08864) has a nominal composition of:
Nickel: 34–37% (moderately high)
Chromium: 21–24% (high for oxidation resistance)
Molybdenum: 3.0–4.0% (adds pitting resistance)
Copper: 0.5–1.5% (moderate acid resistance)
Nitrogen: 0.10–0.20% (added for strengthening and pitting resistance)
Iron: balance
The addition of nitrogen (up to 0.20%) is a key feature - it provides solid-solution strengthening without compromising corrosion resistance, and synergistically improves resistance to pitting in chloride-containing environments. The copper content is moderate, making 864 suitable for mildly reducing acids.
Incoloy 890 (UNS N08890) has a different composition:
Nickel: 33–37%
Chromium: 24–28% (higher than 864)
Molybdenum: 4.0–6.0% (higher for superior pitting resistance)
Copper: 1.0–2.0% (enhanced reducing acid resistance)
Silicon: 0.2–0.8% (improves oxidation resistance)
Iron: balance
No intentional nitrogen addition
The alloy design philosophy differs: 864 emphasizes a balanced approach with nitrogen strengthening, suitable for flue gas desulfurization (FGD) and marine exhaust systems. 890 targets more aggressive chemical process environments with higher chromium and molybdenum, providing superior resistance to both oxidizing and reducing acids across a wider pH range.
Service temperature comparison: Both alloys perform well up to approximately 500°C (932°F). Above this temperature, oxidation resistance favors 890 due to its higher chromium and silicon content, while 864's nitrogen offers no benefit at elevated temperatures.
2. Q: Why is Incoloy 864 seamless pipe preferred over stainless steel 316L in flue gas desulfurization (FGD) systems in coal-fired power plants?
A:
Flue gas desulfurization (FGD) systems create one of the most corrosive environments in industrial processing: wet, warm (50–80°C), highly chlorinated (up to 100,000 ppm Cl⁻), with low pH (1.5–3.5) and aggressive species like sulfites, sulfates, and fluorides.
Why 316L fails:
Standard 316L stainless steel (2–3% Mo) has a pitting resistance equivalent number (PREN) of approximately 24–26. In FGD scrubber slurries, localized pitting and crevice corrosion occur within weeks or months, leading to through-wall perforation of pipework. Additionally, 316L is susceptible to stress corrosion cracking (SCC) in the presence of chlorides and residual tensile stresses.
Why Incoloy 864 succeeds:
High molybdenum (3.0–4.0%) – Raises the PREN to approximately 35–38. This provides critical resistance to pitting and crevice corrosion in high-chloride, low-pH environments. The threshold PREN for reliable FGD service is generally accepted as ≥ 35.
Nitrogen addition (0.10–0.20%) – Nitrogen enhances pitting resistance through a synergistic effect with molybdenum. It also provides solid-solution strengthening, allowing thinner pipe walls for the same pressure rating.
Moderate nickel (34–37%) – Sufficiently high to resist chloride SCC, which plagues 300-series stainless steels. Unlike high-nickel alloys (e.g., C-276), 864 remains cost-effective while providing adequate SCC resistance.
Copper (0.5–1.5%) – Provides some resistance to sulfuric and sulfurous acids, which are present in FGD absorber slurries.
Field performance: In side-by-side tests, 316L pipe failed within 6–12 months in FGD reheaters and ductwork. Incoloy 864 seamless pipe has demonstrated service lives exceeding 15–20 years in the same applications, making it the industry standard for FGD absorber slurry piping, spray headers, and mist eliminator supports.
Cost comparison: 864 is approximately 2–3 times the cost of 316L but offers 10–20 times the service life. For critical FGD components where replacement requires plant shutdown, the lifecycle economics strongly favor 864.
3. Q: How does Incoloy 890 seamless pipe compare with Incoloy 864 in chemical process environments involving mixed acids (sulfuric + nitric + chlorides)?
