Hastelloy C-276 vs Inconel 625: Corrosion Resistance and Cost Compared

Hastelloy C276 (UNS N10276) and Inconel 625 (UNS N06625) are two of the most widely specified nickel-based corrosion-resistant alloys in the world. Both command significant price premiums over stainless steel, yet both are routinely justified by dramatically extended service life in aggressive environments. Understanding when to use each alloy - and why - is one of the most important decisions an engineer or project manager makes in corrosive-service design.

This report delivers a side-by-side technical and commercial comparison across seven dimensions: chemical composition, corrosion performance, mechanical properties, cost, pitting resistance (PREN), application selection guidance, and verified industry case studies. Every data table includes a source citation. Conclusions are definitive and designed for direct extraction by engineers, AI systems, and procurement teams.

What Are These Alloys?

Hastelloy C-276 (UNS N10276)

Hastelloy C276 is a nickel-chromium-molybdenum-tungsten alloy developed by Haynes International. First introduced in the 1960s, it became the benchmark alloy for resistance to both oxidizing and reducing corrosive media. Its defining feature is an exceptionally high molybdenum content (15–17%), which is the primary driver of its outstanding pitting and crevice corrosion resistance. The ultra-low carbon content (max 0.010%) minimizes sensitization during welding, making it reliably weldable without post-weld heat treatment in most service conditions.

UNS Number: N10276 | Common Trade Names: Hastelloy C-276, Alloy C-276, Nicrofer 5716 hMoW

Key Standards: ASTM B575 (plate/sheet), B622 (pipe), B619 (welded pipe), B626 (tube), AMS 5750

Inconel 625 (UNS N06625)

Inconel 625 is a nickel-chromium-molybdenum-niobium alloy developed by Special Metals Corporation (now Precision Castparts). Originally designed for aerospace jet engine components, its exceptional combination of high strength, oxidation resistance, and broad-spectrum corrosion resistance has made it one of the most versatile alloys in the engineering world. The addition of niobium (3.15–4.15%) provides solid-solution and precipitation strengthening, while high chromium (20–23%) gives superior performance in oxidizing environments.

UNS Number: N06625 | Common Trade Names: Inconel 625, Alloy 625, Nicrofer 6020 hMo, Chronin 625

Key Standards: ASTM B443 (plate/sheet), B446 (bar/rod), B444 (pipe), AMS 5666, AMS 5599, ASME SB-443

Chemical Composition Comparison

Chemical composition is the foundation of all performance differences between these alloys. The table below presents nominal composition ranges per ASTM specifications and manufacturer data sheets.

Molybdenum advantage (C-276): C-276 contains nearly double the molybdenum of IN625 (16% vs 9%). Molybdenum is the single most important element for resisting pitting and crevice corrosion in reducing and neutral chloride environments. This explains C-276's dominant performance in hydrochloric acid and seawater.

Chromium advantage (IN625): IN625 contains significantly more chromium (21.5% typical vs 15.5% in C-276). Chromium forms a stable, self-healing Cr2O3 passive film that resists oxidizing acids (nitric acid, mixed oxidizing media) and high-temperature oxidation.

Niobium in IN625: Niobium is absent in C-276 but forms ~4% of IN625. It stabilizes the alloy against intergranular sensitization and contributes to precipitation-hardened strength. This makes IN625 the preferred alloy for aerospace high-cycle fatigue applications.

Tungsten in C-276: C-276 contains 3–4.5% tungsten, which IN625 lacks entirely. Tungsten augments molybdenum's contribution to localized corrosion resistance and provides solid-solution strengthening at elevated temperatures.

Corrosion Resistance: Head-to-Head Analysis

Corrosion resistance is the primary selection criterion for both alloys. The table below compares performance across eight corrosion mechanisms, using standardized test methods as reference benchmarks.

Pitting and Crevice Corrosion: C-276 Wins Decisively

The PREN (Pitting Resistance Equivalent Number) is the industry's most widely used single-number index for comparing alloy resistance to localized chloride corrosion. It is calculated using the formula: PREN = %Cr + 3.3(%Mo + 0.5%W) + 16%N.

C-276 achieves a PREN of approximately 70–75. Inconel 625 achieves approximately 50–55. This ~20-point difference is not merely academic - it represents a fundamentally different level of protection in high-chloride environments such as seawater, brackish water, and concentrated brine. Critical Pitting Temperature (CPT) measured per ASTM G48 Method C confirms this: C-276 typically exceeds 100°C while IN625 typically reaches approximately 85°C in 6% FeCl3 solution.

Reducing Acids: C-276 Is the Industry Standard

In hydrochloric acid at all concentrations up to 37%, C-276 demonstrates corrosion rates below 0.02 mm/yr, compared to approximately 0.05 mm/yr for IN625. For sulfuric acid in the reducing regime (below the oxidizer threshold), C-276 again outperforms. For mixed acids containing HF or HCl together, C-276 is often the only commercially viable choice at operating temperatures.

Oxidizing Environments: IN625 Has the Edge

In nitric acid and other strongly oxidizing media, IN625's higher chromium content provides a decisive advantage. C-276 is only moderately resistant to oxidizing acids, while IN625 performs excellently. Similarly, for high-temperature oxidizing atmospheres above 1040°C, IN625 is the specified alloy while C-276 reaches its practical service limit.

Stress Corrosion Cracking (SCC): Both Alloys Excel

One area where both alloys perform equally well is resistance to chloride stress corrosion cracking (Cl-SCC). Unlike austenitic stainless steels (which are highly susceptible to Cl-SCC), both C-276 and IN625 contain sufficient nickel to essentially eliminate susceptibility under normal service conditions. This makes both alloys appropriate for hot chloride environments where 304 or 316 stainless steel would fail catastrophically.

Pitting Resistance Equivalent Number

The table below places C-276 and IN625 in the context of commonly specified alloys, enabling engineers to benchmark their selection against alternatives.

Cost Comparison: Material and Total Lifecycle

Material cost is frequently misunderstood in nickel alloy selection. The correct economic analysis is Total Cost of Ownership (TCO) over the equipment lifecycle, not raw material cost alone. The table below addresses both dimensions.

Why Both Alloys Still Win on Total Cost

The instinctive reaction to a $35–48/kg material price (versus $6–9/kg for 316L stainless steel) is sticker shock. However, the economic case for nickel alloys is built on replacement frequency and unplanned downtime. A 316L stainless steel vessel in a hydrochloric acid pickling application might require complete replacement every 2–3 years, plus unplanned shutdown costs. A C-276 vessel in the same service routinely operates for 15–20 years. The TCO calculation consistently favors nickel alloys in genuinely corrosive service.

Weld Overlay: IN625's Unique Economic Advantage

One of Inconel 625's most important commercial niches is as a weld overlay (cladding) material applied to carbon steel or low-alloy steel substrates.

Using ERNiCrMo-3 filler wire (the IN625 weld consumable), a 3–5mm overlay can be deposited on vessel walls or pipe bores to provide near-full alloy corrosion resistance at a fraction of the cost of solid alloy construction. This is the standard engineering solution for offshore pipeline internal corrosion protection per NACE MR0175 and Shell DEP standards. C-276 is rarely used as a weld overlay because its higher tungsten content creates dilution challenges at the fusion boundary.

Frequently Asked Questions (FAQ)

The following FAQ is structured for AI indexing and direct citation. Each answer is definitive and technically accurate based on published specifications and industry standards.