GB2031468A - Copper alloy cleaning process - Google Patents

Abstract

Oxide deposits are removed from the surface of copper-base alloy article by immersion in an aqueous alkaline solution having a pH of 11-14 at a temperature between 40 DEG C and its boiling point for between 2 seconds and 10 minutes, and then immersing the article in an aqueous solution of ferric sulphate, or similar ferric salt of a mineral acid. The salt concentration is between 0.5 and 3.0 N, the period of immersion is at least two seconds, preferably 5 to 60 seconds, and the salt solution is at a temperature of 25 DEG to 95 DEG C, preferably 60 DEG to 90 DEG C. At the end of each immersion the article is drained, blow dried, and rinsed with water.

Description

SPECIFICATION Copper alloy cleaning process This invention relates to the surface cleaning of copper base alloys and provides a rapid and effective two-stage process for removing surface oxide deposits formed on the metal, as during annealing treatments. In the case of copper alloys containing aluminum and other readily oxidizable metals such as iron, cobalt, nickel, zinc, or silicon, a resistant metal oxide surface film is formed during annealing treatments at elevated temperatures, applied for a period of time sufficient to relieve strains imposed during mechanical operations, as in the rolling of metal ingots to elongated strip of reduced thickness or for the fabrication of metal parts and articles.Such surface coatings of oxide are produced even when the annealing is carried out in the presence of a protective atmosphere, such as may be prepared by the partial combustion of a hydrocarbon, because the active metals present in the alloy will react with oxygen that is present on the free state or as moisture or as an oxide of carbon to form refractory oxides. When various metal oxides are thus formed, they often tend to combine with each other to form complex metal oxides, such as spinels, of refractory nature, which at times are extremely resistant to removal by conventional cleaning methods. Unless such surface oxide films are thoroughly removed, however, subsequent surface finishing treatments, such as soldering, electroplating, or pressure bonding with other metals, wili generally yield unsatisfactory results because of the resulting poor adhesion.

While numerous proposals have been made in the past for single and multi-stage cleaning treatments, they have generally proved ineffective for the successful cleaning of copper alloys containing aluminum and other metals tending to form refractory oxides in thick surface layers. Although some of these procedures may, if prolonged, finally result in the removal of such layers, an undesirable extent of pitting and etching of etching of the metal surface is found to occur simultaneously during such extended treatments, which may render them unsuitable for the intended purpose.Such unsatisfactory results characterize attempts to remove thick refractory oxide layers from various copper alloys containing aluminum and other active metals by single treatments with aqueous pickling solutions, such as aqueous solutions of sulfuric acid, sulfuric acid and alkali dichromate, sulfuric acid and ferric sulfate, as described on Pages 308-309 of "The Chemical Formulary", Vol.

IX, 1951, Chemical Publishing Co., Inc., Brooklyn, N. Y.

In accordance with one aspect of the present invention, a process for cleaning the surface of a copper base alloy comprises the following two-step treatment, neither step alone being effective to produce a clean metal surface satisfactory for soldering or other surface treating process. The oxide-coated copper-base article or strip is first immersed in a stirred aqueous alkaline solution, as of sodium, potassium or lithium hydroxide, having a pH above 10 i.e., between 11 and 14, for at least two seconds at a temperature of about 40"C to boiling point, and, after draining and rinsing with water, is then submerged in a stirred aqueous solution of ferric sulfate, or similar ferric salt of a mineral acid, at a ferric salt concentration of 0.5 to 3.0 N for at least two seconds, preferably for five to sixty seconds, at a temperature of 25 to 95"C, preferably at 60 to 90"C. Each step should be followed by draining the solution from the article or subjecting it to blowing with air streams, and removing the residual solution by rinsing with water.

The need for such improved cleaning precess arose during the use of the duplex process of U.S. Patent 3,646,946, issued March 7, 1972, consisting of first immersing annealed metal in a hot alkaline solution and then treating with a hot solution of mineral acid, preferably at 12% by volume aqueous sulfuric acid solution. Effective results were attained thereby in removing complex oxide films and producing a surface displaying excellent solderability in a variety of annealed copper base alloys, such as Cu, Fe and Al; Cu, Al, Si, Co and P; and C.D.A. Alloy 668 containing 22.7% Zn, 3.4% Al, 0.4% Co, and balance essentially Cu. The above duplex treatment accomplished the substantially complete removal of complex oxide film in annealed metal shown by capacitance measurements to have surface oxide films up to about 105 in thickness.However, when the latter alloy was annealed under more severe conditions and having surface oxide films up to 1 40 to 1 60 in thickness, such as resulted from bell annealing treatments which were prolonged or carried out in a less protective atmosphere, the above duplex cleaning process displayed substantially decreased effectiveness. Thereby, the need for an improved cleaning precedure arose as an urgent problem, which has now been solved through the provision of the novel second step of the duplex treatment described below.

