Blackening treatment solution for black Cr—Co alloy plating film

Abstract

The present invention provides a blackening treatment method for a black CrCo alloy plating film, the method comprising bringing a black CrCo alloy plating film having a Cr content of 1 to 15 wt. % into contact with a blackening treatment solution comprising an aqueous solution with a pH value of 1 to 5. According to the present invention, the blackish color of a less blackish plating film formed from a plating bath containing trivalent chromium is enhanced to further improve decorativeness. Furthermore, the corrosion resistance of the film can be more improved by performing electrolytic chromate treatment after blackening treatment.

Claims

1. A blackening treatment method for a black CrCo alloy plating film, the method comprising bringing a black CrCo alloy plating film having a Cr content of 1 to 15 wt. % into contact with a blackening treatment solution comprising an aqueous solution with a pH value of 1 to 5.

2. The blackening treatment method according to claim 1, wherein the black CrCo alloy plating film is a film formed by electroplating from a CrCo alloy plating bath containing a cobalt compound and a compound containing trivalent chromium at a weight ratio Cr/Co of 2 to 60.

3. The blackening treatment method according to claim 1, wherein the blackening treatment solution comprising an aqueous solution with a pH value of 1 to 5 contains an organic acid and/or inorganic acid as an acid component.

4. The method according to claim 1, wherein the step of bringing a black CrCo alloy plating film into contact with a blackening treatment solution is a step of immersing an article having the black CrCo alloy plating film in the blackening treatment solution.

5. The method according to claim 1, wherein the black CrCo alloy plating film contains 5 to 15 wt. % of phosphorus.

6. A blackening treatment method for a black CrCo alloy plating film, the method comprising performing electrolytic chromate treatment after performing blackening treatment of a black CrCo alloy plating film by the method according to claim 1.

7. The blackening treatment method according to claim 1, wherein the blackening treatment solution comprising an aqueous solution with a pH value of 1 to 5 contains an inorganic acid as an acid component, and the inorganic acid is at least one member selected from the group consisting of hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, hydrofluoric acid, and boric acid.

8. The blackening treatment method according to claim 1, wherein the blackening treatment solution comprising an aqueous solution with a pH value of 1 to 5 contains an organic acid as an acid component, and the organic acid is at least one member selected from the group consisting of aliphatic monocarboxylic acids, aliphatic dicarboxylic acids, aliphatic hydroxy monocarboxylic acids, aliphatic hydroxy dicarboxylic acids, and aliphatic hydroxy tricarboxylic acids.

9. The blackening treatment method according to claim 3, wherein the amount of the organic acid and/or inorganic acid in the blackening treatment solution is 1 to 100 g/L.

10. The blackening treatment method according to claim 7, wherein the amount of the inorganic acid in the blackening treatment solution is 1 to 100 g/L.

11. The blackening treatment method according to claim 8, wherein the amount of the organic acid in the blackening treatment solution is 1 to 100 g/L.

12. The method according to claim 1, wherein the black CrCo alloy plating film contains phosphorus.

13. The method according to claim 1, wherein the black CrCo alloy plating film after blackening treatment has a reflectance of 20% or less measured by a colorimeter in the wavelength range of 400 to 700 nm.

Description

DESCRIPTION OF EMBODIMENTS

(1) The present invention is described in more detail below with reference to Examples.

EXAMPLE 1

(2) Brass plates with a bright nickel plating film having a film thickness of 5 m formed thereon were used as test samples, and black CrCo alloy plating films were formed using CrCo alloy plating solutions having the compositions shown in Table 1 below.

(3) The reflectance of the formed CrCo alloy plating films was measured by a colorimeter. The reflectance was measured in the wavelength range of 400 to 700 nm. The reflectance measurement results were shown as ranges from the minimum reflectance to the maximum reflectance in the wavelength range of 400 to 700 nm.

(4) Thereafter, the test samples were immersed in blackening treatment solutions having the compositions shown in Table 1 to perform blackening treatment. The treatment conditions are as shown in Table 1 below. NaOH was used when the pH of the blackening treatment solutions was increased.

(5) Subsequently, the reflectance of each test sample after blackening treatment was measured by a colorimeter in the same manner as in the above method. It is indicated that the lower the reflectance value is, the more blackish the plating film is.

