Method for electrolytically passivating an outermost chromium or outermost chromium alloy layer to increase corrosion resistance thereof

Abstract

A method for electrolytically passivating an outermost chromium or chromium alloy layer to increase corrosion resistance thereof, including steps of (i) providing a substrate comprising said outermost chromium or chromium alloy layer, (ii) providing or manufacturing an aqueous, acidic passivation solution comprising trivalent chromium ions, phosphate ions, one or more organic acid residue anion, (iii) contacting the substrate with the passivation solution and passing an electrical current between the substrate as a cathode and an anode in the passivation solution such that a passivation layer is deposited onto the outermost layer, wherein the trivalent chromium ions are obtained by chemically reducing hexavalent chromium in presence of phosphoric acid and at least one reducing agent, with the proviso that during or after the chemical reducing the one or more than one organic acid residue anion is present for the first time in the passivation solution.

Claims

1. A method for electrolytically passivating an outermost chromium or outermost chromium alloy layer to increase corrosion resistance thereof, the method comprising the steps of (i) providing a substrate comprising said outermost chromium or outermost chromium alloy layer, (ii) providing or manufacturing an aqueous, acidic passivation solution, the solution comprising trivalent chromium ions, phosphate ions, one or more than one organic acid residue anion, and (iii) contacting the substrate with the passivation solution and passing an electrical current between the substrate as a cathode and an anode in the passivation solution such that a passivation layer is deposited onto the outermost chromium or outermost chromium alloy layer, wherein in the passivation solution said trivalent chromium ions are obtained by chemically reducing hexavalent chromium in the presence of phosphoric acid through at least one reducing agent selected from the group consisting of hydrogen peroxide and organic reducing agents, with the proviso that during or after the chemical reducing the one or more than one organic acid residue anion is present for the first time in the passivation solution, and wherein the chemical reducing is carried out and started in the presence of phosphoric acid and is started in the absence of the one or more than one organic acid residue anion, said one or more than one organic acid residue anion being present for the first time after the start of the chemical reducing.

2. The method of claim 1, wherein in step (i) the outermost layer is (a) directly on a surface of a base-substrate to form the substrate as defined in step (i), or (b) a layer of a layer stack, the layer stack being on a surface of a base-substrate.

3. The method of claim 1, wherein the outermost layer has a maximum layer thickness of 500 nm or less.

4. The method of claim 1, wherein in step (i) the outermost layer is obtained from electrolytically deposited trivalent chromium ions.

5. The method of claim 1, wherein in step (i) the outermost chromium alloy layer comprises a total amount of chromium of 45 atom-% or more, based on the total amount of atoms in the outermost chromium alloy layer.

6. The method of claim 1, wherein the one or more than one organic acid residue anion in the aqueous, acidic passivation solution is selected from the group consisting of organic acid residue anions having one carboxylic moiety, carboxylic acid residue anions having two carboxylic moieties, and carboxylic acid residue anions having three carboxylic moieties.

7. The method of claim 1, wherein the aqueous, acidic passivation solution does not contain boric acid.

8. The method of claim 1, wherein the aqueous, acidic passivation solution does not contain thiocyanate.

9. The method of claim 1, wherein said trivalent chromium ions are obtained by chemically reducing chromium trioxide.

10. The method of claim 1, wherein the at least one reducing agent is or at least comprises hydrogen peroxide.

11. The method of claim 1, wherein the one or more than one organic acid residue anion is obtained from corresponding carboxylic acids.

12. A method for electrolytically passivating an outermost chromium or outermost chromium alloy layer to increase corrosion resistance thereof, the method comprising the steps of (i) providing a substrate comprising said outermost chromium or outermost chromium alloy layer, (ii) providing or manufacturing an aqueous, acidic passivation solution, the solution comprising trivalent chromium ions, phosphate ions, one or more than one organic acid residue anion, and (iii) contacting the substrate with the passivation solution and passing an electrical current between the substrate as a cathode and an anode in the passivation solution such that a passivation layer is deposited onto the outermost chromium or outermost chromium alloy layer, wherein in the passivation solution said trivalent chromium ions are obtained by chemically reducing hexavalent chromium in the presence of phosphoric acid through at least one reducing agent selected from the group consisting of hydrogen peroxide and organic reducing agents, with the proviso that during or after the chemical reducing the one or more than one organic acid residue anion is present for the first time in the passivation solution, wherein the aqueous, acidic passivation solution comprises oxalate, and the chemical reducing is carried out and started in the presence of phosphoric acid and is started in the absence of oxalate, oxalate being present for the first time after the start of the chemical reducing.

