How Maskant Works in Aerospace Chemical Milling
Chemical milling is one of the most technically demanding applications for maskant in all of manufacturing. It defines the shape of aerospace structural components — fuselage skins, wing panels, bulkheads — by selectively removing material through controlled chemical etching. The maskant is not incidental to this process; it is the tool that determines where material is removed and where it is not, and formulations for this use are commonly qualified against specifications such as SAE AMS-C-81769. Understanding how maskant functions through the process cycle explains why these maskants, discussed more broadly in our overview of maskant in industrial surface protection, are engineered to tolerances general-purpose masking materials cannot meet. The Chemical Milling Process Overview Chemical milling removes metal by immersing a masked part in an etchant solution that dissolves exposed metal at a controlled rate. The sequence runs: prepare the surface (clean and deoxidize to remove oils, oxide layers, and contaminants that would prevent maskant adhesion or create variable etch rates); apply maskant (brushed, sprayed, or dip-applied to the entire part, then cured); scribe the pattern (cut along the design boundary and peel maskant from the areas to be etched); etch (exposed metal dissolves at a calibrated rate while masked metal stays protected); and rinse and strip (after the specified etch depth is reached, remaining maskant is stripped from the protected areas). Each step has specific maskant requirements, and performance through the entire sequence determines whether the finished part meets dimensional specifications. How Maskant Resists Etchant Chemistry Aerospace chemical milling uses different etchant chemistries for different alloys. Aluminum alloys are chemically milled in sodium hydroxide (caustic soda) solution, typically at 70–85°C, with etch rate controlled by NaOH concentration and temperature — for aluminum removed at 0.025 mm per minute, a typical production rate, the bath is aggressive enough to attack most organic materials not specifically formulated to resist alkaline solutions. Aerospace chemical milling maskants for aluminum are typically neoprene (polychloroprene) rubber compounds, which resist alkaline chemistry well at elevated temperature because the polymer backbone lacks the ester or ether linkages that are susceptible to hydrolysis under alkaline attack. The maskant holds its integrity — no swelling that would allow etchant penetration, no adhesion loss that would allow undercutting — for etch cycles that may run several hours. Titanium alloys, by contrast, are milled in hydrofluoric acid / nitric acid mixtures — a chemistry far more aggressive toward polymer maskants than alkaline aluminum etchant — so titanium chemical milling maskants use butyl rubber or proprietary synthetic rubber compounds with demonstrated resistance to HF/nitric acid at production concentrations and temperatures. Our comparison of maskant types for metal etching covers how neoprene, butyl, and silicone chemistries stack up across these and other etchant systems. The Role of Scribing in Pattern Definition The etch pattern is defined not by applying maskant in the pattern shape, but by applying maskant everywhere and then scribing (cutting) and peeling the maskant from the areas to be etched. This approach achieves pattern edge accuracy that direct application cannot…