Nickel alloys have two main properties: good resistance to corrosion and high-temperature strength. There are alloys for medium- and low-temperature applications and for high-temperature conditions in which creep resistance is of main importance [24].
The standard quality of commercially pure nickel (nickel 99% minimum, carbon 0.15% maximum; nickel 200/201) can be readily welded and fabricated in all wrought forms and as castings. However, it is restricted to special applications for wliich nickel alloys are not adequate (for example, for equipment used in the production of caustic soda where it is not subject to stress corrosion cracking in hot caustic soda solutions)
Unalloyed nickel is used where it is necessary to eliminate iron and copper contamination (nickel 200 up to 300°C and nickel 201 above 300°C).
Nickel/Copper (Alloy 400)Alloy 400 has good mechanical properties and is easy to fabricate in all wrought forms and castings. K-500 is a modified version of this alloy and can be thermally treated and is suitable for items requiring strength, as well as corrosion resistance. Alloy 400 has immunity to stress corrosion cracking and pitting in chlorides and caustic alkali solutions.
Alloy 400 is also adequate for equipment processing of dry halogen gases and chlorinated hydrocarbons and can be used in reducing environments.
Nickel/Molybdenum
This alloy has a nominal composition of 65% nickel, 28% molybdenum and 6% iron. It is generally used in reducing conditions. It is intended to work in very severely corrosive situations after post-weld heat treatment to prevent intergranular corrosion. These alloys have outstanding resistance to all concentrations of hydrochloric acid up to boiling-point temperatures and in boiling sulfuric acid solutions up to 60% concentration.
Nickel/Molybdenum/Chromium
The composition of this alloy (54% nickel, 15% molybdenum, 15% chromium, 5% tungsten and 5% iron) is less susceptible to intergranular corrosion at welds. The presence of chromium in this alloy gives it better resistance to oxidizing conditions than the nickel/molybdenum alloy, particularly for durability in wet chlorine and concentrated hypochlorite solutions, and has many applications in chlorination processes. In cases in which hydrochloric and sulfuric acid solutions contain oxidizing agents such as ferric and cupric ions, it is better to use the nickel/molybdenum/ chromium alloy than the nickel/molybdenum alloy.
Nickel/ Chromium/ Molybdenum/Iron
Because the composition of this alloy (47% nickel, 22% chromium, 7% molybdenum and 17% iron) has a higher iron content it cannot withstand such aggressive corrosion conditions as nickel/molybdenum and nickel/
molybdenum/chromium alloys. It is, however, less expensive. The nickel makes these alloys immune to stress corrosion cracking and also superior to stainless steels with respect to pitting in chloride solutions. Because of these properties, their greater cost over stainless steel is justified.
Nickel/Chromium/Molybdenum/Copper
These alloys (50/60% nickel, 20/30% chromium, 5/8% molybdenum, and 5/7% copper) have very good resistance to hot sulfuric acid solutions and similar environments. They are only available as castings but can be hardened by heat treatment. The castings are suitable for parts requiring cutting edges and good wear resistance under corrosion conditions, but should not be used in contact with halogens, halogen acids, and halogen salt solutions.
Nickel/Silicon
Nickel/silicon alloy (10% silicon, 3% copper, and 87% nickel) is fabricated only as castings and is rather brittle, although it is superior to the iron/silicon alloy with respect to strength and resistance to thermal and mechanical shock. It is comparable to the iron/silicon alloy in corrosion resistance to boiling sulfuric acid solutions at concentrations above 60%. Therefore, it is chosen for this and other arduous duties where its resistance to thermal shock justifies its much higher price compared with iron/silicon alloys.