Why Is a Titanium Heater Unsuitable for Heating Phosphoric Acid at Temperatures Above 120°C Despite Its Excellent Performance at 80°C?

For a process engineer specifying a titanium immersion heater for phosphoric acid service such as in fertilizer manufacturing, metal treatment or food processing, the performance of Grade 2 titanium changes substantially between 80°C and 120°C. Titanium exhibits outstanding corrosion resistance in phosphoric acid at 80°C up to 30–40% concentration and corrosion rates less than 0.05 mm/year. But in dilute phosphoric acid (10-20%) even at 120°C the corrosion rate climbs to 0.5-2.0 mm/year - a 10- to 40-fold increase. This rapid acceleration is ascribed to the fact that the protective TiO2 passive coating becomes soluble in phosphoric acid at elevated temperatures, especially in the presence of fluoride impurities or reducing circumstances. Knowledge of this temperature limit allows the engineer to choose alternate materials (tantalum, zirconium, or high‐silicon iron) for phosphoric acid service at high temperature.

Titanium Corrosion Mechanism in Hot Phosphoric Acid
Phosphoric acid ( H3PO4 ) is a mild , non-oxidizing acid. Unlike sulfuric or hydrochloric acid, it does not create a significant cathodic reaction to sustain the titanium passive film. At temperatures below 80 °C, the solubility of TiO 2 in phosphoric acid is low and so the passive film is stable. Above 100°C the solubility increases rapidly. The reaction TiO₂ + 4H⁺ + 4H₃PO₄ → Ti(HPO₄)₂·2H₂O + 2H₂O is proceeding at an increasing rate. The titanium phosphate corrosion product is non-protective, and commonly precipitates as a sludge, exposing fresh metal to continuous attack. Additionally, commercial phosphoric acid contains fluoride impurities (from phosphate rock) or sulfate impurities (from wet-process manufacture) that further increase the corrosion rate of titanium by complexing titanium ions and blocking repassivation.

Temperature Dependent Corrosion Rate of Titanium in Phosphoric Acid – Quantification
Phosphoric Acid Concentration Temperature (°C) Corrosion Rate of Grade 2 Titanium (mm/year)Suitability Recommended Alternative
10% 80 0.02–0.05 Excellent None needed
10% 100 0.10-0.20 Marginal Monitor closely 10% 120 0.50-1.00 Poor Tantalum or zirconium
30% 80 0.03-0.08 Excellent Not required
30% 100 0.15 - 0.30 Marginal Grade 7 upgrade 30% 120 0.80 - 2.00 Unsuitable Tantalum 50% 80 0.05 - 0.15 Acceptable Grade 7 recommended 50% 100 0.30 - 0.80 Poor Tantalum or zirconia
50% 120 2.00–5.00 Unsuitable Tantalum 85% (concentrated) 80 0.10–0.30 Marginal Grade 7 needed 85% 100 1.00–3.00 Unsuitable Tantalum or graphite
A Scenario-Based Guide for Selection of Phosphoric Acid Heaters
Application and Conditions of UseTitanium Suitability Recommended Material Expected Life with Grade 2 (if utilized)
Storage of phosphoric acid (ambient, 25 °C)Excellent Grade 2 titanium 20+ years
Phosphoric acid, dilute 10% (70°C)Grade 2 titanium Excellent 15-20 years
Dilute phosphoric acid (10 %, 100 °C)Marginal Grade 7 titanium or wall thickness 1.5 mm 3-5 years Grade 2 with annual inspection
Dilute phosphoric acid (10%, 120 °C) Unsuitable Tantalum or zirconium < 1 year (Grade 2)
Wet process phosphoric acid (30%, 80°C, fluorides) Poor Zirconium or graphite < 2 years (Grade 2)
Phosphoric acid (85%, 80 °C) Concentrated Marginal Grade 7 titanium or tantalum 2–4 years (Grade 2)
Phosphoric acid with oxidizing impurities (Fe3+, HNO3) OK to 100°C Grade 2 titanium Oxidizers stabilize passive film, increase range
Phosphoric acid containing reducing impurities (H2, sulfides)Not suitable over 60°C Zirconium or tantalumAttack speeds up on lower circumstances
Engineering mitigations for moderate temperature phosphoric acid (90-110 °C)
For the operating temperature in the marginal region (90–110°C) and when replacement of the heater by tantalum is not practical, there are many mitigations that can extend the life of Grade 2 titanium. The best way to counteract this is to add an oxidant to the phosphoric acid (100–200 ppm Fe3+ or 50 ppm hydrogen peroxide). Oxidizers drive the corrosion potential into the passive region and drastically reduce the corrosion rates-by a factor of 5 to 10 at 100°C. A second mitigation is an upgrade to Grade 7 titanium (Ti-0.15% Pd). The inclusion of palladium shifts the corrosion potential to more noble values and Grade 7 can be used at 120°C in 30% phosphoric acid with corrosion rates less than 0.1 mm/year. A third mitigation is to lower the content of phosphoric acid. The corrosion rate at 100°C decreases by 50-70% with the reduction of the operating acid concentration from 30% to 20%.

Conclusion Titanium not suited for phosphoric acid at 120°C and above
Grade 2 titanium is not suited for heating phosphoric acid above 120°C, even at low concentrations (10-20%). Titanium will corrode at rates below 0.08 mm per year at phosphoric acid concentrations of up to 50% at 80°C. The solubility of TiO2 in hot phosphoric acid increases exponentially with temperature. For example, the identical material at 120°C corrodes at 0.5 to 2.0 mm/year, a 10 to 40 fold increase. For applications requiring heating of phosphoric acid above 100°C, Grade 7 titanium (palladium-alloyed) extends the useful range to 110–120°C. Tantalum or zirconium heaters are required above 120 °C. For phosphoric acid service over 80 °C, the supplier needs to be supplied with the specific acid concentration, maximum temperature, and presence of fluoride or oxidizing contaminants to allow proper material choice and avoid early failure.