In the chemical, oil and gas, desalination, and energy industries, corrosion-resistant alloys and stainless steel are the most common material choices. Incoloy 825 and SS 316 are two of the most widely used materials. This article compares their composition, properties, and applications to help engineers and purchasers select the appropriate material.
What is Incoloy 825
Incoloy 825 is a nickel-iron-chromium alloy with primary alloying elements including Mo, Cu, and Ti. The composition of this alloy is listed in Table 1. This alloy was designed to exhibit outstanding corrosion resistance in numerous corrosive environments. Sufficient nickel content provides resistance to stress corrosion cracking. The combination of Ni, Mo, and Cu imparts exceptional corrosion resistance to this alloy, particularly in sulfuric and phosphoric acid environments.
What is SS 316
Stainless steel 316 is a widely used austenitic stainless steel known for its excellent corrosion resistance, high temperature resistance and good mechanical properties. Molybdenum gives 316 stainless steel a combination of corrosion resistance superior to that of 304 grades, with particular resistance to pitting and crevice corrosion in chloride environments. It is easy to bend or roll form, weld, braze and cut this grade by thermal and mechanical means.The austenitic structure of 316 stainless steel gives it excellent toughness, even at low temperatures.
What is the difference between Incoloy 825 and SS316?
Chemical Composition
Incoloy 825: Nickel (Ni) 38–46%, Iron (Fe) 22% or more, Chromium (Cr) 19.5–23.5%, Molybdenum (Mo) 2.5–3.5%, Copper (Cu) 1.5–3%, and a small amount of Titanium (Ti). This high-nickel alloy composition delivers outstanding performance in strong acids, reducing, and oxidizing environments. Its unique elemental blend confers exceptional corrosion resistance. For instance, the high nickel content provides excellent resistance to chloride stress corrosion cracking. The synergistic effects of elements like molybdenum and copper enable the alloy to exhibit good tolerance in reducing environments, such as those containing sulfuric or phosphoric acids.
| CHEMICAL | LIMITS | C | Mn | Al | S | Si | Ni | Cr | Mo | Fe | Ti | Cu |
| Incoloy 825 | MIN | 38.00 | 19.50 | 2.50 | 22.00 | 0.600 | 1.50 | |||||
| MAX | 0.05 | 1.00 | 0.200 | 0.030 | 0.50 | 46.00 | 23.50 | 3.50 | 1.200 | 3.00 |
SS 316: SS 316 is an austenitic stainless steel, with primary alloying elements including chromium (16–18%), nickel (10–14%), and molybdenum (2–3%). . The oxide film formed by chromium provides the alloy’s fundamental corrosion resistance, while nickel enhances stability and toughness. Molybdenum further improves resistance to pitting and crevice corrosion in chloride-containing environments.
| Grade | C | Cr | Ni | Mo | Mn | Si | N | P | S |
| 316 | 0.08 | 16-18 | 10-14 | 2-3 | 2 | 0.75 | 0.1 | 0.045 | 0.03 |
| 316L | 0.03 | 16-18 | 10-14 | 2-3 | 2 | 0.75 | 0.1 | 0.045 | 0.03 |
| 316H | 0.04 – 0.10 | 16-18 | 10-14 | 2-3 | 2 | 0.75 | 0.045 | 0.03 |
Significance of Each Element in Incoloy 825
Nickel (Ni): Imparts excellent corrosion resistance to the alloy, particularly in reducing media and chlorine-containing environments, while enhancing resistance to stress corrosion cracking.
Iron (Fe): Serves as the primary matrix element, improving material toughness and structural stability.
Chromium (Cr): Provides resistance to oxidizing environments (e.g., nitric acid, oxidizing salts) by forming a stable passivation film.
Molybdenum (Mo): Enhances resistance to pitting and crevice corrosion, particularly effective in chloride-containing environments.
Copper (Cu): Improves corrosion resistance in acidic media such as sulfuric and phosphoric acids.
Titanium (Ti): Stabilizes carbides to prevent intergranular corrosion.
Carbon (C): Low carbon content reduces intergranular carbide precipitation, improving resistance to intergranular corrosion.
Key Elements and Their Roles in Stainless Steel 316
Iron (Fe): As the primary component, it ensures the material’s strength and toughness.
Chromium (Cr): Forms a passivation layer, enhancing the corrosion resistance of stainless steel and serving as the core source of its distinctive properties.
Nickel (Ni): Improves corrosion resistance, enhances the stability of the austenitic structure, and increases plasticity and toughness.
