Incoloy 028 Alloy Steel Pipe

INCOLOY® alloy 028 (UNS N08028/W. Nr. 1.4563) is a highly alloyed austenitic stainless steel offering resistance to a variety of corrosive media. By virtue of its contents of chromium and molybdenum, the alloy offers resistance to both oxidizing and reducing acids and salts. The presence of copper increases its resistance to sulfuric acid. The alloy is used in the chemical and petrochemical processing industry. Alloy tubes are cold worked to high strength levels for downhole service in moderately corrosive deep sour gas wells.

Incoloy 028 is a nickel-iron-chromium alloy with molybdenum and copper added. Incoloy 028 has excellent resistance to reducing or oxidizing acids, stress corrosion cracking, and corrosion in the use environment, such as pitting corrosion and crevice corrosion. The alloy is particularly resistant to corrosion by sulfuric acid and phosphoric acid. Used in chemical processing, pollution control equipment, pipelines for oil and gas wells, nuclear fuel reprocessing, acid production and pickling equipment.

With the development of the petroleum industry, more and more sour oil and gas fields containing H2S/CO2 have been developed. The harsh environment of the oilfield puts forward higher requirements for the corrosion resistance of the materials used in oil and gas wells. Nickel-based alloys have excellent resistance to acid oil and gas corrosion and have been widely used in the production of oil and gas industries. As a high-chromium-iron-nickel-based alloy, Incoloy 028 alloy has good resistance to corrosion by oxidizing media, and also has good resistance to stress corrosion, pitting and crevice corrosion. It is an excellent material for oil well pipe selection.

Incoloy 028 alloy undergoes a long heating process during hot working (including forging and hot extrusion), and the heating temperature is between 700 and 1000°C. In this temperature range, Incoloy 028 alloy is easy to form precipitated phases. The types of different precipitated phases in the alloy and the shape, quantity and distribution of different precipitated phases will have various effects on the corrosion resistance of the alloy. Therefore, studying the formation of precipitated phases in alloys and their influence on corrosion behavior is of great significance for ensuring excellent corrosion resistance of alloys. The generation of precipitated phases in the alloy will adversely affect the corrosion resistance of the alloy, and the heat treatment process of the alloy will directly affect the structure of the alloy.

The thermodynamic calculation of Incoloy 028 alloy was carried out by JMatPro thermodynamic calculation software and different sensitization treatment systems were formulated, and then a series of sensitization treatment tests were carried out and tested with metallurgical microscope, X-ray diffractometer, scanning electron microscope, transmission electron microscope, etc. The method characterized the types, morphology, distribution, structure and composition of the precipitated phases in the rolled microstructure of Incoloy 028 alloy under different sensitization treatment temperatures and sensitization treatment times, and proposed the formation mechanism of the precipitated phases in Incoloy 028 alloy. At the same time, the influence of different sensitization treatment temperature and sensitization treatment time on the corrosion resistance of the alloy was studied by the constant potential method to measure the anodic polarization curve, the oxalic acid electrolytic corrosion test, and the sulfuric acid-iron sulfate corrosion test.

The results of metallographic analysis showed that Incoloy 028 alloy has obvious second phase precipitation during the sensitization treatment at 800℃~1000℃. When sensitized at 900℃ for a long time, a large amount of second phase precipitation is produced in the matrix structure. Transmission electron microscopy (TEM) analysis shows that in the initial stage of the sensitization treatment at a sensitization temperature of 900°C, needle-like σ phase and granular MC will be precipitated in the alloy matrix, and M23C6 will be distributed in series at the grain boundary. . X-ray diffraction analysis (XRD) results show that σ phase will be precipitated in the alloy after 900℃5h and then air-cooled sensitized treatment. Scanning electron microscopy (SEM) analysis shows that the precipitated phases after 2h sensitization treatment are mainly σ phase. Energy spectrum analysis (EDS) shows that the content of alloying elements Cr and Mo in these σ phases is higher than that of the matrix, while Fe and The content of Ni is lower than that of the matrix.

The precipitation of Incoloy 028 alloy after sensitization treatment leads to the reduction of the content of alloying elements such as Cr and Mo in the surrounding area, and it is easy to form poor Cr and Mo areas, which leads to preferential corrosion pits and trenches in these areas, resulting in local corrosion. The accumulation of Cl- ions in the pits leads to acidification of the medium environment in the pits, and increased local corrosion, which makes the pits expand. At the same time, the surface parts of the σ phase and γ phase grains rich in Cr and Mo are in a passive state, and the metal anode dissolution current density is very small, while the surface part of the Cr and Mo-poor grain boundary zone is actively dissolved at a large current density. The active corrosion state of the alloy causes the preferential dissolution of the grain boundary zone, and the corrosion continues to deepen along the grain boundary zone, resulting in the decline of the alloy's resistance to intergranular corrosion.

Incoloy is a nickel-chromium-iron alloy designed to resist oxidation and carbonization when the temperature rises. There are many types of Incoloy alloys: common ones such as Incoloy800, Incoloy800H, Incoloy800HT, Incoloy825, Incoloy840, Incoloy901, Incoloy925, Incoloy20, Incoloy330, Incoloy 25-6Mo and so on.

 Incoloy is suitable for various concentrations of sulfuric acid at low temperatures; in a caustic alkali (such as NaOH) solution with a concentration of 50% to 70%, it has good corrosion resistance and does not produce stress corrosion cracking. Inconel is suitable for various solutions and acids containing Cl-, and many media such as caustic with a concentration of <70%.

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