Alloy 625 Inconel® (UNS N06625) Nickel-Chromium Alloy
is used for its high strength, excellent fabricability (including joining), and outstanding corrosion resistance. Service temperatures range from cryogenic to 1800°F. The strength of Inconel® Alloy 625 is derived from the stiffening effect of molybdenum and columbium on its nickel-chromium matrix; thus precipitation-hardening treatments are not required. This combination of elements also is responsible for superior resistance to a wide range of corrosive environments of unusual severity as well as to high-temperature effects such as oxidation and carburization.
The outstanding and versatile corrosion resistance of Alloy 625 under a wide range of temperatures and pressures is a primary reason for its wide acceptance in the chemical processing field. Because of its ease of fabrication, it is made into a variety of components for plant equipment. Its high strength enables it to be used, for example, in thinner-walled vessels or tubing than possible with other materials, thus improving heat transfer and saving weight. Some applications requiring the combination of strength and corrosion resistance offered by Alloy 625 are bubble caps, tubing, reaction vessels, distillation columns, heat exchangers, transfer piping, and valves. In the nuclear field, Alloy 625 may be used for reactor-core and control-rod components in nuclear water reactors. The material can be selected because of its high strength, excellent uniform corrosion resistance, resistance to stress cracking, and excellent pitting resistance in 500°-600°F (260-316°C) water. Alloy 625 is also being considered in advanced reactor concepts because of its high allowable design strength at elevated temperatures, especially between 1200°-1400°F (649-760°C). The high alloy content of Alloy 625 enables it to withstand a wide variety of severe corrosive environments. In mild environments such as the atmosphere, fresh and seawater, neutral salts, and alkaline media there is almost no attack. In more severe corrosive environments the combination of nickel and chromium provides resistance to oxidizing chemicals, whereas the high nickel and molybdenum contents supply resistance to non-oxidizing environments. The high molybdenum content also makes this alloy very resistant to pitting and crevice corrosion and niobium acts to stabilize The alloy against sensitization during welding, thereby preventing subsequent intergranular cracking. Also, the high nickel content provides freedom from chloride ion stress corrosion cracking.
Alloy 625 Characteristics
This combination of characteristics makes Alloy 625 useful over a broad spectrum of corrosive conditions. For instance, it has been recommended as a material of construction for a storage tank to handle chemical wastes, including hydrochloric and nitric acids – chemicals that represent directly opposite types of corrosion problems. Materials that resist either one of these acids are normally severely attacked by the other. Alloy 625 has good resistance to oxidation and scaling at high temperatures.