Hastelloy G-30 in Phosphoric Acid Evaporators: Heat Exchanger Tube Performance Data

Why Is Hastelloy G-30 Specified for Phosphoric Acid Evaporator Tubes?

G-30's chemistry - high chromium with moderate molybdenum and a copper addition - was deliberately engineered to handle the specific combination of contaminants found in wet-process phosphoric acid, and Haynes International names P2O5 evaporator tubes as one of the alloy's two headline applications.

Source: Haynes International nominal composition data sheet, HASTELLOY G-30 alloy.

High chromium content gives G-30 strong resistance to the oxidizing character of wet-process acid and to nitric acid and nitric-acid mixtures, while its molybdenum and copper additions give it moderate resistance to reducing acids such as hydrochloric and sulfuric - both of which are present as impurities in commercial phosphoric acid. Haynes International also notes that G-30 is less susceptible to chloride-induced stress-corrosion cracking than the austenitic stainless steels it commonly replaces in this service, which matters directly for tube bundles that see repeated thermal cycling during evaporator startup and shutdown.

What Makes Wet-Process Phosphoric Acid So Corrosive to Evaporator Tubing?

Wet-process phosphoric acid is corrosive primarily because of what is dissolved in it, not the phosphoric acid itself: unreacted sulfuric acid, metallic ions, fluoride ions, chloride ions, and suspended particulates such as silica all ride along with the P2O5, and of these, chloride ions are the dominant driver of the electrochemical attack that determines tube life.

"Wet-process" phosphoric acid is made by reacting phosphate rock with sulfuric acid, and as produced it contains only about 30% P2O5 because of the large volume of rinse water needed to separate it from the calcium sulfate byproduct. Fluoride ions tend to form stable complexes with metallic ions in solution and are therefore less aggressive toward tube materials than chloride ions, which strongly influence the electrochemical reactions between the acid and the metal surface. This is why alloy selection for evaporator tubing is driven more by chloride-pitting and crevice-corrosion resistance than by the acid's nominal phosphoric acid strength alone.

During the evaporation process, P2O5 concentration is typically raised from roughly 30% up to about 54% across a series of evaporator stages. Higher concentration alone would suggest higher corrosivity, but Haynes International notes this effect is partly offset because impurity levels - including the chlorides that matter most - tend to drop as the acid becomes more concentrated.

How Does G-30 Perform in Real Wet-Process Acid, Not Just Reagent-Grade Acid?

Haynes International backs its reagent-grade laboratory data with field-representative testing conducted in actual wet-process acid supplied by a Florida phosphate producer at three real production concentrations (36%, 48%, and 54% P2O5) and 121°C (250°F), directly comparing G-30 against competitive stainless steels under conditions much closer to an operating evaporator than a reagent-grade beaker test.

This distinction matters for anyone specifying tube material from a data sheet: reagent-grade corrosion tables (used throughout this article for their precision and repeatability) isolate the effect of acid concentration and temperature alone, while real wet-process acid adds the fluoride, chloride, sulfate, and particulate load discussed above. Haynes International's decision to run a dedicated real-acid comparison at production-relevant concentrations is a meaningful piece of due diligence, and it is why G-30 carries a genuine, field-validated track record in this application rather than a purely theoretical one.

Independent field and pilot-plant data reported in phosphoric-acid process patents adds a further, longer-term data point for the broader Hastelloy G alloy family: in phosphate rock digestion trials, a Hastelloy G alloy impeller recorded a corrosion rate of 6.67 mils per year against Alloy 625's 8.35 mils per year under one set of digester conditions, and 11.71 mpy against 15.77 mpy for 625 under a higher-oxidation-potential condition. These figures describe the general G-family alloy rather than G-30 specifically, but they corroborate the same directional conclusion Haynes' own data shows: G-family nickel-chromium-iron alloys hold up well against 625 in real phosphoric acid production environments, not just in the laboratory.

How Does G-30's Corrosion Rate Change as Acid Concentration Rises During Evaporation?

G-30's corrosion rate in boiling phosphoric acid rises steadily from 50% through 85% concentration, but stays low and manageable (0.01-0.84 mm/y) across the entire range an evaporator train will see, including the roughly 54% P2O5 endpoint typical of commercial wet-process concentration.

Source: Haynes International reagent-grade phosphoric acid corrosion table, HASTELLOY G-30 alloy data sheet.

Since most commercial wet-process trains target a final concentration around 54% P2O5, tube designers can expect G-30's reagent-grade corrosion rate to sit in the 0.1-0.2 mm/y range at that endpoint - well within a comfortable corrosion allowance for a multi-decade tube design life, before accounting for the additional localized-corrosion risk from chlorides discussed below.

How Does G-30 Compare to 316L, 254SMO, and Alloy 625 in Evaporator-Relevant Acids?

Haynes International's own comparative plots place G-30 ahead of 316L and 254SMO stainless steel in both hydrochloric and sulfuric acid, and ahead of Alloy 625 specifically in hydrochloric acid - but Haynes states directly that G-30's 0.1 mm/y line sits slightly higher than 625's in sulfuric acid, meaning 625 holds a small edge wherever sulfuric acid contamination is the dominant concern.

