Method of making abrasion resistant coating for aluminum base alloy

Abstract

The method for applying an abrasion resistant coating on the surface of an aluminum or aluminum alloy conductor. After cleaning the conductor surface to remove dirt, grease, and oxides and to make the surface uniformly active, a plurality of coatings of metals are applied including an inner coating of tin, intermediate coatings of bronze and copper, and an outer coating of tin or silver.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an abrasion resistant coating for an aluminum and aluminum alloy conductor and to a method for applying the coating.

2. Description of the Prior Art

The application of protective coatings for various purposes on the surface of members composed of aluminum and aluminum alloys is well-known in the art such as disclosed in U.S. Pat. Nos. 2,377,606; 2,947,639; and 3,274,021. Generally, those patents disclosed a method for providing a tightly adherent tin coating on an aluminum surface.

Where specific environmental problems are incurred, additional protection is necessary. In the past a coating consisting of layers of zincate, copper, and silver plating on aluminum bus bars has left much to be desired for environmental protection; improved corrosion resistance in specialized areas has been indicated.

Associated with the foregoing has been a problem of fretting due to either mechanical wear or corrosion. In one form fretting is wear resulting from oscillating or vibratory motion of a limited amplitude. Another form of fretting results from actual wear, frictional changes, variations in electrical resistances, electrical noise, and fatigue failure. Aluminum is particularly susceptible to fretting corrosion which is the formation of hard abrasive oxides (Al.sub.2 O.sub.3) as debris and by an excessive rate of oxygen absorption. As a result, there has been a problem of providing aluminum members, such as bus bars or conductors, with a protective coating of a suitable type by which abrasion and corrosion are avoided.

SUMMARY OF THE INVENTION

Generally, it has been found in accordance with this invention that the foregoing problems may be overcome by providing an aluminum conductor or bus bar with a multiple layered coating of various metals applied in a given sequence, which sequence consists essentially of an inner layer of tin, intermediate layers of bronze and copper, and an outer layer of either tin or silver.

The invention also comprises a method for applying the several layers in the indicated sequence.

DESCRIPTION OF THE PREFERRED EMBODIMENT

According to the present invention in practice the new method is carried out in the following sequential manner:

1. Preliminarily cleaning the surface of the member to be coated with a caustic solution to remove oxides, dirt, grease, and the like;

2. Rinsing the surface in water;

3. Cleaning the surface with an acid;

4. Rinsing the member in water;

5. Applying an inner coating of tin to the surface;

6. Applying a copper-base alloy coating on the tin coating;

7. Rinsing the member in water;

8. Applying a first coating of copper on the copper-base alloy coating;

9. Applying a second coating of copper on the first coating of copper;

10. Rinsing the member in water and

11. Applying an outer coating of tin or silver on the second coating of copper.

The method is applicable to pure aluminum and a wide range of aluminum alloys such as aluminum alloy numbers 6061, 6101, 2024, 5052, 7075, 356, 360, and 380.

The first step comprises cleaning the surface with a caustic solution to remove oxides, dirt and the like, and removes about 1/2 to 1 mil of aluminum surface. The preferred solution for this purpose is about 8 ounces of sodium hydroxide per gallon of water at 180.degree.F which is applied to the aluminum surface for a time of from about one-half to one minute. Other alkali metal hydroxides such as potassium hydroxide may be used. A chelating agent is preferably added to the solution to slow down the action. A common chelating agent is ethylenediaminetetraacetic acid (EDTA). The advantage of the chelating agent is to maintain the material removed from the aluminum surface in solution.

The second step is a water rinse to remove all traces of the caustic solution of the first step because the third step involves an acid solution.

The third step is a surface cleaning operation with an acid solution such as a 1:1 mixture of nitric acid (67%) in water for the purpose of deoxidizing and removing impurities in aluminum such as iron and manganese. Although it is not necessary, about 1 to 5% hydrofluoric acid is preferably added to make the aluminum surface more uniformly active and to prevent blistering of subsequently deposited metal. The blistering often occurs due to poor metal preparation.

The fourth step is another rinse in water to remove all traces of acid of the previous rinse, because the next step involves an alkaline bath.

