COMPOSITION FOR CHROMIUM PLATING A SUBSTRATE AND CHROMIUM PLATING PROCESS USING SUCH A COMPOSITION

Abstract

An aqueous composition for the electrolytic deposition of a chromium coating on the surface of a substrate is disclosed, in which it has a pH of between 0 and 1 and which contains a trivalent chromium salt, glycine, an alkali metal salt, an aluminium salt and, optionally, an ammonium salt. The chromium plating process according to the invention comprises the immersion of the substrate to be treated in this composition and the application of a current between this substrate and an anode. This process makes it possible to form a high-quality chromium coating on the entire surface of the substrate.

Claims

1. An aqueous liquid composition for the electrolytic deposition of a chromium coating on the surface of a substrate, said composition containing 0.4 to 0.9 mol/l of trivalent chromium salt, 0.6 to 0.9 mol/l of glycine, an alkali metal salt, an aluminium salt and optionally an ammonium salt, and having a pH of between 0 and 1.

2. The composition according to claim 1, wherein the concentration of trivalent chromium salt is between 0.7 and 0.9 mol/l.

3. The composition according to claim 1, wherein at least one of said trivalent chromium salt, said alkali metal salt, said aluminium salt and, where applicable, said ammonium salt, is selected from a group consisting of chlorides, iodides, fluorides, carboxylates, carbonates, nitrates, nitrites, phosphates, phosphites, acetates, bromides, sulphates, sulphites, sulfamates, sulfonates, thiocyanates, or any one of the mixtures thereof.

4. The composition according to claim 1, wherein the concentration of aluminium salt is between 0.06 and 0.7 mol/l.

5. The composition according to claim 1, wherein the concentration of alkali metal salt is between 0.2 and 1.9 mol/l.

6. The composition according to claim 1, wherein the concentration of ammonium salt is between 0 and 1.0 mol/l.

7. The composition according to claim 1, wherein the concentration of ammonium salt is between 0.5 and 0.8 mol/l and the concentration of alkali metal salt is between 0.2 and 1.3 mol/l.

8. The composition according to claim 1, wherein the composition is devoid of ammonium salt and the concentration of alkali metal salt is between 1.5 and 1.9 mol/l.

9. A method for the electrolytic deposition of a chromium coating on the surface of a substrate, comprising: immersing said substrate and anode in a bath of an aqueous composition according to claim 1, and applying a current between said substrate and said anode.

10. The method according to claim 9, further comprising applying a continuous current between the substrate and the anode.

11. The method according to claim 9, wherein the temperature of said bath is between 20° C. and 80° C.

12. The method according to claim 9, wherein applying a current between the substrate and the anode is carried out for a suitable duration for forming on the surface of the substrate a chromium coating with a thickness of between 5 and 500 μm.

13. The composition according to claim 3, wherein a plurality of said trivalent chromium salt, said alkali metal salt, said aluminium salt and, where applicable, said ammonium salt, are selected in the group consisting of chlorides, iodides, fluorides, carboxylates, carbonates, nitrates, nitrites, phosphates, phosphites, acetates, bromides, sulphates, sulphites, sulfamates, sulfonates, thiocyanates, or any one of the mixtures thereof.

14. The composition according to claim 4, wherein the concentration of aluminium salt is between 0.2 and 0.3 mol/l.

15. The method according to claim 10, wherein the density of said continuous current is between 10 and 100 A/dm2.

Description

[0124] The features and advantages of the invention will emerge more clearly in the light of the example embodiments below, provided simply for illustration and in no way limitative of the invention, with the support of FIGS. 1 to 4, wherein:

[0125] FIG. 1 shows a photograph of steel substrates treated by a chromium plating method according to the invention, the aqueous composition used having a pH of 0.5 and being at various ageing stages (expressed in Ah/L), the lower part of the substrates treated having been rubbed with abrasive paper;

