RU2606364C1 - Method of protective coating producing - Google Patents

Abstract

FIELD: machine building.

SUBSTANCE: invention relates to machine building, namely, to galvanic coatings producing methods with subsequent thermal treatment for corrosion protection of steel items. Method involves successive zinc layer electrolytic application on part, and then tin layer with subsequent thermal treatment of part, wherein on said tin layer zinc layer or zinc layer and subsequent tin layer are additionally applied, and part heat treatment is carried out in single stage at temperature of not less than for 18.5 °C below than zinc and tin eutectic melting temperature.

EFFECT: technical result is improvement of coating protective ability and prevention of corrosion products formation.

5 cl, 2 tbl, 1 dwg, 5 ex

Description

The invention relates to the field of engineering, and in particular to methods for producing galvanic coatings followed by heat treatment to protect steel products from corrosion. The method is suitable for corrosion protection of parts with precise tolerances and / or with threads.

It is known that cadmium and zinc coatings are most widely used for corrosion protection. Due to the toxicity of cadmium compounds and the insufficient corrosion resistance of zinc, it became necessary to use alloy coatings.

The industry knows methods for producing doped galvanic zinc coating with metals such as nickel, tin, cobalt directly from electrolytes, which allows to increase the protective ability of the zinc coating (Vyacheslavov P.M. Electrolytic deposition of alloys. - L .: Engineering, Leningrad. Department - Niy, 1986 .-- 112 s.).

A known method of producing a multilayer protective coating consisting of successively applied coating layers with zinc-nickel and tin-zinc alloys containing 10-16% nickel and 8-35% zinc, respectively. The total coating thickness is 10-15 microns (US patent No. 5989735, publ. 23.11.1999).

According to the protective properties of the coatings, zinc-based alloys approach cadmium.

The disadvantages of the known methods for producing protective coatings are the difficulty in maintaining the necessary concentration of the alloying component in the electrolyte, the special technological conditions of the anode process and the inability to obtain coatings uniform in composition on complex profiled parts due to the uneven distribution of current density in different parts of the surface, which can lead to electrochemical heterogeneity and cause amplification corrosion in various parts of the part.

In the literature there is information about the prospects of using multilayer coatings for the replacement of cadmium. So, in US patent No. 6613452, publ. July 18, 2002, a multilayer protective coating is proposed that provides protection of the base metal (for example, steel) from corrosion and hydrogen embrittlement. The known coating consists of two layers: the 1st layer, for example, nickel, is close in its electrochemical properties to the base metal in this medium, and the 2nd one contains at least 50 wt.% Metal anode to the base metal, for example zinc. To form a diffusion interfacial layer between the coating layers, heat treatment is provided.

The disadvantage of this method is the insignificant thickness of the interfacial layer between the coating layers due to the low diffusion rate of zinc in nickel, which, taking into account porosity or local destruction of the coating, leads to significant corrosion of both the upper zinc and carbon steel bases due to contact corrosion. The protective ability of the coating is lower than the protective ability of the cadmium coating.

A known method of applying a combined protective coating on a steel sheet, comprising applying a galvanic zinc coating from an electrolyte containing zinc ions, and then applying a chemical or electrochemical tin coating by immersion in a solution containing tin ions. After applying layers of zinc and tin, the coating undergoes the usual processing in a solution of sodium dichromate and subsequent heat treatment at a temperature above the melting point of tin (232-400 ° C) for a short time of 0.5-10 s (Canadian patent No. 1211407, publ. 16.09. 1986).

The disadvantage of this method is that it is suitable only for coating sheet metal parts, cannot be implemented for parts of complex configuration, including those with threads. Moreover, the coating has a low protective ability due to the formation for such a short period of time (0.5-10 s) of a diffuse layer of small thickness, not able to exclude the occurrence of interlayer contact corrosion, first a zinc layer, then a steel base.