It was theorized that a solution having proper oxidative power and solvent action on complex oxide films might be found through measurements of the rate of weight loss of the basis metal therein. This mode of attack has been successful in establishing an effective second treatment, although the generality of such parallel behavior is quite uncertain and unproved. A study of weight loss measurements was carried out in the above Alloy 688 at 50"C and 65"C immersed for periods of 10, 20, and 30 seconds in a 1 N. aqueous solution of ferric sulfate containing no added acid, and comparison solutions containing 1 N. ferric sulfate and added sulfuric acid at concentrations of 1 N., 3N., and 5 N.At each temperature, the loss in weight per unit area, expressed as micrograms per square centimeter, was linear with time, in seconds, and yielded the values for dissolving rate listed in Table I.

Table I Dissolving Rate (11 grams per cm2 per sec.) Solution 50"C 65"C 1 N.Fe(S04)3 45.0 62.8 1 N.Fe2(SO4)3 + 1 N.H2S04 44.4 58.0 1 N.Fe2(SO4)3 + 3N.H2S04 41.3 53.8 1 N.Fe2(SO4)3 + 5 N.H2S04 36.4 48.3 These results showed quite unexpectedly that the aqueous ferric sulfate solution with no added acid, at both temperatures, dissolves the metal at a higher rate than any of the solutions containing added acid, and further, that the dissolving rate at both temperatures decreases as the concentration of added acid is increased.

It was subsequently shown in duplex cleaning treatments of annealed metal having a surface oxide film up to 1 60 A thick, wherein the treatment consisted of a first immersion in a hot alkaline solution, followed by a second immersion in a hot ferric sulfate solution containing no added acid, that the removal of oxide film was accomplished effectively, as substantiated by the applicable tests, including the solderability test. The cleaning action is as thorough and complete, or better, than when added sulfuric acid is present in the ferric sulfate solution, thereby accomplishing at least equivalent results more conveniently and at decreased expense.

The initial treatment with hot alkaline solution is carried out at a temperature of about 40"C to the boiling point and preferably, at a temperature of about 70"C to the boiling point, the solution having a pH above 10, preferably 11 to 14. The solution is preferably of caustic soda, but other alkali hydroxides as of potassium or lithium, or mixtures may be used within the above-stated pH range. Immersion of the copper alloy should be for at least two seconds and preferably, for five to sixty seconds. The treatment time may be prolonged, particularly at temperatures near the lower limit of the above range, but generally should be for ten minutes or less, as longer times usually provide no added advantages. Optimum treating times and conditions may be determined in accordance with the particular alloy and the results desired.

The second step of the duplex treatment is best effected by immersion of the copper alloy strip or article, after the alkaline solution has been drained therefrom and rinsed with water, in a hot 0.5 to 3 N., best 1.25 to 2 N., solution of ferric sulfate, without added acid at a temperature of about 25' to 95"C, best 65"to 90"C, for at least two seconds, and preferably for five to sixty seconds, and generally for not over ten minutes.

Ferric sulfate for such treatment may be replaced in whole or in part by other soluble ferric salts, as for example by ferric ammonium sulfate or ferric nitrate, generally with the obtainment of equivalent results in the obtainment of clean and solderable copper alloy surfaces. It may be noted that aqueous solutions of ferric sulfate, typical of soluble ferric salts of strong acids, display a pH of about 1 at 0.5 N. solution, decreasing, close to-linearly, to about 0.5 at 2 N. and to about 0.35 at 3 N.

The efficacy of a duplex treatment in accordance with this invention, as above outlined, is illustrated in the following specific example, contrasting its successful results with the ineffective cleaning provided by the process taught in U.S. Patent 3,646,946, the closest prior art duplex treatment.

EXAMPLE I Sheets of C.D. Alloy 688 (containing 22.7% Zn, 3.4% Al, 0.4% Co, and balance essentially Cu) were bell annealed at about 600"C in a closed furnace containing an atmosphere produced by the partial combustion of hydrocarbons to produce two lots of annealed sheet, the first having a surface oxide layer ranging 95 to 105 A in thickness and the second having a surface oxide layer ranging 140 to 1 60 A in thickness.

Samples of the two lots were given a first immersion treatment in boiling caustic soda solution having a pH of 14 for twenty seconds, drained, and washed with water.

Samples of the two lots, treated as above, were then subjected to immersion for twenty seconds in one of the following three solutions: (1) 1.5 N. aqueous ferric sulfate solution, containing no added acid, at 65"C.

(2) 12% by vol. aqueous sulfuric acid solution at 65"C.

(3) 12% by vol. aqueous sulfuric acid solution containing 40z. per gallon of sodium dichromate.