This is a useful, honest data point for specification decisions: G-30 is not universally ahead of every competing alloy in every acid an evaporator might see. Where the dominant impurity is sulfuric acid carried over from the rock-digestion step, Alloy 625 (or the higher-chromium HASTELLOY G-35, covered in a companion article in this series) may offer a small additional margin. Where the acid is closer to the pure wet-process chemistry with its characteristic chloride load, G-30's balance of chromium, molybdenum, and copper gives it a real advantage over both the stainless steels and 625 in hydrochloric acid specifically.

What Is G-30's Resistance to Under-Deposit Pitting and Crevice Corrosion?

Under the two standard accelerated tests used to simulate chloride-fouled conditions - Green Death and Yellow Death - G-30 first shows pitting at 55°C and crevice attack at 45°C (Green Death) or 25°C (Yellow Death), which is exactly the failure mode evaporator tube bundles are most exposed to as scale and solids build up on the tube surface.

Source: Haynes International pitting and crevice corrosion data (24-hour tests), HASTELLOY G-30 alloy data sheet.

This is the single most operationally relevant data set in this article. Bulk acid resistance, as shown in the concentration tables above, describes what happens to a clean tube surface exposed uniformly to flowing acid. Real evaporator tubes accumulate scale, calcium sulfate deposits, and stagnant zones at baffles and support plates - conditions that concentrate chlorides locally and can trigger pitting or crevice attack well below the bulk acid's nominal operating temperature. G-30's crevice-corrosion threshold of 25-45°C is a meaningful, quantified margin above typical evaporator operating temperatures for many designs, but it also underscores why periodic mechanical cleaning and inspection under deposits remains standard practice regardless of alloy selection.

Is G-30 Resistant to Chloride Stress-Corrosion Cracking in Evaporator Service?

Yes - in the standard boiling 45% magnesium chloride test (ASTM G36), G-30 resisted cracking for 168 hours, roughly seven times longer than 254SMO and 84 times longer than 316L, and outperformed two comparison nickel alloys ('28' and '31') tested alongside it.

Source: Haynes International stress-corrosion cracking data (boiling 45% MgCl2, ASTM G36), HASTELLOY G-30 alloy data sheet.

SCC resistance matters for evaporator tube bundles because they combine two of the three ingredients SCC requires - a susceptible material under tensile stress (from thermal cycling, rolling into the tubesheet, or residual weld stress) exposed to a chloride-bearing environment. G-30's roughly 168-hour threshold in a test far more severe than typical service conditions gives tube designers a substantial, quantified safety margin over the stainless steels most often considered as lower-cost alternatives.

How Does Welding Affect Tube-Seam and Tube-to-Tubesheet Corrosion Resistance?

G-30's all-weld-metal corrosion rates stay close to wrought base metal across most sulfuric and hydrochloric acid conditions relevant to evaporator service, with the exception of 90% sulfuric acid, where both weld metal and wrought base metal show a sharp, matched rise - meaning the increase there reflects a genuine chemistry limit rather than a weld-specific weakness.

Source: Haynes International all-weld-metal (multi-pass GMAW cruciform) vs. wrought base-metal corrosion data, HASTELLOY G-30 alloy data sheet.

For evaporator tube bundles fabricated from welded tube (ASTM B619/B626) or requiring tube-to-tubesheet welds, this data supports specifying G-30 without a blanket derating for weld zones in most of the acid conditions the alloy is intended for. The 90% sulfuric acid result is a reminder that G-30, like G-35, is not the right choice if a plant's sulfuric acid carryover regularly reaches that concentration at the tube wall - a condition where 625 or a higher-chromium G-35 tube would be the more conservative specification.

How Does G-30's Thermal Conductivity Affect Heat Exchanger Tube Design?

G-30's thermal conductivity (about 10 W/m·°C at room temperature, rising to roughly 19-21 W/m·°C at 400-600°C) is well below that of copper alloys and even carbon steel, so heat-transfer area and tube-wall thickness must be sized with the alloy's conductivity in mind - but this trade-off is substantially offset by the very thin corrosion allowance G-30 permits compared with the thicker walls a less-resistant, cheaper alloy would need.

Source: Haynes International physical property data, HASTELLOY G-30 alloy data sheet.

This is a genuine engineering trade-off, not a marketing point to gloss over: a nickel-chromium-iron tube wall will conduct heat noticeably less efficiently than a comparable-thickness copper alloy or carbon steel tube, and heat exchanger thermal design (tube count, length, or wall thickness) needs to account for that directly in the heat-transfer calculation. In practice, evaporator designers offset this by taking advantage of G-30's corrosion resistance to specify a thinner tube wall than a carbon-steel or lower-alloy design would require, which partially recovers the heat-transfer penalty from lower conductivity while still delivering a multi-decade service life without the wall-thickness margin a corrosion allowance on a less-resistant material would demand.

What Tube Product Forms and Standards Apply to G-30 Evaporator Tubing?

