When Heating Phosphoric Acid (30-50%) at 100°C, Why Does Dissolved Iron Contamination Accelerate Titanium Corrosion Beyond Acceptable Rates?

Titanium heating equipment in phosphoric acid service exhibits contradictory behavior. Titanium grade 2 in chemically pure 30-50% phosphoric acid at 100°C has a low corrosion rate of 0.05-0.10 mm per year, which is well within acceptable limits for heater tubes. However, industrial wet-process phosphoric acid (WPA) obtained from the digestion of phosphate rock contains many impurities, and dissolved iron (Fe2+ and Fe3+) has the most devastating effect on titanium corrosion resistance among them. Even 0.5% iron pollution can increase titanium corrosion rates by a factor of 10 to 50, leading to tube perforation in months rather than years. This article describes the electrochemical mechanism of iron-induced accelerating corrosion and determines safe limits of iron content for the operation of titanium heaters.

Corrosion acceleration mechanism via iron
The passive film on titanium in phosphoric acid is mainly composed of TiO 2 with integrated phosphate species. The film is stable between a range of electrochemical potentials. The dissolved iron ions, mainly Fe³⁺, are redox shuttles cycling between Fe³⁺ and Fe²⁺ at the titanium surface. Each Fe³⁺ ion that contacts the surface takes an electron from titanium (reducing to Fe²⁺), then diffuses away and is reoxidized to Fe³⁺ by dissolved oxygen. This constant redox cycling successfully transfers electrons from the titanium metal to the bulk solution, so bypassing the protective passive coating.

Electrochemical measurements show the magnitude of this effect. The corrosion potential of titanium Grade 2 in pure 40% H₃PO₄ at 100°C stabilizes between +150 and +250 mV vs. SCE with a passive current density of 5 µA/cm². The addition of 0.5% Fe3+ (in the form of ferric phosphate) raises the corrosion potential to +400 to +550 mV vs. SCE and boosts the passive current density to 50-100 µA/cm2. At 1.0% iron the current density is 200-300 µA/cm 2 and corresponds to a corrosion rate of 0.5-0.8 mm per year. At 2.0% iron, rates are greater than 1.5 mm/year.

**Heater Service Critical Iron Limit**
The long-term exposure test of titanium Grade 2 in phosphoric acid at 100°C finds a practical limit for iron. At dissolved iron below 0.2% corrosion rates are below 0.10 mm/year, sufficient for 10 years heater life with 1.5 mm wall thickness. The rates are 0.15-0.30 mm/year at 0.2 to 0.5% iron which would reduce the projected life to 3-5 years with typical wall thickness. Corrosion rates above 0.5% iron are too high (more than 0.5 mm per year) and involve localized pitting which contributes to additional metal loss.

Wet-process phosphoric acid usually contains 0.8-1.5% dissolved iron from phosphate rock impurities. WPA evaporators and concentrators use titanium heaters, which corrode rapidly in the absence of iron removal pretreatment or upgrading to a more resistant titanium alloy. Grade 7 titanium (Ti-0.15Pd) improves iron tolerance, with rates below 0.15 mm per year at 0.8% iron, however even Grade 7 fails above 1.2% iron.

Application Matrix and Mitigation Strategies
Source of Phosphoric AcidTypical Iron Content Recommended Heater Material Expected Corrosion Rate Service Life (1.65 mm wall)
Purified (thermal process) Grade 2 titanium <0.05% 0.05-0.08 mm/year 15-20 years WPA, solvent-extracted Grade 2 titanium (1.65-2.0 mm) 0.2-0.4% 0.10-0.15 mm/year 8-12 years WPA, untreated (low iron rock) Grade 7 titanium (2.0 mm) 0.5-0.8% 0.12-0.20 mm/year 6-10 years WPA, untreated (high iron rock) Grade 7 + iron removal system 0.8-1.5% 0.20-0.40 mm/year 3-5 years
Superphosphoric acid (>70% P₂O₅) Variable Zirconium or tantalum N/A Titanium not recommended
Summary of Phosphoric Acid Heater Spec
Dissolved iron contamination more than 0.2% in phosphoric acid at 100°C promotes the corrosion of titanium by a redox cycling mechanism that circumvents the protection afforded by the passive film. For wet process acid with typical iron content of 0.8 to 1.5 %, Grade 2 titanium heaters will fail in 2 to 3 years. Engineers are faced with the choice of specifying Grade 7 titanium (which pushes the iron tolerance to approximately 0.8%) or adding iron removal steps (solvent extraction, ion exchange) upstream of the heater. When buying titanium heaters for phosphoric acid service, always ask for a corrosion test coupon that has been subjected to the actual process acid, and demand a minimum wall thickness of 2.0 mm for Grade 2 or 1.65 mm for Grade 7 in iron-bearing acid.