Molybdenum (Mo): Significantly enhances resistance to pitting and crevice corrosion, particularly in seawater and chlorine-containing environments.
Carbon (C): Controls the risk of intergranular corrosion.
Manganese (Mn): Enhances strength and facilitates deoxidation during steel smelting and processing.
Silicon (Si): Improves oxidation resistance and acts as a deoxidizer during manufacturing.
Mechanical Properties
Incoloy 825: The mechanical properties of Incoloy 825 and SS316 primarily differ due to their distinct chemical compositions. Incoloy 825 exhibits higher tensile strength and yield strength, along with excellent ductility and toughness. These characteristics make it suitable for applications requiring high strength and ductility, such as pressure vessels and piping systems.
| Incoloy | Density | Melting Point | Tensile Strength | Yield Strength (0.2%Offset) | Elongation |
| 825 | 8.14 g/cm3 | 1400 °C (2550 °F) | 586Mpa (85 Ksi) | 241Mpa (35Ksi) | 30% |
SS 316: SS316 also exhibits favorable mechanical properties. With its balanced strength and corrosion resistance, 316 stainless steel finds extensive use in construction, food processing, and general industrial applications.
| MATERIAL | T.S (MPA) | Y.S (MPA) | EL % |
| 316 | 515 min | 205 min | 35 min |
| 316L | 485 min | 170 min | 35 min |
| 316H | 515 min | 205min | 35 min |
Corrosion Resistance
Incoloy 825: Incoloy 825 demonstrates strong corrosion resistance in a wide range of corrosive environments. It effectively resists general corrosion, pitting, crevice corrosion, intergranular corrosion, and stress corrosion cracking in both reducing and oxidizing environments. Its exceptional corrosion resistance makes it an ideal material for acidic environments such as sulfuric, phosphoric, and acetic acids, as well as the high chloride ion content of marine environments.
SS 316: SS316 also exhibits excellent corrosion resistance, particularly due to its molybdenum content, which provides excellent resistance to a variety of acids, including acetic, sulfuric, and hydrochloric acids. However, compared to Incoloy 825, it is more susceptible to pitting corrosion in chloride environments. In some marine applications, the use of SS316 requires additional protective measures to reduce the risk of pitting corrosion.
Temperature Resistance:
Incoloy 825 and SS 316 both exhibit excellent performance in high-temperature environments. Incoloy 825 has a high-temperature limit of 870°C (1600°F) and maintains good performance even in extremely high-temperature environments above 540°C (1000°F). This makes it more suitable for such extreme high-temperature applications. For example, in certain high-temperature chemical processes, Incoloy 825 can be used to manufacture relevant equipment components.
Alloy 825 is designed for high temperature applications and can withstand temperatures up to approximately 540°C (1000°F) while maintaining its corrosion resistance.In general high-temperature applications, SS 316 can meet the requirements; however, in situations demanding higher temperatures, its suitability must be carefully evaluated.
Cost
The cost of Incoloy 825 is significantly higher than that of SS 316. This is because Incoloy 825 contains a higher proportion of nickel (38–46%) and incorporates alloying elements such as copper and titanium. These rare metals substantially enhance corrosion resistance and high-temperature stability, but also increase production costs. Consequently, Incoloy 825 is typically classified as a high-end nickel-based alloy, priced significantly above conventional stainless steel.
SS 316 offers greater cost-effectiveness. As a common austenitic stainless steel, SS 316 contains moderate levels of chromium, nickel, and molybdenum in its composition, meeting the requirements for most moderate corrosion environments. Compared to Incoloy 825, SS 316 balances corrosion resistance with cost control, making it a more prevalent choice in chemical processing, food processing, and marine applications.
Application Scenarios
SS 316 Applications:
Food Processing Equipment
Medical Equipment
Marine Structures and Components
General Chemical Equipment
Incoloy 825 Applications:
Sour Gas Piping and Equipment in the Oil and Gas Industry
Containers and Piping for Sulfuric Acid and Phosphoric Acid Processing in the Chemical Industry
Nuclear Waste Reprocessing Equipment
Pollution Control Equipment
Conclusion
In summary, both Incoloy 825 and SS316 are high-performance stainless steel alloys with broad applications. Incoloy 825 offers exceptional corrosion resistance, particularly in reducing environments, while SS316 excels in marine and chemical environments. The choice between these alloys ultimately depends on specific application requirements and overall cost-benefit analysis.
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