The fith step has the dual purpose of activating the aluminum surface and depositing a thin layer of tin. In this step the aluminum member is immersed in an aqueous solution the primary constituent of which is at least one tin-containing compound selected from the group consisting of a stannate of an alkali metal such as sodium stannate and potassium stannate, together with a chelating agent such as EDTA. The composition of the solution is shown in Table I as follows:

Table I ______________________________________ Activating & Tin Plating Bath Constituent Preferred Range Composition* Composition* ______________________________________ Na.sub.2 SnO.sub.3.sup.. H.sub.2 O 11 8-14 and/or K.sub.2 SnO.sub.3.sup.. H.sub.2 O Chelating Agent 6 1-10 (EDTA) ______________________________________ *Ounces per gallon of water

The constituents sodium stannate and potassium stannate may be used either alone or mixted to make up the required composition. Sodium stannate and potassium stannate used here and in subsequent steps may be used either in the anhydrous or the trihydrate form; it being immaterial, because the operating conditions require the solution to be maintained at an elevated temperature of from 75.degree. to 85.degree.F with a preferred temperature of 80.degree.F. The immersion time may range from 10 to 60 seconds with a preferred time of 30 seconds. A partial or wholly substitute chelating agent is preferably used for stabilizing the tin to prevent it from settling to the bottom of the container. It is a mixture of zinc acetate, Zn (C.sub.2 H.sub.3 O.sub.2).sub.2, in an amount of about 2 ounces per gallon of water and m-cresolsulthonic acid in the amount of about 4 ounces per gallon of water. Thus, by step 5 the surface of the aluminum member is completely activated and the protective layer of tin is deposited. The weight of tin deposited varies from about 0.1 to 0.3 mg/in..sup.2.

The sixth step is the application of a coating of a copper-base alloy, such as a bronze strike, applied directly on the tin coating. A rinse between steps 5 and 6 is unnecessary because the solutions of both steps are alkaline. The sixth step is accomplished by immersing the aluminum member into an electrolyte consisting essentially of a stannate of an alkali metal such as sodium stannate and copper cyanide. A partial or complete substitute for sodium stannate may be potassium stannate. The constituents and the amounts used for the bronze strike are shown in Table II as follows:

Table II ______________________________________ Bronze Strike Ounces per gallon of water Constituent Preferred Range Composition Composition ______________________________________ Na.sub.2 SnO.sub.3 4.5 3.5-5.5 and/or K.sub.2 SnO.sub.3 CuCN 2.0 1.5-2.5 KCN (Free) 3.0 2.5-3.5 KOH (Free) 1.0 0.5-1.3 ______________________________________

The electrolyte solution consisting of the constituents listed in Table II includes potassium cyanide (KCN) and potassium hydroxide (KOH) which increase the conductivity and efficiency of the electrolyte and improve the erosion of the copper and tin anodes, thereby reducing the current loss and maintaining the desired electrolyte temperature. In addition, by improving anode corrosion, the amount of makeup of the constituents sodium stannate and potassium stannate as well as copper cyanide is reduced.

The operating conditions of the bronze strike step are shown in Table III, as follows:

Table III ______________________________________ Operating Conditions Condition Preferred Range ______________________________________ Time 4 mins. 3 to 10 mins. Temperature 80.degree.F 75.degree. - 85.degree.F Cathode current density 30 ASF* 20 to 80 ASF Anode current density 15 ASF 10 to 20 ASF ______________________________________ *Amperes per square foot

The thickness of the bronze coating may vary from about 0.00001 to 0.00003 inch.

The seventh step is a water rinse for the purpose of removing the bronze solution and for preventing contamination of the copper strike solution which is the eighth step.

The eighth step, being a copper strike for the application of a relatively thin layer of copper on the bronze strike, has a purpose of providing better adhesion of the copper plate application of step 9. The aluminum member is immersed in electrolyte having the constituents listed in Table IV as follows:

Table IV ______________________________________ Copper Strike Ounces per gallon of water Constituent Preferred Range ______________________________________ CuCN 3.5 3-4 KCN and/or NaCN 6.4 6-7 Additive (tartral) 5% by vol. 4-6% by vol. ______________________________________

The additive (tartral) is preferably a Rochelle salt such as potassium-sodium tartrate, which serves to improve anode and cathode characteristics. The thickness of the copper strike ranges from about 0.00001 to 0.00003 inch, the preferred thickness being 0.00002 inch. The operating conditions for the electrolyte bath are listed in Table V as follows:

Table V ______________________________________ Operating Conditions pH 9.8-10.2 Free Potassium Cyanide 1.0 to 1.5 oz./gallon Temperature .degree.F 105-115 Voltage 9 volt supply; approx. 6 volts across work Current 10-30 ASF ______________________________________

Although the current range is from 10 to 30 amperes per square foot the preferred current is 25 amperes per square foot for 60 to 120 seconds to obtain the above-indicated coating thickness.

