Method for increasing the corrosion resistance of a chrome-plated substrate

Abstract

The present invention relates to a method for increasing the corrosion resistance of a chrome-plated substrate wherein at least one part of a chrome-plated surface of a chrome-plated substrate is dipped into an electrolyte comprising trivalent chromium ions, at least one conducting salt and at least one reducing agent, and afterwards, a trivalent chromium oxide film is formed on the at least one part of the chrome-plated surface by applying a pulse reverse current between the chrome-plated surface and a counter electrode electrically connected with the chrome-plated surface through the electrolyte. Furthermore, the present invention relates to a chrome-plated substrate obtainable by this method.

Claims

1. A method for increasing the corrosion resistance of a chrome-plated substrate, the method comprising: a) dipping at least one part of a chrome-plated surface of a chrome-plated substrate into an electrolyte, the electrolyte comprising trivalent chromium ions, wherein the concentration of the trivalent chromium ions in the electrolyte is in the range of 0.001 to 0.1 M, at least one conducting salt, wherein the concentration of the at least one conducting salt in the electrolyte is in the range of 2 to 50 g/L, and at least one reducing agent, wherein the concentration of the at least one reducing agent in the electrolyte is in the range of 0.1 to 50 g/L, and b) forming a trivalent chromium oxide film on the at least one part of the chrome-plated surface by applying a pulse reverse current between the chrome-plated surface and a counter electrode electrically connected with the chrome-plated surface through the electrolyte; wherein the pulse reverse current is applied for a time period from 30 to 300 seconds.

2. The method according to claim 1, wherein the applied pulse reverse current has a frequency in the range of 0.1 to 1000 Hz, a current density in the range of 0.01 to 10 A/dm.sup.2, and/or a duty cycle in the range of 40 to 95%.

3. The method according to claim 1, wherein the chrome-plated surface of the substrate has been obtained by trivalent chromium electroplating.

4. The method according to claim 1, wherein the substrate comprises a main part made of plastic and at least one under layer arranged on the main part, wherein the at least one under layer is composed of a deposit selected from a metal, a metal alloy or mixtures thereof.

5. The method according to claim 4, wherein the deposit is selected from the group consisting of nickel, alloys of nickel, copper, alloys of copper, and mixtures thereof.

6. The method according to claim 1, wherein the concentration of the trivalent chromium ions in the electrolyte is in the range of 0.002 to 0.08 M.

7. The method according to claim 1, wherein the electrolyte comprises at least one trivalent chromium salt comprising the trivalent chromium ions.

8. The method according to claim 7, wherein the at least one trivalent chromium salt comprising the trivalent chromium ions is selected from the group consisting of chromium sulfate, chromium potassium sulfate, chromium chloride, and mixtures thereof.

9. The method according to claim 1, wherein the at least one conducting salt is selected from the group consisting of sulfates, nitrates, phosphates, carbonates, bicarbonates, acetates, chlorides, and mixtures thereof.

10. The method according to claim 1, wherein the at least one reducing agent is selected from the group consisting of sulfites, metabisulfites, thiosulfates, hydrosulfites, hydrazine, hydroxylamine, hydroxylammonium salts, ascorbic acid and its Na and K salts, formic acid and its Na and K salts, glyoxylic acid and its Na and K salts, glyoxal, glucose, sorbitol, and mixtures thereof.

11. The method according to claim 1, wherein the electrolyte comprises at least one Cr(III) complexing agent.

12. The method according to claim 1, wherein the pH value of the electrolyte is in the range of 2 to 10.

13. The method according to claim 1, wherein the counter electrode is made of stainless steel, graphite, or titanium.

14. The method according to claim 13, wherein the counter electrode is covered by a mixed metal oxide or platinum.

Description

DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a graph that explains schematically the general wave shape for the pulse reverse current used in the method according to the invention. T (=1/f) is the cycle time.

(2) FIG. 2 is a graph that shows the results of XPS profile analysis in the chrome-plated surface obtained by an electrolyte based on trivalent chromium chloride without post treatment as described in example 1.

(3) FIG. 3 is a graph that shows the results of XPS profile analysis in the chrome-plated surface obtained by an electrolyte based on trivalent chromium chloride, which was treated with a cathodic post treatment based on hexavalent chromium as described in example 4.

(4) FIG. 4 is a graph that shows the results of XPS profile analysis in the chrome-plated surface obtained by an electrolyte based on trivalent chromium chloride, which was treated with a cathodic post treatment based on trivalent chromium as described in example 10.

(5) FIG. 5 is a graph that shows the results of XPS profile analysis in the chrome-plated surface obtained by an electrolyte based on trivalent chromium chloride, which was treated with a pulse reverse current, in a post treatment based on Trivalent Chromium as described in example 13.

EXAMPLES

(6) ABS parts having all the same shape and size have been preliminarily treated to make the surface conductive, suitable for electroplating.

