For Pigment Production (TiO₂ via Chloride Process) Heating Ferric Chloride Solution (25%) at 70°C, Which Titanium Alloy Resists Abrasion-Corrosion from Solid TiO₂ Particles?

The chloride procedure for the synthesis of titanium dioxide (TiO 2 ) pigment uses a 25% ferric chloride (FeCl 3 ) solution at 70°C as a chlorinating agent. The solution contains suspended TiO2 particles (1-10 µm diameter) that are very abrasive. The heating tubes immersed in this slurry are simultaneously exposed to abrasion (mechanical removal of material by solid particles) and corrosion (electrochemical dissolving of exposed metal). This synergy-abrasion-corrosion leads to material loss rates significantly higher than either mechanism alone. The study evaluates the abrasion-corrosion resistance of titanium grades (2, 7, 12 and 24) in TiO 2-laden ferric chloride and defines the optimum alloy for this service.

Abrasion Corrosion Mechanism
On titanium, the passive protective coating is continually destroyed by particle impact in a corrosive slurry with particles. The aggressive FeCl3 solution exposes fresh bare titanium which corroded rapidly and then repassivated, only to have the new film removed by the next particle impact. The synergy factor (total wear/sum of pure abrasion and pure corrosion) is between 2 and 10 for titanium alloys.

Critical parameters : Particle hardness (TiO2 is hard, Mohs 6-7) Particle shape (Angular particles are more abrasive) Velocity (Higher velocity increases impact energy) Angle of impact (Normal impacts cause more damage than glancing)

Comparative Tests
Slurry erosion-corrosion controlled tests with a spinning disc (linear velocity 2 m/s, TiO2 particles 10 wt%, FeCl3 25%, 70°C, 100 h):

Grade 2 (commercially pure): Passive film removed quickly. Corrosion rate at abrasion 0.8-1.2 mm/year. Deep gouges and pits on surface. After 100 h the weight loss equates to a penetration of 0.1 mm. Grade 2 doesn't work.

Grade 7 (Ti-0.15Pd): Palladium produces cathodic modification marginally improving corrosion resistance but not increasing hardness. The rate of abrasion-corrosion is 0.5-0.9 mm/year. It's more than 2nd class, but still high. Soft matrix (~180 HV hardness) is easily worn out.

Grade 12 ( Ti-0.3Mo-0.8Ni) Hardness is raised to about 200-220 HV by molybdenum and nickel. The rate of abrasion-corrosion is 0.3-0.6 mm per year. Good for moderate service but not the best. The improved performance is due to tougher surface not increased corrosion resistance.

Grade 24 (Ti-6Al-4V-0.1Ru): alpha-beta alloy with a hardness of 320-360 HV (annealed), heat-treatable to 380-400 HV. Ruthenium (0.1%) gives corrosion resistance similar to Grade 7. Rate of abrasion-corrosion: 0.08-0.15 mm/year, 5-10 times better than Grade 2. The hard surface resists particle impact and so maintains the passive film longer. After 100 h only superficial scratching was observed, no scratching.

Hardness as a Critical Parameter
Titanium Grade Hardness (HV, annealed) Abrasion-Corrosion Rate (mm/year)Relative Life Factor Cost Factor
Grade 2 150-180 0.8-1.2 1x 1.0x
Grade 7 150-180 0.5-0.9 1.3-1.6x 1.4x Grade 12 200-220 0.3-0.6 2-3x 1.3x
Grade 24 (annealed) 320-360 0.08-0.15 6-12x 2.0-2.5x
Grade 24 (aged) 380-400 0.05-0.10 10-15 times 2.2-2.8 times
Hastelloy C-276 (for reference) 250-300 0.08-0.12 Similar to Grade 24 3.5-4.0x
Application Matrix for TiO2 Slurry Heaters
Service Condition Particles/cm3Particle Size Flow Velocity Recommended Titanium Alloy Expected Life (1.65 mm wall)
Low solids, low velocity <5 wt% <5 µm <1 m/sec Grade 12 3-5 years
Typical moderate solids 5-15 wt %5-15 µm 1-2 m/s Grade 24 (annealed) 5-8 years old
High Solids 15-25 wt% 10-50 µm 2-3 m/sec Grade 24 (Aged) 4-6 years
Any service with big angular particles >50 µm 3 m/s Grade 24 with thicker wall (2.5 mm) 5-7 years
Severe service (reboiler, high turbidity)Any Any >3 m/s Grade 24 or Hastelloy C-276 Design specified
Practical implications
Grade 24 titanium (Ti-6Al-4V-0.1Ru) offers the best resistance to both abrasion and corrosion in the production of pigments by heating ferric chloride (25%) with suspended TiO₂ particles at 70°C. Its high hardness (320-400 HV) prevents particle impact, while the ruthenium gives it corrosion resistance comparable to Grade 7. Expected life with 1.65 mm wall is 5-8 years-6-12 times longer than Grade 2. Grade 12 is adequate for low velocity or low solids service but will fail prematurely under normal conditions. No particle service is suggested in this environment grade 2 and grade 7. For Grade 24, order annealed or aged condition as required by hardness specifications. For a new installation, consider Grade 24 tubes with 2.0 mm wall for additional safety margin. - Establish an annual ultrasonic thickness monitoring program at bends and inlet zones where particle impact is most severe. If you have a Grade 2 or Grade 12 heater that fails early, consider upgrading to Grade 24 at the time of the next replacement. The increased cost of grade 24 (2-2.5 times more than grade 2) is compensated by greater service life and less heater replacement down time.