The C family of alloys, the original being Hastelloy alloy C (1930s) was an innovative optimization of Ni-Cr alloys having good resistance to oxidizing corrosive media and Ni-Mo alloys with superior resistance to reducing corrosive media. This combination resulted in the most versatile corrosion-resistant alloy in the "Ni-Cr-Mo" alloy family with exceptional corrosion-resistance in a wide variety of severe corrosive environments typically encountered in Chemical Process Industry (CPI) and other industries. The alloy also exhibited excellent resistance to pitting and crevice corrosion attack in low pH, high chloride oxidizing environments and had total immunity to chloride stress corrosion cracking. These properties allowed this alloy to serve the industrial needs for many years although it had some limitations. The decade of the 1960s (alloy C-276), 1970s (alloy C-4), 1980s (alloy C-22 and 622), and 1990s (alloy 59, alloy 686, alloy C-2000, and MAT 21) saw newer alloy developments with improvements in corrosion resistance, which not only overcame the limitations of alloy C, but further expanded the horizons of applications as the needs of CPI became more critical, severe, and demanding.

Today the original alloy C of the 1930s is practically obsolete except for some usage in the form of castings. The chronology of the various corrosion-resistant Ni-Cr-Mo alloy developments during the twentieth century with special emphasis on the last 40 years of evolution in the C family of Ni-Cr-Mo alloys and their applications is presented below.

Prior to the 1950s the alloy choices available to material engineers for combatting corrosion were very limited. The latter half of the last century saw a phenomenal growth in the development of new alloys including the high-performance C family alloys. Table 7.1 gives a brief listing of some alloys developed during the pre-1950s period and the last five decades. As is evident from this listing of austenitic alloys, today's corrosion/material engineers have a much wider selection of alloys to meet their specific needs. The next few sections describe in detail the historical development of the C family of alloys, their corrosion resistance characteristics, both uniform corrosion and localized corrosion resistance, their thermal stability behavior, and the many industrial applications, where only the alloys of this family have provided reliable, safe, and cost-effective performance.

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