752 Alloy 625 1960s to Present

One of the "severe oxidizing media" limitation of alloy C was overcome by increasing the chromium content from 16 to 22% in alloy 625, an alloy developed in the late 1950s and commercialized in the 1960s. However, the molybdenum content was reduced to 9% and columbium was added for stabilization against intergranular attack, which permitted the use of this alloy in the as-welded condition without the need for a solution anneal as was the case with alloy C. The increased chromium improved corrosion resistance in a number of strongly oxidizing corrosive media, while maintaining adequate resistance to many reducing corrosive media. This alloy had a good balance of corrosion-resistant properties but was not as versatile in "reducing acid media" as alloy C, due to the lower molybdenum level in alloy 625. Also its localized corrosion resistance was significantly lower than alloy C.

This alloy is resistant to corrosion and pitting in seawater and has useful resistance to wet chlorine, hypochlorites, and oxidizing chlorides at ambient temperatures. It is resistant to various concentrations of hydrofluoric acid, even in the aerated condition, and to such acid mixtures as nitric-hydrofluoric, sulfuric-hydrofluoric, and phosphoric-hydrofluoric acids under most conditions encountered in industrial practice. Alloy 625 has good strength and resistance to scaling in air up to 980°C and many uses exist in high-temperature applications.

An alloy with lower nickel content, similar chromium and molybdenum content, alloy X (47Ni, 22Cr, 18.5Fe, 9Mo, 0,10C, 0.6 W) was developed having good high-temperature strength and resistance to oxidation and scaling up to ~1100°C. This alloy is mostly used for high-temperature applications in the heat treatment industry and for flying and land-based gas turbine components.

Another alloy, alloy N (69.5Ni, 7Cr, 16.5Mo, 5Fe) was developed for use with molten fluoride salts at 815°C in nuclear applications. Chromium was reduced to 7% to prevent intergranular attack and mass transfer of chromium in this environment.

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