For a PFA (Perfluoroalkoxy) Coated Titanium Heater Used in a 10% Hydrofluoric Acid Solution at 60°C, How Does the Coating Thickness (0.3 mm vs. 0.8 mm) Change the Time to Diffusion-Limited Attack of the Titanium Substrate After a Pinhole Develops?

PFA-coated titanium heaters in 10% hydrofluoric acid at 60°C depend on the coating for protection of the substrate. If a pinhole develops, HF will diffuse laterally under the coating and attack the titanium. Formation of a pinhole will give 500 – 1,000 hours protection with a 0.3 mm coating and 2,000 – 4,000 hours with a 0.8 mm coating. The increase in lateral diffusion path length due to thicker coating delays the perforation of the substrate . Mechanism of Undercoating Corrosion in HF A pinhole allows HF to reach the titanium substrate. The titanium is corroding: Ti + 6HF = TiF 6 2- + 3H 2 + 2H +. The corrosion product is soluble and does not close the pinhole. HF migrates sideways underneath the PFA covering, which is undermined. The time needed to reduce down the distance L is proportional to L2. Quantitative Undercutting Time as a Function of Coating Thickness Coating Thickness (mm) Undercut Rate (mm/100 h) Time to 1mm' Undercut (hrs) Time to perforation of substrate (hours) 0.2 0.5-1.0 100-200 150-300 0.3 0.3-0.6 150-300 500-1,000 0.5 0.1-0.3 300-1,000 1,000-2,500 0.8 0.05-0.15 600-2,000 2,000-4,000 1.0 0.03-0.10 1,000-3,000 3,000-6,000 Coating Thickness Selection Guide for HF Service Desired Service Life After Pinhole (hours) Minimum Coating Thickness (mm) Suggested 500 0.3 Restricted 1,000 Passable 0.5 2,000 0.8 Sì 4,000 1.0 Yes Informed Specification Specify a coating thickness of 0.8-1.0 mm for PFA-coated titanium heater in 10% HF at 60°C to maximize service life once coating problems emerge. The engineer controls the thickness of the coating to widen the safe operation window.