13. The method of claim 1, wherein in step (iii) the cathodic current density of the electrical current is in the range from 0.1 to 8 A/dm.sup.2.

14. The method of claim 1, wherein the aqueous, acidic passivation solution does not contain sulfur containing compounds comprising a sulfur atom having an oxidation state below +6.

15. The method of claim 1, wherein the one or more than one organic acid residue anion is obtained from carboxylic acids.

16. The method of claim 12 wherein the oxalate is present for the first time after at least 90% of the hexavalent chromium is chemically reduced, based on the total molar amount of hexavalent chromium in the passivation solution at the start of the chemical reducing.

Description

EXAMPLES

(1) ABS base-substrates of identical size and each with a layer-stack on its surface were used throughout all examples, the layer stack comprising a copper layer, a semi-bright nickel layer, a bright nickel layer, a non-conductive particle containing nickel layer (microporous nickel layer), and a bright chromium layer as the outermost layer. Thus, a substrate as defined in step (i) of the method of the present invention was provided.

(2) If a passivation step was carried out, identical insoluble, mixed metal oxide coated anodes were utilized throughout respective examples.

(3) In order to evaluate corrosion resistance, in each example, neutral salt spray tests (NSS-test) were carried out according to ISO 9227 with varying time lengths. Typical time lengths are for example 240, 480, and 720 hours. The results for respective time lengths are summarized in Table 1 in the text below.

(4) Prior to and after each NSS-test, the optical appearance of the outermost layer was visually and systematically inspected.

(5) After each NSS-test, the substrates were rinsed with water, dried and visually inspected in order to determine/quantify the change of optical appearance (expressed as an area of defects determined by means of a caliber plate). If no change of optical appearance (including a change of optical appearance in up to 0.1% of the entire surface of the outermost layer) was observed, a test was considered as passed. In contrast, if a change of optical appearance in more than 0.1% of the entire surface of the outermost layer was observed, the test was considered as failed.

Example 1 (Comparative)

(6) A substrate as defined above was subjected to the above mentioned NSS-test. No pre-treatment and no contacting with a passivation solution as for example defined in step (iii) of the method of the present invention were carried out.

Example 2 (Comparative)

(7) Pre-treatment (i.e. immersion without an electrical current prior to passivation treatment): no pre-treatment was carried out

(8) Passivation step (i.e. including an electrical current):

(9) Passivation solution (not according to the invention): 5 g/L Cr.sup.3+, 28.5 g/L PO.sub.4.sup.3, 10 g/L oxalate Temperature: 25 C., pH: 3.5

(10) electrical current: 1 A/dm.sup.2 for 30 seconds, the substrate being the cathode

(11) The passivation solution was made up by dissolving chromium-(III) phosphate and oxalic acid with subsequent mixing for 3 hours at 80 C. and a final pH-adjustment with sodium hydroxide.

(12) The optical appearance of the outermost layer did not change due to the passivation treatment.

(13) Example 2 is based on JP 2009-235456 A and JP 2010-209456 A, respectively. Our results obtained for Example 2 confirm the results disclosed in JP-2009 and JP-2010.

Example 3 (Comparative)

(14) Pre-treatment (i.e. immersion without an electrical current prior to passivation treatment):

(15) Aqueous immersion treatment solution: 10 g/L Cr.sup.3+, 80 g/L PO.sub.4.sup.3, 15 g/L malic acid Temperature: 25 C., pH: 1.3

(16) immersion for 10 seconds

(17) Passivation step (i.e. including an electrical current): identical to Example 2

(18) The optical appearance of the pre-treated outermost layer did not change due to the passivation treatment.

(19) Example 3 is based on JP 2010-209456 A. Our results obtained for Example 3 confirm the results disclosed in JP-2010, in particular of Embodiment 14 in JP-2010.

Example 4 (Comparative)

(20) Pre-treatment (i.e. immersion without an electrical current prior to passivation treatment): no pre-treatment was carried out

(21) Passivation step (i.e. including an electrical current):

(22) Passivation solution (not according to the invention): 4.4 g/L Cr.sup.3+, 9.9 g/L PO.sub.4.sup.3, 9.7 g/L oxalate Temperature: 25 C., pH: 3.5

(23) electrical current: 1 A/dm.sup.2 for 30 seconds, the substrate being the cathode

(24) The passivation solution was made up by dissolving chromium-(III) phosphate and chromium-(III) oxalate with subsequent mixing for 3 hours at 80 C. and a final pH-adjustment with sodium hydroxide.

