Alloys and Alloying Elements

Many of the superalloys are nickel-based or have a high nickel content, and have a good resistance to corrosion. Several of the key alloying elements and their impact on the corrosion resistance of nickel are reviewed in the following sections. In addition, some nickel-based alloys are considered as well.

Chromium

The addition of chromium enhances the resistance of nickel to high temperature corrosion. Chromium additions improve the resistance to oxidation at high temperatures and the resistance to oxidizing acids such as nitric and chromic acids. Furthermore, chromium improves the resistance of nickel to carburization and sulfidation at higher temperatures, but negatively impacts the resistance to corrosion in high temperature environments containing nitrogen or fluorine. Chromium forms a passive film on the nickel alloy in these types of environments. It also provides resistance to corrosion in liquid environments at lower temperatures, and to SCC, pitting and crevice corrosion. The maximum corrosion resistance is achieved with a chromium content of approximately 20%, and corrosion resistant superalloys usually contain 15-30% Cr.

Nickel-Chromium-Iron Alloys

Inconel 600 is a Ni-Cr-Fe alloy that is very resistant to corrosion in organic acids, caustic soda, and alkalis, but is only moderately resistant to corrosion in mineral acids. It is also resistant to atmospheric corrosion, high temperature corrosion, SCC, oxidation, carburization and nitridation. Hastelloy G-30 has excellent resistance to corrosion in nitric acid, and also resistant to sulfuric acid, phosphoric acid, fluorides, and oxidizing acids in general. Inconel 690 exhibits excellent resistance to oxidizing agents, sulfuric acid, and nitric acid. It is also very resistant to high temperature corrosion.

Nickel-Chromium-Molybdenum Alloys

Inconel 617 is a nickel-chromium-molybdenum alloy that exhibits excellent resistance to oxidation. Inconel 625 is resistant to pitting, crevice corrosion and oxidation at high temperatures, as well as to highly corrosive environments. It also shows resistance to corrosion in halides, as well as to carburization, which can cause corrosive degradation of the material. Hastelloys C-276 and C-4 offer resistance to localized corrosion as well as SCC. Inconel 625 and Hastelloy C-276 can be resistant to hydrochloric acid even in the presence of oxidizing agents. Hastelloy C-22 provides superior resistance to oxidation, as well as excellent resistance to SCC and localized corrosion. Hastelloy C-2000 offers very good resistance to uniform corrosion in a wide range of environments, as well as very good resistance to SCC and localized corrosion.

Nickel-Chromium-Iron-Molybdenum Alloys

Incoloy 825 is a nickel-chromium-iron-molybdenum alloy that exhibits excellent resistance to sulfuric acid and phosphoric acid, moderate resistance to hydrochloric acid, and less resistance to corrosion in alkalis and halogens. Incoloy 825 is resistant to SCC, pitting and intergranular corrosion. Hastelloy G and Hastelloy G-3 are suitable for service in sulfuric acid and phosphoric acid. Hastelloy G-30 is resistant to corrosion in phosphoric acid, sulfuric acid, nitric acid, fluorides, and oxidizing acids. Hastelloy D-205 exhibits excellent corrosion resistance in sulfuric acid at high temperatures and to oxidizing agents. Most of the alloys in this group are very resistant to atmospheric corrosion.

Copper

Nickel-copper alloys have excellent resistance to corrosion in seawater, some acids, alkalis, and halides. Additions of copper typically improve nickel’s resistance to nonaerated, nonoxidizing acids. For example, additions of 30-40% Cu typically result in nickel having a good resistance to sulfuric acid and an excellent resistance to hydrofluoric acid. Copper is the main alloying element in Monel superalloys, which contain approximately 70% Ni and 30% Cu and have a good resistance to hydrofluoric acid. Copper can be added to Ni-Cr-Mo-Fe alloys to improve their resistance to hydrochloric, sulfuric and phosphoric acids.

Nickel-Copper Alloys

Nickel-copper alloys possess corrosion resistance similar to that of pure nickel, that is, they are resistant to corrosion in a broad range of environments. They are also similar to nickel in that they are susceptible to corrosion in oxidizing environments. Nickel-copper alloys have a good resistance to corrosion in sulfuric acid, seawater, and halogens.

Monel 400 is a nickel copper alloy with additional alloying elements, and is very resistant to seawater, sulfuric acid, alkalis, and halogen acids, including hydrofluoric acid as long as oxygen is not present in significant quantities. The resistance of Monel 400 to corrosion in low concentrations of nonoxidizing hydrochloric acid is very good even at higher temperatures (up to 200°C). It is much more susceptible to corrosion in hydrochloric acid containing oxidizing agents. Monel 400 is also very resistant to atmospheric corrosion and to corrosion in flowing seawater. Monel 400 exhibits very good resistance to erosion corrosion in seawater, but is susceptible to pitting and crevice corrosion in stagnant or low-flow velocity seawater. Monel K- 500 has corrosion characteristics similar to Monel 400.