(7) Therefore, the very ones have been treated with conventional electroplating processes such as copper, semi-bright nickel, bright nickel, microporous nickel and chrome.

(8) Different chrome deposits have been tested, all of them coming from trivalent chromium electrolytes: one electrolyte based on chlorides to obtain a clear Cr deposit; one electrolyte based on sulfates to obtain a clear Cr deposit, too; one electrolyte based on Chlorides formulated to obtain a dark Chrome deposit.

(9) Examples from 1 to 6 have been taken as a reference to establish the exact corrosion resistance, according to ISO9227 NSST or ASTM b117 standards, of parts without any treatment or of parts that underwent a conventional Cr(VI) treatment.

(10) In the examples from 13 to 15 a post treatment according to the method of the present invention has been used.

(11) All parts have been subjected to the neutral salt spray test according to the above-mentioned standards. Parts have been inspected every 120 h, rinsing parts with demineralized water and drying them to highlight possible corrosion points. Parts have been considered as conform when there were no spots for more than 5% of the whole surface. If spots exceeded the herein value, parts were considered as not conform (“No” in table 1).

(12) In addition to that, after treating parts, the Cr(VI) presence in the electrolyte used in post-treatment has been checked. 1,5-Diphenylcarbazide has been used as reactive agent, to highlight Cr(VI) presence according to IRSA-CNR 3150 Chromium method C.

(13) In addition, for examples 1, 4, 10 and 13, the chrome-plated surface has been analyzed after the post-treatment to determine its film thickness and type. The samples have been analyzed by XPS. Argon gun profiles have been performed to evaluate the thickness of the top surface chromium oxide layer. The XPS profile has been obtained (in atomic %) for the different elements depending on depth. The estimated chromium oxide layer on the surface of the samples is measured at the half of maximum oxygen concentration. XPS analysis profiles are shown in FIGS. 2, 3, 4 and 5.

(14) All samples were analyzed by XPS using a ESCA-5000 (Physical Electronics) Versa Probe system. The following X ray settings were used: beam size diameter: 200 μm; beam power: 50 W; voltage: 15 kV. The pressure in the analysis chamber was typically 2.10-6 Pa. The XPS data were collected using monochromatic AlKalpha radiation at 1486.6 eV. Photoelectrons were collected at take-off angle of 45° (normal detection) to the surface normal. For all samples, argon profile was made (Ar.sup.+ 500 V sputtered area: 2×2 cm.sup.2). The sputter rate on SiO.sub.2 was measured to be 0.9 nm/min (measured just before samples profiling). The profiles were performed with a step of 0.9 nm depth (1 min sputtering between 2 acquisitions). At each step, the elements were analyzed with a pass energy of 23.5 eV (high resolution spectra): Atomic compositions were derived from peak areas using photoionisation cross-sections calculated by Scofield, corrected for the dependence of the escape depth on the kinetic energy of the electrons and corrected for the analyzer transmission function of our spectrometer. Atomic compositions were derived from peak areas after a Shirley background subtraction.

(15) The measurements were performed by Materia Nova Materials R&D centre in Mons (Be).

Example 1 (Reference)

(16) A clear chromium deposit obtained from trivalent chromium chloride based electrolyte without post treatment.

Example 2 (Reference)

(17) A clear chromium deposit obtained from trivalent chromium sulfate based electrolyte without post treatment.

Example 3 (Reference)

(18) A dark chromium deposit obtained from trivalent chromium chloride based electrolyte without post treatment.

Example 4 (Reference)

(19) A clear chromium deposit obtained from trivalent chromium chloride based electrolyte, which was treated with a cathodic post treatment based on hexavalent chromium.

Example 5 (Reference)

(20) A clear chromium deposit obtained from trivalent chromium sulfate based electrolyte, which was treated with a cathodic post treatment based on hexavalent chromium.

Example 6 (Reference)

(21) A dark chromium deposit obtained from trivalent chromium chloride based electrolyte, which was treated with a cathodic post treatment based on hexavalent chromium.

Example 7

(22) A clear chromium deposit obtained from trivalent chromium chloride based electrolyte, which was treated with a cathodic post treatment based on trivalent chromium.

(23) Electrolyte:

(24) 0.05 M Cr(III) introduced as basic chromium sulfate

(25) 0.02 g/L sodium gluconate

(26) pH=3.5

(27) Parameters of Cathodic Post Treatment:

(28) j=0.5 A/dm.sup.2; t=120 sec; ⊖=25° C.

Example 8

(29) A clear chromium deposit obtained from trivalent chromium sulfate based electrolyte, which was treated with the cathodic post treatment based on trivalent chromium that has been previously mentioned on the example 7.

Example 9

(30) A dark chromium deposit obtained from trivalent chromium chloride based electrolyte, which was treated with the cathodic post treatment based on trivalent chromium that has been previously mentioned on the example 7.

