For Acidic Zinc Plating Baths (pH 3.5) at 55°C, How Does the Presence of Fluoride Ions Above 50 ppm Necessitate a Shift from Titanium to Hastelloy?

Titanium Grade 2, in most instances, manages an acidic zinc plating solution - generally chloride based or sulfate chloride formulations working at pH 3.5 and 55°C - which is a somewhat harsh environment. However, many modern high speed zinc plating operations add fluoride or fluoroborate ions to improve conductivity, increase throwing power or activate difficult to plate substrates. At concentrations of fluoride ions beyond 50 ppm, the corrosion behavior of titanium is fundamentally changed, leading to rapid disintegration of the passive layer and uniform attack that can perforate a standard gauge heater tube in weeks. This article provides a quantification of the threshold fluoride concentration at which titanium is not useable and defines the parameters under which Hastelloy C-276 or C-22 is the needed heater sheath material.

Mechanism of attack of titanium by fluoride
Titanium has excellent corrosion resistance due to its stable, adhering passive film of TiO 2. This film is attacked by fluoride ions in a chemical dissolution reaction . TiO 2 + 6F − + 4H + → TiF 6 2 − + 2H 2 O The hexafluorotitanate compound is soluble and so the passive coating is chemically eliminated and not just penetrated. As the layer dissolves, the underlying titanium metal is exposed to the acid bath and corrodes readily. Fluoride assault is widespread and homogeneous, unlike localized pitting by chlorides, where the passive coating between pits remains intact.

The rate of passive film dissolution is controlled by three factors: fluoride concentration, pH and temperature. The critical fluoride concentration for Grade 2 titanium is about 30 ppm at pH 3.5 and 55°C . When the potential is below 30 ppm, the passive film dissolves slowly and a steady-state thickness is maintained via repassivation. In the 30-50 ppm range, the film dissolution exceeds the repassivation and the corrosion rate increases to 0.1-0.3 mm per year. Above 50 ppm the film cannot renew and the titanium actively corrodes at rates of 1-5 mm per year depending on exact conditions. Grade 2 titanium will corrode at about 3 mm per year in 100 ppm fluoride - enough to put a hole in a 1.65 mm wall tube in six to eight months.

The Synergy of Temperature and pH
High temperature exponentially accelerates fluoride attack. At 55°C, and 50 ppm fluoride, Grade 2 has a corrosion rate of about 0.4 mm per year. At 65 °C the rate climbs to 1.2 mm per year at the same fluoride content. Zinc plating baths run at 55°C are already close to the limit in terms of temperature sensitivity, and any excursion to 60-65°C can rapidly increase the rate of attack.

Since hydrogen ions are consumed in the dissolving event, the attack of fluoride is worse at low pH. The reaction is moderate at pH 3.5. for pH 2.5 the rate is about doubled for the same fluoride concentration. Zinc plating baths are normally operated at pH 3.5-4.0, but localized pH drops can be observed near the anode or in stagnant zones.

And Others, Grade 7, Titanium Alloys
Grade 7 titanium (Ti-0.15Pd) has better resistance to reducing acids but is not resistant to fluoride attack. The presence of palladium increases the kinetics of the cathodic reaction and promotes passivation in acids where the passive film is preserved, but it cannot prevent the chemical dissolution of TiO2 by fluoride ions. Tests indicate that Grade 7 will corrode at around 70% of the rate of Grade 2 in fluoride-containing solutions -an improvement, but not enough to offer an acceptable service life above 50 ppm fluoride. For example Grade 2 will corrode at 0.8 mm per year at 55°C and 75 ppm fluoride and Grade 7 will corrode at 0.5-0.6 mm per year. Both are too high for a 1.65 mm wall tube.

Hastelloy as an Alternative
Nickel-based Hastelloy alloys (C-276 and C-22) resist fluoride attack by a different mechanism. The passive film on Hastelloy C-276 contains less acid-soluble nickel and chromium oxides than TiO2. Corrosion rates of less than 0.05 mm per year were observed for Hastelloy C-276 in 50-200 ppm fluoride at pH 3.5 and 55°C. For fluoride at 100 ppm the rate is less than 0.10 mm/yr. The alloy also exhibits resistance to pitting by chlorides in zinc plating baths.

Heater Zinc Plating Application Matrix
Bath Condition Fluoride Concentration (ppm) Recommended Heater Sheath Material Expected Corrosion Rate Estimated Life (1.65 mm wall)
Zinc chloride standard (no fluoride) <10 ppmGrade 2 titanium < 0.02 mm/year > 10 years
Low-fluoride additive package 10-30 ppm Grade 2 titanium, weekly monitor 0.03-0.08 mm/year 5-10 years
Moderate fluoride (standard fluoroborate) 30-50 ppm Grade 7 titanium or Hastelloy 0.10-0.25 mm/yr 3-6 years
High-fluoride (high speed bath) 50-100 ppm Hastelloy C-276 0.05-0.10 8-12 years mm/year
Very high fluoride (>100 ppm) 100 to 500 ppmHastelloy c-22 or Tantalum 0.10-0.20 mm/yr 5-8 years
Zinc Plating Heater Specification Conclusion
The replacement of the immersion heater sheath material from titanium to Hastelloy is necessary for fluoride ions exceeding 50 ppm in acidic zinc plating baths at pH 3.5 and 55°C. Grade 2 titanium is the most cost effective above 30 ppm fluoride. Grade 7 titanium could provide sufficient service life in the 30 to 50 ppm range if vigilantly monitored, but many engineers prefer Hastelloy for reliability. Titanium, in all grades, will fail from months to 2 years if chloride content exceeds 50 ppm. Use Hastelloy C-276 or C-22. When ordering heaters for zinc plating lines, ask for a thorough bath chemistry analysis including the fluoride concentration, and select the sheath material according to the highest expected fluoride level, not the nominal figure. This proactive technique avoids abrupt heating failures that stop production and pollute plating baths.