How Does the Wet-Bulb Temperature of the Ambient Air Affect the Corrosion Rate of the Unsubmerged (Vapor Phase) Section of a Titanium Heater?

The corrosion rate of the unsubmerged (vapor phase) section of a titanium immersion heater being operated by a process engineer in an open tank where the top portion is exposed to vapor and ambient air is directly influenced by the wet-bulb temperature of the ambient air. The equilibrium moisture content on the titanium surface is determined by the wet-bulb temperature. If the wet-bulb temperature is high (e.g., >25°C, high humidity), moisture will condense on the colder titanium surface (if the surface temperature is below the dew point). This layer of condensation contains dissolved gases (oxygen, carbon dioxide, chlorides) and can be corrosive. If the wet-bulb temperature is low (e.g. < 15°C, dry air), the titanium surface stays dry and passive and corrosion is insignificant. For example, a titanium heater in a hot (80°C) bath of acidified water at ambient wet-bulb temperature of 28°C can suffer pitting corrosion at rates of 0.05-0.2 mm/year in the unsubmerged section, whereas the identical heater in a dry climate (wet-bulb 10°C) exhibits no significant corrosion.

Vapor Phase Corrosion Mechanism of Titanium
The unsubmerged part of a titanium heater is exposed to the vapor from the process liquid and the ambient air. The vapor consists of volatile species (HCl, mist of H2SO4, organic acids or water vapor). If the ambient wet-bulb temperature is high and the surface temperature is lower than the dew point of the vapor-air mixture, moisture will condense on the titanium surface. The liquefied condensate produces a thin electrolyte coating (usually 1-100 µm thick) which absorbs corrosive gases (CO₂, SO₂, Cl₂) from the air and volatile acids from the vapor. The film is very hostile because of its large surface area to volume ratio and because it concentrates corrosive species as it evaporates. Under these conditions it is possible to disrupt the passive film of titanium and to initiate pitting or crevice corrosion at the vapor-liquid interface.

Quantification of vapor phase corrosion rate vs wet-bulb temperature
Ambient Wet-Bulb Temp (°C) Relative Humidity (%) Condensation on Ti Surface (at 40°C Surface Temp)Vapor Phase Corrosion Rate (mm/yr)Dominant Species Level of risk < 10 < 30 None (dry) < 0.005 None NegligibleOccasional (intermittent) 0.005-0.01 Mist of diluted acidLow 15-20 50-70 Frequent (thin film) 0.01-0.05 Acid mist + chlorides20-25 70-85 Continuous (thin film) 0.05-0.15 Concentrated acid + chlorides High 25-30 85-95 Continuous (thicker film) 0.15-0.30 Highly concentrated speciesSevere > 30 > 95 Flooding (dripping) 0.10-0.20 (washed) Dilution by dripping Moderate (washed)
A Scenario Based Guide to Vapor Phase Corrosion Control
Vapor Composition Liquid ProcessAmbient Wet-Bulb Temp (°C) Corrosion Risk in Unsubmerged PortionMitigation recommended
Hot water, neutralWater vapor only Any Low No action needed
Dilute HCl (5%) HCl mist + water vapor 15 Low Titanium vaporizes (no effect)
HCl mist + water vapour Dilute HCl (5%) 25 Moderate Vapour phase section coated with PTFE
Concentrated HCl (20%) HCl vapour + water vapour 25 High Use sealed tank with vapour extraction
Hot brine (20% NaCl) Water vapor + salt mist 28 High (pitting) Section of the electropolish vapor phase. Add water spray to avoid salt build up.
50% Sulfuric acidH₂SO₄ mist + water vapor 25 Moderate to high Lengthen heated portion to keep surface above dew point
Chromic acid (plating bath) Chromic acid vapour 20 Low (chromate inhibits)No action required (Chromates passivate)
Organic solvent (no water) Solvent vapor Any Very low No condensation of water. Surface is dry.
Engineering Approaches to Reduce Vapor Phase Corrosion
When vapour phase corrosion takes place on the unsubmerged portion of a titanium heater due to high wet-bulb temperatures ( > 20°C), four mitigations are effective. The first mitigation is to raise the heated part higher above the liquid level. The temperature of the titanium surface must be maintained above the dew point (e.g. > 45°C for 25°C wet-bulb) for condensation to be avoided. This needs the heater to be constructed so that the heating element extends into the vapor phase region, keeping the titanium sheath heated. The second mitigation is a PTFE or fluoropolymer covering on the above water area. The coating stops the condensed electrolyte from attacking the titanium surface. The final mitigation is to install a vapor extraction device (exhaust hood) to remove corrosive vapors and minimize humidity over the tank. Condensation is avoided if the wet-bulb temperature is reduced below 15°C. The fourth mitigation is the employment of a water spray or mist eliminator to continuously cleanse the unsubmerged portion to prevent concentration of corrosive species.

Conclusion: Vapor phase corrosion up to 0.3 mm/year is due to high wet-bulb temperature (> 25°C)
The wet-bulb temperature of the ambient air directly impacts the corrosion rate of the unsubmerged (vapor phase) portion of a titanium heater. If the wet-bulb temperature is below 15°C (dry conditions), the titanium surface will stay dry and non-corrosive. When the wet-bulb temperature is over 25 °C (humid conditions), moisture condenses on the cooler titanium surface and forms a thin film of electrolyte, which absorbs corrosive gasses and concentrates species by evaporation. In harsh environments (e.g. hot HCl or brine with high ambient humidity), vapor phase corrosion rates can be as high as 0.15-0.30 mm/year, leading to pitting and perforation of the unsubmerged part in 2-5 years. Mitigations include heating the part above the dew point that is not submerged, applying PTFE coatings or implementing vapor extraction to minimize the wet-bulb temperature. For any application with corrosive vapors and high ambient humidity, specifying a titanium heater will require the supplier to be provided with the projected wet-bulb temperature range so that the unsubmerged part can be designed appropriately (extended heating, coating or material upgrading).