Inconel 625 vs. Incoloy 825 for Offshore Piping: A Material Selection Guide for Project Engineers

Introduction

Inconel 625 and Incoloy 825 sit in the same commercial bracket of nickel-based alloys, are both listed in NACE MR0175, and both appear on offshore piping specifications for chloride-containing service. Yet they solve fundamentally different corrosion problems, and swapping one for the other without reviewing the service conditions is an expensive and recurring mistake.

The two alloys differ in nickel content by roughly 15 percentage points, in molybdenum content by a factor of three, and in cost by 30–50% on a per-kilogram basis. Their maximum service temperatures differ by over 400°C. Their strengths differ by a factor of nearly two. This guide quantifies those differences and provides a decision framework that project engineers can apply directly to offshore piping specifications.

The Core Chemistry: Different Alloys, Different Problems

The most important distinction is that Incoloy 825 is not a “cheap Inconel 625.” It is a nickel-iron-chromium alloy with deliberate additions of copper and titanium, designed around resistance to sulfuric and phosphoric acid environments. Inconel 625 is a nickel-chromium-molybdenum-niobium superalloy with no intentional copper, designed for high-temperature strength and broad-spectrum corrosion resistance in the most aggressive chloride and mixed-acid environments.

Data from ASTM B444/B446 product specifications and published alloy datasheets.

The PREN gap — from ~31 to ~51 — is the single most consequential number for offshore service. A PREN of 40 is the informal threshold for sustained seawater exposure. Incoloy 825 sits well below that threshold. This does not mean 825 fails in seawater — it has good resistance to chloride stress corrosion cracking and is used in seawater-cooled exchangers — but its pitting and crevice corrosion resistance is a different order of magnitude from 625.

Corrosion Performance: Where Each Alloy Wins and Loses

Seawater and Chloride Environments

Inconel 625 is the more capable alloy in chloride service. The molybdenum content of 8–10% provides exceptional resistance to pitting, and the PREN of ~51 places it well above the threshold for full seawater immersion. Incoloy 825, with 2.5–3.5% molybdenum, has a PREN in the low 30s — adequate for moderate chloride service but not in the same class.

However, a critical qualification applies to both alloys: Inconel 625 is susceptible to crevice corrosion in chlorinated seawater. This is counterintuitive because 625 has excellent pitting resistance. The crevice corrosion susceptibility arises in oxidizing environments — specifically chlorinated seawater where chlorine injection creates a more aggressive localized chemistry. In natural, unchlorinated seawater, 625 performs well, but in chlorinated seawater systems, the alloy has been found to be susceptible to crevice attack. This is a design consideration that offshore engineers must address through joint design, gasket selection, and where possible, minimizing crevice geometries.

Incoloy 825’s high nickel content (38–46%) gives it excellent resistance to chloride stress corrosion cracking — superior to 304 or 316 stainless steels. The alloy has shown excellent resistance to SCC in sour (H₂S/Cl⁻) environments and in chloride-containing solutions. Where 825 falls short is in pitting resistance at elevated temperatures: the relatively low molybdenum content limits its resistance to pitting above approximately 60°C.

Acid Service

This is where Incoloy 825 justifies its existence. The copper addition (1.5–3.0%) is specifically designed to form a protective film in sulfuric acid environments where chromium oxide dissolves. 825 is the workhorse alloy for sulfuric acid coolers, phosphoric acid production, and pickling systems. Inconel 625 also performs well in sulfuric acid but at significantly higher cost.

For hydrochloric acid, the picture reverses. 625 has good resistance to dilute hydrochloric acid; 825 has limited resistance. The molybdenum content that gives 625 its chloride resistance is what provides HCl resistance — a chemistry that 825 simply does not possess at the same level.

Sour Service (H₂S + CO₂ + Chlorides)

Both alloys are listed in NACE MR0175 / ISO 15156, but their acceptable operating windows differ. Incoloy 825 is approved at Level V; Inconel 625 at Level VII. The practical implication is that 625 is approved for higher H₂S partial pressures, higher chloride concentrations, and higher temperatures than 825.

For offshore sour service, this means that 825 can be used where the H₂S partial pressure and chloride levels fall within its NACE listing — but the design envelope is narrower. Above 538°C, 825 is out of the race entirely, and 625 is the only option.

Cost Analysis: The Real Numbers

Material cost for Incoloy 825 typically ranges from $25–35/kg**, while Inconel 625 ranges from **$35–45/kg — roughly a 30% premium for 625. Some sources place the spread higher, with 825 at $15–40/kg and 625 at $35–65/kg depending on product form and market conditions.

