In Jet Fuel Storage Tanks Heating Recovered Water (200 ppm Chloride, pH 6) to 80°C, What Minimum Titanium Grade 2 Wall Prevents Pitting After 5 Years?

Jet fuel storage tanks can collect water at the bottom from condensation and fuel handling. The recovered water is generally saline (100 to 300 ppm of chlorides from air and marine sources), includes dissolved oxygen and trace hydrocarbons. Generally, the water layer is heated to 65-80 °C using titanium immersion heaters to prevent microbial growth and ice formation. Titanium Grade 2 is resistant to pitting in chloride solutions up to a critical pitting temperature (CPT) which is dependent on the chloride content. Grade 2 is operated at its CPT limit and may undergo pitting for prolonged periods of time at 80 °C and 200 ppm Cl⁻. The wall thickness gives a corrosion allowance - pits must permeate the entire wall to trigger failure. This paper determines the minimal wall thickness to avoid pitting perforation throughout a 5-year service life.

200ppm Cl{sup -} Pitting Behavior
A chloride concentration of 200 ppm (0.02%) is modest compared to saltwater (19,000 ppm) or brine. The CPT of Grade 2 in 200 ppm Cl⁻ is about 85-90°C. At 80°C, the alloy is below its CPT and thus, under ideal conditions, pitting should not initiate. However, in jet fuel tank service, the effective CPT is reduced by many factors:

Stagnant conditions: No or low water flow causes local chloride accumulation at the heater surface.

Deposits Crevice-like conditions under deposits produced by microbial slime or hydrocarbon films.

Thermal cycling: Turning the heater on and off can disrupt the passive film.

Low oxygen – Biological activity can use up dissolved oxygen and reduce the ability to re-passivate.

The actual CPT under these field conditions may be 5 to 10 °C lower than the value obtained in the laboratory. However, at 80°C pitting may occur on prolonged exposure (months to years) despite low bulk chloride concentrations .

Comparison of Propagation Rates
Once pits start, they propagate at a rate that is dependent on the concentration of chloride, temperature and available oxygen. For Grade 2: At 80°C in 200 ppm Cl-:

Time to first stable pit (pit start time): 1,000-3,000 hours (6 weeks to 4 months) in field circumstances. Under clean laboratory circumstances initiation may not happen at all.

Pit propagation rate after initiation: 0.05 to 0.15 mm/year at 80°C in 200 ppm Cl⁻. The pace is somewhat sluggish, as the chloride concentration is low; For comparison, propagation rates in saltwater (19,000 ppm Cl-) are 0.5-2.0 mm/year.

For a 1.65 mm wall tube time from pit initiation to perforation = wall thickness / propagation rate = 1.65 / 0.10 = 16.5 years. This assumes, however, a single pit propagating continuously. Indeed, several pits can initiate and the propagation rate of pits can increase with pit depth due to local acidification. The estimated time from initiation to perforation is conservatively 8 to 12 years.

Wall Thickness versus 5 Year Survival
Wall Thickness (mm) Expected Pit InitiationTime to Perforation After Initiation Probability of 5-Year Survival 0.9 mm Yes (1,000-3,000 hours)5-8 years 70-80% (some chance of)
1.2 mm Yes (1,000 to 3,000 hours)7-12 years 85-95% 1.65 mm (BWG 14)Delayed, but possible 10-15 years 95-99%
2.0 mm Likely but delayed 12-18 years >99%
The minimum wall thickness required to achieve 95%+ probability of no pitting perforation over 5 years is 1.65 mm. At 1.2 mm, there is a 5-15% chance that a pit that initiates early may propagate through the wall within 5 years.

JET FUEL TANK WATER HEATERS Application Matrix Service Condition Chloride Concentration Temperature Recommended Wall Thickness (Grade 2) Expected 5-Year Survival
Good circulation, clean recovered water 100-200 ppm 65-70 °C 1.2 mm >95 %
Typical jet fuel tank (stagnant, some deposits) 150-250 ppm 75-80°C 1.65 mm 95–99%
Marine environment (high chloride incursion) 200-400 ppm 80C 2.0 mm >95%
Poor maintenance (biofilms, sludge)200-300 ppm 80°C 2.0 mm + Grade 7 99%+ Any service with chlorides >500 ppm >500 ppm 80°C Not Grade 2; use Grade 7 Grade 7 necessary
Practical Use Suggestions
In a 200 ppm Cl- recovered water at 80°C jet fuel storage tank situation, a Grade 2 titanium tube (BWG 14, min. wall thickness 1.65 mm) will not be pitting perforated in a 5 year service life with 95-99% confidence. Thinner walls (1.2 mm) may survive, but the risk is greater. The low chloride concentration leads to sluggish pit growth rates (0.05-0.15 mm per year) and even if pits form, a 1.65 mm wall leaves a significant safety buffer. For tanks in coastal conditions or with chloride levels exceeding 300 ppm specify 2.0 mm wall or upgrade to Grade 7. Ask for pickled surface polish on purchase (removes iron pollution that might start pitting). Perform an annual inspection. Remove the heater. Visually inspect for pits (10x magnification). Measure any pits using a depth gauge. For pits > 0.3 mm deep, replace heater every 5 years. Grade 2 provides a cost-effective, reliable service in jet fuel tank water heating applications with proper wall thickness selection and periodic inspection. For new installations, specify 2.0 mm wall thickness or Grade 7 with 1.65 mm wall, if 10 year life is desired without inspection.