Excerpt:
Hastelloy G-35 (UNS N06035, W.Nr. 2.4643) technical guide from Shanghai Hangbo Alloy Group. 33% chromium / 8% molybdenum Ni-Cr-Mo alloy with a 2% iron ceiling for the most severe wet-process phosphoric acid and HNO3/HCl mixed-acid service; ASTM B581/B582; VIM+ESR melted and used as-welded without post-weld heat treatment.
Hastelloy G-35 (UNS N06035) Technical Guide: The 33% Chromium Answer to Wet-Process Phosphoric Acid Service | Hangbo Alloy
Introduction
Wet-process phosphoric acid — the merchant-grade acid produced by digesting phosphate rock with sulfuric acid — carries a contaminant load that laboratory-grade acid never shows. Chlorides, fluorides, silica, sulfates, and residual gypsum solids travel through evaporators, heat exchangers, and acid coolers at 90–130 °C, attacking equipment by uniform corrosion, under-deposit pitting, and crevice attack all at once. Hastelloy G-30 (UNS N06030) set the modern performance benchmark in that service by raising chromium to roughly 30%, but the most severe acid circuits — high-chloride, high-fluoride rock sources, and evaporator trains pushed to maximum concentration — demanded one more step. Hastelloy G-35 (UNS N06035, W.Nr. 2.4643) is that step: a nickel-chromium-molybdenum alloy carrying approximately 33% chromium and 8% molybdenum with iron deliberately limited to 2% maximum, engineered specifically for wet-process phosphoric acid and for the mixed nitric/hydrochloric acid environments of the broader chemical industry.
G-35 is not a tweak of G-30; it is a rebalancing of the entire alloy system. Chromium climbs from the 28–31.5% band of G-30 to 32.25–34.25%, molybdenum rises from 4–6% to 7.6–9.0%, tungsten is essentially removed, and iron collapses from 13–17% to a 2% ceiling. The result is an alloy with a stronger, more repairable passive film in hot oxidizing acid, higher resistance to localized attack beneath gypsum and scale deposits, and a leaner microstructure that resists the precipitation of secondary phases during welding and thermal exposure. Shanghai Hangbo Alloy Group manufactures and supplies Hastelloy G-35 to ASTM B581 (rod, bar, and wire) and ASTM B582 (plate, sheet, and strip), with seamless and welded pipe and tube to the applicable ASTM product standards, and ships every heat with EN 10204 3.1 documentation, 100% PMI verification, and full traceability back to the melt. This guide explains the metallurgy, the corrosion logic, and the application economics of G-35.
Chemical Composition
The composition limits below apply to G-35 mill products supplied by Hangbo Alloy to ASTM B581 and B582. Nickel is the balance element.
| Element | Min % | Max % |
|---|---|---|
| Nickel (Ni) | Balance | — |
| Chromium (Cr) | 32.25 | 34.25 |
| Molybdenum (Mo) | 7.6 | 9.0 |
| Iron (Fe) | — | 2.0 |
| Carbon (C) | — | 0.05 |
| Silicon (Si) | — | 0.60 |
| Manganese (Mn) | — | 1.0 |
| Tungsten (W) | — | 0.60 |
| Cobalt (Co) | — | 1.0 |
| Copper (Cu) | — | 0.30 |
| Niobium (Nb) | — | 0.50 |
| Phosphorus (P) | — | 0.03 |
| Sulfur (S) | — | 0.015 |
Three features of this specification explain the alloy's behavior. First, chromium at 32.25–34.25% — the highest of any commercial Ni-Cr-Mo alloy — supplies the oxidizing power that carries wet-process acid duty: the passive film forms faster, repairs faster, and tolerates far more chloride and fluoride contamination before breaking down. Second, molybdenum at 7.6–9.0% provides the defense in locally reducing regions, under deposits, and in crevices where oxygen is consumed and the passive film cannot be maintained. Third, the 2% iron ceiling is the metallurgical masterstroke: low iron suppresses the precipitation of chromium- and molybdenum-rich secondary phases that can otherwise form in the heat-affected zone of welds and at service temperatures, so the full corrosion resistance of the alloy survives fabrication. Carbon is held to 0.05% maximum and niobium to 0.50% maximum to further stabilize the as-welded microstructure.
