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Inconel alloys for application in offshore plant conditions for economical service

phosphoric acid plants

The strength level in age hardened materials is becoming widely important, specifically for offshore applications that are exploiting high pressure deep well reserves, where weight factors can significantly affect the economic feasibility of the project. The material selection for down hole and wellhead equipment like hangers, sub surface safety valves, pumps etc should be made of age –hardenable alloys to achieve required strength in heavier cross sections which cannot be reinforced by cold processing. Nickel alloys such as Inconel 718 for offshore applications, is used in oil country applications.

Age hardened alloys are used at different strength levels based on the application. Inconel 718 has 827 Mps as its minimum level of yield strength. The improved strength of Inconel alloys is possible by suitable heat and mechanical processing during production.

General pitting and crevice

Conventional corrosion resistant alloys are evaluated by their pitting resistance equivalent number to determine their pitting resistance. The critical pitting temperature for an alloy is calculated by using the samples to an acidified 6% ferric chloride solution. The temperature is then increased in significant magnitudes until the pitting occurs. The tests are performed up to 85oC because with increase in temperatures the solution becomes unstable.

Galvanic compatibility

Galvanic corrosion occurs when different materials are placed in a contact of conductive fluid. Inconel alloys are noble materials and hence they prevent galvanic corrosion. When Inconel 625 is used in seawater at ambient temperature for almost 92 days, it shows excellent galvanic compatibility.

General Corrosion in sour conditions

In mineral acids Inconel 625 shows good performance. Different chemicals used as inhibitors and dispersants for offering good general corrosion resistance as is demanded in sour wells. Weight loss tests in hydrogen sulfide conditions are performed. Inconel 625 offers good resistance to general corrosion in the test condition and significantly higher performance as compare to Inconel 718.

Environmental cracking

It is essential that wellhead and downhole components should resist stress corrosion cracking. The compatibility for scc becomes essential with higher temperature and concentration of hydrogen sulfide and the presence of chloride ions and elemental sulfur.

Major damage in services is also caused by lower temperature hydrogen embrittlement and sulfide stress cracking. These conditions may be speed up through galvanic corrosion combined with acidic conditions and dissolved hydrogen sulfide.

Sulfide stress cracking and hydrogen embrittlement

Generally the resistance to stress corrosion cracking and hydrogen embrittlement enhances with increase in concentration of nickel, chromium, molybdenum, tungsten and niobium in an alloy. Inconel 625 shows excellent performance even after 10 days of service in sour brine condition.

In order to receive suitable level of strength, ductility, toughness and cracking resistance, the maximum hardness levels are specified for each alloy. The significant effect of alloy nickel concentration on chloride based stress corrosion cracking resistance is well known. Inconel 625 and other grades comprising 42% or more nickel offer supreme performance to stress corrosion cracking in water based chlorides. If you are looking for more assistance in evaluating the performance of these high strength grades, Heanjia will help you decide the best suited alloy for your application.