Aluminum

Aluminum additions help to provide resistance to oxidation, sulfidation (which can cause corrosive degradation) and carburization at high temperatures, but may also make nickel more susceptible to high temperature corrosion in nitriding environments. With greater than 4% Al content, an oxidation inhibiting aluminum oxide film is capable of forming on the surface of the nickel alloy; however, it only occurs at high temperatures (>870°C). Once the film is formed it will protect against lower temperature oxidation too, but if it is abraded or removed, the alloy will no longer have the same oxidation resistance. Aluminum may result in a degradation of the hot corrosion resistance in superalloys, but it is also dependent on Cr content and the temperature of the environment.

Titanium

Additions of titanium to nickel are not typically used in nickel alloys intended for use in lower temperature applications. Titanium may provide some improvement in nickel’s resistance to hot corrosion, but it may also degrade the resistance to SCC, if carbon, oxygen or nitrogen is present. Titanium additions are used in superalloys with aluminum to improve the strength, and a high titanium to aluminum content ratio results in improved hot corrosion resistance.

Molybdenum

Additions of molybdenum improve the resistance of nickel to crevice corrosion, pitting corrosion in seawater, and to corrosion in nonoxidizing acids. Up to 28% Mo is used for nonoxidizing environments of hydrochloric, phosphoric, hydrofluoric and sulfuric acids. However, molybdenum also degrades nickel’s resistance to hot corrosion and to nitric acid, and decreases the resistance of nickel to oxidation at high temperatures. Furthermore, nickel-molybdenum alloys are susceptible to corrosion in oxidizing acid environments.

Nickel-Molybdenum Alloys

Nickel-molybdenum alloys exhibit excellent corrosion resistance in hydrochloric, sulfuric, and phosphoric acid environments. They are however, more susceptible to corrosion in oxidizing environments, and are especially susceptible to corrosion in nitric acid environments.

Hastelloy B and Hastelloy B-2 exhibit good resistance to corrosion in hydrofluoric acid environments at low temperatures. Welded components made from Hastelloy B may be susceptible to intergranular corrosion. Hastelloy B-2 is a nickel-molybdenum alloy that has excellent resistance to aluminum-chloride environments, while Hastelloy B-3 exhibits good resistance to SCC. Both of these alloys exhibit superior resistance to corrosion in hydrochloric acid compared to all of the nickel-based alloys, but they are more susceptible to this environment if oxidizing agents are present.

Tungsten

Tungsten additions improve the resistance of nickel to localized corrosion and corrosion in the presence of nonoxidizing acids. Tungsten can quickly increase the density of a nickel alloy, however, because of its relatively high atomic weight. When used with 13-16% Mo in amounts of 3-4%, tungsten provides extra local corrosion resistance. However, alloying tungsten with nickel superalloys can result in a poorer resistance to hot corrosion. Tungsten may negatively affect the resistance of nickel to oxidation at high temperatures.

Silicon

Nickel alloys containing silicon often have it in small amounts as a contaminant from a processing step during fabrication. Silicon additions however, are sometimes intentionally used (typically 9-11%) to provide hot corrosion resistance in concentrated sulfuric acid environments. Moreover, silicon additions improve the resistance of nickel to high temperature corrosion; specifically oxidation, nitridation, sulfidation and carburization.

Iron

Additions of iron can be used to reduce the cost of nickel, since nickel is more expensive than iron, but it does not offer much in terms of corrosion resistance. Iron additions, though, can improve nickel’s resistance to sulfuric acid, and may also improve the resistance to carburization at high temperatures.

Cobalt

Cobalt is not typically used in significant amounts since it has corrosion resistance characteristics very similar to nickel and is more expensive. It does however, provide improvement to high temperature carburization and sulfidation resistance in nickel alloys.

Other Alloying Elements

Yttrium, lanthanum, and other elements may also improve the corrosion resistance of nickel and its alloys. Yttrium generally improves the resistance to high temperature oxidation, sulfidation and carburization. Tantalum and niobium can improve the corrosion resistance at higher temperatures and the resistance of nickel alloys to intergranular corrosion. Niobium may
increase the resistance of nickel to carburization at high temperatures, but may also decrease the resistance of nickel to nitridation at high temperatures. Carbon can improve the resistance of nickel to nitridation and carburization at high temperatures, but decreases the resistance to high temperature oxidation. Manganese typically reduces the resistance of nickel to high temperature oxidation and nitridation.