The closest in technical essence to the claimed one is a method of applying a combined protective coating with a zinc-tin galvanothermal alloy, according to which an immersion tin coating is applied to a zinc galvanic coating, then heat treatment is carried out in two stages: 1st stage - temperature 140-160 ° C for 4 hours, 2nd stage - 180-200 ° C for 2-24 hours, as a result of which a thin layer of an alloy with enhanced protective properties is formed on the surface of the zinc coating (RF patent 2427671, publ. 27.08.20 eleven).

The disadvantage of this method is that the immersion tin coating has a small and unregulated thickness, which leads to the formation of a thin alloy layer of less than 1 μm during heat treatment on the coating surface. The coating in its protective ability exceeds the cadmium coating only in the initial period of operation before the appearance of traces of "white" corrosion, that is, before the corrosion of the zinc coating. The full protective ability of the coating is inferior to the protective ability of the cadmium coating.

The advantage of a single-stage heat treatment according to the proposed method is shown in Figure 1. When heat treatments are close to the melting point of the zinc-tin eutectic (198.5 ° C) (figure 1), there may be a violation of the continuity of the tin layers, which is manifested in the appearance of foci of corrosion of the substrate. At temperatures above the melting of the zinc-tin eutectic, a violation of the layered structure of the coating is observed and leads to selective corrosion of the coating during corrosion tests.

An object of the present invention is to provide a method for producing a protective coating on parts made of ferrous metals and their alloys, including with exact tolerances and / or having a thread, which has high protective properties when operating parts in all climatic conditions.

The technical result of the present invention is to increase the protective ability of the coating and prevent the formation of corrosion products.

The technical result is achieved using the method of applying a protective coating to steel parts, including sequential electrolytic deposition of a zinc layer on the part, and then the tin layer, followed by heat treatment of the part, while the zinc layer or the zinc layer is additionally applied to the said tin layer, followed by application tin layer, and the heat treatment of the part is carried out in one stage at a temperature of not less than 18.5 ° C below the melting point of the zinc-tin eutectic.

Preferably, the heat treatment of the part is carried out at a temperature of from 150 to 180 ° C for 4-24 hours

To reduce contact corrosion of the mating part, a three-layer zinc-tin-zinc coating is applied, and with increased requirements for protective ability, a four-layer zinc-tin-zinc-tin coating is applied to the part.

Preferably, the protective coating is applied to steel parts with precise tolerances and / or having threads.

Preferably, after heat treatment, the coated steel parts can be further processed in one of the passivating solutions: chromation, phosphating, oxidation.

A distinctive feature of the invention is the application of layers of both zinc and tin by the electrolytic method, the number of layers applied alternately, starting with zinc from the base, is three or four, the thickness of each layer is regulated depending on the subsequent heat treatment. It is recommended to heat treatment at a temperature of 150 to 180 ° C for 4-24 hours.

The present invention is illustrated in figure 1, which shows the effect of heat treatment of a four-layer coating of zinc-tin-zinc-tin at temperatures: Fig. 1A) 160-180 ° C, FIG. 1B) 185 ° C, FIG. 1B) more than 200 ° C and the appearance of the coating after accelerated corrosion tests in a salt spray chamber.

The authors of the present invention found that ensuring high protective properties of the coating is achieved by forming a smooth change in the electrochemical behavior of the coating in thickness according to the type of "base-anode-diffusion zone-cathode", or "base-anode-diffusion zone-cathode-diffusion zone-anode" or “base-anode-diffusion zone-cathode-diffusion zone-anode-diffusion zone-cathode” by applying a crystalline continuous tin layer by electrolysis, adjusting the number and thickness of zinc and tin layers alternately, starting from the base, depending on the heat treatment conditions at a temperature not less than 18,5 ° C below the eutectic melting temperature (i.e. up to 180 ° C).

Zinc and tin layers are applied from any galvanizing and tin electrolyte, respectively, depending on the conditions of galvanic production and the need for uniform distribution of the coating over the part profile. First, a zinc layer is applied to the base, then tin and then alternately.

