Shot material for mechanical plating, and high corrosion resistant coating using same

Abstract

Shot material for mechanical plating is provided, comprising steel core particles clad with an alloy comprising 1 to 5% by weight of aluminum, 3 to 15% by weight of magnesium, preferably 5 to 15% by weight of magnesium, and the balance of zinc and unavoidable impurities. The alloy may contain a total of 1% by weight of impurities. An Fe—Zn alloy layer may be provided between the alloy cladding and the steel core. The shot material may contain 3 to 80% by weight of iron. The preferable average diameter of the shot particles is 100 to 600 μm.

Description

FIELD OF THE INVENTION [0001] The present invention relates to a shot material used in mechanical plating for forming a high corrosion resistant coating on a metal material surface, and to a high corrosion resistant coating formed using the shot material. DESCRIPTION OF THE PRIOR ART [0002] Forming a zinc or zinc alloy coating (referred to hereinbelow as “zinc based coating”) on iron-based metal materials is a widely-used method of improving the corrosion resistance of the iron-based metal. Specific technologies for this that are in practical industrial use include hot dipping, phosphate plating, electroplating, and mechanical plating. [0003] If zinc based coating formed by zinc plating or the like is exposed to the air without first being treated, it can give rise to white rust of the zinc in a relatively short time, expediting deterioration of the coating. This is especially pronounced in an outside environment. To prevent that happening, a treatment such as chromating is used to ...

AI Technical Summary

Benefits of technology

[0023] To adequately manifest the characteristic effect of the invention, the ratio between the steel particle core and the zinc based alloy cladding layer should be controlled so that the content of the iron in the shot material is from 3 to 80% by weight. If the iron content is lower than that, it is difficult to obtain a sufficient impact energy, while a higher content will result in a relatively small amount of cladding material, shortening the life of the shot material during the blasting process of the mechanical plating.

[0024] It is desirable to use steel cores having a hardness in the order of 200 to 700 HV. At least 95% by weight of all shot particles should have a particle diameter that falls within the range 10 to 800 μm, and an average diameter of 100 to 600 μm. Coating Thickness

[0025] The coating formed on the surface of the subject metal material has to be at least 2 μm thick. However, using mechanical plating to form a coating with a heavy coating weight that exceeds 15 μm in thickness is uneconomical. In accordance with the present invention a good corrosion resistance effect can be obtained by controlling the coating thickness to be within the range of about 2 to 15 μm. Method of Manufacturing the Shot Material

[0026] The shot material of the invention can be manufactured by the steps of preparing a zinc alloy melt having a composition corresponding to the material used to clad the steel core particles, charging the steel core particles into the zinc alloy melt, stirring the mixture, removing it when it cools into a semi-solidified state, crushing and screening. For this, the amount of the zinc alloy melt and the amount of the steel particles charged into the melt should be controlled to bring the iron content in the shot particles to within the range of 3 to 80% by weight. When the zinc alloy is adhered around the steel particles by the above process, an Fe—Zn alloy layer forms at the interface between the steel particles and the zinc alloy. The Fe—Zn alloy layer is relatively brittle, so when the shot material is impacted on the surface of subject material to be treated in the blasting process, the microparticles of zinc alloy sheared-off finely in the portion of the Fe—Zn alloy layer are pressed onto the surface of the subject material, increasing the uniformity of the coating.

[0027] Commercially available steel shot can be used for the steel particles. As stated in the foregoing, the alloy-clad shot particles should have a diameter within the range 10 to 800 μm. The average particle diameter should be within the range 100 to 600 μm, and may be within the range 100 to 400 μm or 150 to 300 μm. Since the iron content of the shot particles is preferably from about 3 to 80% by weight, the size and quantity of the steel particles used should be set in accordance with those target values. First EmbodimentINVENTIVE

Problems solved by technology

If zinc based coating formed by zinc plating or the like is exposed to the air without first being treated, it can give rise to white rust of the zinc in a relatively short time, expediting deterioration of the coating.

However, the solution used for the treatment contains hexavalent chromium, which is toxic.

While there are also methods of providing protection using a trivalent chromium coating, polymer chelate coating, silicate-based inorganic coating and the like, the protective effect of such coatings is inferior to that provided by a hexavalent chromate coating.

Another drawback with methods that do not use hexavalent chromium is that the administration of the treatment solution is complicated and can readily give rise to post-treatment variations in characteristics, making the treatment quite costly compared to the hexavalent chromate treatment.