1. Field of the Invention
The present invention relates to a fuel tank and a fuel-filler tube for supplying such fuel as gasoline or methanol-containing gasoline to an engine of a motor vehicle.
2. Description of the Related Art
A fuel tank is installed in an automobile or a motorcycle for storage of gasoline or the like. If the fuel tank is poor of airtightness, gasoline evaporated in the fuel tank diffuses to the open air. Diffusion of gasoline also occurs from a fuel-filler tube poor of airtightness.
In order to inhibit diffusion of gasoline harmful on the environment, the fuel tank and the fuel-filler tube shall have an airtight structure. But, airtightness of a conventional fuel tank made of synthetic resin becomes worse as the lapse of time. Although use of a steel sheet coated with a Zn or Al plating layer is proposed as material for a fuel tank, the plating layer often flaked or cracked during forming the steel sheet to a product shape. Such defects as flakes and cracks act as starting points of corrosion, when a fuel tank is exposed to a corrosive atmosphere containing organic acids generated by degradation of gasoline. If the corrosion leads to pinholes in the fuel tank, the airtight structure of the fuel tank is broken. An external surface of a fuel tank or a fuel-filler tube is also suffered from corrosion, since it is exposed to a severely corrosive atmosphere especially in the cold region where a deicing salt is scattered on roads.
Applicability of stainless steel, i.e. representative corrosion-resistant material, for a fuel tank or fuel-filler tube has been researched and examined in order to maintain an airtight structure over a long term. But, stainless steel is often cracked during forming it to a product shape, due to its worse formability than plain steel. Cracks degrade airtightness of the fuel tank or fuel-filler tube, resulting in diffusion of gasoline to the open air.
Pitting corrosion sometimes occurs at work-induced defects and leads to pinholes especially at an inner surface part of the fuel tank or fuel-filler tube in contact with sour gasoline.
Crevice corrosion is also one of problems in a fuel tank or fuel-filler tube made of stainless steel. Since a fuel tank or fuel-filler tube is fabricated by seam-welding a steel sheet and fixing various members thereto, formation of narrow gaps cannot be avoided. When rainwater invades to the narrow gaps, stainless steel is suffered from crevice corrosion. Especially in the region where a deicing salt is scattered on roads, the narrow gaps are attacked by crevice corrosion due to invasion and condensation of the deicing salt. The crevice corrosion also causes pinholes and substantially damages faculties of the fuel tank or fuel-filler tube.
The present invention aims at provision of a fuel tank and a fuel-filler tube, which maintains an airtight structure without corrosion over a long term, made of a stainless steel sheet good of formability.
The fuel tank or fuel-filler tube proposed by the present invention is made of a ferritic stainless steel sheet, which is coated with a corrosion-preventing layer. The stainless steel sheet has elongation of 30% or more after fracture by a uniaxial tensile test and minimum Lankford value (value-rmin) of 1.3 or more. The corrosion-preventing layer may be a cathodic electrodeposition paint film or a plating layer of Al, Zn or a Zn alloy containing one or more of Fe, Ni, Co, Mg, Cr, Sn and Al. When the fuel tank or fuel-filler tube is fabricated by welding steel sheets, Zn-rich paint is preferably applied to a welded part in prior to electrodeposition of the paint film.
Preferable composition of the ferritic stainless steel comprises of C up to 0.015 mass %, Si up to 0.5 mass %, 11.0-25.0 mass % of Cr, N up to 0.020 mass %, 0.05-0.50 mass % of Ti, 0.10-0.50 mass % of Nb, B up to 0.010 mass %, optionally one or more of Mo up to 3.0 mass %, Ni up to 2.0 mass %, Cu up to 2.0 mass % and Al up to 4.0 mass %, and the balance being Fe except inevitable impurities.