RU2529328C1 - Electrolyte for anode treatment of aluminium and alloys thereof before copper plating - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: electrolyte contains orthophosphoric acid 15 vol%, sulphuric acid 15 vol%, fluorine-containing inorganic substance selected from a group comprising ammonium bifluoride, bifluoric acid, sodium fluoride 4-15 g/l and water - the balance.

EFFECT: low power and material consumption, shorter process with high quality of coating.

2 tbl, 2 dwg, 2 ex

Description

The invention relates to an electrochemical method for coating products from aluminum and its alloys.

Aluminum alloys are the main structural material in aviation, automotive, electrical industry. Now there is a tendency to expand their scope in other industries. This is facilitated by applying various galvanic coatings to aluminum products, which increase their resistance to corrosion and mechanical wear, increase surface conductivity, improve solderability, and exhibit decorative properties. But the deposition of galvanic coatings on products from this metal is associated with a number of specific difficulties, for example, the presence on their surface of a natural oxide film that impedes the strong adhesion of the coating to the substrate. In addition, the high electronegative value of the aluminum potential leads to contact displacement of the coated metal ions before the passage of electric current through the electrolyte solution, which also disrupts the adhesion between the coating and the base. Overcoming these difficulties is achieved by special methods of preparing the surface of the coated products (zinc treatment, anodizing, chemical oxidation).

The treatment in zincate solutions used in modern galvanic production has several disadvantages: the process is not stable enough and leads to a certain percentage of poor-quality coatings, adhesion with this method is not high enough, galvanic coatings should be used only in light and medium operating conditions [1]. Chemical oxidation is a simple and cheap method of processing aluminum, which is used to obtain primer for paint coatings. Films obtained by chemical oxidation are significantly inferior in terms of protective and mechanical properties to oxide films obtained by the electrochemical method. Therefore, chemical oxidation of aluminum is of limited use. In addition, these electrolytes have a limited resource [2].

Anodizing makes it possible to obtain a more reliable adhesion of galvanic coatings with an aluminum base in comparison with other methods [3]. Films obtained during anodizing have high hardness. Therefore, anodizing many times increases the resistance of aluminum products to mechanical stress (abrasion, scratching, etc.).

It is known that the anodizing of aluminum and its alloys before plating is carried out in a solution of phosphoric acid, which gives positive results in a wide range of concentrations. However, a significant drawback of this process is its great sensitivity to the smallest changes in the composition of the processed alloys. When anodized in phosphoric acid, the more difficulties there are, the purer the aluminum. Anodization in orthophosphoric acid does not give the desired effect in the galvanic coating of cast alloys [4].

Universal for the anodizing of aluminum and its alloys in this regard is an electrolyte containing 15% (vol.) H ₂ SO ₄ and 15% (vol.) H ₃ PO ₄ [5]. The authors showed that the film obtained by anodizing in a mixture of acids (figa), is quite dense (porosity is only 7.3%). This leads to slow coating at high current densities (1-2 A / dm ² ) to not covering the surface of the alloy. In this case, to apply a high-quality galvanic coating, it is necessary to reduce the cathodic current density (up to 0.5-0.8 A / dm ² ), which significantly increases the time of the process of applying electroplating.

A known method [6] applying a copper plating on a part of aluminum and its alloys, anodized in a solution containing a mixture of acids (sulfuric and phosphoric), followed by applying a sublayer of copper from a solution of the following composition:

CuSO ₄ · 5H ₂ O 200 ÷ 250 g / l H ₂ SO ₄ 50 ÷ 70 g / l Hf 10 ÷ 15 g / l Water rest

at the cathodic current density j _k - 1 ÷ 2 A / dm ² for 2 ÷ 3 min and room temperature.

Then, after washing, the copper coating grows to the required thickness from a standard sulfate electrolyte:

CuSO ₄ · 5H ₂ O 200 ÷ 250 g / l H ₂ SO ₄ 50 ÷ 70 g / l C ₂ H ₅ OH 7 ÷ 10 ml / l Water rest

at the cathodic current density j _k - 1 ÷ 2 A / dm ² for 2 ÷ 3 min and room temperature.

But with this method of obtaining high-quality electroplating, additional material (applying a sublayer and water consumption for leaching) and energy costs are necessary. These shortcomings are eliminated by the proposed solution.

The task is to increase the manufacturability of the method.

EFFECT: reduction of energy and material costs, reduction of technological process time with high quality coating.

