Nickel Corrosion
While nickel makes up 0.007 per cent of the earth's crust and is today one of our most useful metals, it was virtually unknown until the 1600s and was not isolated as an element until 1751. Early chemists called it Kupfer-Nickel (kupfer for copper), because nickel was originally found in association with copper ores, and nickel, after the German "Old Nick" or Satan, because it was initially so difficult to extract from those ores.

Nickel is malleable, resists corrosion, and maintains its mechanical and physical characteristics under extreme temperatures. Once it was isolated and these valuable properties were recognized, nickel assumed a growing role in commerce and industry. By the early 1800s, nickel was being used to make a nickel-copper-zinc alloy, which could be silver plated. The technique of nickel plating was developed in about 1870. Moreover, the first pure nickel coin was struck in 1881. Each of these uses continues today.

Corrosion of Nickel Alloys

A distinction is often made between those alloys that are primarily used for high-temperature strength, commonly referred to as superalloys, and those that are primarily used for corrosion resistance. High performance alloys are defined, in the present context, as Ni, Ni-Fe and Co base alloys able to operate at higher temperatures than 550oC and high pressures. Fe, Co and Ni are transition metals with consecutive positions in the Periodic Table. The relative order of abundance decreases as Fe, Ni, Co.

The Fe-Ni-base high performance alloys are an extension of stainless steel technology and generally are wrought, whereas Co-base and Ni-base high performance alloys may be wrought or cast depending upon the application or composition involved. Appropriate compositions of all high performance alloy base metals can be forged, rolled to sheet or otherwise formed into a variety of shapes. The more highly alloyed compositions normally are processed as castings. Typical high performance alloys have modulus of elasticity around and above of 200 GPa.

The principal microstructural variables of high performance alloys are:

• The precipitate amount and its morphology
• Grain size and shape
• Carbide distribution

Monel

Monel (also called Monell) is a patented Nickel/Copper alloy (66/29) plus significant additions of cobalt, manganese, iron, silicon, carbon and aluminum. It is named after the Canadian metallurgist Ambrose Monell who developed it. (Mr. Monell died in 1921.) Its resistance to air oxidation and reaction with carbon dioxide makes it reasonably worry free. When corrosion does occur it is at the boundary where the crystals of nickel and copper join. This would suggest that some batches of Monel might have better resistant to corrosion than others. Monel is also a very hard and wear resistant alloy.