The minimum layer thicknesses are determined by the need for spatial separation of the cathode, anode and diffusion layers along the thickness of the total coating, as well as the technological capabilities of electrochemical technologies to ensure the deposition of a thin metal layer uniformly distributed over the part profile.

The maximum thicknesses of the layers are determined by the required thickness of the total coating, determined by the operating conditions and the category of placement of the part.

The need for a final zinc layer (i.e., a three-layer zinc-tin-zinc coating) is determined by the need to eliminate or significantly reduce contact corrosion in mating parts from various materials or with different coatings.

The need to apply a four-layer zinc-tin-zinc-tin coating is dictated by increased requirements for protective ability, especially in case of violation of the integrity of the coating during operation.

The formation of a uniform diffusion zone between the anode and cathode layers of the coating is ensured by a one-step heat treatment of the coating, preferably at a temperature of from 150 to 180 ° C for 4-24 hours. In this case, the heat treatment should not lead to the formation of a phase of the eutectic composition, which due to its increased mobility near the melting temperature (198.5 ° C) during the heat treatment is able to "grow" through the tin layer, thereby sharply reducing the protective ability of the coating (see figa), B), C)). For this, the maximum temperature should not exceed the temperature by at least 18.5 ° C below the melting point of the eutectic, i.e. up to 180 ° C. This allows diffusion processes to form a continuous surface front of diffusion penetration of zinc into the tin layer without the formation of a eutectic.

The maximum duration of heat treatment at the appropriate temperature is limited to prevent the coating composition from completely equalizing in thickness.

Additional processing of the resulting coating is allowed in any passivating solutions: chromation, phosphating, oxidation, etc.

Example 1. Details of a bracket type made of medium strength 30KhGSA steel were electrochemically degreased in a standard phosphate-alkaline solution and then, after washing in warm and cold water, they were activated in a solution of hydrochloric acid. After washing in cold water, a zinc coating 2 μm thick was applied to the parts from an electrolyte of the following composition, g / l:

zinc sulfate 85 ammonium chloride 200 sodium acetate 40 NF dispersant liquid (grade A) 100 ml / l

the pH of the solution is 4.5,

at a temperature of 20 ° C and a cathode current density of 3 A / dm ² .

After washing in cold water, a tin coating 2 μm thick of tin electrolyte of the following composition was applied to galvanized steel parts, g / l:

tin dichloride 40 sodium fluoride fifty hydrochloric acid 3 gelatin 2

the pH of the solution is 1.5,

at a temperature of 23 ° C and a cathode current density of 1 A / dm ² .

After washing in cold water, a zinc coating 2 μm thick was applied to the parts from an electrolyte of the following composition, g / l:

zinc sulfate 85 ammonium chloride 200 sodium acetate 40 NF dispersant liquid (grade A) 100 ml / l

the pH of the solution is 4.5,

at a temperature of 20 ° C and a cathode current density of 3 A / dm ² .

After washing in cold water, a tin coating with a thickness of 1 μm was made from a tin electrolyte of the following composition, g / l:

tin dichloride 40 sodium fluoride fifty hydrochloric acid 3 gelatin 2

the pH of the solution is 1.5,

at a temperature of 23 ° C and a cathode current density of 1 A / dm ² .

After washing and drying, parts with a four-layer coating of C2. O2. C2. O1 (designation according to GOST 9.306-85) were placed in an air oven at a temperature of 150 ° C for 2 hours.

After heat treatment, parts with a four-layer coating were activated in a sulfuric acid solution, washed, subjected to additional chromate treatment in a standard solution of the composition (g / l): sodium dichromate 180, sulfuric acid 10 at a temperature of 25 ° C and a holding time of 20 seconds, followed by washing and drying.

Example 2. Details of the type of strap made of high-strength steel 30KhGSN2A (σ _in = 1800 MPa) after grinding and blowing with corundum sand were chemically degreased in a standard phosphate-alkaline solution and then, after washing in warm and cold water, they were activated in a solution of hydrochloric acid. After washing in cold water, a zinc coating 4 microns thick was applied to the parts from an electrolyte of the following composition, g / l:

zinc sulfate 55 ammonium chloride 205 boric acid fifteen TsKN-3 40 ml / l

pH of solution 5,

at a temperature of 20 ° C and a cathode current density of 2 A / dm ² .