The technical result is achieved by the fact that in the anodizing electrolyte containing phosphoric and sulfuric acids, an additional fluorine-containing inorganic substance from the group: ammonium bifluoride, boron fluoride acid, sodium fluoride is additionally introduced:

Orthophosphoric acid 15% vol. Sulphuric acid 15% vol. Inorganic fluoride substance 4-15 g / l Water rest

The electrocrystallization of the deposited coating metal begins, first of all, in the pores of the oxide film, which are filled with metal, as a result of which conditions are provided for strong adhesion of the precipitate to the base. Therefore, to apply electroplating with a high degree of adhesion, it is necessary to obtain a more porous film firmly bonded to the base during anodizing on the surface of an aluminum alloy [4]. For this purpose, substances containing F ^- ions in their composition capable of increasing the porosity of the oxide film formed on the surface of aluminum during anodization were additionally introduced into the anodizing electrolyte (Fig. 2b).

The positive effect of hydrofluoric acid on the process of obtaining porous oxide on the surface of parts of any shape made of a titanium-aluminum alloy was noted in [7]. However, taking into account the high toxicity of hydrofluoric acid and its increased consumption during the anodization process, which leads to a deterioration in the quality of the copper coating during copper plating, other fluorine-containing inorganic substances were also chosen for research: sodium fluoride NaF, ammonium bifluoride NH ₄ HF ₂ , and borofluoric acid HBF ₄ . The results are presented in table 1.

The reduction of energy consumption occurs due to a decrease in the voltage on the bath (approximately 2-4 times), and material - due to the elimination of the bath applying a copper sublayer (according to the prototype) and reducing the water consumption for washing. At the same time, the quality of the coating remains high, especially for complex parts. In addition, the possibility of non-covering in hard-to-reach places is excluded.

The method is as follows. Complicated parts made of aluminum alloys of the following grades AMts, AD0, AD1, AD31, AK4, AK9ch, AL2 were coated. A sketch of these details is presented in figure 1. Previously, the parts were degreased, clarified in a solution of HNO ₃ : HF = 3: 1. Further, the part is anodized in a solution of 15% H ₂ SO ₄ + 15% H ₃ PO ₄ + fluorine-containing inorganic substance with a solution of 15% H ₂ SO ₄ + 15% H ₃ PO ₄ + fluorine-containing inorganic substance at room temperature and anode current density of 2 A / dm ² . After anodizing and thorough washing, a copper coating of a standard sulfate electrolyte was applied to the part. The thickness of the copper coating is 9 μm.

After coating, the parts were heated in a vacuum oven at a temperature of 200-230 ° C for 1 hour (standard method). The adhesion strength of the coating to the base was determined by the method of grids, the method of quartzization with copper brushes and by monitoring the delamination of the electroplated coatings after annealing. Prototypes passed all tests. Peeling of the coatings was not observed.

Examples

Example 1. A part made of an alloy of grade AD1 was anodically oxidized in an electrolyte 15% H ₂ SO ₄ + 15% H ₃ PO ₄ , water — the rest (according to the prototype) at room temperature and current density of 2 A / dm ² for 5-6 minutes . The voltage on the bath was 19-21 V. Next, a copper coating was applied from a standard sulfate electrolyte of the following composition:

CuSO ₄ · 5H ₂ O 200 ÷ 250 g / l H ₂ SO ₄ 50 ÷ 70 g / l C ₂ H ₅ OH 7 ÷ 10 ml / l Water rest

at a cathodic current density of 14-2 A / dm ² . During visual inspection of the coating, non-coating over the entire surface of the parts was observed.

Example 2. A part made of an alloy of the grade AD1 was anodically oxidized in an electrolyte 15% H ₂ SO ₄ + 15% H ₃ PO ₄ + 4 ÷ 6 g / l ammonium bifluoride, water - the rest at room temperature and current density 2 A / dm ² in for 5-6 minutes. The voltage on the bath in this case was 6 ÷ 9 V. Next, a copper coating was applied from a standard sulfate electrolyte of the following composition:

CuSO ₄ · 5H ₂ O 200 ÷ 250 g / l H ₂ SO ₄ 50 ÷ 70 g / l C ₂ H ₅ OH 7 ÷ 10 ml / l Water rest

at a cathodic current density of 1 ÷ 2 A / dm ² . Upon visual inspection, the surface of the part is completely covered. The coating turned out to be crystalline, semi-brilliant, non-porous.

With a decrease in the concentration of ammonium bifluoride in the anodizing electrolyte to 1-2 g / l, a high coating quality is maintained. However, there is a significant increase in voltage on the anodizing bath to 11-12 V. An increase in the concentration of this additive to 10-15 g / l leads to a decrease in voltage on the bath to 1-3 V. The quality of the copper coating remains the same. A further increase in the concentration of the additive does not lead to a significant decrease in stress.

