Why Phosphoric Acid Eats Metal

Phosphoric acid is unique in its corrosivity because the industrial product is not the pure reagent-grade H₃PO₄ you can buy from a lab supplier. Wet-process phosphoric acid (WPA) — produced by digesting phosphate rock with sulfuric acid — contains a long list of contaminants that make it far more aggressive than pure H₃PO₄:

• Free sulfuric acid (1–4% H₂SO₄): the digestion acid that was not fully consumed.
• Fluorides (1–3% F⁻ as HF, H₂SiF₆, fluorosilicates): from the fluorapatite mineral in the rock.
• Chlorides (0.1–1% Cl⁻): from the rock and from the sulfuric acid.
• Suspended solids (gypsum, silica, undigested rock): 10–30% by weight: high-velocity erosion.
• Aluminum, iron, magnesium, sodium, potassium, uranium: from the rock, present as dissolved ions.

The result is an acid that is hot (60–110°C), concentrated (28–54% P₂O₅), aerated, chloride- and fluoride-bearing, with high solids content. 316L stainless fails in months. 904L fails in 1–2 years. Alloy 20 lasts 5–10 years. Hastelloy G-30 typically delivers 15+ years of service life in the most aggressive positions in a DAP/MAP plant.

Hastelloy G-30: Designed for Wet-Process H₃PO₄

G-30 (UNS N06030) was developed by Haynes International in the 1980s specifically as an upgrade to G-3 (UNS N06985) for the phosphate fertilizer industry. The alloy is a Ni-Fe-Cr-Mo-Cu-W alloy with the right balance of Cr (for oxidizing-acid resistance), Mo (for reducing-acid resistance), and Cu (for sulfuric acid resistance) — without the tungsten content of C276 that makes welding more difficult.

Chemical Composition (UNS N06030)

Three features stand out: 30% Cr (very high — gives the alloy resistance to oxidizing conditions like concentrated HNO₃ and F⁻-rich acid), 2% Cu (resistance to H₂SO₄ in the digestion acid), and 5% Mo + 2.5% W (resistance to Cl⁻ and reducing acid).

ASTM B575 / B622 / B626 Product Forms

ASME “SB” versions are required for Section VIII pressure-vessel service. Welding electrode per SFA-5.11 is ENiCrMo-11; per SFA-5.14 is ERNiCrMo-11. Both are available from Special Metals, Haynes, and select European mills.

G-30 vs C276: Head-to-Head

Where G-30 wins

• Wet-process phosphoric acid (any concentration, any temperature, with F⁻ + Cl⁻)
• Concentrated HNO₃ + HF pickling (semiconductor, stainless)
• Sulfuric + nitric mixed acid (metal finishing)
• Phosphate fertilizer evaporator tubes

Where C276 wins

• Hot concentrated HCl (any concentration above 10%)
• Wet FGD scrubber with reducing-acid conditions
• Severe chloride with no oxidizer
• Any environment where the Cr is consumed and only Mo+W can save you

G-30 vs Alloy 20: When to Switch

Alloy 20 (UNS N08020) is the previous-generation WPA material. The upgrade rule of thumb:

G-30 vs G-35: The New Generation

Hastelloy G-35 (UNS N06035) is the latest-generation upgrade to G-30, with even higher Cr (33%) and lower Fe (≤ 2%) to handle the new “cleaner acid” WPA chemistries used in high-purity phosphate applications (lithium-iron-phosphate batteries, food-grade phosphates).

For new DAP/MAP plant construction, G-35 is the modern standard. For repair and replacement of existing G-30 equipment, G-30 remains cost-effective and readily available.

Corrosion Data in Wet-Process Acid

Applications in Phosphate Fertilizer Plants

Detailed Corrosion Rates: G-30 in the Full WPA Process Stream

The wet-process phosphoric acid (WPA) plant contains multiple unit operations, each with its own corrosive fingerprint. A single alloy rarely covers the entire plant — G-30 is specified for the most aggressive sections and a less expensive alloy (Alloy 20, 904L, or 316L) for the rest:

Fabrication of Hastelloy G-30: Welding, Forming & Machining Guide

G-30 is a fully austenitic solid-solution alloy, which makes it fundamentally easier to weld than precipitation-hardenable or duplex grades — but it has its own specific requirements:

Welding

• Processes: GTAW (TIG) preferred for root passes and thin sections; GMAW (MIG) for fill passes on thick sections; SMAW (stick) for field repair — all standard.
• Filler metal: ERNiCrMo-11 (AWS A5.14) for TIG — this is the matching filler. ERNiCrMo-3 (Inconel 625 filler) is an acceptable alternative for dissimilar-metal welds to carbon steel or 316L but NOT for matching G-30-to-G-30 welds in corrosive service (the lower Mo and absence of W in 625 filler will make the weld metal the weak link).
• Shielding gas: 100% argon for GTAW; 75%Ar/25%He or 100% Ar for GMAW. No hydrogen additions — hydrogen in the shielding gas increases the risk of hot-cracking in high-Ni alloys.
• Preheat: Not required. Interpass temperature: ≤ 150°C. Lower is better — 100°C max for thin sections.
• Post-weld heat treatment: Not required. G-30 is solid-solution strengthened and the as-welded condition meets all ASTM requirements.
• Hot cracking risk: Low compared to precipitation-hardenable nickel alloys, but still a concern in highly restrained, thick-section welds (≥ 25 mm). Use stringer-bead technique and minimize weld-pool size.