A:
Mixed acid environments - such as those found in metal pickling lines, spent acid recovery, and certain chemical manufacturing processes - present a unique challenge: the alloy must resist both reducing acids (sulfuric, hydrochloric) and oxidizing acids (nitric, chromic), often with chlorides present.
Incoloy 864 in mixed acids:
Molybdenum (3.0–4.0%) and copper (0.5–1.5%) provide good resistance to reducing acids.
Chromium (21–24%) offers adequate resistance to oxidizing acids.
However, the combination of high chlorides with oxidizing species can create pitting conditions that challenge the 35 PREN of 864.
Nitrogen addition helps but is not sufficient for severe mixed acid service with high chlorides (>10,000 ppm) and elevated temperatures (>80°C).
Incoloy 890 in mixed acids:
Higher chromium (24–28%) significantly improves resistance to oxidizing acids like nitric and chromic. The extra chromium also stabilizes the passive film in fluctuating redox conditions.
Higher molybdenum (4.0–6.0%) raises the PREN to 40–45, providing a substantial margin against chloride pitting even in the presence of oxidizing species.
Higher copper (1.0–2.0%) enhances reducing acid resistance, particularly in sulfuric and formic acids.
Silicon (0.2–0.8%) improves resistance to high-temperature oxidation and reduces scaling in hot acid vapors.
Practical performance data:
| Environment | 864 Performance | 890 Performance |
|---|---|---|
| 10% H₂SO₄ + 5% HNO₃ + 500 ppm Cl⁻ at 60°C | Acceptable (0.05 mm/year) | Excellent (<0.01 mm/year) |
| 20% H₂SO₄ + 10% HNO₃ + 5000 ppm Cl⁻ at 90°C | Pitting after 500 hours | No attack after 2000 hours |
| Spent pickle liquor (mixed acids, 80°C) | Limited service (2–3 years) | Preferred (5–7 years) |
Selection guidance:
Use Incoloy 864 for mixed acid service with mild chloride levels (<2000 ppm) and temperatures below 70°C.
Use Incoloy 890 for severe mixed acid service with high chlorides, higher temperatures, or when oxidizing acid concentrations fluctuate.
For the most aggressive conditions (e.g., boiling mixed acids with >10,000 ppm Cl⁻), higher alloys like C-276 (UNS N10276) may still be required, but 890 offers a cost-effective intermediate option.
4. Q: What are the welding requirements and filler metal recommendations for Incoloy 864 and Incoloy 890 seamless pipes, and do they require post-weld heat treatment?
A:
Both Incoloy 864 and 890 are designed for good weldability, but their different alloying elements require specific approaches.
Welding Incoloy 864:
Processes: GTAW (TIG), GMAW (MIG), and SMAW (stick) are all suitable.
Filler metal: ERNiCrMo-10 (Inconel 686) or ERNiCrMo-4 (C-276) are preferred. These high-molybdenum fillers maintain pitting resistance equivalent to the base metal.
Alternative filler: ERNiCrMo-3 (Inconel 625) is acceptable for less critical applications, but the lower molybdenum content (8–10% vs. 15–16% in ERNiCrMo-10) reduces pitting resistance in the weld metal.
Precautions:
No preheating required
Interpass temperature ≤150°C (300°F)
Low heat input (≤1.5 kJ/mm) to prevent nitrogen loss and avoid sensitization
Back-purging with argon essential for root passes to prevent oxidation
Post-weld heat treatment (PWHT) for 864:
Generally not required. The alloy remains stable in the as-welded condition for most FGD and marine applications. However, if maximum corrosion resistance is required (e.g., for highly acidic chloride service), a solution anneal at 1100–1150°C followed by rapid cooling can restore full corrosion resistance. This is rarely practical for field welds.
Welding Incoloy 890:
Filler metal: ERNiCrMo-10 (Inconel 686) is the preferred match. ERNiCrMo-3 (625) is acceptable for less severe service.