After washing in cold water, galvanized steel parts were coated with a tin coating 4 μm thick from a tin electrolyte of the following composition, g / l:

tin sulfate 40 sulfuric acid 80 sodium sulfate 40 OS-20 four,

solution pH <1,

at a temperature of 23 ° C and a cathode current density of 1 A / dm ² .

After washing in cold water, a zinc coating 2 μm thick was applied to the parts from an electrolyte of the following composition, g / l:

zinc sulfate 55 ammonium chloride 205 boric acid fifteen TsKN-3 40 ml / l

pH of solution 5,

at a temperature of 20 ° C and a cathode current density of 2 A / dm ² .

After washing in cold water, a tin coating with a thickness of 1 μm was made from a tin electrolyte of the following composition, g / l:

tin sulfate 40 sulfuric acid 80 sodium sulfate 40 OS-20 four,

solution pH <1,

at a temperature of 23 ° C and a cathode current density of 1 A / dm ² .

After washing and drying parts with a four-layer coating C4. O4. C2. O1 were placed in an air oven at 170 ° C for 8 hours.

After heat treatment, parts with a four-layer coating were activated in a sulfuric acid solution, washed, subjected to additional chromate treatment in a standard solution of the composition (g / l): sodium dichromate 180, sulfuric acid 10 at a temperature of 25 ° C and a holding time of 20 seconds, followed by washing and drying.

Example 3. Parts such as a shaft with an external thread and chamfers of medium strength 30KhGSA steel were electrochemically degreased in a standard phosphate-alkaline solution and then, after washing in warm and cold water, activated in a solution of hydrochloric acid. After washing in cold water, a zinc coating 4 microns thick was applied to the parts from an electrolyte of the following composition, g / l:

zinc oxide 12 sodium hydroxide 120 Ekomet Ts1 brand A 4 ml / l

at a temperature of 20 ° C and a cathode current density of 3 A / dm ² .

After washing in cold water, galvanized steel parts were coated with a tin coating 4 μm thick from a tin electrolyte of the following composition, g / l:

sodium stannate 85 caustic soda twenty sodium acetate 17,

the pH of the solution is 9.5,

at a temperature of 75 ° C and a cathode current density of 1 A / dm ² .

After washing in cold water, a zinc coating 2 μm thick was applied to the parts from an electrolyte of the following composition, g / l:

zinc oxide 12 sodium hydroxide 120 Ekomet Ts1 brand A 4 ml / l

at a temperature of 20 ° C and a cathode current density of 3 A / dm ² .

After washing in cold water, a tin coating with a thickness of 1 μm was made from a tin electrolyte of the following composition, g / l:

sodium stannate 85 caustic soda twenty sodium acetate 17,

the pH of the solution is 9.5,

at a temperature of 75 ° C and a cathode current density of 1 A / dm ² .

After washing and drying parts with a four-layer coating C4. O4. C2. O1 were placed in an air oven at a temperature of 150 ° C for 4 hours.

After heat treatment, parts with a four-layer coating were activated in a sulfuric acid solution, washed, subjected to additional chromate treatment in a standard solution of the composition (g / l): sodium dichromate 180, sulfuric acid 10 at a temperature of 25 ° C and a holding time of 20 seconds, followed by washing and drying.

Example 4. Parts of the type of fitting with external and internal threads made of steel St10 were electrochemically degreased in a standard phosphate-alkaline solution and then, after washing in warm and cold water, they were activated in a solution of sulfuric acid. After washing in cold water, a zinc coating 4 microns thick was applied to the parts from an electrolyte of the following composition, g / l:

zinc oxide 12 sodium hydroxide 120 Ekomet Ts1 brand A 4 ml / l

at a temperature of 20 ° C and a cathode current density of 3 A / dm ² .

