Handling & Storage of Nitric Acid (HNO3)

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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.

Nitric Acid Plant GraphicAs 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.

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 PointLow corrosion rate
2205≤ 65%Acid Boiling PointSimilar performance to 304/304L
31610%90°C Can be used with 60%HNO3 + 2%HCl at 50°C - 122°C
316L65.3%Acid Boiling Point304/304L performs better due to an absence of Molybdenum
C27610%90°C Can be used with 60%HNO3 + 2%HCl at 50°C - 122°C (performs better than 316 under the same conditions)
62510%Acid Boiling PointHigh corrosion rate for mixed acid i.e. 3%HF
690≥ 70%≤ 80°CExcellent for use with 10%-15%HNO3 + 3%HF and 20%HNO3 + 2%HF at 60°C
310≥ 70%Subject to TestingLow carbon grade (S31002)
G3010%Acid Boiling PointHigh 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).

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