Alloys for Performance in Phosphoric acid
Pure phosphoric acid has no influential oxidizing power and is categorized as a non-oxidizing acid as dilute sulfuric acid. Industrial phosphoric acid, although often contains contaminants like fluorides and chlorides that remarkably improve its corrosivity. Oxidizing compounds like ferric salts may also be available to influence corrosion. Probably due to change in magnitudes of impurity of acids analyzed, the corrosion rates do not always match. The commonly used nickel based alloys in processes involving pure phosphoric acid are Incoloy alloy 825, 020 and 25-6MO and Inconel alloy G-3. For rigorous, hot phosphoric acid conditions, particularly those containing halides, Inconel alloy 625, 622 and C-276 are chosen.
Nickel alloy 200
Nickel alloy 200 has confined use in the conditions containing phosphoric acid. In the pure, unaerated acid, corrosion rates are small for all contents at natural temperatures. In hot or high content acid, the corrosion rates are very high for using commercial pure nickel.
Monel alloy 400
Monel alloy 400 offers outstanding resistance to pure phosphoric acid. The corrosion rates are lower than 10 mpy or 0.25mm/a for all contents at temperature limits lower than 176oF or 80oC. The elevated temperatures and aeration can apparently increase the corrosion rates as shown in the following figure:
The corrosion rates in crude phosphoric acid solutions are probably due to presence of high oxidizing salts even a slight 4% ferric ion may increase the corrosion rates of an alloy by any limit.
Incoloy alloy 825, 020 &25-6MO:
Incoloy alloy 825 offers outstanding resistance to pure phosphoric acid at all concentrations and temperature limits about 85% boiling acid. The lab and plant corrosion test outcomes are described in the following table:
| Content % | Test conditions | Temp, oF or oC | Test time, days | Corrosion rate mpy, mm/a |
| 45 | Partly immersed in liquid and vapor while boiling in open beaker. Concentration changed with evaporation and inclusion of water | 145 to 155 or 63 to 68 | 30 | 0.6 or 0.02 |
| 60 | Submerged in boiling solution in flask equipped with reflux condenser, average of three 20 hours tests | Boiling | 0.8 | 6.8 or 0.17 |
| 70 | Submerged in boiling solution in flask equipped with reflux condenser | Boiling | 1 | 8.6 or 0.22 |
| 70 | Submerged in boiling solution in flask equipped with reflux condenser. Average of 4 tests of 20 hours | Boiling | 0.8 | 7.1 or 0.18 |
| 75 | Immersed in solution of commercial acid. velocity for 800 to 1000 ft per min or 244 to 305 m/min | 172 or 78
185 or 85 194 or 90 203 or 95 221 or 105 |
30
30 30 30 30 |
0.2 or 0.005
0.3 or 0.008 0.3 or0.008 0.5 or 0.013 1.3 or 0.03 |
| 75 | Half immersed in liquid and vapor while boiling in open beaker. Concentration changed with evaporation and inclusion of water | 240 to 260
115 to 127 |
30 | 3.9 or 0.10 |
| 85 | Submerged in boiling solution in flask comprised of reflux condenser. Average of triple tests of 24 hours | Boiling | 1 | 36 or 0.91 |
| 85 | Submerged in boiling solution in flask equipped with reflux condenser. Average of four 24 hours tests | Boiling | 0.8 | 50 or 1.27 |
In the conditions of commercial grades of phosphoric acid, considerable pitting or crevice corrosion would limit the application of Incoloy alloy 825.
Inconel alloy 600
Inconel alloy 600 offers resistance to phosphoric acid of all concentrations at room temperature. Although, corrosion rates increase quickly with temperature therefore alloy 600 is not suitable for service in hot acid.
Inconel alloy 622, 625, 686, C276 & G3
Due to presence of high magnitudes of chromium and molybdenum and small inclusions of niobium or tungsten, Inconel alloys 622, 625, 686, C276 & G3 have outstanding resistance to phosphoric acid. In pure, concentrated, boiling acid, these alloys are not superior to Incoloy alloy 825.Although, they provide outstanding resistance in the presence of significant extents of chlorides and fluorides, that may cause pitting of Incoloy alloy 825. The wet process acid evaporators are instances of such a condition. The general resistance of Inconel alloys G-3 and C-276 is described in the following diagrams:
The corrosion resistance in commercially pure phosphoric acid in the decreasing order:
Inconel alloy 686> C276> Inconel alloy 625>Inconel alloy G-3>Incoloy alloy 825>Monel alloy 400