Why G-35 Wins in Wet-Process Phosphoric Acid
Corrosion in wet-process phosphoric acid is governed by the same electrochemistry as pitting in chloride media, but intensified by temperature, fluorides, and solids. Fluoride ions attack the passive film chemically; chlorides destabilize it locally; hot concentrated acid accelerates both; and scale deposits create crevices where the acid deoxygenates and becomes aggressively reducing. An alloy survives only if its passive film is robust enough to resist chemical attack and reform quickly after localized breakdown — which is precisely what high chromium buys — and if its bulk composition can resist the reducing acid that develops under deposits — which is what high molybdenum buys. G-35 is the only commercial Ni-Cr-Mo alloy that maximizes both at once.
The table below positions G-35 against the alloys it displaces in phosphoric acid evaporator and exchanger service, based on published comparative test programs and plant experience in commercial wet-process acid.
| Alloy | Cr, typical % | Mo + W, typical % | Relative resistance in severe wet-process H₃PO₄ | Typical role |
|---|---|---|---|---|
| 316L stainless | 17 | 2.1 | Baseline | Digestion, mild duty only |
| Alloy 904L | 20 | 4.3 | Moderate | Limited evaporator service |
| Hastelloy G-3 | 22 | 7 | Good | Older evaporator circuits |
| Alloy 625 | 21.5 | 9 | Moderate-good | Historically over-specified |
| Hastelloy G-30 | 29.5 | 7 | Excellent | Standard modern evaporator alloy |
| Hastelloy G-35 | 33.2 | 8.1 | Superior (high Cl/F acid) | Most severe modern acid circuits |
G-35 was developed directly from G-30 service experience in fertilizer complexes, where coupon programs in real evaporator acid repeatedly showed that the residual corrosion of G-30 scaled with chloride and fluoride loading. In acid containing extreme chloride-plus-fluoride burdens — several thousand ppm and above — G-35 routinely extends tube and plate life by a further factor over G-30, which is why new builds on the most corrosive rock sources and retrofit programs on the hottest evaporator effects specify G-35. Where acid analysis is less severe, G-30 remains the cost-optimized choice, and Hangbo Alloy supplies both grades so that fertilizer producers can run side-by-side coupon verification in their own acid.
G-35 versus G-30: Element-by-Element
| Element / Feature | Hastelloy G-30 (N06030) | Hastelloy G-35 (N06035) | Consequence of the change |
|---|---|---|---|
| Chromium | 28.0–31.5% | 32.25–34.25% | Stronger passive film in oxidizing acid |
| Molybdenum | 4.0–6.0% | 7.6–9.0% | Better under-deposit and crevice defense |
| Iron | 13.0–17.0% | 2.0% max | Far fewer secondary-phase precipitates |
| Tungsten | 1.5–4.0% | 0.60% max | Simplified phase balance |
| Copper | 1.0–2.4% | 0.30% max | Re-focused chemistry for oxidizing service |
| Niobium | 0.3–1.5% (Nb+Ta) | 0.50% max | Stability in as-welded condition |
| Design target | Commercial phosphoric acid, oxidizing acids | Severe wet-process H₃PO₄, HNO₃/HCl mixtures | Performance headroom in extreme acid |
The engineering message of this comparison is that G-35 is not universally "better" than G-30 — it is better where the acid is oxidizing, hot, and contaminated with chlorides and fluorides, which is exactly the wet-process evaporator envelope. In mildly contaminated phosphoric acid, or in applications where G-30's higher iron content offers an advantage in reducing sulfuric acid service, the older alloy remains appropriate. Selection should follow coupon testing in the actual acid, not habit.
Broader Corrosion Resistance
Because its ~33% chromium and ~8% molybdenum make it comfortable across a wide redox range, G-35's envelope extends well beyond phosphoric acid:
| Environment | G-35 Performance |
|---|---|
| Wet-process phosphoric acid (severe, high Cl/F) | Excellent — the design target |
| Reagent phosphoric acid, all concentrations | Excellent, to boiling |
| Nitric acid | Excellent across a wide concentration range |
| Nitric + hydrochloric acid mixtures | Excellent — a signature G-35 environment |
| Nitric + hydrofluoric acid (pickling liquors) | Good — used in stainless pickling systems |
| Sulfuric acid | Good in middle and high concentrations at moderate temperature |
| Oxidizing chloride media (chlorine dioxide, hypochlorite) | Excellent pitting and crevice resistance |
| Hot concentrated pure hydrochloric acid | Limited — B-family alloys remain the reference |
| Caustic service | Good, with low caustic de-alloying tendency |
G-35's resistance to chloride stress-corrosion cracking, as a fully austenitic high-nickel alloy, is essentially total, which matters in evaporator vapor bodies, acid mist zones, and anywhere chloride-laden condensate can reach stressed components.