Analogously to example 2, borofluoric acid and sodium fluoride were tested as additives in the anodizing electrolyte. The test results are presented in table.2. From table 2 it is seen that the most optimal additive is ammonium bifluoride. Activating treatment in a mixture of acids with the addition of ammonium bifluoride allows you to apply a copper coating to any aluminum alloys such as wrought alloys AMts, AD0, AD1, AD31, AK4, silumin AK9ch, as well as cast alloy AL2. The use of the proposed anodizing electrolyte can significantly reduce material, energy and time costs.

Figure 1 shows a sketch of the used aluminum parts for plating.

On figa - the structure of the oxide film obtained by anodizing in the base electrolyte.

On figb - the structure of the oxide film obtained by anodizing in the base electrolyte with the addition of ammonium bifluoride.

Information sources

1. Mamaev V.I. On the causes of marriage of matte nickel coatings on aluminum // Electroplating and surface treatment. - 2012. - No. 3. - S.22-25.

2. Denker I.I., Kuleshova I.D. Protection of aluminum and its alloys with paintwork. 2nd ed., Revised. and additional M: Chemistry. - 1985. - P.23-27.

3. Khudyakov V.L. The experience of using anodic oxide films in the chromium plating of aluminum // In: Anodic protection of metals: Dokl. 1st inter-university. conf. / ed. Epiphany A.F. M.: Mechanical Engineering, 1964. - S.292-309.

4. Liner V.I. Galvanic coatings of light alloys. M .: Metallurgizdat, 1959 - p.21.

5. Devyatkina TI, Bolshakova OA, Rogozhin VV, Mikhalenko MG Deposition of copper plating on parts made of aluminum and its alloys // Proceedings of the XI International Youth Scientific and Technical Conference "The Future of Technical Science". 2012. - P.297.

6. Devyatkina TI, Rogozhin VV, Bolshakova OA, Dumitrash OV, Mikhalenko MG The method of applying a copper plating on parts of aluminum and its alloys // RF Patent No. 2471020, C25D 5/44, C25D 11/20, publ. 12/27/2012.

7. Kokatev AN, Hanina E.Ya., Chupakhina EA, Yakovlev AN, Yakovleva NM The method of formation of porous oxide on a titanium-aluminum alloy // Application No. 20111114311/02. Stated April 12, 2011; publ. 10/20/2012.

Table 1 Parameters of oxide films obtained by anodizing in electrolytes with various additives. Anodizing mode: j _a = 2 A / dm ² , t = 18 ÷ 20 ° C, τ = 5 ÷ 6 min. Anodizing electrolyte δ, μm Porosity,% Voltage on the anodizing bath, V 15% H ₂ SO ₄ + 15% H ₃ PO ₄ 2.14 7.3 19 ÷ 21 15% H ₂ SO ₄ + 15% H ₃ PO ₄ + NaF 1,62 19 12 ÷ 13 15% H ₂ SO ₄ + 15% H ₃ PO ₄ + HBF ₄ 1.24 16 9 ÷ 11 15% H ₂ SO ₄ + 15% H ₃ PO ₄ + NH ₄ HF ₂ 1,242 twenty 6 ÷ 9

table 2 Visual assessment of the quality of the copper coating. Anodizing mode: j _a = 2 A / dm ² , t = 18 ÷ 20 ° C, τ = 5 ÷ 6 min. Copper electrolyte: standard sulfate. Copper mode: j _k = 1.5A / dm ² , t = 18 ÷ 20 ° C, τ = 27 min. Anodizing electrolyte Coating quality 15% H ₂ SO ₄ + 15% H ₃ PO ₄ Not spread over the entire area of the part 15% H ₂ SO ₄ + 15% H ₃ PO ₄ + NaF Semi-shiny fine-crystalline coating, no coverings. At a 500-fold increase, an insignificant number of pores per 1 mm ² surface is observed on alloys of the AK9ch and AL2 grades. 15% H ₂ SO ₄ + 15% H ₃ PO ₄ + HBF ₄ Semi-shiny fine-crystalline coating, no coverings. At a 500-fold increase, an insignificant number of pores per 1 mm ² surface is observed on alloys of grades AK4, AL2 and AD. 15% H ₂ SO + 15% H ₃ PO ₄ + NH ₄ HF ₂ Semi-shiny fine-crystalline coating, no coverings. Non-porous coating on all alloys.

Claims (1)

An electrolyte for anodizing aluminum and its alloys before applying copper plating, including phosphoric acid, sulfuric acid and water, characterized in that it additionally contains fluorine-containing inorganic substance from the group: ammonium bifluoride, bifluoride acid, sodium fluoride in the following ratio of components:
orthophosphoric acid 15 vol.% sulphuric acid 15 vol.% Fluorine-containing inorganic substance 4-15 g / l water rest

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