The ninth step is the application of the second coating, or the so-called copper plating. For that purpose the aluminum member is immersed in an electrolyte for a period of 4 minutes at 100% cathode efficiency to provide a copper plate thickness of from about 0.00018 to 0.00022 inch, and a preferred thickness of 0.00020 inch. The formula of the electrolyte bath and the operating condition for the bath are listed in the following Tables VI and VII:

Table VI ______________________________________ Copper Plate Ounces per gallon of water Constituent Preferred Range Composition Composition ______________________________________ CuCN 9 8-10 KCN and/or NaCN 15 14-16 Additive (tartral) 6% by vol. 5-7% by vol. ______________________________________

Table VII ______________________________________ Operating Conditions Free KCN and/or 0.5-1.5 oz./gallon NaCN Temperature 165.degree.F Voltage 9 volts Current Density 15-35 ASF (Preferred 25 ASF) Plating Time for .0002" Cu 4 min. at 25 ASF Agitation Filtered air Filtration Continuous and sufficient to turn over tank volume approx. 3 times per hour. ______________________________________

Step 10: After the copper plate or coating has been applied, the member is ready for the outer coating of tin or silver as preferred. However, a rinse in two baths of cold water is required before subsequent plating of tin or silver.

Step 11: Where the choice is tin, the copper plated aluminum member is immersed into the plating solution having the formula and operating conditions set forth in Tables VIII and IX as follows:

Table VIII ______________________________________ Tin Plate Ounces per gallon of water Constituent Preferred Range Composition Composition ______________________________________ KOH 2 1-4 (K.sub.2 SnO.sub.3) and/or Sodium Stannate (Na.sub.2 SmO.sub.3) 28 26-28 Tin metal equivalent 10.6 10-11 ______________________________________

Table IX ______________________________________ Operating Conditions Range Preferred ______________________________________ Temperature .degree.F 130-180 140.degree.F Voltage 3-6 Current density 30-160 ASF 75 Anode to Cathode ratio Never more than 1:2 Time to deposit .0001 to .0006 in. of plating at recommended current 2-4 2 density, minutes ______________________________________

The thickness of the outer tin plate or coating may range from 0.0001 to 0.0003 inch, the preferred thickness being 0.0002 inch.

Where the outer plating or coating is silver instead of tin, the copper-plated aluminum member is placed in the silver plating electrolyte having a formula and control conditions as set forth in Tables X and XI as follows:

Table X ______________________________________ Silver Plate Ounces per gallon of water Constituent Range Composition ______________________________________ AgCN 2.5-3.5 KCN 8.0-9.0 KCO.sub.3 6.0-10.0 ______________________________________

Table XI ______________________________________ Operating Conditions Range Preferred ______________________________________ Time 60-70 seconds Temperature 70.degree.-100.degree.F 85.degree.F Voltage 2-6 volts Current Density 5-10 ASF Anode to Cathode ratio 1:1 ______________________________________

A silver plating or coating thickness of from 0.00003 to 0.00005 inch is normally deposited in about 70 seconds for the indicated current density. As in the preceding deposition such as for the copper and tin, the cathode is the aluminum member held stationary in the electrolyte. The anode is composed of copper, tin, or silver depending upon the metal being deposited.

The resulting protective coating provided in the method set forth above provides a corrosion and abrasion resistant plating for stabs for bus ducts when bus bars are subjected to 30,000 consecutive abrasion tests, i.e., involving sliding wear. More specifically, a prior art problem of wear through aluminum resulted in the formation of high resistance due to globules of aluminum oxide (Al.sub.2 O.sub.3) which resistance caused excess heating at the contact points and resulted in plastic flow in the form of fretting and mechanical weldings of the stab and the contact which in turn resulted in arcing. The multiple layered coating of this invention satisfies the foregoing problem of fretting with attendent problems. The bronze plating provides a suitable substrate for the copper strike which in turn sets up a bond for the copper plating. The copper plating has a tri-fold advantage including the establishment of a base for the silver or tin which is subsequently applied, preventing corrosion, and providing a hard base for sliding friction on the stab surface. The outer coating of tin prevents oxidation of the copper. Silver likewise prevents copper oxidation but in addition resists fretting better than tin and does it with a thinner coating.