Following the twenty second immersion, the samples were drained, washed with water, and dried.

Solderability tests applied to the treated samples of the first lot (95 to 105 A oxide layer) revealed that those treated with solution (1) and (3) were solderable, while those treated with solution (2) could not be soldered effectively.

Solderability tests applied to the treated samples of the second lot (140 to 1 60 A oxide layer) revealed that those treated with solution (1) were solderable, while those treated with solutions (2) and (3) could not be effectively soldered.

Furthermore, capacitance tests on the above-treated samples revealed that the oxide layer had been substantially completely removed from the samples which displayed acceptable solderability [Lot 1, treated with (1) or (3)] and [Lot 2, treated with (1)] and [Lot 2, treated with (2) and (3)] were determined to have retained about 20 to 30% of the initial oxide layer.

Thus, the only duplex treatment which was completely successful in the above example was the one utilizing for the second treatment the immersion in aqueous ferric sulfate solution with no added acid.

The above test for solderability corresponds to that described in Report W 72-51.2, American Society for Metals, Metals Park, Ohio. The sample is immersed in a rosin flux solution, then vertically dipped into a 60 Sn: 40Pb molten solder at 230"C, held in the bath for five seconds, withdrawn, and examined after cooling. The coating is rated according to appearance, ranging from "ideal" for a bright, smooth deposit of uniform thickness to that in which there has been no solder adherence to the metal surface.

The capacitance test referred to above has been described by J. J. McMullen and M. J.

Pryor in "First International Congress in Metallic Corrosion", pages 53-54, 1961 (Butterworth's London), as amplified by Beck, Heine, Keir, van Rooyen, and Pryor in "International Journal of Corrosion Science", Vol. 2, pages 136 and 144-145, 1962.

The mechanism of the duplex cleaning treament of this invention is believed to differ in a significant way from the mechanism of the duplex cleaning treatment of U.S. Patent 3,646,946. The first treatment in the aqueous alkaline solution is believed to hydrolyze the refractory oxides to convert them to a gelatinous condition. In the patented process the immersion in the mineral acid solution serves to dissolve the copper oxides and to help break up the refractory oxides without significant metal removal. In the process of the present invention the immersion in the ferric sulfate solution provides a cleaning action through removal of underlying metal beneath the oxide which undermines the oxide layer. The gelatinous condition of the oxide layer from the alkaline treatment allows the ferric sulfate solution to penetrate to the metal surface to provide the metal removal.

It has been observed for certain silicon containing copper alloys that an oxide pene tration of from about 6 to 10 microns occurs at the grain boundaries. The prior art duplex treatment is not effective to remove the oxides at the grain boundaries because of insufficient removal of the adjacent metal. The process of this invention is highly effective for removing such grain boundary oxides because it attacks the metal surface. The process of this inven tion is designed to remove oxides which are insoluble or resistant to normal pickling agents including ferric sulfate.

The effect of added mineral acid on the potency of the ferric sulfate cleaning bath as demonstrated in Table I has also been verified as applied to samples with an oxide coating.

Therefore, it is an important aspect of this invention that the ferric sulfate bath have no added mineral acid to thereby provide im proved cleaning efficiency.

Claims (11)

1. A process for cleaning the surface of a copper base alloy having a surface layer of thermally formed metal oxide, comprising im mersing the said surface- in an aqueous alkal ine solution having a pH above 10 and heated to temperature of about 40"C to its boiling point for a period of at least two seconds and preferably not more than ten minutes, and then immersing the said surface in 0.5 to 3 N. aqueous solution of ferric sulfate at a temperature of 25 to 95"C for a period of at least two seconds and preferably not more than five minutes.

2. A process according to claim 1 wherein the said alloy contains Al and at least one metal selected from the group consisting of Zn, Fe, Co and Si.

3. A process according to claim 1 or 2 wherein the said thermal oxide layer has a thickness of 140 to 1 60 A.

4. A process according to claim 1, 2, or 3 wherein the temperature of the said alkaline solution is at least 70"C.

5. A process according to any of claim 1-4 wherein the said ferric sulfate solution contains no added mineral acid.

6. A process according to any of claims 1-5 wherein the said ferric sulfate solution is at a temperature of 60 to 90"C.

7. A process according to any of claims 1-6 wherein the said surface is immersed in said alkaline solution for a period of five to sixty seconds.

8. A process according to any of claims 1-7 wherein the said surface is immersed in the said ferric sulfate solution for a period of five to sixty seconds.

9. A process according to any of claimsl-8 wherein said surface is immersed in a 1.25 to 2 N. ferric sulfate solution.

1 0. A process according to any of claims 1-9 wherein the said ferric sulfate solution has a pH of 0.35 to 1.

11. A process of cleaning the surface of a copper base alloy substantially as hereinbefore described and with reference to the foregoing example.