[0126] FIG. 2 shows a photograph of steel substrates treated by a chromium plated method according to the invention, the aqueous composition used having a pH of 1 and being at various stages of ageing (expressed in Ah/L), the lower part of the treated substrates having been rubbed with abrasive paper;

[0127] FIG. 3 shows a photograph of a substrate partially covered with a chromium coating at the end of a Hull cell test using an aqueous composition according to the invention, the associated current density values varying between 0 A/dm.sup.2 (on the right in the figure) and more than 100 A/dm.sup.2 (on the left in the figure), a chromium coating being observed for current densities greater than or equal to 13 A/dm.sup.2;

[0128] and FIG. 4 shows photographs of steel substrates treated by a chromium plating method, by means respectively of an aqueous composition comprising a concentration of glycine of 0.75 mol/l (a/), an aqueous composition comprising a glycine concentration of 1 mol/l (b/) and an aqueous composition comprising a glycine concentration of 1.25 mol/l (c/), each aqueous composition used being at various ageing stages (expressed in Ah/L), and the lower part of the substrates treated having been rubbed with abrasive paper.

EXAMPLE 1

[0129] Cylindrical substrates made from XC38 steel, of 20 mm in diameter and 200 mm long, are subjected to the following steps of a chromium plating method according to the invention:

[0130] 1/ Alkaline degreasing, by immersing the substrate in a Presol 7045 composition from Coventya at a temperature of 60° C. for 20 min.

[0131] 2/ Electrolytic pickling in a sulfuric medium, by immersing the substrate in a composition of sulfuric acid and ethylene glycol at ambient temperature, applying for the anodic phase a current density of 40 A/dm.sup.2 for 45 s and for the cathodic phase a current density of 30 A/dm.sup.2 for 4 min.

[0132] 3/ Hard chromium plating

[0133] For this purpose, the substrate is immersed, with an iridium-titanium anode, in a bath of an aqueous composition according to the invention, containing, in solution in water: [0134] 0.79 mol/l of CrCl.sub.3.6H.sub.2O [0135] 0.75 mol/l of glycine [0136] 0.60 mol/l of NaCl [0137] 0.65 mol/l of NH4Cl [0138] 0.26 mol/l of AlCl.sub.3

[0139] The pH of this aqueous composition has been previously adjusted to a value of 0.5 by adding a suitable quantity of hydrochloric acid in the composition.

[0140] The temperature of the aqueous composition is 45° C.

[0141] Various substrates are treated successively in a bath of this aqueous composition.

[0142] For each substrate, a current density of 40 A/dm.sup.2 is imposed between the substrate and the anode for a suitable duration for forming on the surface of the substrate a chromium coating with a thickness of 50 nm, which for each coating corresponds in this precise case to a quantity of electrical load imposed per volume of aqueous composition of between 2.2 and 2.3 Ah/L. Several substrates are thus treated successively in this same bath at various stages of ageing, until a bath ageing of 38.1 Ah/L is reached.

[0143] After this treatment, each substrate is subjected to a degassing step for 3 h at 190° C.

[0144] At the end of this method, for each substrate treated, a chromium coating of uniform thickness is obtained on the surface of the substrate, this coating being of metal grey colour, homogeneous, devoid of any black marks that would testify to the presence of chromium oxides instead of metallic chromium. These observations testify to a good quality of the chromium coating formed on the surface of the substrate.

[0145] For each substrate treated, the adhesion of the coating is evaluated by rubbing the lower part of the substrate with abrasive paper.

[0146] FIG. 1 shows a photograph of the substrates thus obtained. In this figure, the values expressed in Ah/L associated with each substrate correspond to the stages of ageing of the aqueous composition at the end of the treatment of this substrate, the various substrates having been successively treated in the composition.

[0147] As can be clearly observed, no change in appearance has occurred at the lower parts of the substrates that have been rubbed. The metallic coatings are all adherent to the substrate, even when they have been formed in an electrolytic bath wherein the equivalent of 38.1 Ah/L has been imposed, and this without having added in the bath additional quantities of one or several of the constituents thereof.

