Hastelloy G-30 Pipe Fittings: The Optimised Grade for Phosphoric Acid and Fertiliser Service
Hastelloy G-30 (UNS N06030) is a nickel alloy specifically developed for the wet-process phosphoric acid (WPA) environment — one of the most corrosive combinations in the fertiliser and chemical industry. Unlike the C-family alloys (C-276, C-22) which are optimised for hydrochloric and sulphuric acid, G-30 adds 5% Co and 2.5% Cu to create a composition tuned to the specific corrosive conditions of phosphoric acid plants. When Alloy 20 and 904L underperform in phosphoric acid service, G-30 is the next step before resorting to C-276 or titanium.
What Makes WPA Corrosive
Wet-process phosphoric acid (H₃PO₄ produced by reacting phosphate rock with sulphuric acid) is not a pure acid — it contains fluorides (HF, SiF₄), sulphates, chlorides, and organic impurities at significant concentrations. These contaminants are far more aggressive than reagent-grade phosphoric acid, and they eliminate materials that would otherwise perform acceptably in pure H₃PO₄. The HF component in particular corrodes passive films and accelerates pitting of standard stainless steels and 904L. At 60–80°C with 30–50% P₂O₅ concentration (typical digestion stage conditions), even Alloy 20 can be marginal.
G-30 vs Other Grades in WPA Service
| Material | WPA Service Rating | Notes |
|---|---|---|
| 316L / 904L | Marginal to poor | HF and Cl⁻ contamination causes pitting in digestion and evaporation stages |
| Alloy 20 (N08020) | Good at moderate conditions | Standard for clean WPA — borderline in high-fluoride or high-temperature stages |
| Incoloy 825 (N08825) | Good — better than 20 | Higher Ni/Mo than Alloy 20 — handles more aggressive WPA stages |
| Hastelloy G-30 (N06030) | Excellent | Optimised for WPA — Co and Cu additions specifically address the WPA impurity mix |
| Hastelloy C-276 (N10276) | Excellent — but over-specified for most WPA | C-276 handles WPA well but costs 30–40% more than G-30 for no additional benefit in standard WPA service |
G-30 Chemistry and Welding
Hastelloy G-30 composition: Ni ~43%, Cr 28–31.5%, Fe 13–17%, Mo 4–6%, Co 4–6%, Cu 1.5–2.5%, W 1.5–4%, Nb+Ta 0.3–1.5%. The high chromium content (28–31%) — significantly higher than C-276 (15%) — is the primary contributor to oxidising acid resistance. The cobalt and copper additions provide additional resistance to the HF/fluoride impurities that characterise WPA.
Welding filler for G-30 is ERNiCrMo-11 (Haynes 230-W or equivalent). This is a different consumable from ERNiCrMo-3 (Inconel 625) and ERNiCrMo-4 (C-276). A separate WPS qualification under ASME IX is required. Preheat is not needed; interpass ≤177°C. Solution anneal is recommended after welding for maximum corrosion resistance in the most severe WPA conditions.
Where G-30 Is Specified in Phosphoric Acid Plants
- Digestion reactor pipework (sulphuric acid + phosphate rock → WPA) — high fluoride, high temperature
- Flash cooler headers and evaporator connections — concentration and temperature combine with fluorides
- Slurry pump connections and transfer headers
- Scrubber connections handling HF off-gas
- Phosphoric acid storage tank nozzles for merchant-grade acid