75 Historical Development Chronology Of C Alloys 751 Alloy C 1930s to 1965

The element nickel has some unique electrochemical properties of its own, thus making unalloyed nickel a suitable choice in certain applications, but more important is its metallurgical compatibility with a number of other important alloying elements such as chromium, molybdenum, tungsten, copper, and iron. This compatibility and optimization between Ni-Cr and Ni-Mo alloys led to the first alloy of the C family, Hastelloy alloy C in the 1930s. The development of this alloy has been well described by McCurdy in 1939 [23].

This alloy was the most versatile corrosion-resistant alloy available in the 1930s through mid-1960s to handle the needs of the chemical process industry. However, the alloy had a few severe drawbacks. When used in the as-welded condition, alloy C was often susceptible to serious intergranular corrosion attack in HAZ in many oxidizing, low-pH, halide-containing environments. This meant that for many applications, vessels fabricated from alloy C had to be solution heat treated to remove the detrimental weld HAZ precipitates. This put a serious limitation on the alloy's usefulness. The CPI during the late 1940s and 1950s was constantly coming up with new processes that needed an alloy without these limitations of "solution heat-treating" after welding. Also in severe oxidizing media, this alloy did not have enough chromium to maintain useful passive behavior, thus exhibiting high uniform corrosion rates.

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