Nickel alloys have been utilized for decades in sulfuric acid service in the chemical processing industry. Laboratory and filed data are assimilated for nickel alloys that have historically been used in sulfuric acid applications. Performance data for recently developed nickel alloys are analyzed. The alloys are ranked for applicability by acid concentration and temperature.
Sulfuric acid is one of the world?s most important industrial chemicals as it is used in a variety of industrial processes. World production exceeds 100 million metric tons per year. Diluted acid is very corrosive while concentrated acid at ambient temperature is customarily stored and processed in carbon steel equipment. The corrosivity of the acid varies with concentration, temperature, its velocity relative to exposed surfaces, and with the nature of possible contaminants.
Dilute acid is very reducing in nature. At greater concentrations, it begins to take on an oxidizing character. Concentrated acid (above 87% by weight) is oxidizing in nature. Thus, dilute and intermediate strengths of the acid can be contained by materials resistant to reducing conditions while stronger concentrations require materials resistant to oxidizing media.
Selection of a metal or alloy for a particular process depends primarily on the reducing or oxidizing nature of the solution as determined by acid concentration, and the corrosive effects of aeration, temperature and the nature of impurities. Selection depends on factors such as velocity, film formation, continuity of exposure, allowable metallic content of the solution, and physical properties of the alloy.
The most commonly used nickel alloys in processes containing dilute sulfuric acid are alloys 25-6MO, 27-7MO and other 6% molybdenum super-austenitic stainless steels, 825, 020, and G-3. For aggressive, hot, sulfuric acid environments, alloys 625, 622, C-276, and 686 are required. For concentrated acid higher chromium alloys such as 690 and 693 are used.
DISCUSSION
Nickel and Its Alloys
Nickel retains a face-centered-cubic (FCC) austenitic crystal structure up to its melting point, providing freedom from ductile-to-brittle transitions and minimizing the fabrication problems that can be encountered with ferritic metals. In the electrochemical series, nickel is more noble than iron but more active than copper. Thus, in reducing environments, nickel is more corrosion resistant than iron, but not as resistant as copper. Alloying with chromium provides resistance to oxidation thus providing a broad spectrum of alloys for optimum corrosion resistance in both reducing and oxidizing environments. Nickel-based alloys have a higher tolerance for alloying elements in solid solution than stainless steels and other iron-based alloys while maintaining good metallurgical stability. These factors have prompted development of nickel-based alloys to provide resistance to a wide variety of corrosive environments.
As seen in Figure 1, many alloying elements can be combined with nickel in single-phase, solid solution over a broad composition range to provide alloys with useful corrosion resistance in a wide variety of environments. For example, molybdenum and tungsten improve resistance to reducing acids. Copper also improves resistance to reducing acids, particularly non-aerated sulfuric acid. Chromium improves resistance to oxidizing corrodents. Alloys containing these elements provide useful engineering properties in the annealed condition without deleterious metallurgical changes resulting from fabrication or thermal processing.
Table 1 exhibits the chemical composition of alloys evaluated in this study.
N