(6) Further, the appearance of the cobalt plating films after blackening treatment was evaluated by visual observation. Films with a uniform black appearance were expressed as A, and films with slight unevenness after blackening treatment were expressed as B.

(7) Table 1 below shows the results.

(8) TABLE-US-00001 TABLE 1 Sample 1 Sample 2 Sample 3 Sample 4 CrCo 40% Chromium 184 ml/L 276 ml/L 244 ml/L alloy (III) sulfate plating Cr.sup.3+ 30 g/L 45 g/L 40 g/L Cobalt Sulfate 23.8 g/L 14.3 g/L 9.5 g/L 19.0 g/L Co 5 g/L 3 g/L 2 g/L 4 g/L Cr/Co in bath 6 15 20 10 Citric acid 40 g/L 80 g/L Ascorbic acid 20 g/L 5 g/L 10 g/L 5 g/L Boric acid 50 g/L 35 g/L 50 g/L Sodium sulfate 150 g/L 250 g/L Current density 10 A/dm.sup.2 .sup.8 A/dm.sup.2 Bath temperature 55 C. 35 C. 45 C. pH 3.5 3.8 3 2.8 Plating time 10 min Cr content of film 2.0 wt % 1.8 wt % 3.5 wt % 3.0 wt % Reflectance of film 20-23% 20-23% 20-23% 20-23% Blackening Sulfuric acid 2 g/L 1 g/L treatment Hydrochloric acid 1 g/L 2 g/L Malic acid 10 g/L Acetic acid 30 g/L 10 g/L pH 2.5 3.5 4.5 1.8 Treatment temperature 50 C. 60 C. 50 C. 20 C. Treatment time 10 min 5 min 5 min 10 min Evaluation Reflectance 8-12% 12-14% 15-20% 12-14% of film Film appearance A A A A

(9) As is clear from the above results, when Samples 1 to 4 were subjected to blackening treatment after formation of CrCo alloy plating films thereon, the reflectance of the plating films was reduced, without impairing the appearance of the films, and black plating films with a uniform appearance were formed.

EXAMPLE 2

(10) Brass plates with a bright nickel plating film having a film thickness of 5 m formed thereon were used as test samples, and plating treatment was performed using a CrCo alloy plating bath comprising an aqueous solution with a pH of 3.5 containing 184 ml/L of 40% chromium sulfate, 140 g/L of potassium sulfate, 14.3 g/L of cobalt sulfate, 40 g/L of boric acid, and 10 g/L of sodium hypophosphite, at a bath temperature of 50 C. at a cathode current density of 10 A/dm.sup.2 for 10 minutes, thereby forming black CrCo alloy plating films having a film thickness of about 1.0 m. The obtained CrCo alloy plating films contained Co: about 80 wt %, Cr: about 2 wt %, P: about 7 wt %, 0: about 7 wt %, and C: about 4 wt %.

(11) Subsequently, aqueous solutions containing 10 g/L of malic acid and having pH values adjusted with sulfuric acid were used as blackening treatment solutions. The samples with a CrCo alloy plating film formed thereon were immersed in the treatment solutions at a solution temperature of 50 C. for 10 minutes to perform blackening treatment.

(12) Each sample after blackening treatment was subjected to electrolytic chromate treatment using a commercially available electrolytic chromate treatment solution (trade name: ECB-Y, produced by Okuno Chemical Industries Co., Ltd.). The main component of the electrolytic chromate treatment solution was sodium dichromate. The treatment conditions were as follows: ECB-Y 100 ml/L solution (chromium concentration: 0.7 g/L), pH of treatment solution: 3.5, bath temperature: 25 C. (RT), cathode current density: 0.5 A/dm.sup.2, and treatment time: 1 minute.

(13) The appearance of each sample after electrolytic chromate treatment was evaluated by visual observation. Samples with a uniform black appearance were expressed as A, samples with yellow discoloration were expressed as B, and samples with severe discoloration and unevenness were expressed as C.

(14) Further, each sample was subjected to a corrosion resistance test (CASS test) according to JIS H8502 using a CASS tester for 24 hours, and the corrosion resistance of each sample after the test was evaluated by the rating number (RN) based on the total corrosion area percent. Samples with no rust on their entire surface are scored as rating number (RN) 10. It is indicated that the lower the rating number (RN) is, the higher the corrosion area percent is. Table 2 below shows the results.

