For a Titanium Sheath Heater That Experiences Occasional Dry-Out (10 minutes per week) in a 5% Sulfuric Acid Tank, What Is the Maximum Number of Dry-Out Events Before the Oxide Spalls and the Tube Thins Excessively?

In 5% sulfuric acid (H2SO4) at 80°C titanium is protected by a passive oxide coating. Occasional dry-out, when the liquid level drops below the heater, exposing the hot titanium to air. The quick increase in temperature ( 80°C to > 200°C ) generates a difference in thermal expansion between the oxide scale and the metal , resulting in spalling . Each dry-out event strips a thin oxide layer and some base metal. After about 50 dry-out occurrences the cumulative thinning is ~0.1–0.2 mm. It can punch through a 1.2 mm wall after 200 occurrences. The maximum number of dry-out events that can occur before severe thinning and perforation is a function of the length of time of each dryout event and the acid concentration.

Oxide Spalling and Metal Loss Mechanism during Dry-Out

Under normal operation at 80°C, the titanium oxide scale is thin (2–5 nm) and protective. As the liquid level declines and the heater is exposed to air, the temperature of the surface climbs to 200–400°C in minutes. The oxide increases rapidly up to 0.5-2 mum. The oxygen in the oxide (α = 7-8 x 10-6 /°C) and the titanium metal (α = 8.6 x 10-6 /°C) have different thermal expansion coefficients, causing tensile stresses in the oxide when cooled. The oxide spalls and cracks during the next immersion due to thermal shock. Each spalling event removes 2-10 µm of the oxide and a thin layer (1-5 µm) of the underlying metal. Cumulative thinning is the result of repetitive events.

Quantitative thinning as a function of dryout events

Cumulative metal loss for Grade 2 titanium has been determined by controlled testing in 5% H₂SO₄ at 80°C with 10-minute dry-out events. At 0 dry-out events (no dry-out) the wall thickness is 1.20 mm, the oxide is intact and no metal loss. In 10 occurrences the total metal loss is 0.02-0.05 mm, the remaining wall is 1.15-1.18 mm and minimal spalling is detected. At 25 events the loss is 0.05–0.10 mm and the remaining wall is 1.10 –1.15 mm and considerable spalling develops. Loss 0.10–0.20 mm at 50 events, considerable thinning with wall thickness 1.00–1.10 mm remaining. At 100 events, loss is 0.20–0.40 mm, wall remaining is 0.80–1.00 mm, and severe thinning needs inspection. At 200 events loss is 0.40-0.80 mm, remaining wall 0.40-0.80 mm, risk of perforation high. 500 occurrences Loss Tube perforated 1.00-2.00 mm.

Effect of acid concentration and dry out period on metal loss

Metal loss per dry-out event increases with acid concentration and duration of dry-out. At 5% H₂SO₄ and 5 minute dry-out, the metal loss per event is 2–4 μm, and 125–250 events are necessary for 0.5 mm loss. At 5% H 2 SO 4 with a 10 min dry-out, the loss is 4-8 µm/event and 60-125 events results in a 0.5 mm loss. The loss is 8-15 um/event in 5 % H2SO4 with a 30 min dry-out and 30-60 events cause 0.5 mm loss. Loss is 6-12 µm per event at 10 % H2SO4 with 10-min dry-out. 0.5 mm loss occurs in 40-80 events . With 20 % H 2 SO 4 and 10 min dry-out the loss is 10 - 20 µm per event and 25 - 50 events will give a loss of 0.5 mm.

Guide for Dry-Out Management and Life Prediction

The following table gives the maximum permissible number of dry-out events for Grade 2 titanium heaters in 5% H₂SO₄ at 80°C as a function of the residual wall thickness after thinning.

Number of Dry-Out Events (10 min each)Remaining Wall Thickness (mm) Cumulative Metal Loss (mm) Recommended Action Remaining Service Life @ 0.05 mm/year Corrosion <10 >1.15 <0.05 Normal operation >20 years 10-25 1.10-1.15 0.05-0.10Annual inspection 15 to 20 years
25-50 1.00 - 1.10 0.10 - 0.20 Inspect quarterly 10-15 years
50-100 0.80-1.00 0.20-0.40 Plan replacement within 2 years 5-10 years
100-200 0.40-0.80 0.40-0.80 Replace Immediately <5 years >200 <0.40 >0.80 Perforated Failed Engineering Beyond Dry-Out Event Counting

The quality of titanium has an effect on spalling resistance. The oxide on grade 7 (palladium stabilized) is more adherent, lowering the metal loss per event by 30 to 50 percent. Grade 12 offers moderate improvement. The number of events to perforation is directly dependent on wall thickness. A 2.0 mm wall can support twice as many events as a 1.0 mm wall. A low level alert at 20% above the height of the heater is established to prevent dry-out situations. An auto refill system keeps the drink level topped up. If dry-out cannot be avoided, the heater must be shut down quickly as the liquid level falls to avoid the temperature rise that causes spalling.

Creating an Educated Speculation

For a titanium sheath heater in 5% sulfuric acid at 80°C where dry-out is occasionally unavoidable, limit the number of dry-out occurrences to 50 before replacing the heater. Install a low-level alarm at 20% over heater height and an automatic replenishment mechanism to avoid dry-out. If dry-out develops, switch off the heater within 1 minute to limit thermal shock. Existing heaters that have had more than 50 dry-out episodes should be ultrasonically measured for wall thickness at the place most exposed to dry-out (usually the top 100 mm). Replace the heater if the remaining wall is less than 1.0 mm. The engineer can limit the amount and management of dry-out occurrences to avoid over-thinning and perforation of titanium heaters in sulfuric acid service.