755Alloy C22 1982 to Present

The expiration of the alloy C-276 patent in the United States in 1982 saw the introduction of a newer development in the C family, alloy C-22. This alloy claimed that the mu-phase control in alloy C-4, which was accomplished by controlling the "electron vacancy" number by omitting tungsten and reducing iron, was done at the expense of reduced corrosion resistance to oxidizing chloride solutions, where tungsten is a beneficial element. In addition, both alloys C-276 and C-4 suffered high corrosion rates in oxidizing, nonhalide solutions, due to their relatively low chromium levels of 16%. Hence, the claim was, that there existed a need for an alloy with higher chromium levels for oxidizing environments with an optimized balance of Cr, Mo, and W, thus yielding an alloy with superior corrosion properties and good thermal stability. This led to the alloy C-22 composition (Table 7.2) with approximately 21% Cr, 13% Mo, 3% W, and 3% Fe with balance nickel. Even though the corrosion resistance of this alloy was superior to alloys C-276 and C-4 in highly oxidizing environments, slightly better pitting corrosion resistance in "green death" solution, its behavior in highly reducing environments and in severe localized crevice corrosion conditions was still inferior to the 16% molybdenum containing alloy C-276. Details on the development of alloy C-22 have been described elsewhere [30-32]. Research efforts during the 1980s at Krupp VDM led to the most advanced alloy development within the Ni-Cr-Mo family, alloy 59 [33, 34], which overcame the shortcomings of both alloys C-22 and C-276. It also provided solutions to the most severe and critical corrosion problems of the CPI, petrochemical, pollution control, and other industries.

0 0

Post a comment