But unit price is not the decision metric. Several factors narrow or widen the effective cost gap:

Strength enables wall thickness reduction. Inconel 625 has a minimum yield strength of 414 MPa versus 241 MPa for 825 — a 72% advantage. On pressure-containing components, this can translate into thinner walls and less material per fitting or pipe section. A 625 pipe that requires 6 mm wall may require 10 mm in 825 for the same pressure rating. The weight reduction partially offsets the per-kilogram premium.

Welding costs differ. Both alloys are weldable by GTAW and GMAW, and both are commonly welded using ERNiCrMo-3 filler metal (the AWS classification for 625 filler). For 825, a matching filler (ERNiFeCr-1) is available, but ERNiCrMo-3 is usually used as an “overmatching” composition to provide better corrosion resistance in the weld zone. This means the weld metal in an 825 system may effectively be 625 composition — a detail that matters for cost and corrosion performance.

Mill availability and lead time. 625 is more widely stocked in offshore piping sizes and has broader mill availability. 825 availability is rated “medium” compared to 625's “high” in material selection matrices for offshore flare piping. On projects with compressed schedules, the lead-time difference can matter more than the per-kilogram cost.

Welding and Fabrication Considerations

Both alloys require controlled welding practices, but the demands differ.

Inconel 625 is commonly applied as a weld overlay on carbon steel pipe and fittings for offshore service. This approach provides corrosion resistance at a fraction of the cost of solid alloy pipe. The welding process for 625 overlay requires “hot” processes — high current, high deposition rate — to produce sound, well-bonded deposits. Solid 625 pipe is welded with ERNiCrMo-3 filler under GTAW or GMAW with argon shielding.

Incoloy 825 has good weldability, and its titanium stabilization minimizes the risk of intergranular corrosion after welding. Solution annealing at 1,800°F (982°C) followed by rapid quenching is required when specified, and interpass temperature should be limited to 300°F (149°C) maximum. The titanium addition is what makes 825 weldable without the sensitization risk that plagues unstabilized austenitic alloys.

A practical point for offshore fabrication: both alloys are commonly welded using 625 filler metal (ERNiCrMo-3). This means a weld shop qualified for 625 welding is also substantially qualified for 825 welding, provided the base metal procedure qualification is performed. The overmatching filler composition gives the weld zone corrosion resistance that exceeds the 825 base metal — a beneficial condition in most service environments.

NACE MR0175 Compliance: Different Windows, Not Different Categories

Both alloys are listed in NACE MR0175 / ISO 15156, but the compliance is conditional. The standard specifies maximum H₂S partial pressures, maximum chloride concentrations, and maximum temperatures for each alloy. Incoloy 825 is approved at a lower level than 625, meaning its acceptable operating envelope is narrower.

For project engineers, this means that “NACE compliant” is not a binary. The material selection must verify that the specific combination of H₂S partial pressure, chloride concentration, and temperature falls within the alloy’s listing. A piping specification that lists “Incoloy 825, NACE MR0175” without specifying the service envelope is incomplete.

Inconel 625’s higher NACE level and broader temperature range make it the safer default for sour service where the operating conditions are near the boundaries of the 825 listing, or where future changes in reservoir conditions could push the system beyond 825’s limits.

A Decision Framework for Offshore Piping

What to Verify on Your Purchase Order

Whichever alloy you specify, the delivered material must match the specification, and the verification requirements differ:

For Inconel 625: Confirm UNS N06625, nickel content ≥58%, molybdenum 8–10%, and niobium + tantalum 3.15–4.15%. Product specifications are ASTM B444 (seamless pipe), ASTM B446 (rod and bar), and ASTM B366 (fittings). For weld overlay, verify the overlay thickness and bond integrity per the applicable overlay specification.

For Incoloy 825: Confirm UNS N08825, nickel 38–46%, molybdenum 2.5–3.5%, copper 1.5–3.0%, and titanium 0.6–1.2%. Product specifications are ASTM B423 (seamless pipe), ASTM B425 (rod and bar), and ASTM B366 (fittings). The titanium content is critical — it is what provides intergranular corrosion resistance after welding, and a heat with titanium below the minimum should be rejected.

For both alloys, require EN 10204 3.1 mill test certificates with full chemical analysis including the minor elements that drive corrosion performance (Nb+Ta for 625, Cu and Ti for 825). For critical offshore service, specify EN 10204 3.2 with third-party inspection.