Nickel alloys combat high-temperature corrosion.

Alloys with high-temperature corrosion resistance have been developed by the careful addition of elements that promote durability in specific environments.

During the last few decades, a better understanding of alloying effects, advances in melting technology, and the development of controlled thermo-mechanical processing have led to new and improved high-temperature alloys. Most such alloys have sufficient amounts of chromium (with or without additions of aluminum or silicon) to form chromium oxide, alumina, and/or silica protective oxide scales, which provide resistance to environmental degradation. However, oxides cannot protect against failure by creep, mechanical or thermal fatigue, thermal shock, or embrittlement. In the real world, failure is typically caused by a combination of two or more attack modes, which synergistically accelerate degradation.

To counter these attacks, two new nickel-base alloys have been developed, in which high-temperature corrosion resistance has been optimized by the careful addition of elements such as chromium, aluminum, silicon, and rare earths. They provide economical and reliable solutions to attack by oxygen, sulfur, halogens, carbon compounds, and nitrogen in a range of high-temperature applications. For example, Alloy 602CA is utilized in heat treating equipment, catalytic automotive parts, and chemical processing apparatus. Alloy 45TM has been successfully used in coal gasification equipment, incinerators, refineries, and process machinery involving severe sulfidizing conditions.

Composition of the alloys

Alloy 602CA combines the beneficial effects of high levels of chromium, aluminum, and carbon with microalloying amounts of titanium, zirconium, and yttrium in a nickel matrix. The relatively high carbon content of approximately 0.2%, in conjunction with 25% chromium, ensures the precipitation of bulky, homogeneously distributed carbides having typical diameters of 5 to 10 [[micro]meter]. Transmission and scanning electron microscopy suggest that these carbides are [M.sub.23][C.sub.6]-type primary precipitates.

Microalloying with titanium and zirconium further allows for finely distributed carbides and carbonitrides. Solution annealing, even at 1200 [degrees] C (2200 [degrees] F), does not dissolve all these stable carbides. Therefore, the alloy maintains relatively high creep strength because of the combination of solid-solution hardening and carbide strengthening. …