The corrosion behavior of TiAlN coatings prepared by PVD in a hydrofluoric gas atmosphere

The corrosion behavior of a TiAlN coated surface in a hydrofluoric gas atmosphere was investigated for its application to the Teflon extrusion process. Teflon is a high temperature thermo-plastic that fills the need for molded parts, electrical insulation or extrusions requiring chemical resistance and good dielectric properties. Its properties are well suited for mechanical, electrical and chemical components and cable insulation. However, Teflon in the molten state is highly corrosive to most metals because of the hydrofluoric (HF) gas generated by decomposition of molten Teflon. Therefore, special corrosion resistant alloys must be used for parts of the equipment which come in contact with molten Teflon. Although a few super-alloys like ‘Hastelloy C-276’ or ‘Monel 400’ are suitable for Teflon processing, these materials have some problems, such as high cost, and poor formability, hardness and mechanical strength. We investigated coating TiAlN onto an extrusion die and nipple for Teflon cable insulation, and the material of the die and nipple was changed from Hastelloy C-276 to alloy steel. TiAlN coating has one of the highest working temperatures (800°C) due to the surface being covered with a stable and passive aluminum oxide layer. When TiAlN is exposed to a HF gas atmosphere in working conditions, it reacts with HF and forms aluminum fluoride (AlF3), which is chemically very stable to various corrosives such as acid, alkaline, alcohol and even HF. The process was quite successful and we applied the new die and nipple made of alloy steel with TiAlN coating to Teflon cable extrusion. No corrosion occurred, and we achieved a higher quality surface of the extruded cable and a much longer life for the die and nipple. The TiAlN coated surface of the die and nipple after Teflon extrusion was investigated by SEM, ESCA and AES in order to confirm the formation of AlF3.