[0148] This demonstrates that the electrolytic bath according to the invention has a long service life, and that the chromium plating method makes it possible to form on the surface of the substrate a metallic chromium coating having good adhesion.

[0149] The other properties of this chromium coating are evaluated as follows, for each of the substrates treated.

[0150] The thickness of the coating, measured by electron microscopy, is between 5 and 500 μm.

[0151] The Vickers hardness, measured for an applied load of 100 g, is greater than 800 Hv.

[0152] No detachment of the coating is observed when the substrate is subjected to the grinding test in accordance with ASTM B571.

[0153] Also, no detachment of the coating is observed following a thermal shock, in particular after the substrate has been subjected to three cycles comprising heating at 300° C. for 1 to 2 h and then cooling by immersion in cold water.

[0154] No detachment of the coating is observed after rectification by a thickness greater than 100 μm.

[0155] The resistance to wear of a coating, measured by the Taber test, in accordance with ASTM D4060 standard, is equivalent to that obtained for the coatings obtained by the methods using hexavalent chrome of the prior art.

[0156] Subjected to the hydrogen weakening test, the substrate treated being under traction, in accordance with ASTM F519 standard, no rupture is observed after 200 hours at 75% of the breaking point.

[0157] All these properties testify to a particularly high performance of the chromium plating method according to the invention.

EXAMPLE 2

[0158] In this example, substrates made from XC38 steel identical to those described in Example 1 are treated, in accordance with the present invention, as indicated in Example 1, except that the pH of the aqueous composition used was adjusted to a value of 1, by adding hydrochloric acid in the composition.

[0159] At the end of this method, a chromium coating of uniform thickness is obtained on the surface of the substrate, this coating being metal grey in colour, homogeneous, devoid of any black marks.

[0160] For each substrate treated, the adhesion of the coating is evaluated by rubbing the lower part of the substrate with abrasive paper.

[0161] FIG. 2 shows a photograph of the substrates thus obtained. In this figure, the values expressed in Ah/L associated with each substrate correspond to the stages of ageing of the aqueous composition at the end of the treatment of this substrate, the various substrates having been successively treated in the composition.

[0162] As can be clearly observed, the metallic coatings are all adherent to the substrate even when they have been formed in an electrolytic bath wherein the equivalent of 22.4 Ah/L has been imposed, and this without having added in the bath additional quantities of one or more of its constituents. A loss of adhesion is found afterwards, as shown by the white arrows, which designate the regions rubbed with abrasive paper. These results, even if they are less good than those obtained with the electrolytic bath of pH 0.5 of Example 1, are all the same very satisfactory with regard to the service life of the bath.

[0163] The properties of the coatings formed on the substrates, at least up to 22.4 Ah/L, are similar to those described above for the coatings of Example 1.

EXAMPLE 3

[0164] A method according to the invention is implemented in accordance with the conditions described in Example 1, for a substrate as described in Example 1.

[0165] The current density imposed between the substrate and the anode is 40 A/dm.sup.2 for 40 min.

[0166] At the end of this method a chromium coating is obtained with a thickness of 50 μm on the surface of the substrate.

[0167] This coating has: [0168] a Vickers hardness (measured with a load of 100 g) of 900 Hv after degassing at 190° C. for 3 h, and of 1300 Hv after such a degassing and then a heat treatment of 300° C. for 2 h; [0169] a wear index, measured by the Taber test, in accordance with ASTM D4060 standard (grinder: CS-10, load: 1000 g on each arm), equal to 3.

[0170] The performances indicated above in Example 1, in terms of adhesion to the substrate and absence of negative effect on the mechanical properties of the substrate, are also achieved.

EXAMPLE 4

[0171] Various aqueous compositions according to the invention (named C1 to C13) or not in accordance with the invention (named C14 to C20) are tested in order to evaluate the current density range in which a chromium plating method using them can function.

