Preparation method of silver coating of aluminum and aluminum alloy

Abstract

The invention belongs to the technical field of aluminum alloy electroplating, and particularly relates to a preparation method of a silver coating of aluminum and aluminum alloy. The preparation method of the silver plating layer of the aluminum and the aluminum alloy comprises the following steps that the aluminum or the aluminum alloy is sequentially subjected to alkali etching, primary acid pickling, primary zinc dipping, secondary acid pickling, secondary zinc dipping, copper plating, silver pre-plating and cyaniding silver plating; wherein during copper plating, a copper plating solution comprises 30 g / L to 40 g / L of CuCN and 50 g / L to 60 g / L of NaCN; a silver pre-plating solution in the silver pre-plating process comprises 2-3 g / L of AgCN and 90-100 g / L of KCN, the current density in the silver pre-plating process is 0.3-0.6 A / dm < 2 >, and the silver pre-plating time is 1-2 min. And by controlling electroplating process parameters of copper plating and silver pre-plating in the electroplating process, the compactness of the plating layer is improved, the capability of resisting water vapor and the like is effectively enhanced, and then the corrosion resistance of the silver plating layer is improved.

Description

Technical Field

[0001] The invention relates to the technical field of aluminum alloy electroplating, and in particular to a method for preparing a silver plating layer of aluminum and aluminum alloy. Background Art

[0002] At present, silver-plated parts for high-voltage electrical appliances are prone to corrosion and silver layer shedding during other processing steps, storage and service. The preparation process of silver plating should be fundamentally improved to enhance the corrosion resistance of the plating and reduce the quality cost of rework caused by product corrosion. At the same time, the conductivity of the plating and the current carrying capacity of high-voltage electrical equipment are not affected, ensuring the safe and stable operation of power grid equipment.

[0003] Traditional cyanide silver plating is widely used in high-end industries such as high-voltage disconnectors in power systems due to its stable plating solution, low cost and excellent plating performance. Silver plating of aluminum and aluminum alloys requires intermediate plating layers to gradually reduce the potential difference. However, the porosity of each deposited plating layer has a great influence on the corrosion resistance of the final silver plating layer. Due to the above-mentioned silver plating characteristics of aluminum and aluminum alloys, before silver plating of aluminum and aluminum alloy substrates, it is necessary to add transition layers such as zinc and copper layers on the surface, and then add a nickel intermediate plating layer on the basis of the copper layer. However, this will affect the flow capacity of aluminum and aluminum alloy parts, and thus affect the conductivity of high-voltage electrical products; while using the traditional cyanide silver plating process, only zinc and copper transition layers are added. After the parts are silver-plated, the silver layer is very likely to corrode and blister during other processing steps, storage and use.

[0004] Therefore, there is an urgent need to develop a silver plating process that improves the corrosion resistance of aluminum and aluminum alloy high-voltage electrical products. Summary of the invention

[0005] The invention provides a method for preparing a silver-plated layer of aluminum and an aluminum alloy, which solves the problem that the silver-plated layer of aluminum and an aluminum alloy obtained by the existing silver-plating process has poor corrosion resistance.

[0006] In order to solve the above technical problems, the technical solution of the method for preparing the silver coating of aluminum and aluminum alloy of the present invention is as follows: A method for preparing a silver-plated layer of aluminum and aluminum alloys, comprising the following steps: sequentially performing alkaline etching, primary pickling, primary zinc immersion, secondary pickling, secondary zinc immersion, copper plating, pre-silver plating, and cyanide silver plating on the aluminum or aluminum alloy; wherein the copper plating solution during copper plating comprises 30-40 g / L of CuCN and 50-60 g / L of NaCN; the pre-silver plating solution during pre-silver plating comprises 2-3 g / L of AgCN and 90-100 g / L of KCN, and the current density during pre-silver plating is 0.3-0.6 A / dm 2 The pre-silver plating time is 1~2min.

[0007] The present invention improves the prior art and provides a method for preparing a silver coating of aluminum and aluminum alloys. Since the silver coating of aluminum and aluminum alloys requires an intermediate coating to gradually reduce the potential difference, the properties of each deposited coating will have an important impact on the final performance of the coating. The present invention controls the electroplating process parameters of the intermediate coating of copper plating and pre-silver plating in the cyanide silver plating process, which is conducive to the smooth and uniform surface of each deposited coating, improves the density of the coating, and can effectively enhance the ability to resist the infiltration of water vapor, etc., thereby improving the corrosion resistance of the silver coating. Using the silver plating process of the present invention, the corrosion resistance level reaches level 10 after 24 hours of neutral salt spray test; the corrosion resistance level can still reach level 9 after 48 hours of neutral salt spray test, and the corrosion resistance is reduced to level 0 after 72 hours. Using the silver plating process of the present invention, by improving the corrosion resistance of the silver coating, the time cost and quality cost of rework of parts caused by corrosion can be greatly reduced, and quality and efficiency can be fundamentally improved. It also reduces the frequency of corrosion problems at the power grid equipment installation site, provides protection for the process stability and production cycle of the switch assembly process, ensures the on-time commissioning rate of high-voltage switchgear, and improves the reliability and stability of power grid operation.