(15) TABLE-US-00002 TABLE 2 After electrolytic chromate treatment pH value of Corrosion blackening resistance Sample treatment Film CASS 24 hr No. solution appearance (RN) 5 2.5 A 4 6 2 A 7.5 7 1.5 A 9 8 1 A 9.5 9 0.5 A 9.5

(16) As is clear from the above results, it was confirmed that the corrosion resistance after electrolytic chromate treatment was significantly improved particularly when blackening treatment was performed using blackening treatment solutions having a pH value of 1 or less.

EXAMPLE 3

(17) Samples with a CrCo alloy plating film formed thereon in the same manner as in Example 2 were subjected to blackening treatment using, as a blackening treatment solution, an aqueous solution with a pH value of -0.1 containing 200 mL/L of 35% hydrochloric acid, at a solution temperature of 40 C., while changing the treatment time from 2 to 10 minutes.

(18) Subsequently, after electrolytic chromate treatment was performed under the same conditions as Example 2, the film appearance and corrosion resistance were evaluated in the same manner as in Example 2. Table 3 below shows the results.

(19) TABLE-US-00003 TABLE 3 After electrolytic chromate treatment Corrosion Blackening resistance Sample treatment Film CASS 24 hr No. time (min) appearance (RN) 10 2 A 7.5 11 4 A 9.5 12 6 A 9.5 13 8 A 9 14 10 C (peeled after treatment)

(20) As is clear from the above results, the corrosion resistance of the films after electrolytic chromate treatment was particularly excellent when the blackening treatment time was within the range of 4 minutes to 6 minutes.

EXAMPLE 4

(21) Samples with a CrCo alloy plating film formed thereon in the same manner as in Example 2 were subjected to blackening treatment using blackening treatment solutions having a pH value adjusted with 98% sulfuric acid within the range of 0.32 to 0.45, at a solution temperature of 40 C., while changing the treatment time from 3.5 to 8 minutes.

(22) Subsequently, after electrolytic chromate treatment was performed under the same conditions as Example 2, the film appearance and corrosion resistance were evaluated in the same manner as in Example 2. Table 4 below shows the results.

(23) TABLE-US-00004 TABLE 4 After electrolytic chromate treatment Blackening treatment Corrosion Sulfuric acid Treatment resistance Sample concentration time CASS 24 hr No. (ml/L) pH (min) Appearance (RN) 15 20 0.45 8 A 9.0 16 40 0.15 5 A 9.5 17 60 0.07 4.5 A 9.5 18 80 0.2 4 A 9.5 19 100 0.32 3.5 A 9.5

(24) As is clear from the above results, the appearance and corrosion resistance of the films after electrolytic chromate treatment were both excellent when blackening treatment solutions having a pH value adjusted with sulfuric acid were used.

EXAMPLE 5

(25) Samples with a CrCo alloy plating film formed thereon in the same manner as in Example 2 were immersed in a blackening treatment solution comprising an aqueous solution with a pH value of 0 containing 50 mL/L of 98% sulfuric acid, at a solution temperature of 40 C. for 5 minutes to perform blackening treatment.

(26) Subsequently, after electrolytic chromate treatment was performed using the same electrolytic chromate treatment solution as that of Example 2 at a cathode current density of 0.5 A/dm.sup.2 at a solution temperature of 25 C. for an electrolytic chromate treatment time of 10 seconds to 3 minutes, the film appearance and corrosion resistance were evaluated in the same manner as in Example 2. Table 5 below shows the results.

(27) TABLE-US-00005 TABLE 5 After electrolytic chromate treatment Corrosion Blackening Electrolytic resistance Sample treatment chromate CASS 24 hr No. conditions (time) Appearance (RN) 20 Sulfuric acid 10 sec.sup. A 2 21 50 ml/L, 30 sec.sup. A 3 22 40 C., 5 min 1 min A 9 23 2 min A 9.5 24 3 min B 7

(28) As is clear from the above results, excellent corrosion resistance was imparted particularly when the treatment time of electrolytic chromate was within the range of 1 to 2 minutes under the treatment conditions in Example 5.