SU945243A1 - Method for diffusion saturation of aluminium and its alloys with zinc - Google Patents

Description

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This invention relates to the chemical heat treatment of aluminum and its alloys.

There is a known method for the diffusion galvanizing of aluminum and its alloys in order to harden the surface and increase the corrosion resistance in the humid air environment, including heating products made of aluminum and its alloys in a mixture of powders, wt .;

Zn20-30

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Saturation is carried out in an airtight container at 250-600 ° C for 1-10 hours 1.

The disadvantage of this method is the long duration of the process, the need for reheating to quench the heat-hardened alloys.

There is a known method of galvanizing aluminum and its alloys, which includes the treatment of the product in the melt of a mixture of alkali metal halides (alkaline earth metals) containing zinc salts. Moreover, the total content of salts and Christmas metals exceeds the content of zinc salts 2j.

The known method has a number of nedos) 0 tatkov. Diffusion galvanizing of aluminum and its alloys in a known manner leads to a decrease in the strength properties due to the melting of grain boundaries, leading to the formation of stress concentrators. A decrease in the strength of alloys occurs due to a decrease in the concentration of quickening elements in the alloy, capable of reducing Zn by

20 reactions ()

2мё- -nZnl - 2Mer t-nZn, (|)

where Me is A1, Mg, Li, etc .; G - halogen. The active interaction of aluminum with a melt containing Zn ions by reaction (1) results in the release of large amounts of zinc on the surface of the product, which causes surface melting and changes in size and configuration due to melting at process temperatures above 420 ° C. The closest to the present invention is method 3 for the production of zinc from molten chlorides, including the electrolysis decomposition of zinc chloride and the release of zinc at the cathode at a current density j of 0.5 A / dm. The melt for electrolysis contains; LiCl 51.3 KC1 35.7 ZnCl 13. The disadvantage of this method is the low corrosion resistance of the products due to the high concentration of zinc in the coating and its sharp decrease upon transition to the base, and also the low quality of the products associated with the surface melting or complete dissolution of the product in the electrolyte as a result of active interaction by the reaction (|) at process temperature over. The aim of the invention is to improve the mechanical properties of the product: surface hardness, strength, corrosion resistance, prevent melting during galvanizing at a temperature of more. The goal is achieved by imposing on the product before exposure a negative electric potential of 1-ЗВ. Imposing a negative electric potential on a product made of aluminum or its alloys, immersed in a molten salt containing more than 10 ZnCI, does not have a noticeable effect on the process of spontaneous release of zinc by the reaction (|) due to the large depth of its flow. At 500 ° C constant. equilibrium of the reaction (I) K-b10 In the concentration range 3 {ZnCl2 | 10, there is a slight increase in the mass of zinc released on the surface with an increase in the absolute value of the potential of the product. At a ZnCT concentration of 3%, the application of potential to a product causes a decrease in the mass of zinc produced with an increase in potential in absolute value. At s -3.8 V and a ZnClj concentration of 0.4, zinc is completely stopped. The phenomenon is explained by the impossibility of the reaction (I) due to the surface passivation, the electrolytic emission of zinc is not observed due to the flow of the Zn + 2e-1n process during the melt, not on the surface. Thus, imposing a potential on a product made of aluminum or its alloy immersed in the melt will either have a weak effect on the formation process of the zinc coating, or cause a decrease in the amount of zinc released and, in the extreme case, complete cessation of coating formation (process is similar to cathodic protection). At a concentration of ZnCl, j, less than 3.0, the electrolysis of ZnCI / is not responsible for the coating formation process. Example. The proposed method is carried out as follows. The product is immersed in a salt melt of a mixture of zinc halides (concentration of not more than 3%) and alkali metals, then a negative potential is applied to the product, maintained for a time allowing the formation of a diffusion layer of the required depth. The saturation of samples of alloys A7 and D1b with a thickness of 4.0 mm is carried out in a bath with a melt of compositions A, B, C. The results are shown in Table. 1. Table 1 At a temperature of 500 ° C for 0} in when applying an electrical potential of 1-3.0 V. At the end of the exposure, the product is hardened in water and subjected to natural aging (h). The properties of the products obtained by zinc plating are given in table. 2

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Claims (2)

S, 9 The effect of strengthening the product, the hardness of its surface and the prevention of surface melting are associated with a decrease in the concentration of zinc in the coating. The overlap of an electric negative potential makes it possible to limit the exchange by reaction (I). This allows saturation at temperatures above k2ffC ((f C T), eliminating melting and dissolving the product, and therefore combining the diffusion saturation process with heating for hardening products from thermally hardened alloys. Corrosion resistance of products subjected to diffusion saturation by the proposed method which is estimated by the gravimetric method, increases by a factor of 2 in comparison with those processed by known methods, owing to the formation of a diffusion layer with a zinc concentration on the surface from 15 to 30 (depending on concentration of ZnCl.-and the potential of the product), gradually decreasing over the depth of the diffusion layer. Thus, the application of the proposed method allows to increase the mechanical properties of the products, subjected to galvanization, eliminate their melting and dissolution at process temperatures over 10 3 and increase their corrosion resistance, claims 1. A method for the diffusion galvanizing of aluminum and its alloys, including heating and holding in a melt of salts containing zinc and alkali metal halides, characterized in that spruce increasing the surface hardness, strength, corrosion resistance and preventing saturation reflow temperature, the product is applied to the exposure value of the negative electric potential-1 pollutants. 2. Method POP.1, characterized in that 8 as a saturating medium, a melt of the following composition is used, weight: potassium chloride, 7-57 lithium chloride, 5-57 zinc chloride, 0.3-1.5 attention during examination 1. USSR author's certificate 561735, cl. C 23 C 9/00, 1977. 2. German Patent No. 63277, class., W B 9/10, published. 1936. Z. RZh Metallurgi, 1977..