Method for forming coating film on constant velocity universal joint
A coating film having excellent adhesion, even without the presence of a chemical conversion film treatment as an undercoat, and a metal automotive part having the coating film. A powder is deposited by powder-coating onto the surface of a metal automotive part that has been quenched after simultaneously forging, and tempering the metal automotive part and bake-hardening the deposited powder to form a skin film on the surface of the metal automotive part. The surface of the metal automotive part before the powder is powder-coated thereon is a work-hardened material surface that has not been subjected to a chemical conversion filming treatment.
1. A method of forming a coating film on a surface of a metal automotive part, comprising:
a powder-coating step in which a powder is deposited by powder-coating on the surface of the metal automotive part that has been quenched after forging, and
a tempering and bake-hardening step in which tempering of the metal automotive part and bake-hardening of the deposited powder are performed simultaneously,
wherein the surface of the metal automotive part before the powder is deposited thereon is a work-hardened basis material surface,
wherein in the powder-coating step, the powder is deposited on a surface which is a material surface that has not been subjected to a chemical conversion film treatment and has been cleaned using an alkaline detergent at a temperature of 50 to 80° C.,
wherein the alkaline detergent is composed of an alkaline aqueous solution, that can remove non-reactive soap layers or reactive soap layers remaining on the surface of the metal automotive part that has been quenched after forging, contains a surfactant and
wherein adhesion of the coating film to the material surface is increased by alkali treatment alone without formation of the chemical conversion film.
2. The method of forming a coating film according to claim 1 , characterized in that the material surface is a surface layer that has been quenched after forging, and a turned face in which a part of the surface layer has been turned.
3. The method of forming a coating film according to claim 1 , characterized in that the forging is cold forging, the surface hardness after cold forging is HRB 90 to 110, and the surface hardness after quenching is HRC 50 to 65.
4. The method of forming a coating film according to claim 1 , characterized in that the quenching is high-frequency quenching, and tempering and powder bake-hardening are carried out by high-frequency induction heating.
5. The method of forming a coating film according to claim 1 , characterized in that the powder is a pulverulent epoxy-based powder coating composition.
6. The method of forming a coating film according to claim 5 , characterized in that the powder is a pulverulent coating composition comprising a bisphenol A epoxy resin, a hydrazide compound, and an inorganic filler.
7. The method of forming a coating film according to claim 6 , characterized in that the hydrazide compound is an organic acid polyhydrazide.
8. The method of forming a coating film according to claim 6 , characterized in that the inorganic filler is barium sulfate.
9. The method of forming a coating film according to claim 1 , wherein the metal automotive part is an outer joint member constituting a constant velocity universal joint, or an intermediate shaft constituting a drive shaft.
10. The method of forming a coating film according to claim 9 , characterized in that the coating film on the outer joint member or the intermediate shaft has a thickness of 40 to 150 μm, a pencil hardness of H to 2H, and a corrosion resistance of 120 hr or greater as determined by a salt water spray test.