For offering outstanding corrosion resistance and good mechanical
characteristics, nickel based alloys are used in diverse applications in broad
range of industries such as chemical, petrochemical processing, pollution
control, oil and gas extraction, marine engineering, power production and pulp
and paper manufacture. The versatility and reliability of these alloys make
them special materials for construction of process vessels, pipes, pumps,
valves and various other applications.
Pure phosphoric acid doesn’t possess any oxidizing power and
is categorized as a non-oxidizing acid similar to dilute sulfuric. Commercial
phosphoric acid although often consists of impurities of fluorides and
chlorides that significantly make it corrosive. Oxidizing materials like ferric
salts, may also be available to influence corrosion. Feasibly due to changes
impurity concentrations of acids evaluated, studied corrosion rates are not
always authenticated. Commonly used nickel alloys in processes of pure
phosphoric acid are Incoloy 825 and Hastelloy G3. In extreme hot phosphoric acid conditions,
particularly those contaminated with halides, Inconel 625 is chosen.
Nickel 200 has limited significance in phosphoric
acid. In pure, unaerated acid, corrosion rates are nominal for all acid
concentrations at atmospheric temperatures. In hot or concentrated acid,
corrosion rates are normally very high for using pure nickel.
Monel 400 offers significant resistance to pure
phosphoric acid. Corrosion rates are below 10 mpy for all concentrations at
temperatures up to 176oF or 80oC. Higher temperatures and aeration can
considerably increase corrosion rates. Corrosion rates in crude phosphoric acid
are very high due to presence of oxidizing salts. Even a nominal 0.4% ferric
ion may accelerate the corrosion of an alloy.
Incoloy 825 offers great resistance to pure
phosphoric acid at all concentrations and temperatures up to boiling 85% acid.
Lab and plant corrosion test results are evaluated. In commercial grade
phosphoric acid conditions, considerable pitting or crevice corrosion would
limit the use of Incoloy alloy 825.
Inconel alloy 600 is resistant to phosphoric acid
concentrations at room temperature. Although corrosion rates increase steeply
with temperature therefore it is not recommended for use in warm acid.
Due to presence of high chromium and molybdenum
concentrations and nominal magnitudes of niobium or tungsten, Alloy Inconel 625 wire
and Hastelloy C276 offer outstanding resistance to phosphoric acid. In pure,
concentrated, boiling acid, these alloys do not serve better than Incoloy 825. Although
they show excellent resistance in the presence of significantly magnitudes of
chlorides and fluorides that may cause pitting of Incoloy 825. Wet process acid
evaporators are examples of these environments. In a solution of 25% phosphoric
and 2% hydrofluoric acid at boiling point, Inconel 625 corroded at 2mpy in 48
hour test. Longer tests in practical operation have shown nominal corrosion
rates and no localized corrosion in evaporating wet process phosphoric acid.
The comparison of nickel based super alloys for corrosion resistance in
commercially pure phosphoric acid in decreasing order is:
Hastelloy C276
Inconel 625
Incoloy 825
Monel 400
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