In Phosphoric Acid Production (Wet Process) at 90°C with 2% Fluorine, Can Titanium Grade 24 (Ti-6Al-4V-0.1Ru) Resist Attack Better Than Grade 7?

The wet-process phosphoric acid (WPA) obtained by the sulfuric acid digestion of phosphate rock contains considerable fluorine impurities (usually 1-3% as HF and H 2 SiF 6 ) which come from the fluorapatite minerals. At the usual WPA concentration of 30-50% P2O5 (around 40-60% H3PO4) and at 80-100°C, fluorine produces a very aggressive environment for metals. Titanium Grade 7 (Ti-0.15Pd) is the standard alloy for WPA heating tubes and evaporators. However, titanium in fluorine-containing acid has a substantial corrosion rate (0.1-0.3 mm/year). Grade 24 (Ti-6Al-4V-0.1Ru) is a higher strength titanium alloy initially created for aerospace uses, amended with 0.1% ruthenium to improve corrosion resistance. The corrosion performance of Grade 24 relative to Grade 7 in simulated WPA with 2% fluorine at 90°C is compared and whether Grade 24 gives any advantage for heater service is established.

Mechanism of Fluorine Attack on Titanium Alloys
In phosphoric acid (H3PO4) with HF or fluosilicic acid (H2SiF6) the major corrosion process is the chemical dissolution of the passive TiO2 film: TiO2 + 6F- + 4H+ TiF62- + 2H2O. Fluorine attack, in contrast to the confined nature of chloride pitting, is universal and uniform. The hexafluorotitanate complex is soluble and the passive film is therefore continuously dissolved and has to be reformed. The steady-state corrosion rate depends on the interplay between film dissolution and repassivation. The addition of alloying materials which stabilize the passive coating or decrease its rate of disintegration will increase corrosion resistance.

In the 7th grade palladium is a cathodic modifier which shifts the corrosion potential noble and reduces the pace of breakdown of the passive layer. However, in the fluorine containing acids the high solubility of TiO 2 does not allow even a noble potential to avoid film dissolution, the film is chemically removed regardless of the electrochemical conditions. Thus the benefit of palladium is lessened in high-fluorine conditions.

Grade 24 contains 6% aluminium, 4% vanadium, and 0.1% ruthenium. The ruthenium has a similar cathodic modification role as does palladium, but may be more successful in fluorine containing acids. Inside the passive film Al and V create their own oxides (Al2O3, V2O5) which may modify the solubility characteristics of the film.

Comparative corrosion tests
Controlled immersion testing of Grade 7 (annealed), Grade 24 (solution treated and aged) and Grade 2 (baseline) was carried out for 1,000 hours in simulated WPA (45% H₃PO₄, 2% F⁻ as H₂SiF₆, 90°C). Findings:

Grade 2 titanium deteriorated at a consistent rate of 0.45 to 0.60 mm/yr. After 1,000 hours (1.4 months), the wall thickness was reduced by 0.05-0.07 mm, a significant reduction but not perforating a conventional tube. 5 year life requires wall thickness >3 mm for longer exposure.

Grade 7 titanium rusted uniformly at 0.12 to 0.18 mm/year. After 1,000 hours the reduction was 0.014 to 0.021 mm. Palladium was 3-4 times better than Grade 2. For 5 years life (43800 hours) corrosion allowance 0.6-0.9 mm is required-possible with 1.65-2.0 mm wall.

Grade 24 titanium corroded uniformly at 0.10–0.14 mm/year. After 1,000 h the reduction was 0.011-0.016 mm. The improvement over Grade 7 is marginal (a corrosion rate reduction of about 15-20%). Metallographic investigation found no evidence of preferential attack on the alpha and beta phases. The two-phase microstructure did not prevent uniform corrosion of the alloy.

Ruthenium content (0.1%) appears to be as effective as palladium (0.15%) in fluorine-containing acid. Aluminum and vanadium don't add value; their oxides are also HF soluble and V_2O_5 can be reduced in the presence of phosphoric acid, which can introduce vanadium ions to the product acid - not desirable for some downstream uses (fertilizer production is tolerant of this; food-grade or battery-grade acid is not).

Strength Differences and Mechanical Considerations
Grade 24 has much higher strength than Grade 7 : yield strength of 830-900 MPa for Grade 24 vs. 275-345 MPa for Grade 7 (annealed) . Grade 24 allows thinner walled heater tubes at the same pressure rating for heater tubes under internal pressure or external pressure (evaporator service). For example, a Grade 7 tube requiring 1.65 mm wall for pressure could be substituted with a Grade 24 tube of 1.0-1.2 mm wall. Thinning the wall can save material and enhance heat transfer, but it limits the corrosion allowance. Fluorine containing acid has homogeneous corrosion therefore the thinner wall would have lower life correspondingly. For similar corrosion allowance (say 0.8 mm for 5 year life), Grade 24 will require 0.8 mm + 0.3 mm structure = 1.1 mm Grade 7 will require 0.8 mm + 0.5 mm structural = 1.3 mm. The overall increase in wall thickness is small.

Grade 24 is stronger which is an advantage where mechanical load (pressure, vibration, thermal cycling) is more important than corrosion. In fluorine dominated corrosion the life is dictated by the rate of uniform corrosion and strength is of secondary importance.

WPA Heater Material Application Matrix (2% F-, 90°C)
Material Rate of Corrosion (mm/year)Factor (Relative Cost)Corrosion Allowance (5 Year)Minimum Wall for 5 Year Life Best Application
Grade 2 0.45-0.60 1.0x 2.25-3.00 mm 2.5-3.5 mm Not economic for 5 year life
Grade 7 0.12-0.18 1.4x 0.60-0.90 mm 1.2-1.5 mm Proven standard choice
24 0.10-0.14 2.0-2.5x 0.50-0.70 mm 1.0-1.2 mm Higher cost, negligible corrosion advantage
Hastelloy C-276 0.02-0.05 3.5-4.0x 0.10-0.25 mm 0.5-0.8 mm Best corrosion resistance, highest price
Zirconium 702 0.01-0.02 8-10x 0.05-0.10 mm 0.4-0.6 mm Premium for longest life
Conclusion for WPA Heater Speciation
Under these conditions, titanium Grade 24 (Ti-6Al-4V-0.1Ru) shows a uniform corrosion rate slightly lower than that of Grade 7 (0.10-0.14 mm per year compared with 0.12-0.18 mm per year) in wet-process phosphoric acid at 90°C with 2% fluorine, an improvement of 15-20%. This small advantage does not worth the much greater cost (2-2.5 times Grade 7) for most WPA heater applications. For use in fluorine-containing phosphoric acid, alloy 7 remains the standard, most economical titanium alloy. Grade 24 can be considered for use where higher mechanical strength is needed (e.g. thin-wall tubing for improved heat transfer, or high-pressure evaporator service) and the corrosion benefit is secondary, where the slightly lower corrosion rate increases the interval between replacements from 5 to 6 years, or where Grade 7 is not available and Grade 24 can be substituted. For fluorine concentrations greater than 3% or temperatures greater than 95°C neither Grade 7 nor Grade 24 will give adequate long term life and Hastelloy C-276 or zirconium should be specified. When purchasing heaters for WPA service, order corrosion test coupons in the actual process acid. This is due to the fact that the fluorine content and phosphate rock impurities vary greatly from source to source. For average WPA with 2% fluorine at 90°C, Grade 7 with a wall thickness of 1.65 mm should be specified for 5-year operation. Grade 24 should be specified only where a particular mechanical or thermal advantage justifies the increased cost.