Physical and Mechanical Properties
Typical annealed minimum values per ASTM B581/B582 for G-35 are listed below; actual heat values are certified on the EN 10204 3.1 documents supplied by Hangbo Alloy.
| Property | Value |
|---|---|
| Density | 8.22 g/cm³ |
| Melting range | ≈ 1,320 – 1,370 °C |
| Tensile strength (annealed, min) | 690 MPa (100 ksi) |
| 0.2% yield strength (annealed, min) | 310 MPa (45 ksi) |
| Elongation in 50 mm (min) | 40% |
| Hardness (annealed, max) | 217 HB |
| Solution annealing temperature | 1,120 – 1,170 °C, rapid water quench |
G-35 is normally supplied in the solution-annealed condition. The low-iron chemistry means the alloy can be re-annealed repeatedly or exposed to service temperatures in the 600–1,000 °C range without the damaging precipitation that historically plagued higher-iron Ni-Cr-Mo alloys, preserving fabricability and corrosion margin over the equipment lifetime.
Fabrication and Welding
G-35 is produced from VIM + ESR melted stock for clean microstructure, hot-worked with finishing above approximately 950 °C, and solution annealed at 1,120–1,170 °C with rapid water quenching. It welds readily by GTAW, GMAW, and SMAW. Matching G-35 filler is preferred where maximum weld-metal corrosion margin is required; over-alloyed Ni-Cr-Mo fillers such as AWS ERNiCrMo-13 (alloy 59 chemistry) are the established alternative used across the industry for severe phosphoric acid and mixed-acid service. Because the low iron and controlled carbon prevent harmful grain-boundary precipitation, G-35 is used in the as-welded condition without post-weld heat treatment — a decisive fabrication advantage for large evaporator shells, exchanger bundles, and field-erected piping. Standard nickel-alloy discipline applies: clean joint surfaces, argon shielding with backing gas, controlled heat input, and interpass temperature limits. Hot forming should finish above approximately 950 °C followed by solution annealing; after severe cold forming (above roughly 7–10% strain) in corrosive service, a re-anneal is recommended.
Applications
- Wet-process phosphoric acid evaporators, concentrators, heat exchangers, reboilers, and agitators in fertilizer complexes handling high-chloride, high-fluoride rock sources.
- Phosphoric acid acid-cooling and storage circuits where merchant-grade acid is concentrated to the aggressive end of the process envelope.
- Nitric/hydrochloric acid mixture service in chemical processing, where few alloys survive at all.
- Stainless steel and specialty-alloy pickling lines using nitric/hydrofluoric acid mixtures.
- Chlorine dioxide generators, bleach towers, and pulp bleaching equipment in chloride-rich oxidizing service.
- Nuclear fuel-cycle equipment handling nitric-acid-based dissolver and recovery solutions.
- Flue-gas desulfurization absorber internals where oxidizing, chloride-laden condensates form.
- Pharmaceutical and fine-chemical reactors and piping for aggressive oxidizing media.
Hangbo Alloy Supply Program
| Product Form | Specification | Typical Size Range |
|---|---|---|
| Plate / Sheet / Strip | ASTM B582 / ASME SB582 | 1 – 50 mm thickness |
| Round Bar / Rod / Wire | ASTM B581 / ASME SB581 | 6 – 400 mm diameter |
| Seamless Pipe & Tube | ASTM B622 | 6 – 219 mm OD |
| Welded Pipe & Tube | ASTM B619 / B626 | 60 – 610 mm OD |
| Forgings & Flanges | ASTM B564 | Custom |
| Fittings | ASTM B366 | 1/2" – 24" |
Hangbo Alloy verifies chromium, molybdenum, nickel, and trace elements by 100% XRF/arc-OES PMI on all delivered G-35 product, and can supply ASTM G28 intergranular-corrosion documentation, ASTM G48 pitting test reports, and controlled-test data from commercial phosphoric acid coupon programs where project specifications require independent evidence.
