The most versatile alloys in Nickel based
alloy matrix is Nickel-Chromium-Molybdenum alloy. The presence of chromium and
nickel offers resistance to oxidizing and reducing acids and also act as
synergistically to offer great resistance to chloride induced cracking of
pitting, crevice and stress corrosion cracking. Other families are
nickel-chromium-iron and nickel-iron-chromium that are made to bridge the
performance and cost gaps between the Nickel-Chromium alloys and austenitic
stainless steels. Their advantages over stainless steels including better
resistance to stress corrosion cracking.
Nickel-Chromium-Molybdenum alloys are
specifically resistant to insidious and variable forms of corrosion resulted by
chlorides, pitting, crevice, and stress corrosion cracking. Chromium causes
passivation in oxidizing acids as it occurs in the stainless steels. Molybdenum
offers resistance to reducing acids specifically to hydrochloric acid. In heat
exchangers, not just steam is essential, in fact cooling medium is also
considered. Cooling waters are usually chlorinated to combat biofouling and sea
water is used as a coolant at various coastal locations.
As Ni-Cr-Mo alloys prevent hydrochloric
acids and related salts, they also prevent the related compounds of bromine and
fluorine. Actually Ni-Cr-Mo alloys are one of the metallic materials that
withstand hot hydrofluoric acid. They resist sulfuric and phosphoric acids and
specific contents of caustic soda and caustic potash.
Hastelloy C
Hastelloy C-276- It was made by introduction of argon-oxygen decarburization during
melting
Hastelloy C-2000- Copper was added to alloy C2000 for resistance to sulfuric and
hydrofluoric acids.
The effect of tungsten in Ni-Cr-Mo alloys
is similar to Molybdenum.
NiCrMo alloys are basically face centered
cubic structure. They are similar to austenitic stainless steels. They are not
fully stable and are hence sensitive towards the production of second phases
when subjected to temperatures in about range 650oC to 1100oC.
The NiCrMo alloys are usually supplied in
the solution annealed form, the annealing temperature for many alloys is about
1120oC. They are usually cooled in water from their annealing temperatures to
lock in their high temperature fcc microstructures. Although they are cooled in
gas if they are annealed in hydrogen.
At low temperature limits, a homogenous,
long range ordering reaction is feasible however this is slow and of no worry
during welding. Nickel alloy Hastelloy C22 wire was designed
particularly to take benefit of the long range ordering reaction in a time
length of 48 hours with just a moderate reduction in its corrosion
functionality. The development of cast Nickel-Chromium-Molybdenum alloys has
followed a similar path with initial efforts followed at enhancing heat
stability and where feasible improved corrosion resistance. The nickel-chromium-molybdenum
alloys are usually supplied in the solution annealed condition, the annealing
temperature for many alloys is around 1120oC. They are usually cooled in water
from their annealing temperature to hold their high temperature fcc microstructures.
Concerns over structural instability are extreme during welding, since weld heat affected regions are subjected to temperatures in the sensitizing limit. The second phases of many concerns are M6C carbides that develop in the limit 650 – 1040oC and micro-phase that occurs in the limit 760oC to 1090oC. These phases develop quickly and differently usually within the alloy grain limits, leaving them to be sensitive to preferential corrosion.