[0172] For this purpose, a Hull cell study is carried out so as to define the current density range corresponding to each composition tested.

[0173] The Hull cell has a trapezoidal shape and makes it possible to position the cathode and the anode, constituting opposite walls of the cell, in a way that is not parallel to each other. The other two walls are parallel and insulating. By making it possible to obtain a wide variation in current density on the cathode surface, this cell makes it possible to evaluate the influence of the latter on the quality of the deposition of chromium.

[0174] For these tests, brass plates (substrates) are used at the cathode and an iridium-titanium grid is used at the anode. The deposit of chromium having a grey colour and brass being golden, it is possible to visually estimate the current density range wherein the deposition can form.

[0175] For these tests, the temperature of the composition is 45° C., and a current density of 8 A is applied for 1 min 30 s.

[0176] The current density applied at a precise point of the cathode can be determined by means of the following formula:

j=I×(5.10-5.24 log d) [0177] wherein [0178] j represents the current density in A/dm.sup.2 at the point in question [0179] I represents the current density in A passing through the cell [0180] d represents the distance in cm between the origin of the cathode and the point in question, the origin of the cathode corresponding to the end of the cathode closest to the anode.

[0181] For the aqueous composition according to the invention described in Example 1, here named C1, extreme current density values are thus for example determined, for the operating current density range associated with the composition, which are equal to 13 A/dm.sup.2 for the low value and greater than 100 A/dm.sup.2 for the high value.

[0182] Between these extreme current density values, a chromium coating of entirely satisfactory quality is obtained on the substrate, as shown in FIG. 3, this coating being metal grey in colour, homogeneous and devoid of any black marks.

[0183] The results obtained by the Hull cell test, for the various aqueous compositions tested, are indicated in Table 1 below.

TABLE-US-00001 TABLE 1 results of the Hull cell test for compositions according to the invention (C1 to C13) and other compositions (C14 to C20) Current Composition density CrCl.sub.3 Glycine NaCl NH.sub.4Cl AlCl.sub.3 (A/dm.sup.2) No (mol/l) (mol/l) (mol/l) (mol/l) (mol/l) pH Max. Min. C1 0.79 0.75 0.60 0.65 0.26 0.5 >100 13 C2 0.41 0.75 0.60 0.65 0.26 0.5 >100 16 C3 0.86 0.75 0.60 0.65 0.26 0.5 >100 16 C4 0.79 0.63 0.60 0.65 0.26 0.5 >100 20 C5 0.79 2.80 0.60 0.65 0.26 0.5 >100 28 C6 0.79 0.75 0.26 0.65 0.26 0.5 >100 24 C7 0.79 0.75 1.28 0.65 0.26 0.5 >100 15 C8 0.79 0.75 0.60 0.00 0.26 0.5 >100 18 C9 0.79 0.75 0.60 0.93 0.26 0.5 >100 15 C10 0.79 0.75 0.60 0.65 0.06 0.5 >100 15 C11 0.79 0.75 0.60 0.65 0.62 0.5 >100 18 C12 0.79 0.75 0.60 0.65 0.26 0 >100 28 C13 0.79 0.75 0.60 0.65 0.26 1.0 >100 24 C14 0.79 0.75 0.60 0.65 0.26 1.5 35 18 C15 0.79 0.75 0.60 0.65 0.00 0.5 60 15 C16 0.79 0.75 0.00 0.65 0.26 0.5 100 40 C17 0.79 3.33 0.60 0.65 0.26 0.5 100 50 C18 0.34 0.75 0.60 0.65 0.26 0.5 70 16 C19 0.94 0.75 0.60 0.65 0.26 0.5 60 24 C20 0.79 0.47 0.60 0.65 0.26 0.5 60 24

[0184] These results clearly show that the compositions according to the present invention (C1 to C13) are all associated with very wide current density ranges, the composition with the highest performance being composition C1.

