Nickel alloys are usually identified as high-performance alloys since they have got high resistance to severe conditions of temperature, pressure and corrosion. Two of the most common ones are Inconel 625 and Inconel 825 that are widely used in the chemical, petrochemical and aerospace industry and also for marine applications.
Even if they seem very similar, there are some specific characteristics that make them more suitable for a corrosive environment rather than another. The question naturally arises: which alloy must be chosen in highly corrosive environments.
Let’s make a comparison to discover their chemical composition and their main characteristics.
| INCONEL 625 | INCONEL 825 |
|---|---|
| Chemical Composition | |
| Nickel + Chromium + Molybdenum + Niobium, providing high resistance to corrosion and high temperatures | Nickel + Chromium + Iron + Molybdenum and especially Copper, which improves corrosion resistance in acidic environments |
| Characteristics | |
| High resistance to corrosion from chlorides, seawater and aggressive environments | Excellent resistance to corrosion from strong acids such as sulfuric and phosphoric acid |
| Excellent resistance to intergranular corrosion | Good resistance to chloride corrosion and oxidizing environments |
| Resistance to high temperatures and oxidation | Higher ductility and toughness compared to other superalloys |
| Does not require hardening treatments to achieve high mechanical properties | Good mechanical performance at moderate temperatures |
| Often the first choice for marine and chemical applications | Often selected for highly acidic corrosive environments |
| Excellent up to 700–800 °C with good mechanical strength | Good properties at moderate temperatures (lower than 625) |
| More versatile |
Applications Examples
INCONEL 625
For heat exchangers in thermoelectric and combined cycle power plants exposed to seawater and chloride-rich condensate.
For aircraft gas turbine engine components, such as exhaust nozzles, combustion chambers, and turbines, where bushings, flanges, and fittings are subjected to extremely high temperatures and repeated thermal cycling.
Equipment in petrochemical, offshore environments and mining sites to combat corrosive phenomena.
Used for lining risers, subsea joints and valves, completion tubing, high-pressure fittings and flanges in sour service (H₂S) injection lines for offshore extraction, subject to continuous saltwater exposure and cyclic loading.
Heat exchangers in Combined Cycle Gas Turbine (CCGT) plants, i.e., Heat Recovery Steam Generators (HRSG), for high-temperature gas-side tubes and headers subjected to thermal cycling.
In thermal power plants, coal, biomass, or waste-to-energy plants where flue gases containing chlorides and sulfur compounds are present. It is used to cover boilers, for superheater tubes and for hot fumes exposed components. Gas and steam turbine components such as high resistance bolts, creep-resistant structural parts and joints exposed to oxidation.
In nuclear power plants, it is used in the secondary circuits, not directly in the core. It is utilized for heat exchangers, parts subjected to corrosive pressurized environments and other secondary circuit components
INCONEL 825
In the chemical industry, it is used to line absorption towers, tanks, and columns in acid gas treatment plants – such as those handling flue gases containing sulfur dioxide (SO2) or carbon dioxide (CO2) due to continuous contact with highly aggressive solutions. It is used to cover columns and tubs.
In desulfurization systems within refineries and oil treatment plants that produce highly corrosive acidic solutions, and in hydrotreating/hydrodesulfurization units for removing sulfur from fuels through acidic compounds. It is used for tanks and components such as tubes, flanges and pumps, exchange pipes, process columns and splitters.
Marine applications with highly corrosive or polluted water in ballast water or wastewater treatment systems containing combined corrosive agents (acids, chlorides, sulfides).
Amine Units that is removal of H₂S and CO₂ using amine solutions. It is used for heat exchangers and solvent recirculation pipes.
Flue Gas Desulfurization (FGD) plants and coal-fired power plants equipped with SO₂ removal system. Scrubbers in desulfurization plants are industrial devices used to remove SO₂ and other acid gases from combustion fumes through a washing liquid.
Geothermal plants operating with geothermal fluids containing chlorides, solphides and carbon dioxide. It is used for fluid transportation piping, in splitters components and heat exchangers.
Cooling systems operating with aggressive industrial water and heavily contaminated cooling water especially in coastal areas. It is used for pipes, tanks and pumps.
To sum up, it can be said that in case of seawater, aggressive chlorides or stress corrosion cracking conditions, but also in high temperature, alkaline and saline environments, Inconel 625 is the best choice. In these cases,it is used for heat exchangers, pipes or components subject to mechanical stress or exposed to saline solutions.
In contexts where corrosion is caused by strong acids such as sulphuric and phosphoric acid and by aggressive solutions with the presence of sulphate ions, Inconel 825 is more performant thanks to the presence of copper and its specific resistance to chemical elements. For these reasons it is used in chemical plants, in sour gas absorption towers and in corrosive solutions storage tanks.
Considering the premises, the choice between Inconel 625 and Inconel 825 depends on the kind of corrosive context and on operating conditions like temperature, pressure, mechanical stress and it is usually determined by a combination of chemical corrosion and mechanical stress.
Due to an increasing demand in use requests for these nickel alloys, Tecra is now equipped to provide a more detailed range of products to better satisfy its customers needs developing partnerships with reliable producers.
At Tecra, we guide you in selecting the perfect material for your requirements.