In plant designing there is not the absolute best material. The choice is always influenced by the context where the product will be used, by the fluids and gases flowing inside the plant, by pressures and temperatures.
In fact, each application requires a careful technical evaluation. Choosing the most suitable alloy means optimizing the plant performance, the safety and the operational costs; for this reason, it is crucial to carefully evaluate the application context before choosing the most suitable material.
Therefore, the choice of the material plays a crucial role especially in particularly severe operational conditions, or when maximum corrosion resistance and mechanical stability are required. In these cases, traditional steels may not be safe enough; on the contrary the use of materials with higher performant characteristics makes the plant reliable over time, ensures safety and reduces maintenance costs.
What is inconel
Among the most appreciated alloys in the most high-technology sectors, there is Inconel, a material designed to ensure excellent performance even in the most demanding environments.
It belongs to the high-performance alloys, made of nickel, and it is widely used in highly corrosive contexts such as marine environments, power plants, refineries etc.
In this category, the benchmark is without doubts Inconel 625 (UNS N06625), similar to Incoloy 825 (UNS N08825) from which it is different in terms of chemical composition and characteristics that make the second alloy more useful for even more extreme contexts.
When talking about Inconel, therefore, the reference is to a family of alloys and not to a single material: alloys sharing the same nickel base but which, depending on the added elements, react differently to the various corrosion mechanisms.
Chemical composition and properties
Inconel 625 is made of nickel, chromium, molybdenum and niobium which give a higher corrosion and mechanical stress resistance.
Each element makes its own specific contribution. Thanks to the high nickel content, the alloy offers an exceptional resistance to the formation of cracks under tension during corrosion processes, a phenomenon known as stress corrosion cracking. Hardness, on the contrary, is given by the presence of molybdenum with the addition of niobium, without the need for thermal hardening processes.
Corrosion and oxidation resistance
Inconel 625 is an alloy highly resistant to both oxidation and corrosion, including pitting and crevice corrosion. It offers, in fact, a good resistance to nitric, phosphoric, sulphuric and hydrochloric acid. For this reason it is used in highly corrosive environments such as seawater or in contact with acid substances.
Behaviour at operating temperatures
It withstands high temperatures up to about 1000 °C and it maintains an excellent mechanical strength up to 700-800 °C. It also performs well at cryogenic temperatures, far below zero.
Machinability and weldability
It is an alloy that lends itself well to being machined and welded, an aspect that is not secondary during installation and maintenance.
Inconel 625 and Incoloy 825: where the choice changes
The two alloys are often evaluated as alternatives, but they meet different needs.
| Inconel 625 | Incoloy 825 | |
|---|---|---|
| Composition | Nickel, chromium, molybdenum and niobium | Nickel, chromium, molybdenum, iron, copper and titanium |
| Main strength | Resistance to corrosion from chlorides, seawater and aggressive environments | Resistance to corrosion from strong acids such as sulphuric and phosphoric |
| Intergranular corrosion | Excellent resistance | Good resistance to chlorides and oxidising environments |
| Temperatures | Excellent up to 700-800 °C with good mechanical strength | Good properties up to moderate temperatures, slightly above 500 °C |
| Mechanical properties | Does not require hardening treatments | Can be strengthened through cold working |
| Typical use | Marine and chemical applications | Strongly acidic corrosive environments |
In presence of seawater, aggressive chlorides, salty or alkaline environments and high temperatures, Inconel 625 is generally the best choice and also the most versatile one.
Where corrosion is instead caused by strong acids and by aggressive solutions containing sulphate ions, Incoloy 825 can offer better performance thanks to the presence of copper.
Fields of application
Inconel is used in the chemical, petrochemical, Oil&Gas, aerospace industry, in marine environments, in biomass power plants, in shipyards, thermoelectric and combine cycle power plants, in offshore plants, in offshore extraction sites and, in general, in salty environments, in presence of seawater, aggressive chlorides and in conditions enhancing corrosion.
In these contexts, it is used, for example, for pipe and fittings coatings, to realize heat exchangers and for aero-engine and steam turbine components.
Some more concrete examples:
- Oil&Gas and offshore: injection lines and tubing in sour wells, therefore in presence of hydrogen sulphide, internal lining of risers, subsea joints and valves, high pressure fittings and flanges;
- Thermoelectric and combined cycle power plants: heat exchanger tubes exposed to seawater and to chloride-rich condensates, gas side tubes and headers subject to thermal cycles;
- Biomass and waste-to-energy plants: boiler lining, superheater tubes and components exposed to hot flue gases containing chlorides and sulphur compounds;
- Aerospace and turbines: combustion chambers and exhaust nozzles of aero-engines, high strength bolting and joining elements exposed to oxidation and to repeated thermal cycles;
- Chemical industry and shipbuilding: piping, fittings and heat exchangers in contact with acid substances or with seawater.
When Inconel is the right choice
Not all applications require a nickel superalloy: in less severe operating conditions traditional steels remain an adequate and economically more balanced solution. Inconel becomes the right choice when one or more of these conditions occur:
- Presence of seawater, aggressive chlorides or salty environments;
- High temperatures with repeated thermal cycles or, on the opposite, cryogenic temperatures;
- Risk of stress corrosion cracking, pitting or crevice corrosion;
- Combination of chemical attack and mechanical stress on the same component;
- Components that are difficult to inspect, for which durability over time is a design requirement.
An investment to be assessed over the life cycle
Unlike traditional steels, the initial investment will be obviously higher, but this cost will be paid off in the long term since the choice will reduce the maintenance and downtime costs increasing the durability of the plant itself.
This is an evaluation to be made over the life cycle and not on the purchase price alone. On a component exposed to seawater or to flue gases containing chlorides, the real cost includes maintenance, replacements, lost production during downtime and interventions on parts of the plant that are difficult to reach, items on which the difference in material cost has a marginal impact.
Why choosing Tecra
The increase in the demand for Inconel 625 solutions over the last years lets Tecra expand its product range specializing and promoting also this kind of material to completely satisfy its customers’ requirements developing synergies with important producers.
A qualified technical support helps identify the solution that offers the best balance between performance, durability and costs. Our team is ready to satisfy our customers’ needs.
Contact us to receive a personalized technical proposal and discover the most suitable solution for your industrial project.
FAQ
What is the difference between Inconel 625 and Incoloy 825?
They differ in chemical composition and, as a consequence, in the type of environment where they perform at their best. Inconel 625 is preferable in presence of seawater, chlorides and high temperatures; Incoloy 825, thanks to the presence of copper, performs better where corrosion is caused by strong acids such as sulphuric and phosphoric acid.
What temperatures does Inconel 625 withstand?
It withstands high temperatures up to about 1000 °C, with an excellent mechanical strength between 700 and 800 °C. It also maintains good characteristics at cryogenic temperatures.
Can Inconel 625 be welded?
Yes, it lends itself well to both machining and welding and it does not require thermal hardening treatments to obtain high mechanical properties.
Why does Inconel cost more than a traditional steel?
The higher cost is linked to the composition of the alloy and to the performance deriving from it. The evaluation has to be made over the life cycle of the plant, considering the reduction of maintenance, downtime and replacements.