Example 10

(31) A clear chromium deposit obtained from trivalent chromium chloride based electrolyte, which was treated with a cathodic current post treatment based on trivalent chromium.

(32) Electrolyte:

(33) 0.002 M Cr(III) introduced as basic chromium sulfate

(34) 0.01 M etidronic acid or 1-hydroxyethane 1.1-diphosphonic acid (HEDP):

(35) 15 g/L sodium bicarbonate

(36) 1 g/L ascorbic acid

(37) pH=9.5

(38) Parameters of cathodic current post treatment:

(39) j=0.1 A/dm.sup.2; t=120 sec; ⊖=25° C.

Example 11

(40) A clear chromium deposit obtained from trivalent chromium sulfate based electrolyte, which was treated with the cathodic current post treatment mentioned on the example 10.

Example 12

(41) A dark chromium deposit obtained from trivalent chromium chloride based electrolyte, which was treated with the cathodic current post treatment mentioned on the example 10.

Example 13

(42) A clear chromium deposit obtained from trivalent chromium chloride based electrolyte, which was treated with a pulse reverse current post treatment based on Trivalent Chromium.

(43) Electrolyte:

(44) 0.002 M Cr(III) introduced as basic chromium sulfate

(45) 0.01 M etidronic acid or 1-hydroxyethane 1.1-diphosphonic acid

(46) 15 g/L sodium bicarbonate

(47) 1 g/L ascorbic acid

(48) pH=9.5

(49) Parameters of Pulse Reverse Current Post Treatment:

(50) j.sub.ano=0.1 A/dm.sup.2; j.sub.cat=0.1 A/dm.sup.2; f=5 Hz;

(51) duty cycle γ=t.sub.cat/(t.sub.cat+t.sub.ano)=50%; t=120 sec; ⊖=25° C.

Example 14

(52) A clear chromium deposit obtained from trivalent chromium sulfate based electrolyte, which was treated with the pulse reverse current post treatment mentioned on the example 13.

Example 15

(53) A dark chromium deposit obtained from trivalent chromium chloride based electrolyte, which was treated with the pulse reverse current post treatment mentioned on the example 13.

(54) Table 1 summarizes the tests and analysis results. Examples 13, 14 and 15 have been performed according to the method of the present invention while examples 1 to 12 are reference samples. Therefore, the mentioned examples achieved the targeted goal to obtain a corrosion resistance, according to the ISO 9227 NSST Standard, comparable or higher than a post-treatment done using hexavalent chromium, even if Cr deposit type or Cr alloy varies and avoiding the hexavalent chromium formation into the post-treatment electrolyte. The goal achievement has been confirmed by XPS analysis profile, which highlighted how the use of pulse reverse current on the same electrolyte allows to form a thicker Cr(III) oxide film.

(55) FIG. 1 highlights the pulsed reverse current type applied on the above mentioned examples, leading to the achievement of the objective of the present invention.

(56) TABLE-US-00001 TABLE 1 Summary of post treatment performances Cr (VI) Thick- presence ness Chromium Kind of after use by Exam- Chromium electrolyte post in the XPS ples color based treatment electrolyte 120 h 240 h 360 h 480 h 600 h 720 h 840 h 960 h (nm) 1 Bright Chloride — — Ok No No — — — — — 2.7 2 Bright Sulfate — — Ok No No — — — — — — 3 Dark Chloride — — Ok No No — — — — — — 4 Bright Chloride Cathodic Cr (VI) Yes Ok Ok Ok Ok Ok Ok Ok Ok 4.3 5 Bright Sulfate Cathodic Cr (VI) Yes Ok Ok Ok Ok Ok Ok Ok Ok — 6 Dark Chloride Cathodic Cr (VI) Yes Ok Ok Ok Ok Ok Ok Ok Ok — 7 Bright Chloride Cathodic Cr (III) Yes Ok Ok Ok No No — — — — 8 Bright Sulfate Cathodic Cr (III) Yes Ok Ok Ok No No — — — — 9 Dark Chloride Cathodic Cr (III) Yes Ok Ok Ok No No — — — — 10 Bright Chloride Cathodic Cr (III) No Ok Ok Ok No No — — — 3.7 11 Bright Sulfate Cathodic Cr (III) No Ok Ok Ok No No — — — — 12 Dark Chloride Cathodic Cr (III) No Ok Ok Ok No No — — — — 13 Bright Chloride Pulse Current Cr (III) No Ok Ok Ok Ok Ok Ok Ok Ok 9.9 14 Bright Sulfate Pulse Current Cr (III) No Ok Ok Ok Ok Ok Ok Ok Ok — 15 Dark Chloride Pulse Current Cr (III) No Ok Ok Ok Ok Ok Ok Ok Ok —