Handling & Storage of Nitric Acid (HNO3)
The second largest commodity mineral acid, nitric acid (HNO3) is widely used in the manufacture of fertilisers and explosives. It is also a major constituent for pickling solutions used in stainless steels and nickel alloys production.
As a strong oxidising agent, nitric acid can be very corrosive depending on its concentration and temperature. Therefore, materials selected for the manufacture of its storage and handling equipment have to be readily available, easy to fabricate and most importantly corrosion resistant to HNO3 at a specific range of concentrations and temperatures.
The following table shows suggested alloys for storage and handling equipment of HNO3 in various conditions.
| Alloy | Concentration | Temperature | Note |
|---|---|---|---|
| 304/304L | ≤ 60% | Acid Boiling Point | “L” grades refer to stabilised grades. They are used to prevent intergranular attack from HNO3 at weld joints (chromium depleted area due to carbide formation). |
| 17-4PH | ≤ 25% | Acid Boiling Point | Low corrosion rate |
| 2205 | ≤ 65% | Acid Boiling Point | Similar performance to 304/304L |
| 316 | 10% | 90°C | Can be used with 60%HNO3 + 2%HCl at 50°C - 122°C |
| 316L | 65.3% | Acid Boiling Point | 304/304L performs better due to an absence of Molybdenum |
| C276 | 10% | 90°C | Can be used with 60%HNO3 + 2%HCl at 50°C - 122°C (performs better than 316 under the same conditions) |
| 625 | 10% | Acid Boiling Point | High corrosion rate for mixed acid i.e. 3%HF |
| 690 | ≥ 70% | ≤ 80°C | Excellent for use with 10%-15%HNO3 + 3%HF and 20%HNO3 + 2%HF at 60°C |
| 310 | ≥ 70% | Subject to Testing | Low carbon grade (S31002) |
| G30 | 10% | Acid Boiling Point | High corrosion rate for 60%-65% concentration and for mixed acid i.e. 1-3%HF |
| Titanium Gr 2 & 7 | ≤ 10% & 65%-90% | Subject to Testing | |
| Aluminium Alloys e.g. A91100 & A95052 | ≥ 80% | Room Temperature | |
| Aluminium Alloys e.g. A91100 & A95052 | ≥ 93% | ≤ 43°C |
Molybdenum bearing grades such as PRODEC 316/316L or duplex 2205 offer no additional advantage over 304L in pure acid. This is because the sigma formation and other molybdenum transitional phases, commonly associated to the alloys, are prone to HNO3 attack. However, subject to coupon testing, duplex and super duplex may become more suitable when it comes to mixed acid (HNO3+HF and HNO3+HCl) or acid containing metallic ions (pickling solution).