Forming

• Cold forming: G-30 work-hardens at a moderate rate — about 1.5× that of 316L. Intermediate annealing is recommended after ~20% cold reduction for complex shapes.
• Hot forming: 870–1,150°C, followed by re-solution-anneal at 1,121°C and water quench. Hot forming below 870°C risks sigma-phase formation (G-30 has ~6% Mo, and sigma can precipitate in the 650–900°C range if held too long).

Machining

• Machinability rating: ~20% of B1112 free-cutting steel — similar to 316L but with slightly higher tool wear.
• Use rigid setups, positive-rake carbide tools (C-2 or C-3 grade), and abundant sulfurized cutting oil.
• Speeds: 12–18 m/min for HSS tools, 30–45 m/min for carbide — 30–50% lower than for 316L to control work-hardening.

G-30 vs G-35: When to Upgrade to the Next Generation

Hastelloy G-35 (UNS N06035) was developed by Haynes International in the early 2000s as an improvement on G-30 for the most aggressive WPA and mixed-acid environments. Key differences:

Recommendation: Specify G-35 when the evaporator acid temperature exceeds 110°C, when the chloride content is > 2%, or when the plant design life exceeds 25 years. For most standard WPA plants, G-30 remains the proven, cost-effective choice — and is available from stock at Huaxiao Alloy in plate, sheet, pipe, and welding consumables.

Welding Filler Metal Selection for G-30 — The Complete Guide

Selecting the right filler metal for Hastelloy G-30 welding is critical — using the wrong filler can create a weld that is 10–100× more corrosion-prone than the base metal. Here is the complete filler metal guide:

G-30 Equipment Examples in Real Fertilizer Plants

Several case studies from operating phosphate fertilizer plants illustrate where G-30 is specified vs where it is not needed:

Phosphate Fertilizer Market Trends Driving G-30 Demand

The global phosphate fertilizer market is shifting toward lower-grade, higher-contaminant phosphate rock as high-grade reserves in Florida, Morocco, and China are depleted. This trend will increase demand for G-30 and G-35 because:

• Higher chloride in the rock: Lower-grade rock contains more NaCl and KCl (from seawater intrusion in marine-origin deposits). Chloride in the WPA stream increases from 0.5% (Florida 68 BPL rock) to 2–3% (lower-grade 60–64 BPL). Alloy 20 cannot handle > 1.5% Cl⁻ in WPA; G-30 is required above ~1% Cl⁻.
• Higher fluoride: Lower-grade rock also contains more fluorine. G-30’s higher Cr content provides marginally better F⁻ resistance.
• Higher solids: Lower-grade rock means more gypsum byproduct per ton of P₂O₅ produced — increasing the erosion-corrosion component. G-30’s higher hardness (~200 HB, vs ~160 HB for Alloy 20) improves erosion resistance.

Why Choose Huaxiao Alloy for Your Hastelloy G-30 Procurement

G-30 Stock Availability and Procurement Lead Times

G-30 is a specialty grade — not a commodity like 316L or even Alloy 20. Procurement teams should plan for 3–4 month lead times for mill-order material and always maintain an in-plant spare inventory of the most critical G-30 components (pump impellers, agitator blades, critical flanges) to avoid a single-point failure during an unplanned outage.

FAQs About G-30 Procurement and Service

Extended FAQ — G-30 Selection and Service

G-30 Alternatives: When to Use Each Alloy in WPA Service

G-30 is not the only option for phosphoric acid service, and it’s not always the best economic choice. Here is a decision matrix that matches the service condition to the lowest-cost alloy that meets the corrosion requirements:

Source Hastelloy G-30 from Huaxiao Alloy

We stock G-30 plate (3–25 mm thick), sheet (0.8–2.0 mm), and ERNiCrMo-11 welding wire. For pipe, tube, bar, and large-plate orders, we arrange mill-direct delivery with full EN 10204 certification. Our metallurgists can review your WPA or sulfuric acid service conditions and confirm whether G-30 is the right choice — or whether a lower-cost alternative (Alloy 20, 825) or a higher-performance option (G-35, C276) is more appropriate.

Frequently Asked Questions