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Nickel Alloys.
Many nickel alloys have been developed specifically for their high
resistance to corrosion. The alloys in this group are among the most corrosion
resistant materials in many environments and some of the alloys are essentially
immune to attack. Nickel.
Nickel, as usually applied as a plating material, is resistant to corrosion
in mildly corrosive environments but is not resistant to corrosion in chloride
containing environments. Except in chemical processing, nickel is seldom used as
a structural material. Nickel-Copper Alloys.
The most common nickel-copper alloy is Monel 400, which is nominally 70%
nickel and 30% copper. Monel 400 is highly resistant to corrosion in many
environments but is not totally resistant to corrosion in seawater. It is
subject to pitting attack in seawater if exposed without coupling to other
metals. The incubation time for pitting in seawater is rather long and ranges
from 1 to 2 years. Monel 400 is subject to metal ion concentration attack and
the corrosion of this alloy is usually reduced within crevices. Monel is
cathodic to most other common alloys. Monel K-500 is an age hardenable
nickel-copper alloy and has higher strength than Monel 400 but has comparable
corrosion characteristics. Other Nickel Alloys.
Nickel alloys such as Inconel 625 and Hastelloy C-276 are essentially immune
to corrosion in many environments. Inconel alloy 718 has high strength in the
order of 150 ksi and has good resistance to corrosion in many environments and
usually only exhibits small amounts of crevice attack. A-286 is a nickel
chromium- titanium-molybdenum alloy that is widely used as a high strength
fastener alloy. While subject to crevice corrosion with initiation times and
propagation rates similar to Type 316 stainless steel, it has been successfully
used in many applications. Its practical corrosion resistance is improved due to
its cathodic potential with respect to many materials and the significant
improvement in its corrosion resistance when coupled to more anodic materials.
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