[0185] The current density ranges determined for the compositions that are not in accordance with the present invention (C14 to C20) are, in comparison, much narrower.

[0186] In particular, the results obtained, in terms of amplitude of the current density range, are significantly inferior when the pH of the aqueous composition is greater than 1, than when the pH is between 0 and 1 as recommended by the present invention.

[0187] A Hull cell test is also carried out, under the operating conditions described above, but with a temperature of the composition of 50° C. or 55° C., for a composition according to the invention containing, in solution in water: [0188] 0.79 mol/l of CrCl.sub.3.6H.sub.2O [0189] 0.75 mol/l of glycine [0190] 1.71 mol/l of NaCl [0191] et 0.26 mol/l of AlCl.sub.3.

[0192] This solution is devoid of ammonium salt. The pH thereof has previously been adjusted to a value of 0.5 by adding a suitable quantity of hydrochloric acid in the composition.

[0193] The following operating current density ranges are obtained: [0194] for the composition at 50° C.: 13 to 100 A/dm.sup.2 [0195] for the composition at 55° C.: 16 to 100 A/dm.sup.2

[0196] There also, these results are particularly good.

EXAMPLE 5

[0197] In this example, the influence, on the ageing of the bath, of the glycine concentration in the composition is studied. Three compositions are tested: a concentration equal to 0.75 ml/l (composition B1), a concentration equal to 1 mol/l (composition B2) and a concentration equal to 1.25 mol/l (composition B3).

[0198] For substrates as described in Example 1, a method according to the conditions described in Example 1 is implemented, with the exception of the value of the pH, which is equal to 1, and the glycine concentration, which is equal to 0.75 mol/l for composition B1, to 1.00 mol/l for composition B2 or to 1.25 mol/l for composition B3.

[0199] For each composition B1, B2 and B3, the following experiment is carried out.

[0200] For each substrate, a current density of 40 A/dm.sup.2 is imposed between the substrate and the anode for a suitable period for forming on the surface of the substrate a chromium coating 50 μm thick, which, for each coating, corresponds to a quantity of electrical load imposed per volume of aqueous composition of between 2.2 and 2.3 Ah/L. A plurality of substrates are thus treated successively in the same bath at various stages of ageing, until an ageing of the bath of 33.6 Ah/L is reached.

[0201] After this treatment, each substrate is subjected to a degassing step for 3 h at 190° C.

[0202] For each of the compositions, with the new bath, homogeneous, metallic and adherent deposits are obtained. For each of the substrates treated, the adhesion of the coating is evaluated by rubbing the lower part of the substrate with abrasive paper.

[0203] FIG. 4 shows photographs of the substrates thus obtained, respectively at a/ for composition B1, at b/ for composition B2 and at c/ for composition B3.

[0204] As can be observed, for composition B1, according to the invention, a loss of adhesion of the metallic coating (indicated by a white arrow on the figure) is observed at the region rubbed with abrasive paper for the metallic coatings that were formed in the electrolytic baths in which the equivalent of 24.6 Ah/L and more has been imposed. For the baths in which the equivalent of 22.4 Ah/L or less has been imposed, the metallic coating remains adherent.

[0205] For composition B2, the loss of adhesion of the coating is observed at much shorter ageing times, as soon as after 15.6 Ah/L (loss of adhesion indicated by a black arrow).

[0206] For composition B3, the loss of adhesion occurs after having imposed in the bath an even lower electrical load, equivalent to 6.8 Ah/L.

[0207] This demonstrates that, entirely surprisingly, the electrolytic baths based on compositions containing 1 mol/l of glycine and more have a greatly reduced service life compared with baths formed from compositions according to the invention containing no more than 0.9 mol/l of glycine. The latter advantageously have a long service life, during which they make it possible to form, on the surface of the substrate, a metallic chromium coating having good adhesion.