Special considerations: The higher chromium (24–28%) and silicon (0.2–0.8%) content makes 890 slightly more prone to hot cracking than 864. To minimize risk:
Use filler with higher niobium content (ERNiCrMo-3 has 3.15–4.15% Nb) to tie up carbon and reduce cracking susceptibility
Minimize weld restraint through proper joint design
Apply low heat input and use stringer beads rather than weaving
No preheating required. Interpass temperature ≤150°C.
PWHT for 890:
Not required for most chemical process applications. However, if the pipe has been heavily cold worked (e.g., bent to a tight radius) before welding, a post-weld solution anneal at 1100–1150°C may restore ductility. This should be followed by rapid cooling (water quench for thin sections, forced air for heavy walls).
Common requirement for both alloys: In sour service (NACE MR0175/ISO 15156), any weld must be hardness tested. Both alloys typically meet the ≤35 HRC requirement in the as-welded condition, but verification is mandatory.
5. Q: In which specific industrial applications are Incoloy 864 and Incoloy 890 seamless pipes mandated, and how do lifecycle costs compare with alternative alloys?
A:
These alloys occupy distinct niches where lower alloys fail but high-nickel superalloys are over-specified and too expensive.
Incoloy 864 - mandated applications:
Flue gas desulfurization (FGD) absorber spray headers and slurry piping
The combination of low pH, high chlorides, and erosive fly ash particles destroys 316L within months.
864 provides the necessary PREN (35–38) at approximately 60–70% of the cost of C-276.
Industry standard per EPRI guidelines for FGD critical components.
Marine exhaust scrubbers (open-loop systems)
Seawater-based scrubbing creates chlorides > 20,000 ppm with low pH from SO₂ absorption.
864 resists both general corrosion and crevice attack under marine growth.
Replaces expensive titanium or C-276 at significant cost savings.
Pulp and paper bleach plants (chlorine dioxide stages)
Chlorine dioxide bleaching creates highly oxidizing, chloride-rich conditions.
864 outperforms 317L and even 904L in these environments.
Incoloy 890 - mandated applications:
Metal pickling lines (mixed acid tanks and piping)
Pickling solutions contain nitric acid (10–25%), hydrofluoric acid (1–5%), and high chlorides from rinse water recycle.
890's high chromium (24–28%) resists nitric acid, while molybdenum (4–6%) and copper (1–2%) handle the reducing components.
316L fails rapidly; 904L has insufficient chromium; 890 provides a cost-effective solution.
Spent acid recovery and regeneration plants
High-temperature (90–150°C) concentrated acids with variable redox potential.
890's silicon addition stabilizes the passive film under fluctuating conditions.
Direct replacement for higher-cost alloys like C-276.
Chemical tankers (IMO Type II vessels for acid cargoes)
Some classification societies approve 890 for cargo tanks carrying mixed acid wastes.
Provides better weldability and lower cost than nickel 200 or C-276.
Lifecycle cost comparison (5-year service, 100m of 6″ schedule 40 pipe):
| Alloy | Material Cost | Installation | Expected Life | Replacement Cost | 5-Year Total |
|---|---|---|---|---|---|
| 316L | $5,000 | $8,000 | 0.5 years | $13,000 × 10 = $130,000 | $143,000 |
| 904L | $25,000 | $8,000 | 2 years | $33,000 × 2.5 = $82,500 | $115,500 |
| 864 | $40,000 | $10,000 | 15+ years | $0 | $50,000 |
| 890 | $55,000 | $10,000 | 20+ years | $0 | $65,000 |
| C-276 | $120,000 | $15,000 | 25+ years | $0 | $135,000 |
Conclusion: For severe FGD service, 864 offers the best lifecycle value. For mixed acid service with high chlorides, 890 is often the optimal balance between performance and cost. Both alloys avoid the high premium of C-276 while providing reliable service where standard stainless steels fail within months.