Technical FAQ
1. What makes Hastelloy G-35 different from G-30?
G-35 raises chromium to 32.25–34.25%, raises molybdenum to 7.6–9.0%, and cuts iron to 2% maximum, versus G-30's 28–31.5% Cr, 4–6% Mo, and 13–17% Fe. The higher chromium and molybdenum improve corrosion resistance in severe wet-process phosphoric acid and mixed acids, and the low iron suppresses secondary-phase precipitation that could otherwise degrade weld and service performance.
2. Why is wet-process phosphoric acid so corrosive?
It is produced by digesting phosphate rock with sulfuric acid and therefore contains chlorides, fluorides, silica, sulfates, and suspended gypsum. Hot and concentrated, it attacks both by general corrosion and by localized pitting and crevice attack beneath scale deposits, defeating alloys that handle pure reagent-grade acid easily.
3. Which ASTM standards cover G-35 plate and bar?
Plate, sheet, and strip are covered by ASTM B582 (ASME SB582); rod, bar, and wire by ASTM B581 (ASME SB581). Seamless tubing is supplied to ASTM B622, welded pipe and tube to ASTM B619/B626, fittings to ASTM B366, and forgings to ASTM B564.
4. Can G-35 handle hydrochloric acid?
It performs well in dilute hydrochloric acid and, notably, in mixed nitric/hydrochloric acid solutions where many Ni-Cr-Mo alloys fail. For hot, concentrated, purely reducing hydrochloric acid, the B-family alloys (B-2, B-3) remain the reference materials.
5. Does G-35 need post-weld heat treatment?
No. The low iron content, 0.05% carbon ceiling, and controlled niobium keep the heat-affected zone free of harmful precipitates, so G-35 weldments retain full corrosion resistance in the as-welded condition. This simplifies fabrication of large evaporator shells and exchanger bundles.
6. What filler metal is used to weld G-35?
Matching G-35 filler is preferred where maximum corrosion margin is required. Over-alloyed AWS ERNiCrMo-13 (alloy 59 chemistry) filler is the widely used alternative. GTAW with argon backing is recommended for the highest-integrity, highest-corrosion-resistance welds.
7. What is the maximum service temperature of G-35?
For continuous corrosion service, approximately 400 °C is the practical ceiling; above this range, long-term thermal exposure can begin to affect the microstructure. In phosphoric acid evaporators, operating temperatures of 90–130 °C leave enormous margin, and corrosion resistance — not temperature capability — governs design.
8. How does G-35 compare with alloy 625 and C-276 in phosphoric acid?
Alloy 625, historically used in evaporators, exhibits corrosion rates many times higher than G-35 in aggressive commercial acid and has been displaced across the fertilizer industry. C-276 retains advantages in strongly reducing hydrochloric systems, but in oxidizing, chloride- and fluoride-laden phosphoric acid, G-35 is clearly superior.
9. When should I choose G-35 instead of G-30?
Select G-35 when your acid analysis shows high chloride and fluoride loadings, when evaporator effects run at the aggressive end of the temperature/concentration envelope, or when G-30 coupon corrosion rates in your own acid exceed the design allowance. Hangbo Alloy can supply both grades for direct comparative coupon testing in your process acid.
10. How is G-35 quality verified on delivery?
Every Hangbo Alloy G-35 shipment is supplied with EN 10204 3.1 mill certification, 100% PMI verification of key elements, tensile and hardness testing, and where specified, ultrasonic examination, ASTM G28 and G48 corrosion testing, and liquid penetrant inspection. Full traceability to the melt is maintained throughout.
This page is part of the technical guide series published by Hangbo Alloy (Shanghai Hangbo Alloy Group, nickel-alloy.com). Data presented are typical engineering values compiled from recognized industry and producer sources and are provided for material selection guidance; the governing documents for any purchase are the applicable ASTM/ASME/AMS specifications and the certified mill test report. Contact Hangbo Alloy for current stock, mill certificates, PMI verification, and application engineering support.