[0008] In order to further improve the compactness of the copper plating layer, preferably, the temperature during copper plating is 40-50°C and the current density is 0.8-1.2A / dm 2 The copper plating time is 10~15min.

[0009] In order to further improve the compactness of the pre-silver plating layer, preferably, the temperature during the pre-silver plating is 20-35°C.

[0010] In order to further improve the compactness of the silver-plated layer, preferably, the silver-plating solution during cyanide silver plating includes 60-70 g / L KAg(CN)2, 115-135 g / L KCN, and 10-30 g / L K2CO3. In order to further improve the glossiness and oxidation resistance of the silver-plated layer, more preferably, the silver-plating solution during cyanide silver plating includes 60-70 g / L KAg(CN)2, 115-135 g / L KCN, 10-30 g / L K2CO3, and 10-20 mL / L brightener.

[0011] In order to further improve the corrosion resistance of the silver-plated layer, preferably, the temperature during cyanide silver plating is 20-35°C and the current density is 0.5-0.8A / dm 2 The time for cyanide silver plating is 40~60min.

[0012] In order to further improve the bonding strength of the coating, preferably, the zinc dipping solution during the primary zinc dipping and the secondary zinc dipping includes 220-250 g / L NaOH, 22-28 g / L ZnO, 30-50 g / L potassium sodium tartrate, 5-8 g / L FeCl3·6H2O, 0.6-0.8 g / L CuSO4·5H2O, and 8-10 g / L NaCN.

[0013] In order to further improve the deposition quality of the zinc layer, preferably, the temperature during the first zinc immersion and the second zinc immersion is 20-26° C., and the time is 1-2 min.

[0014] In order to activate the surface of aluminum or aluminum alloy and facilitate subsequent metal deposition, preferably, the pickling solution of the first pickling includes 250-300 ml / L nitric acid and 100-120 ml / L phosphoric acid; the pickling solution of the second pickling includes 300-400 ml / L nitric acid. More preferably, the pickling solution of the first pickling includes 250-300 ml / L nitric acid, 100-120 ml / L phosphoric acid, and 200-300 ml / L descaling agent.

[0015] In order to further improve the activation effect, preferably, the first pickling and the second pickling are both carried out at room temperature, the first pickling time is 15 s ~ 1 min, and the second pickling time is 30 s ~ 1 min.

[0016] In order to effectively remove the oxide film on the surface of aluminum or aluminum alloy and remove the surface oil stains, preferably, the alkali solution used in the alkali etching includes 50-60 g / L of sodium hydroxide and 20-25 g / L of sodium phosphate; the temperature during the alkali etching is 60-70°C and the time is 30s-2min.

[0017] After cyanide silver plating, neutralization and hot water washing are performed. The neutralization is performed by washing with 5-7% industrial nitric acid at room temperature for 5-15 seconds. The hot water washing is performed at 80-85° C. for 5-10 minutes. DETAILED DESCRIPTION

[0018] The technical concept of the method for preparing the silver-plated layer of aluminum and aluminum alloy of the present invention is as follows: Due to the silver-plating characteristics of aluminum and aluminum alloys, when the prior art adopts the cyanide silver-plating process, adding a nickel intermediate plating layer on the basis of the zinc layer and the copper layer transition layer will affect the conductivity of aluminum or aluminum alloy parts, which is not conducive to the application of aluminum or aluminum alloys in the field of high-voltage disconnectors, but only the zinc layer and the copper layer transition layer will damage the corrosion resistance of the workpiece. The present invention can improve the density of the plating layer and thus greatly improve the corrosion resistance of the workpiece by controlling the electroplating process parameters of copper plating and pre-silver plating in the cyanide silver-plating process, and there is no need to add a nickel intermediate plating layer.

[0019] The embodiments of the present invention are further described below in conjunction with specific examples. The chemical reagents involved in the following examples are all commercially available conventional products unless otherwise specified. Nitric acid and phosphoric acid are both commercially available nitric acid and commercially available phosphoric acid, with the mass fraction of commercially available nitric acid being 65% and the mass fraction of commercially available phosphoric acid being 85%.

[0020] 1. Specific embodiments of the method for preparing the silver-plated layer of aluminum and aluminum alloy of the present invention Example 1 The silver coating preparation method of the aluminum alloy of the present embodiment is as follows: 1) Alkali etching: The aluminum alloy was etched in an alkaline solution at 60°C for 2 min. The alkaline solution consisted of 50 g / L sodium hydroxide, 20 g / L sodium phosphate and water.