G-30 evaporator tubing is available as both seamless tube under ASTM/ASME B622/SB622 and welded tube under ASTM/ASME B619/B626 and SB619/SB626, is supplied mill-annealed, and should be re-annealed after any cold-forming or tube-bending operation that produces 7% or more outer-fiber elongation to restore full corrosion resistance.

Source: Haynes International "Specifications and Codes" table, HASTELLOY G-30 alloy data sheet.

The annealing temperature for G-30 is 1,177°C (2,150°F) with water quenching advised, though rapid air cooling is feasible for structures thinner than 10 mm (0.375 in) - a category that includes most evaporator tube-wall thicknesses. G-30 is stiffer than the austenitic stainless steels and work-hardens more readily, which matters directly for tube-bending and expansion operations during bundle fabrication and should be planned into the manufacturing sequence.

What Are the Practical Design and Operating Limits for G-30 Evaporator Tubes?

G-30 performs reliably across the full 30-54% P2O5 concentration range and up to 121°C in real wet-process acid, but designers should treat localized chloride contamination - not bulk acid strength - as the governing design constraint, and should watch specifically for the sharp corrosion-rate spikes G-30's own hydrochloric acid data shows at certain intermediate concentration-and-temperature combinations.

Haynes International's hydrochloric acid table shows corrosion rates rising sharply at some intermediate conditions - for example, 2.5% HCl at 93°C (200°F) jumps to 4.23 mm/y, and boiling 2-2.5% HCl reaches 9.47-12.67 mm/y - far above the well-behaved rates at both lower and higher concentrations nearby. This kind of non-monotonic behavior is a reminder that iso-corrosion diagrams and concentration tables should be read at the actual, local acid chemistry expected at the tube wall (including any concentration effects from boiling or evaporation right at the surface), not just the nominal bulk process concentration.

• Design and operating guidance: Track chloride and free-sulfuric-acid impurity levels in the process acid, since these - not the nominal P2O5 concentration - are what actually drive tube corrosion risk.
• Design and operating guidance: Schedule periodic mechanical cleaning and inspection under scale and deposits, since G-30's crevice-corrosion threshold (25-45°C in accelerated tests) can be approached wherever fouling concentrates chlorides locally.
• Design and operating guidance: Avoid designs that concentrate hydrochloric acid locally into the 2-3% range at elevated temperature, where G-30's own data shows a disproportionate corrosion-rate spike.
• Design and operating guidance: Where sulfuric acid carryover is unusually high or regularly approaches 90% concentration at the tube wall, evaluate Alloy 625 or HASTELLOY G-35 as a more conservative alternative.
• Design and operating guidance: Re-anneal tubing after cold-forming or bending operations exceeding about 7% outer-fiber elongation to fully restore corrosion resistance before commissioning.

Frequently Asked Questions

Q: Why is Hastelloy G-30 used for phosphoric acid evaporator tubes?

A: G-30's high chromium content resists the oxidizing character of wet-process phosphoric acid and nitric-acid contamination, while its molybdenum and copper content resist the sulfuric acid impurity present in every wet-process stream. Haynes International names P2O5 evaporator tubes as one of the alloy's two primary applications, backed by both reagent-grade and real wet-process acid testing.

Q: Does G-30's corrosion resistance hold up at the full 54% P2O5 concentration used in commercial evaporators?

A: Yes. Reagent-grade boiling phosphoric acid data shows G-30's corrosion rate rising only gradually across the concentration range, staying in the 0.1-0.2 mm/y range around 54-60% P2O5 - well within a comfortable design allowance.

Q: What is the biggest corrosion risk for G-30 tubes in evaporator service?

A: Chloride-driven pitting and crevice corrosion beneath scale and deposits, not uniform acid attack on a clean surface. Standard accelerated tests show G-30's pitting and crevice-corrosion thresholds at 25-55°C, which is why periodic cleaning and inspection under fouling remains important regardless of alloy choice.

Q: Is Hastelloy G-30 better than Alloy 625 for phosphoric acid evaporators?

A: It depends on which impurity dominates. Haynes International's own comparative data shows G-30 ahead of 625 in hydrochloric acid, but 625 holding a slight edge in sulfuric acid. Where sulfuric acid carryover is unusually high, 625 or the higher-chromium HASTELLOY G-35 may be the more conservative specification.

Q: Does welding reduce G-30's corrosion resistance in evaporator tube fabrication?

A: Generally no. Published weld-metal-versus-wrought data shows only a small difference across most sulfuric and hydrochloric acid conditions relevant to evaporator service, with both weld metal and wrought base metal showing a matched, chemistry-driven rise only at the most severe condition tested (90% sulfuric acid).

Q: How does G-30's thermal conductivity compare to other heat exchanger tube materials?

A: G-30's thermal conductivity (about 10 W/m·°C at room temperature) is well below copper alloys and carbon steel, so heat-transfer area must be sized accordingly. In practice this is partly offset because G-30's corrosion resistance allows a thinner tube wall than a less-resistant alloy would need for the same service life.