Nickel based super alloys are widely used in the chemical
plants in the targeted applications. For instance, applications of Hastelloywire grade B2 is in handling hot reducing acids as it offers nominal
corrosion rates in this media. Commercial nickel grade Ni 200 is used to handle
hot caustic solutions. Other grades of Hastelloy family such as C276 containing
Ni, Cr and Mo are versatile and can be used in almost each condition, although
their function in hot reducing acids would be lower than Nickel-Molybdenum
alloys and in hot caustic it would have a higher corrosion rate as compare to
Nickel 200. Unlike austenitic stainless steels, nickel alloys prevent stress
corrosion cracking in hot chloride conditions. Although nickel alloys may
experience stress corrosion cracking in conditions of hot caustic and dilute
hydrofluoric acid conditions.
Caustic Conditions
Caustic conditions involve highly concentrated solutions of
sodium hydroxide or caustic soda, potassium hydroxide or caustic potash and
calcium hydroxide or caustic lime that may be seen in the industries of oil
refineries, pulp and paper. It is likely that the cracking sensitivity of
Nickel alloys is related to a dealloying phenomenon.
Cross section of 0.6mm thick sheet of C276 that was in use
for ten months in a heat exchanger between water and 50% NaOH and traces of
perchlorate at temperatures about 100oC. Cracking occurred in the dealloyed
layer subjected to the caustic solution. The supreme performing material in
caustic conditions is commercially pure Nickel. Large magnitudes of molybdenum in
nickel alloys are detrimental and chromium seems to be an advantageous element
in high concentrations. During dealloying chromium and molybdenum dissolve
leaving behind a porous pure nickel layer even also the alloy is plated on the surface
with pure nickel. In slow strain rate conditions, Hastelloy C276 was prone to
transgranular cracking in 50% NaOH at 147oC. Mill annealed and aged for 24 hour
at 677oC, Hastelloy C22 resist cracking when kept in 50% NaOH solution at 147oC
for 720 hours.
Inconel 600 experiences stress corrosion cracking in hot
caustic solutions. Lab SCC test was conducted by using cylindrical slow strain
rate samples and spring loaded bend beam samples of grades 600 and 800 in
deaerated 10% sodium hydroxide solution at 550oF. Stress corrosion racking was
noticed in both alloys, although alloy 600 offered better resistant to cracking
as compare to alloy 800, feasibly due to higher nickel concentration. It is
found that resistance to stress corrosion cracking increases with magnitude of
Nickel in tan alloy, however there was an extensive variation in results
depending on the hydroxide concentration and temperature.
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