[0021] 2) Pickling once: The aluminum alloy after alkaline etching in step 1) is pickled once in a pickling solution composed of the following components: 300 ml / L nitric acid, 120 ml / L phosphoric acid, 300 ml / L descaling agent and water, and the pickling is carried out at room temperature for 1 minute.

[0022] 3) Single zinc immersion: The aluminum alloy after the single pickling in step 2) is subjected to a single zinc immersion in a zinc immersion solution composed of the following components: 240 g / L NaOH, 25 g / L ZnO, 40 g / L potassium sodium tartrate, 7 g / L FeCl3·6H2O, 0.8 g / L CuSO4·5H2O, 10 g / L NaCN and water, and the zinc immersion is carried out at 23°C for 1 min.

[0023] 4) Secondary pickling: The aluminum alloy after the zinc immersion in step 3) is pickled for a second time in a pickling solution consisting of 400 ml / L of commercially available nitric acid and water, and the pickling is carried out at room temperature for 1 minute.

[0024] 5) Secondary zinc immersion: The aluminum alloy after the primary zinc immersion in step 4) is subjected to secondary zinc immersion in a zinc immersion solution consisting of the following components: 250 g / L NaOH, 28 g / L ZnO, 50 g / L potassium sodium tartrate, 8 g / L FeCl3·6H2O, 0.8 g / L CuSO4·5H2O, 10 g / L NaCN and water. The secondary zinc immersion is carried out at 23°C for 1 min.

[0025] 6) Copper plating: The aluminum alloy after the secondary zinc immersion in step 5) is copper plated in a copper plating solution, the copper plating solution consists of the following components: 30 g / L CuCN, 50 g / L NaCN (including 15 g / L free NaCN) and water, the temperature during copper plating is 40 °C, and the current density is 1 A / dm 2 , the copper plating time is 10min.

[0026] 7) Pre-silver plating: The aluminum alloy after copper plating in step 6) is pre-silver plated in a pre-silver plating solution, which consists of the following components: 2g / L AgCN, 90g / L KCN and water. The temperature during pre-silver plating is 20°C and the current density is 0.3A / dm 2 , the pre-silver plating time is 1min.

[0027] 8) Cyanide silver plating: The aluminum alloy pre-silvered in step 7) is subjected to cyanide silver plating in a silver plating solution. The silver plating solution consists of the following components: 65 g / L KAg(CN)2, 130 g / L KCN, 20 g / L K2CO3, 10 mL / L brightener and water. The temperature during cyanide silver plating is 20°C and the current density is 0.5 A / dm 2 The cyanide silver plating time is 40 minutes.

[0028] 9) Neutralization and hot water washing: The aluminum alloy after cyanide silver plating in step 8) is cleaned with 5% industrial nitric acid at room temperature for 10 seconds, and then washed with water at 80°C for 5 minutes.

[0029] Example 2 The method for preparing the silver plating layer of the aluminum alloy in this embodiment is basically the same as that in Example 1, except that the copper plating solution in step (6) consists of the following components: 32 g / L CuCN, 50 g / L NaCN (including 15 g / L free NaCN) and water.

[0030] 2. Comparison Comparative Example 1 The method for preparing the silver-plated layer of the aluminum alloy in this comparative example is basically the same as that in Example 1, except that: the copper plating solution in step 6) is composed of the following components: 55 g / L CuCN, 70 g / L NaCN (including 10 g / L free NaCN) and water, the temperature during copper plating is 40 °C, and the current density is 1 A / dm 2 The copper plating time is 10 min; the pre-silver plating solution in step 7) is composed of the following components: 5 g / L AgCN, 125 g / L KCN and water, and the current density during pre-silver plating is 1.0 A / dm 2 , the pre-silver plating time is 15s.

[0031] Comparative Example 2 The method for preparing the silver-plated layer of the aluminum alloy in this comparative example is basically the same as that in Example 1, except that the copper plating solution in step 6) is composed of the following components: 40 g / L CuCN, 57 g / L NaCN (including 13 g / L free NaCN) and water, the temperature during copper plating is 40 °C, and the current density is 1 A / dm 2The copper plating time is 10 min; the pre-silver plating solution in step 7) is composed of the following components: 4 g / L AgCN, 140 g / L KCN and water, and the current density during pre-silver plating is 1.0 A / dm 2 , the pre-silver plating time is 15s.

[0032] Comparative Example 3 The method for preparing the silver-plated layer of the aluminum alloy in this comparative example is basically the same as that in Example 1, except that the pre-silver plating solution in step 7) is composed of the following components: 1.8 g / L AgCN, 140 g / L KCN and water, and the current density during the pre-silver plating is 1.0 A / dm 2 , the pre-silver plating time is 15s.