(25) The optical appearance of the outermost layer did not change due to the passivation treatment.

Example 5 (Comparative)

(26) Pre-treatment (i.e. immersion without an electrical current prior to passivation treatment): identical to Example 3

(27) Passivation step (i.e. including an electrical current): identical to Example 4

(28) The optical appearance of the pre-treated outermost layer did not change due to the passivation treatment.

Example 6 (Comparative)

(29) Pre-treatment (i.e. immersion without an electrical current prior to passivation treatment): identical to Example 3

(30) Passivation step (i.e. including an electrical current):

(31) Passivation solution (not according to the invention): 5 g/L Cr.sup.3+, 13 g/L PO.sub.4.sup.3, 10 g/L oxalate, 13 g/L SO.sub.4.sup.2 Temperature: 25 C., pH: 3.5

(32) electrical current: 0.2 A/dm.sup.2 for 30 seconds, the substrate being the cathode

(33) The optical appearance of the pre-treated outermost layer became slightly darker due to the passivation treatment.

(34) The passivation solution was made up by dissolving chrometane (basic chromium sulfate), phosphoric acid and oxalic acid with subsequent mixing for 3 hours at 80 C. and a final pH-adjustment with sodium hydroxide.

Example 7 (According to the Invention)

(35) Pre-treatment (i.e. immersion without an electrical current prior to passivation treatment): no pre-treatment was carried out

(36) Passivation step (i.e. including an electrical current):

(37) Passivation solution (according to the invention): 4.9 g/L Cr.sup.3+, 9.5 g/L PO.sub.4.sup.3, 7.5 g/L oxalate Temperature: 25 C., pH: 3.5

(38) electrical current: 1 A/dm.sup.2 for 30 seconds, the substrate being the cathode

(39) The passivation solution (as defined in step (ii) of the method of the present invention) was made up by reducing CrO.sub.3 with H.sub.2O.sub.2 and subsequent addition of oxalic acid and a final pH-adjustment with sodium hydroxide.

(40) The optical appearance of the outermost layer did not change due to the passivation treatment.

Example 8 (According to the Invention)

(41) Pre-treatment (i.e. immersion without an electrical current prior to passivation treatment): identical to Example 3

(42) Passivation step (i.e. including an electrical current): identical to Example 7

(43) The optical appearance of the outermost layer did not change due to the passivation treatment.

Example 9 (According to the Invention)

(44) Pre-treatment (i.e. immersion without an electrical current prior to passivation treatment): no pre-treatment was carried out

(45) Passivation step (i.e. including an electrical current):

(46) Passivation solution (according to the invention): 4.9 g/L Cr.sup.3+, 47 g/L PO.sub.4.sup.3, 7.5 g/L oxalate Temperature: 25 C., pH: 3.5

(47) electrical current: 1 A/dm.sup.2 for 30 seconds, the substrate being the cathode

(48) The passivation solution (as defined in step (ii) of the method of the present invention) was made up by reducing CrO.sub.3 with H.sub.2O.sub.2 and subsequent addition of oxalic acid and a final pH-adjustment with sodium hydroxide.

(49) The optical appearance of the outermost layer did not change due to the passivation treatment.

Example 10 (According to the Invention)

(50) Pre-treatment (i.e. immersion without an electrical current prior to passivation treatment): identical to Example 3

(51) Passivation step (i.e. including an electrical current): identical to Example 9

(52) The optical appearance of the outermost layer did not change due to the passivation treatment.

(53) In Table 1 all experimental results are summarized.

(54) TABLE-US-00001 TABLE 1 Summary of experimental results Length of Example NSS-test [hours] area of defects [%] Test passed 1 240 5 to 10 NO 2 240 1 to 2.5 NO 3 240 <0.1 YES 480 0.25 to 0.5 NO 4 240 2.5 to 5 NO 5 240 2.5 to 5 NO 6 240 10 to 25 NO 7 240 <0.1 YES 480 <0.1 YES 720 <0.1 YES 1008 <0.1 YES 8 240 <0.1 YES 480 <0.1 YES 720 <0.1 YES 1008 <0.1 YES 9 240 <0.1 YES 480 <0.1 YES 720 <0.1 YES 10 240 <0.1 YES 480 <0.1 YES 720 <0.1 YES

(55) According to own experiments, corrosion resistance in neutral salt spray tests is significantly increased utilizing the method of the present invention compared to known methods.