Hastelloy C-276 vs Monel 400: Extreme Corrosion Resistance Compared

C-276 is the go-to nickel alloy for mixed acid environments and oxidizing-reducing conditions. Monel 400 excels in reducing acids and seawater. Understanding which corrosion mechanism dominates guides the choice.

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## Composition and Structure Both are nickel-based alloys, but their compositions target different corrosion mechanisms. | Element | C-276 (UNS N10276) | Monel 400 (UNS N04400) | |---------|---------------------|------------------------| | Nickel | 57% (bal) | 63% (min) | | Chromium | 14.5-16.5% | None | | Molybdenum | 15-17% | None | | Copper | None | 28-34% | | Carbon (max) | 0.01% | 0.3% | C-276 is a Ni-Cr-Mo alloy designed for resistance to both oxidizing and reducing environments. Monel 400 is a Ni-Cu alloy designed specifically for reducing environments. ## Mechanical Properties | Property | C-276 (Annealed) | Monel 400 (Annealed) | |----------|------------------|---------------------| | Tensile Strength | 790 MPa | 517 MPa | | Yield Strength | 365 MPa | 172 MPa | | Elongation | 40% | 48% | ## Corrosion Resistance ### C-276 Strengths Resists both oxidizing and reducing acids simultaneously, handles wet chlorine gas and hypochlorite, withstands hot contaminated sulfuric acid and hydrochloric acid. PREN > 70 makes it essentially immune to seawater pitting. ### Monel 400 Strengths Excellent in hydrofluoric acid (HF) at all concentrations and temperatures. Outstanding in seawater with velocities up to 30 m/s. Immune to chloride stress corrosion cracking. ### Limitations Monel 400 fails in oxidizing acids (nitric acid, hot concentrated sulfuric). C-276 handles oxidizing conditions but costs substantially more. ## Cost C-276 costs approximately 50-80 USD/kg, one of the most expensive common nickel alloys. Monel 400 costs approximately 25-40 USD/kg. ## When to Choose Each **Choose C-276 when**: The environment contains mixed oxidizing and reducing conditions, wet chlorine or hypochlorite is present, or maximum corrosion resistance is needed regardless of cost. Key applications include chemical reactor vessels and pharmaceutical processing. **Choose Monel 400 when**: The primary corrosive is hydrofluoric acid, the environment is purely reducing, flowing seawater service is required, or cost is a concern. Key applications include HF alkylation units and marine fasteners.