A steel sheet for a fuel tank includes: a Zn—Ni alloy plated layer which is placed on one surface or each of both surfaces of a base metal and formed on at least one surface; and an inorganic chromate-free chemical conversion coating film which is placed over the Zn—Ni alloy plated layer. The Zn—Ni alloy plated layer has a crack starting from an interface between the Zn—Ni alloy plated layer and the inorganic chromate-free chemical conversion coating film and reaching an interface between the Zn—Ni alloy plated layer and the steel sheet, and a water contact angle on a surface of the inorganic chromate-free chemical conversion coating film is more than or equal to 50 degrees.

The present invention relates to a hot-rolled steel sheet for a high strength galvanized steel sheet, having excellent surface quality, and a method for producing the same, the hot-rolled steel sheet comprising, by weight %: C: 0.05 to 0.15%, Si: 0.03 to 0.10%, Mn: 0.7 to 1.39%, P: 0.001 to 0.05%, S: 0.001 to 0.03%, Al: 0.002 to 0.035%, and the remainder being Fe and unavoidable impurities. The weight ratio of Mn/Si is 15 to 25, the weight ratio of C/Si is 1 to 5, and the weight ratio of Si/P is 3 to 10. The hot-rolled steel sheet has a microstructure consisting of, in area fraction, 10 to 40% of bainite, 20 to 30% of pearlite and 40 to 60% of ferrite, and includes a ternary eutectic compound of FeO, Fe.sub.2SiO.sub.4 and Fe.sub.3(PO).sub.4 formed within 50 μm from the surface.

Provided is a jewelry ring comprising a substrate, a first coating of a metallic nitride or a metallic boride, and an external metallic coating. Also provided is a metallic article comprising a substrate comprising tungsten carbide, cobalt, tungsten, titanium, titanium carbide, zirconium, tantalum or aluminum; a first coating of a metallic nitride or a metallic boride; and an external metallic coating. A method for making a jewelry ring comprising a substrate, a first coating of a metallic nitride or a metallic boride, and an external metallic coating is additionally provided. Further provided is a method for making a metallic article comprising a substrate comprising tungsten carbide, cobalt, tungsten, titanium, titanium carbide, zirconium, tantalum or aluminum; a first coating of a metallic nitride or a metallic boride; and an external metallic coating.

Provided is a black coupling member for vehicles, comprising a trivalent chromium black chemical conversion film and a black coating film upon the trivalent chromium black chemical conversion film, on the surface of a zinc-plated metal substrate. The trivalent chromium black chemical conversion film is formed using a hexavalent chromium-free trivalent chromium black chemical conversion treatment solution having a Zn ion concentration of no more than 20 g/L. The lightness (L) of the trivalent chromium black chemical conversion film is no more than 33. The black coating film includes a black chemical conversion component, a modified organopolysiloxane, and a friction coefficient-adjustment component. The black chemical conversion component content is 2-25 wt % relative to 100 wt % of the black coating film. The lightness (L) is no more than 28.

A self-drilling screw (10) having a head (20), a shaft (30) at least partially wearing a thread (35) and a drill point (40). The base material of the screw (10) including the drill point (40) is integrally manufactured from an austenitic 300 series steel with a surface hardness (uncoated) of 400-600 HV 0.3. The surface of the screw has a top coating of Zn—Ni with a Ni-content between 12-15% deposited on the austenitic base material. This self-drilling screw (10) is manufactured from a blank of raw austenite 300 series steel which is initially squeezed by cold forming to reduce its diameter in a first operation and, in following cold forming operations the head, the drill point and a thread are formed.

A trivalent-chromium chemical conversion coating from which substantially no hexavalent chromium is released. The trivalent-chromium chemical conversion coating is one formed on the surface of a zinc or zinc-alloy deposit. In a brine spray test, the chemical conversion coating has unsusceptibility to corrosion (time required for white-rust formation) of 96 hours or longer. The chemical conversion coating has a hexavalent-chromium concentration less than 0.01 μg/cm.sup.2 in terms of metal atom amount. The amount of hexavalent chromium released after 30-day standing in a thermo-hygrostatic chamber at a temperature of 80° C. and a humidity of 95% (amount of hexavalent chromium released when the coating is immersed in 100° C. water for 10 minutes) is smaller than 0.05 μg/cm.sup.2.

The metal connector or adaptor for hydraulic or oil dynamic application at high pressure, comprises electrolytic passivated zinc plating free from hexavalent chromium, the zinc plating comprising an external trivalent chromium passivation layer, said passivation layer having a distinctive unique coloration among high corrosion resistance processes.

Provided is a multilayer zinc alloy plated steel material comprising a base steel material and multiple plating layers formed on the base steel material, wherein each of the multiple plating layers includes one of a Zn plating layer, a Mg plating layer, and a Zn—Mg alloy plating layer, and the ratio of the weight of Mg contained in the multiple plating layers to the total weight of the multiple plating layers is from 0.13 to 0.24.

Provided is a surface treatment solution composition comprising: 30 to 51 wt % of a trivalent chromium compound comprising chromium phosphate (A) and chromium nitrate (B) and having a content ratio of A/(A+B) that satisfies 0.3 to 0.6; 5 to 15 wt % of silane coupling agent; 0.2 to 3 wt % of vanadium-based anti-corrosive rust inhibitor; 3 to 12 wt % of colloidal silica; 0.5 to 5 wt % of polysiloxane copolymer; and 14 to 61.3 wt % of water, a hot dip galvanized steel sheet surface-treated using same, and a manufacturing method thereof. The hot dip galvanized steel sheet treated with the surface treatment solution composition containing trivalent chromium has an excellent corrosion resistance, blackening resistance, pipe-forming oil reactivity, and alkali resistance.

Provided is a hot-dip plating method that achieves good plating wettability between a metal material and a hot-dip plating bath and that makes it possible to reduce the amount of consumed energy as compared to conventional techniques. In a plating step included in the hot-dip plating method, vibration is applied to a hot-dip plating bath such that the ratio of the average sound pressure level (excluding noise) over ranges each lying between sound pressure peaks at harmonic frequencies of a fundamental frequency to the average sound pressure level (excluding noise) over the measured frequency range in an acoustic spectrum is greater than 0.2.