015 002

The two main alloys, commercially pure alloy 200 and alloy 201, have useful resistance at low to moderate temperatures to corrosion by dilute unaerated solution of the common nonoxidizing mineral acid such as HCl, H2SO4, or H3PO4. The reason for its good behavior is the fact that the standard reduction potential of nickel is more noble than that of iron and less noble than copper. Because of nickel's high overpotential for hydrogen evolution, there is no easy discharge of hydrogen from any of the common nonoxidizing acids and a supply of oxygen is necessary for rapid corrosion to occur. Hence in the presence of oxidizing species such as ferric or cupric ions, nitrates, peroxides, or oxygen, nickel can corrode rapidly. Nickel's outstanding corrosion resistance to alkalies has led to its successful use as caustic evaporator tubes. At boiling temperatures and concentration of up to 50% NaOH, the corrosion rate is less than 0.005 mm/year. The iso-corrosion diagram for nickel 200 and 201 in sodium hydroxide clearly shows its superiority and usefulness even at higher concentrations and temperatures [9]. However, when nickel is to be utilized at temperatures above 316°C (600°F) in these applications, the low-carbon version (alloy 201) is recommended to guard against the phenomenon of graphitization occurring at the grain boundaries, which leads to possible loss of ductility causing embrittlement.

Nickel is very resistant to chloride stress corrosion cracking resistance but may be susceptible to caustic cracking in aerated solution in severely stressed conditions. Use of Ni-Cr-Fe such as alloy 600 may be more resistant under such conditions. Nickel has a high resistance to corrosion by most natural freshwaters and rapidly flowing seawater. However, under stagnant or crevice conditions, severe pitting attack may occur. While nickel's corrosion resistance to oxidizing acids such as nitric acid, is poor, it is sufficiently resistant to most nonaer-ated organic acids and organic compounds. Nickel is not attacked by anhydrous ammonia or very dilute ammonium hydroxide solution (<2%). Higher concentrations cause rapid attack due to formation of a soluble (Ni-NH4) complex corrosion product.

Nickel's good resistance to halogenic environments at elevated temperatures such as in chlorination or fluorination reactions has been utilized in many modern-day chemical processes, largely due to the fact that the nickel-halide films formed on the nickel surface have relatively low vapor pressures and high melting points.

Nickel has been successful in production of high purity caustic in 50-75% concentration range, petrochemical industry, chemical process industry, handling of food and food industry, and production of synthetic fibers. Other useful applications are due to its magnetic and magnetostrictive properties, high thermal and electrical conductivities, and low vapor pressure.

7.4.2 Ni-Cu Alloys

Alloy/UNS #

Ni + Co

Cu

Fe

Mn

0 0

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