[0033] 3. Experimental Examples According to "GB / T 6461 Metal and other inorganic coatings on metal substrates" and "GB / T 10125-2012 Artificial atmosphere corrosion test", the aluminum and aluminum alloy with silver-plated layer obtained in the examples and comparative examples were tested for corrosion resistance. The results show that the corrosion resistance rating of the aluminum alloy silver-plated layer of comparative example 1 is 10 after 2 hours of neutral salt spray test; the corrosion resistance rating drops to 0 after 4 hours of neutral salt spray test. The corrosion resistance rating of the aluminum alloy silver-plated layer of comparative example 2 is 10 after 2 hours of neutral salt spray test; the corrosion resistance rating drops to 9 after 4 hours of neutral salt spray test; the corrosion resistance rating drops to 0 after 8 hours of neutral salt spray test. The corrosion resistance rating of the aluminum alloy silver-plated layer of comparative example 3 is 10 after 4 hours of neutral salt spray test; the corrosion resistance rating drops to 9 after 8 hours of neutral salt spray test; the corrosion resistance rating drops to 0 after 24 hours of neutral salt spray test. However, after 24 hours of neutral salt spray test, the corrosion resistance rating of the aluminum alloy silver-plated layer of Example 1 can still reach level 10; after 48 hours of neutral salt spray test, the corrosion resistance rating drops to level 9; after 72 hours of neutral salt spray test, the corrosion resistance rating drops to level 0. The corrosion resistance of the aluminum alloy silver-plated layer of Example 2 is equivalent to that of Example 1.

[0034] Finally, it should be noted that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for those skilled in the art to modify the technical solutions described in the aforementioned embodiments or to make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing a silver coating of aluminum and aluminum alloys, characterized in that: The following steps are involved: The aluminum or aluminum alloy is sequentially subjected to alkaline etching, primary pickling, primary zinc immersion, secondary pickling, secondary zinc immersion, copper plating, pre-silver plating, and cyanide silver plating; wherein the copper plating solution during copper plating includes 30-40 g / L CuCN and 50-60 g / L NaCN; the pre-silver plating solution during pre-silver plating includes 2-3 g / L AgCN and 90-100 g / L KCN, and the current density during pre-silver plating is 0.3-0.6 A / dm 2 The pre-silver plating time is 1~2min.

2. The method for preparing a silver-plated layer of aluminum and aluminum alloy according to claim 1, characterized in that: The temperature during copper plating is 40~50℃ and the current density is 0.8~1.2A / dm 2 The copper plating time is 10~15min.

3. The method for preparing a silver-plated layer of aluminum and aluminum alloy according to claim 1, characterized in that: The temperature during pre-silver plating is 20~35℃.

4. The method for preparing a silver-plated layer of aluminum and aluminum alloy according to claim 1, characterized in that: The silver plating solution during cyanide silver plating includes 60~70 g / L KAg(CN)2, 115~135 g / L KCN, and 10~30 g / L K2CO3.

5. The method for preparing a silver-plated layer of aluminum and aluminum alloy according to claim 1 or 4, characterized in that: The temperature during cyanide silver plating is 20~35℃ and the current density is 0.5~0.8A / dm 2 The time for cyanide silver plating is 40~60min.

6. The method for preparing a silver-plated layer of aluminum and aluminum alloy according to claim 1, characterized in that: The zinc immersion solution during the primary zinc immersion and the secondary zinc immersion includes 220~250g / L NaOH, 22~28g / L ZnO, 30~50g / L potassium sodium tartrate, 5~8g / L FeCl3·6H2O, 0.6~0.8g / L CuSO4·5H2O, and 8~10g / L NaCN.

7. The method for preparing a silver-plated layer of aluminum and aluminum alloy according to claim 1 or 6, characterized in that: The temperature for the first and second zinc immersion is 20~26℃, and the time is 1~2min.

8. The method for preparing a silver-plated layer of aluminum and aluminum alloy according to claim 1, characterized in that: The pickling solution for the first pickling includes 250~300 ml / L nitric acid and 100~120 ml / L phosphoric acid; the pickling solution for the second pickling includes 300~400 ml / L nitric acid.

9. The method for preparing a silver-plated layer of aluminum and aluminum alloy according to claim 1 or 8, characterized in that: Both the primary pickling and the secondary pickling are carried out at room temperature. The primary pickling time is 15 s~1 min, and the secondary pickling time is 30 s~1 min.

10. The method for preparing a silver-plated layer of aluminum and aluminum alloy according to claim 1, characterized in that: The alkali solution used in alkali etching includes 50~60g / L sodium hydroxide and 20~25g / L sodium phosphate; the temperature during alkali etching is 60~70℃, and the time is 30s~2min.