RU2148109C1 - Method of preparing heat-resistant chrome coatings - Google Patents

Abstract

FIELD: electroplating, more particularly reinforcing the working surface of tools, machine components and mechanisms. SUBSTANCE: method comprises electrochemical deposition of coatings from saturated solution of potassium silicon fluoride in aqueous solution of chromic anhydride and sulfuric acid. Concentration of sulfuric acid as 1.125-1.25 wt parts with respect to chromic anhydride. Wear resistance of coatings prepared by the claimed method increases by 2 or 3 times as compared with conventional chrome coatings, heat treatment temperature at which microhardness of coatings decreases increases from 450-500 C to 650-700 C. EFFECT: more efficient preparation method. 2 dwg

Description

 The invention relates to electroplating, and more specifically to the production of chromium coatings from electrolytes containing partially soluble salts in aqueous solutions of chromic anhydride, for example: potassium silicofluoride, sodium silicofluoride, sodium fluoroaluminate, etc. The field of application of heat-resistant chromium coatings coincides with the field of application conventional chrome coatings: coatings can be recommended for hardening the working surface of a tool, tooling, machine parts and mechanisms, especially in those cases when Strengthened firs are operated at elevated temperatures. For example: a press tool for powder metallurgy, parts of a piston group of internal combustion engines, etc.

 A known method for producing composite coatings [1], in which ultrafine diamonds (UDA-B) are introduced into the chromium electrolyte containing potassium silicofluoride, which leads to increased wear resistance of coatings due to a change in their structure [2]. The use of UDA-B significantly increases the cost of coatings and complicates the technology of operation of the electrolyte.

 Known methods for producing chromium coatings from self-regulating electrolytes [3], containing partially soluble salts. However, the wear resistance and heat resistance of coatings obtained from such electrolytes approximately corresponds to the characteristics of coatings from conventional industrial bathtubs.

 A known method of producing coatings from the so-called self-regulating barium electrolyte [4]. In this electrolyte, the lack of concentration of acid anions introduced in the form of potassium silicofluoride is compensated for by adding sulfuric acid to the electrolyte. The electrolyte contains barium sulfate. Due to the low solubility, this component practically does not affect the concentration of anions. Compared to a standard electrolyte, a self-regulating barium electrolyte provides for the deposition of specular precipitates, and unlike self-regulating electrolytes, it has a wider range of operating current densities. However, the wear resistance and heat resistance of coatings of self-regulating barium electrolyte approximately correspond to the characteristics of coatings from conventional industrial bathtubs.

In the proposed solution, in order to increase the heat resistance and wear resistance of the coatings, the concentration of sulfuric acid in the self-regulating barium electrolyte increases from 1/1000 to 1/500 wt. hours in relation to chromic anhydride [4] to 1/125 - 1/25 wt. hours in relation to CrO ₃ .

 When studying the possibilities of improving the performance of coatings from various electrolytes intended for the deposition of chromium coatings, an unexpectedly strong effect of the concentration of sulfuric acid on the heat resistance and wear resistance of coatings obtained from electrolytes representing a saturated solution of potassium silicofluoride in an aqueous solution of chromic anhydride was found.

 In FIG. Figure 1 shows the dependence of the relative wear of the coatings on the mass fraction of sulfuric acid for electrolytes containing various amounts of chromic anhydride.

The coatings were deposited at a current density of 40 A / dm ² and an electrolyte temperature of 50-55 ^° C from a saturated solution of potassium silicofluoride. The volumetric wear of the coatings was determined in accordance with [5] with dry friction of the coating sample at a speed of 0.76 m / s in a counterbody made of X12M steel. As follows from the above data, in the range of sulfuric acid concentration 1/125 to 1/25 wt. hours in relation to chromic anhydride there is a sharp increase in the wear resistance of coatings (relative wear equal to 1, in Fig. 1 corresponds to the average level of characteristics of coatings made of self-regulating barium electrolyte). The increase in heat resistance of the proposed coatings can be judged by the results of measuring their microhardness after heat treatment for 2 hours at different temperatures in comparison with coatings of self-regulating barium electrolyte (see Fig. 2). Coating samples were obtained from an electrolyte containing 250 g / l of chromic anhydride at a sulfuric acid concentration of 5 g / l, an electrolyte temperature of 50-55 ^o C, a current density of 40 A / dm ² . The microhardness of the coatings was determined on a PMT-3 microhardness meter with a load of 100 g and a coating thickness of at least 50 μm. The coatings obtained from the electrolyte in accordance with the proposed solution have a relatively high level of hardness of 600 kg / mm ² up to temperatures of 650-700 ^o C, in contrast to conventional coatings, the microhardness of which decreases significantly after heat treatment temperatures of 450-500 ^o C.

To obtain coatings in accordance with the described method, an electrolyte with a concentration of chromic anhydride corresponding to a standard electrolyte is recommended. Using well-known methods [3] prepare the electrolyte composition, g / l:
Chromic Anhydride - 200-250
Potassium silicofluoride - up to 20
Sulfuric acid - 2-10
The working temperature of the electrolyte is maintained in the range of 50 - 55 ^o C and special attention is paid to the dissolution of potassium silicofluoride to form a saturated solution. For this, the prepared electrolyte is maintained at a working temperature and continuous stirring for at least 4-5 hours. The electrolyte is worked out in accordance with the known [3] recommendations at the rate of 5-6 A h / l. Coatings are precipitated at a current density of 10 to 60 A / dm ² . The characteristics of the coatings are as shown in FIG. 12.

 The described method is used in the development of technological documentation of the process of applying heat-resistant chrome coatings. This process was implemented at Sibpribarmash software in Biysk and is used to harden the working surfaces of metal-cutting and pressing tools.

Literature
1. A method of producing coatings based on chromium. International application PCT / SU / 88/00230, international publication number WO 89/07668 of 04.24.89.

 2. Vereshchagin A. L., Zolotukhina I.I. et al. The effect of the diamond-like phase of carbon on the microstructure of the electrodepositive chromium coating. Superhard Materials, 1991, N1, p. 46-49.

 3. M. B. Cherkez, L.Ya. Bogorad. Chrome plating. Leningrad: Mechanical Engineering, 1978, p. 13-16, p. 58-63.

 4. S. A. Lobanov. Practical tips for electroplating. Leningrad: Mechanical Engineering, 1983, p. 227-233.

 5. Halling J. A cross cylinder wear machine and its use in the study of severe wear of brass on mild steel. Wear, 4, 1961, pp. 22-31.

Claims (1)

 A method of obtaining heat-resistant chromium coatings, including electrochemical deposition of coatings from a saturated solution of potassium silicofluoride in an aqueous solution of chromic anhydride and sulfuric acid, characterized in that the concentration of sulfuric acid is 1 / 125-1 / 25 wt.h. in relation to chromic anhydride.

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