In a Titanium Electric Heater Used to Prevent Freezing of a 25% Potassium Acetate Runway Deicer at -10°C Ambient, Why Does the Heater Corrode Faster During Standby (No Flow) Than During Active Heating?

Potassium acetate (KAc) runway deicers are employed at 25% concentration at temperatures down to -10°C. Storage tanks are equipped with titanium electric heaters to prevent freezing. During active heating the heater is at 40-60°C and the fluid is liquid and has a natural convection flow. During the off time of the heater the solution cools to ambient temperatures of −10 °C . The potassium acetate solution may become supersaturated and partially freeze or concentrate. Electrical conductivity is greater and pH is lower (hydrolysis) in the concentrated acetate solution. Crevice corrosion is found under ice or under layers of concentrated solution. In idle, the corrosion rate may be 5-10 times higher than during active heating.

The Accelerated Standby Corrosion Process

Active heating . The solution is warm and fluid, oxygen levels are moderate and the titanium passive film is stable . On standby, the solution cools and ice crystals form, concentrating the remaining liquid to 40-60% potassium acetate. The pH of the concentrated solution is 7-8 (vs. 9-10 at 25°C) due to hydrolysis of acetate. High concentration and low pH make the passive film unstable. Also, differential aeration cells are formed between ice-covered and liquid-covered locations. The restricted geometry imposed by ice or concentrated solution films leads to crevice conditions. "Flow is necessary for the dilution of aggressive species.

Quantitative corrosion rates during active heating and standby

Controlled tests in 25% potassium acetate at various temperatures have determined corrosion rates for grade 2 titanium. Corrosion rate is 0.005-0.010 mm/year, passive film is steady and with active heating at 50°C with flow (natural convection) The rate is 0.008-0.015 mm per year in a steady operation at 25°C with flow. When cooled to 0 °C with some ice development, the rate increases to 0.03–0.08 mm per year. The corrosion rate under standby conditions at -10°C with ice (50% ice, 50% concentrated solution) is 0.08-0.15 mm/year, which is 10-20 times higher than the active heating rate. At standby at -10°C and full freezing (no liquid phase) the corrosion rate is reduced to 0.01-0.03 mm/year as there is no electrolyte.

Standby corrosion of cold rolled steel with regard to the effect of freeze-thaw cycles and solution concentration

The degree of standby corrosion is related to the number of freeze-thaw cycles and the concentration of the residual fluid. In one freeze-thaw cycle, the maximum concentration is 45-50% KAc. The corrosion rate in the thawed period is 0.05-0.10 mm per year. The solution is further concentrated to 55–60% KAc after 10 freeze-thaw cycles, and the corrosion rate increases to 0.10–0.20 mm per year. The corrosion rate at 0 °C is 0.15–0.30 mm/year at a concentration of 60% KAc with a pH of 7.0. The corrosion rate at 10°C at 70% concentration (close to saturation) is 0.20-0.40 mm/year.

Runway Deicer Heaters Standby Corrosion Mitigation Guidance

Recommendations for preventing rapid corrosion during standby in 25% potassium acetate runway deicer storage tanks are summarized in the accompanying table.

Standby Time (hours) Ambient Temperature (°C)Recommended Action Expected Corrosion Rate (mm/year)
<24 -5 None (minimum freezing) <0.02 24–72 -5 to -15 Heater on low power (10% duty) <0.02 >72 -5 to -15 Periodic solution circulation 0.02–0.04
Any -15 to -25 Insulate tank + trace heating 0.01-0.03
Any <0.01 Use heated enclosure or indoor storage
Engineering Beyond Standby

The titanium grade impacts the stand-by corrosion resistance. Grade 7 (palladium stabilized) has a corrosion rate 3-5 times lower in concentrated acetate solutions, lowering standby attack by 70-80%. Grade 12 does intermediate improvement. Corrosion allowance is wall thickness . If wall is 2.0 mm and corrosion allowance is 0.10 mm per year in standby , it will last 20 years . When the heater is installed horizontally, there will be no fissure in which the solution might pool concentrated. A modest recirculation pump is used during standby to prevent concentration gradients. A heating cable wrapped around the tank keeps the solution temperature above 0°C, so that no freezing occurs at all.

Making a Well-informed Specification

Design a titanium electric heater to be used as a runway deicer (25% potassium acetate) at an ambient temperature of -10°C. The heater should be maintained on low power (10% to 20% of rated power) during standby to maintain the surface temperature above 0°C and prevent ice formation. For existing heaters that suffer from standby corrosion, install a recirculation pump that operates for 10 minutes every hour during standby. Use Grade 7 titanium for new installations. When operating , watch the current through the heater . If the voltage stays constant but the current increases , ice is forming and must be dealt with immediately . The engineer prevents ice formation by low-power standby operation to eliminate the accelerated corrosion that occurs during passive standby in potassium acetate runway deicer service.