Polyamide coatings are generally known in the art and which exhibit high wear resistant properties along with hydrophobic (water repelling) characteristics. Such coating thicknesses can vary however application thicknesses in the area of 100 micrometers to 1 mm are not uncommon. Such coatings can further include both thermoset and thermoplastic compositions and are often used for finishing and protection of metal surfaces.
The prior art is documented with examples of automotive fluid tubing utilizing such coatings to provide varying performance characteristics. A first example of this is shown by the automotive fluid tubing of Picco et al., U.S. Pat. No. 6,915,820 which is configured for carrying any of gasoline/diesel fuel or hydraulic fluid and is composed of a metal with a coating of aluminum, over which is extrusion coated a polyamide 12 layer and for improving the wear-resistance and corrosion-resistance of the tubing.
A further example of the prior art is depicted in Berger et al., U.S. Pat. No. 9,556,358 which teaches a method for coating of a metallic article, in which the metal surface is coated with a polymer or a two-component system that reacts to form a polymer following application to the metal surface. The composition includes a 70-2700 meq/kg olefinic double bonds which leads to stronger adhesion and to increased corrosion resistance.
US 2018/00453257, to Kawai et al., teaches a multi-layer coated film applied to a metal pipe and which covers an outer circumferential surface of the pipe. The coating film includes a chemical conversion layer containing a zirconium oxide and/or zirconium hydroxide. A primate layer contains a polyamide imide and/or an epoxy resin.
US 2018/0119871, also to Kawai, teaches a coated metal pipe in which the multilayered coating includes a chemical conversation layer and a primer layer which further includes a polyamide imide and at least one kind of additive component selected from a polyamide, a fluorine resin, a silane coupling agent, and an epoxy resin.