After washing in cold water, galvanized steel parts were coated with a tin coating 4 μm thick from a tin electrolyte of the following composition, g / l:

tin dichloride 136 potassium pyrophosphate 550 gelatin four methylene blue indicator 0.25,

the pH of the solution is 8.0,

at a temperature of 65 ° C and a cathode current density of 1 A / dm ² .

After washing in cold water, a zinc coating 2 μm thick was applied to the parts from an electrolyte of the following composition, g / l:

zinc oxide 12 sodium hydroxide 120 Ekomet Ts1 brand A 4 ml / l

at a temperature of 20 ° C and a cathode current density of 3 A / dm ² .

After washing and drying parts with a three-layer coating C4. O4. C2 was placed in an air oven at 160 ° C for 6 hours.

After heat treatment, parts with a three-layer coating were subjected to additional processing by chromatography in a standard solution of the composition (g / l): sodium dichromate 30, nitric acid 6, sodium sulfate 13 at a temperature of 25 ° C and a holding time of 10 seconds, followed by washing and drying.

Example 5. Details of the key type made of steel U8 were electrochemically degreased in a standard phosphate-alkaline solution and then, after washing in warm and cold water, they were activated in a solution of hydrochloric acid. After washing in cold water, a zinc coating 5 microns thick was applied to the parts from an electrolyte of the following composition, g / l:

zinc sulfate 55 ammonium chloride 205 boric acid fifteen TsKN-3 40 ml / l

pH of solution 5,

at a temperature of 20 ° C and a cathode current density of 2 A / dm ² .

After washing in cold water, galvanized steel parts were coated with a tin coating of 3 μm thickness from tin electrolyte of the following composition, g / l:

tin sulfate 40 sulfuric acid 80 sodium sulfate 40 OS-20 four,

solution pH <1,

at a temperature of 23 ° C and a cathode current density of 1 A / dm ² .

After washing and drying, parts with a two-layer coating C5. O3 was placed in an air oven at 150 ° C for 8 hours.

For comparative corrosion tests, sheet samples of 5 pieces were made. each type of coating is coated with a protective coating according to the proposed method with a thickness of 6-9 and 9-12 microns. Comparative corrosion tests of coatings formed by the proposed method, in comparison with the coating of the prototype in a salt spray chamber (SST) in accordance with GOST 9.308 with constant spraying of a 5% neutral solution of sodium chloride and a temperature of 35 ° C for 5040 hours, were carried out.

Table 1 shows the results of comparative corrosion tests of coatings formed by the proposed method and the prototype.

Table 2 shows the results of comparative corrosion tests of coatings formed by the proposed method and the prototype, with violations caused by a cross-cutting of the coating to the base and scratching with a mounting Phillips screwdriver.

Thus, the proposed method allows to obtain on the parts of ferrous metals and their alloys, including with accurate tolerances and / or having a thread, a protective coating with enhanced protective properties.

Claims (5)

1. The method of applying a protective coating to a steel part, including sequential electrolytic deposition of a zinc layer on the part, and then a tin layer followed by heat treatment of the part, characterized in that the zinc layer or zinc layer and the subsequent tin layer are additionally applied to said tin layer, and heat treatment of the part is carried out in one stage at a temperature of not less than 18.5 ° C below the melting point of the zinc-tin eutectic. 2. The method according to claim 1, characterized in that the heat treatment of the part is carried out at a temperature of from 150 to 180 ° C for 4-24 hours 3. The method according to p. 1, characterized in that to reduce contact corrosion of the mating part, a three-layer zinc-tin-zinc coating is applied, and with increased requirements for protective ability, a four-layer zinc-tin-zinc-tin coating is applied to the part. 4. The method according to p. 1, characterized in that the protective coating is applied to the steel part with an exact tolerance and / or having a thread. 5. The method according to p. 1, characterized in that after heat treatment, the coated steel part is further processed in one of the passivating solutions of chromating, phosphating or oxidizing.

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