Pipe Fittings for Nitric Acid (HNO₃) Service: Why the Oxidising Environment Changes Everything
Nitric acid service is one of the few environments where the grade selection logic for pipe fittings is the reverse of most acid services: highly alloyed Ni-Mo grades (C-276, B-3) that excel in reducing acids (HCl, H₂SO₄) fail rapidly in HNO₃, while lower-alloy stainless grades (304L, 316L) perform adequately and are the standard choice. Understanding why oxidising acids behave differently, where the grade boundaries are, and which speciality grades are needed for the most aggressive HNO₃ conditions prevents a costly and dangerous misapplication.
Why HNO₃ Is an Oxidising Acid
Hydrochloric and sulphuric acids (in dilute/moderate concentration) are reducing acids — they provide no oxidising species and corrode metals by dissolving them in the hydrogen evolution reaction. Nitric acid is an oxidising acid — NO₃⁻ itself is the oxidising agent. In oxidising conditions, chromium-rich passive films (Cr₂O₃) are enhanced and stabilised. This is why stainless steel, which passivates in oxidising conditions, performs well in HNO₃. Ni-Mo alloys (C-276, B-3) lack sufficient chromium to form a protective passive layer under the oxidising attack of HNO₃ — they corrode rapidly, often at rates comparable to or worse than mild steel.
Grade Selection by HNO₃ Concentration and Temperature
| HNO₃ Concentration | Temperature | Fitting Grade |
|---|---|---|
| 0–65% | Ambient to 60°C | 304L or 316L — standard choice |
| 0–65% | 60–90°C | 316L — Mo provides incremental improvement |
| 65–90% | Any | 304L preferred — higher Ni of 316L can increase corrosion rate slightly |
| >90% (fuming) | Any | Low-silicon 304L (Si ≤0.10%) or EN 1.4306 — standard 304L attacks above 90% |
| Boiling all concentrations | Boiling point | High-silicon stainless (Uranus 65, EN 1.4361, 4–5% Si) for continuous boiling service |
| Any | Any | C-276, B-3, Monel — absolutely NOT suitable |
The Huey Test — Verification for HNO₃ Service
The Huey test (ASTM A262 Practice C) is the standard corrosion test for intergranular attack in nitric acid — specimens are immersed in boiling 65% HNO₃ for five 48-hour periods, with the corrosion rate averaged. A Huey test result ≤0.50 mm/year is typically acceptable for 304L in HNO₃ service. When specifying fittings for boiling or concentrated HNO₃ service, the PO should include: "Material to pass ASTM A262 Practice C (Huey test). Corrosion rate to be reported on EN 10204 3.1 certificate. Maximum acceptable rate: [XX mm/year per project specification]." For ordinary 316L in sub-boiling dilute HNO₃, the Huey test is usually not required but can be specified as an additional quality control check.