Claims

What is claimed is:

1. A method for applying an abrasion resistant coating on the surface of an aluminum-base alloy member comprising the steps of:

1. cleaning the surface of the member;

2. rinsing the surface of the member;

3. applying an aqueous deoxidizing solution to the surface of the member;

4. rinsing the surface of the member;

5. applying a coating of tin by immersing the member in an aqueous solution consisting essentially of, for each gallon of water, from about 8 to about 14 ounces of at least one tin-containing compound selected from the group consisting of sodium stannate and potassium stannate, and of from about 1 to 10 ounces of a chelating agent;

6. electroplating a coating of a copperbase alloy on the tin coating by immersing the member in an electrolyte consisting essentially of, for each gallon of water, from about 3.5 to about 5.5 ounces of at least one compound selected from the group consisting of sodium stannate and potassium stannate, from about 1.5 to about 2.5 ounces of copper cyanide, from about 2.5 to about 3.5 ounces of potassium cyanide, and from about 0.5 to about 1.3 ounces of potassium hydroxide;

7. rinsing the surface of the member;

8. electroplating a first coating of copper on the copper-base alloy coating by immersing the member in an electrolyte consisting essentially of, for each gallon of water, from about 3 to about 4 ounces of copper cyanide, of from about 6 to about 7 ounces of at least one compound selected from the group consisting of sodium cyanide and potassium cyanide, and of from about 4 to about 6% of potassium-sodium tartrate;

9. electroplating a second coating of copper on the first coating of copper by immersing the member in an electrolyte consisting essentially of, for each gallon of water, from about 8 to about 10 ounces of copper cyanide, of from about 14 to about 16 ounces of at least one compound selected from the group consisting of sodium cyanide and potassium cyanide; and

10. electroplating an outer coating of a metal selected from a group consisting of silver and tin to the second coating of copper.

2. The method of claim 1 wherein between steps 9 and 10 the member is rinsed and in step 10 the outer coating is tin.

3. The method of claim 2 wherein in step 10 the electrolyte is an aqueous solution consisting essentially of, for each gallon of water, from about 1 to 4 ounces of potassium hydroxide, from about 26 to 28 ounces of potassium stannate, and about 10 ounces of tin metal.

4. The method of claim 3 wherein the operating conditions comprise a temperature of from about 130.degree. to 180.degree.F, a voltage of from about 3 to 6 volts, a current density of from about 30 to 160 amperes per square foot, and an anode to cathode ratio of up to 1:2.

5. The method of claim 4 in which the temperature is about 140.degree.F and the current density is 75 amperes per square foot.

6. The method of claim 3 wherein there are about 2 ounces of potassium hydroxide, about 28 ounces of potassium stannate, and about 10.6 ounces of tin metal.

7. The method of claim 2 wherein the tin applied by step 5 has a weight of from about 0.1 to 0.3 mg/in.sup.2, the copper base alloy coating has a thickness ranging from about 0.00001 to 0.00003 inch, the first coating of copper has a thickness of from about 0.00001 to 0.00003 inch, the second coating of copper having a thickness ranging from about 0.00018 to 0.00022, and the outer coating of tin having a thickness of from about 0.0001 to 0.0003 inch.

8. The method of claim 1 wherein the outer coating is silver.

9. The method of claim 8 wherein the electrolyte in step 10 consists essentially of an aqueous solution of, for each gallon of water, about 2.5 to 3.5 ounces of silver cyanide, from about 8 to 9 ounces of potassium cyanide, and from about 6 to 10 ounces of potassium carbonate.

10. The method of claim 9 wherein a current density of 10 amperes per square foot is provided.

11. The method of claim 1 wherein in step 6 the cathode current density is from about 20 to 80 amperes per square foot, the anode current density is from about 10 to 20 amperes per square foot, the temperature range is from about 75.degree. to 85.degree.F, and the time of exposure of the member is from about 3 to 10 minutes.

12. The method of claim 11 wherein the cathode current density is about 30 amperes per square foot, the anode current density is about 15 amperes per square foot, the temperature of the electrolyte is about 80.degree.F, and the time of exposure of the member is about 4 minutes.

13. The method of claim 1 wherein in step 8 the electrolyte consists essentially of, for each gallon of water, about 3.5 ounces of copper cyanide, about 6.4 ounces of at least one compound selected from the group consisting of sodium cyanide and potassium cyanide, and of about 5% by volume of potassium-sodium tartrate.

14. The method of claim 13 wherein the operating conditions of the electrolyte consists essentially of a pH of from about 9.8 to 10.2, a temperature ranging from about 105.degree. to 115.degree.F, and a voltage supply of about 9 volts.

15. The method of claim 1 wherein in step 9 the electrolyte consists essentially of about 9 ounces of copper cyanide, about 15 ounces of at least one compound selected from the group consisting of sodium cyanide and potassium cyanide, about 6% by volume of potassium-sodium tartrate.

16. The method of claim 15 wherein the operating conditions of step 9 consist essentially of an electrolyte temperature of about 165.degree.F, a supply voltage of about 9 volts, and a plating time of about 4 minutes at 25 amperes per square foot.

17. The method of claim 1 wherein in step (5) there is about 11 ounces of the tin-coating compound.