NL8300090A - CHROMATING MIXTURE AND METHOD FOR TREATING ZINC / NICKEL ALLOYS. - Google Patents

Description

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Chromation mixture and method of treating zinc / nickel alloys.

The invention relates to a mixture and a method of applying colored chromate coatings to zinc / nickel alloy surfaces and in particular to mixtures and methods of applying colored chromate coatings to electrolytically deposited zinc / nickel alloy surfaces to form of a composite layer with improved corrosion resistance compared to that obtained with electrolytically deposited zinc.

The use of electrolytically deposited zinc on iron or steel to thereby obtain improved protection against corrosion has long been in use. Although such electrolytic coatings with zinc greatly improve the corrosion resistance of the iron or steel, the zinc itself forms a white "rust", which may eventually lead to corrosion of the iron or steel itself, resulting in a red rust . To reduce this and improve the corrosion resistance of electrolytically zinc coated substrates, it is common to treat such surfaces with an acidic solution containing hexavalent chromium to apply a visible or colored passivating chromate layer to the surface, typical methods and mixtures for such passivating chromate 20 layers are described in U.S. Pat. Nos. 2,021,592, 2,106,904, 2,288,007, 2,376,158, 2,939,664, 2,610,133, 2,760,891, 3,090,910, 3,404,046 and 3,895,969.

Recently, a considerable amount of work has been done to improve the corrosion resistance of electrolytically zinc coated substrates by replacing the electrolytically deposited zinc with electrolytically deposited zinc alloys. Although different metals have been used together with the zinc in such alloys, particularly good results have been obtained with zinc / nickel alloys, due to the improvement in corrosion resistance and surface gloss. Depending on the nickel content of such electrolytically precipitated zinc / nickel alloys, the time that elapses before red rust is formed during a salt water spray test may be 5-10 times the time with electrolytically deposited zinc alone.

8300090> * 2

Nevertheless, the formation of white rust and the corrosion of such zinc / nickel hedges are still a problem, which should be alleviated. by applying a passivating chromate layer or other passivating layer.

However, it has been found that the formation of a colored or visible chromate layer with high corrosion resistance on such zinc / nickel alloys is much more difficult than on electrolytically deposited zinc. In the various patents mentioned above, no special mention is made of the treatment of electrolytically precipitated zinc / nickel alloy, although this generally refers to the treatment of precipitated zinc alloys. In general, the chromating mixtures of those patents have not been able to form a satisfactory corrosion resistant chromate layer on electrolytically deposited zinc / nickel alloy. Thus, U.S. Patent 2,106,904 discloses a solution of chromic acid and sulfuric acid, which is about 13-104 g / l contains hexavalent chromium and about 1.8 - 144 g / l SO ^, while the weight ratio of hexavalent chromium / SO ^ is 0.09-59: 1. This solution is reported to have a pH not greater than 1.0, and when this solution is used to treat a zinc / nickel alloy, the resulting chromate layer has a rather low corrosion resistance. This difficulty in forming colored chromate layer with high corrosion resistance on an electrolytically deposited zinc / nickel alloy is one of the reasons why such layers have not been more widely used as a substitute for electrolytically deposited zinc in the manufacture of decorative and corrosion resistant layers on iron and steel.

The object of the invention is therefore to provide an improved chromating solution which forms excellent colored, corrosion-resistant layers on electrolytically deposited zinc / nickel alloy.

Another object of the invention is to provide an improved method of forming colored, corrosion resistant chromate layers on electrolytically deposited zinc / nickel alloys.

35 These and other purposes will become apparent from the description below. It has been found that excellent colored, corrosion 8300090 3

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Resistant layers can be formed on electrolytically precipitated zinc / nickel alloys containing up to about 15% nickel by weight by treating such electrolytically deposited alloys with an aqueous acid solution of pH 1.3-2.7, which solution Hexavalent chromium contains in an amount of at least +6 0.5 g / l and sulfate ions in a SO 2 / CR weight ratio of about 0.025-1.5 / 1. Typically, such electrolytic zinc / nickel layers are immersed in this aqueous acidic chromatography solution for sufficient time to form the desired corrosion resistant chromate layer 10 on the surface of the alloy. Also, the improved corrosion resistant colored chromate layer can be formed by electrolysis of the electrolytic zinc / nickel alloy layer in the chromation solution, using a relatively weak electric current and with the zinc / nickel alloy as anode, before the immersion is started. In some instances, it has also been found to be electrolytic to apply a thin film of zinc to the zinc / nickel alloy prior to treatment with the chromating solution.

The chromate layer thus obtained was found to have excellent corrosion resistance, both against white rust and red rust, on zinc / nickel alloy electrolytically coated substrates. It has further been found that the heat resistance to fading of the color of such layers is also improved.

The improved chromation solutions of the invention are aqueous acidic solutions with a pH of about 1.3-2.7 +6 containing at least 0.5 g / l hexavalent chromium (Cr) and sulfate ions (SO4 + 2 ^ ^ a SO4 / Cr weight ratio of about 0.025-1.5 / 1 Although the maximum concentration of the hexavalent chromium in this solution has not been found to be critical, amounts can be used up to the saturation limit of hexavalent chromium in the solution, but when large Chromium concentrations are used, the cost of the process increases and additional waste water or rinse water treatment is often required to comply with environmental laws. In general, the use of hexavalent chromium concentrations above about 100 g / l has not been shown to be of any further significance. improves the corrosion resistance of the chromate film, therefore, for practical reasons, preference is given to chromium concentrations of about 8300090 4 0.5-100 g / l. When chromium concentrations below 0.5 g / l are used, the chromate film obtained is relatively thin and usually no sufficient corrosion resistance is obtained.

It has been found important in the chromating solution according to the invention to keep the SO 2 / Cr weight ratio within the indicated range of 0.025-1.5 / 1 and preferably 0.05-1.0 / 1. When this ratio becomes less than about 0.025, the desired colored chromate layer is not completely formed on the entire electrolytically deposited zinc / nickel alloy. When the ratio of sulfate to hexavalent chromium is greater than about 1.5, the resulting chromate layer is relatively thin, and its corrosion resistance is undesirably low.

Likewise, it is important to keep the pH of the chromation solution within the stated range of 1.3-2.7 and preferably 1.4-2.2. It has been found that a pH of the solution less than 1.3 results in a chromate layer with relatively small amount of chromium attached, so that the. corrosion resistance is undesirably poor and the color unsatisfactory. This result is quite surprising because in the treatment of electrolytically precipitated zinc instead of an alloy of zinc and nickel, the use of a conventional chromating solution with 100 g / 1 CrO 3 and 5 g / 1 and a pH of 0.5 leads to a colored chromate film with good appearance and good corrosion resistance. Furthermore, it has been found that at a pH of the currently proposed chromation solutions above about 2.7, the reactivity of the chromation solution is reduced and a colored chromate film having good appearance and good corrosion resistance is not formed on the electrolytically deposited zinc / nickel.

The improved chromation solutions of the invention can be formulated with any bath-soluble hexavalent chromium compound and sulfate compound, the anions of which, respectively. the cations are not harmful to the chromating solution or the chromate layer to be formed on the zinc / nickel alloy. Typical of the compounds to be used are chromic acid, sulfuric acid, alkali metal chromates and dichromates, metal sulfates such as zinc sulfate and chromium sulfate, and the like. In the latter regard, it has been found that in some cases addition of trivalent chromium, which is typically added as chromium sulfate, may be beneficial for the formation of the desired corrosion resistant chromate film. Even if trivalent chromium is not added to the bath upon initial preparation, deux will typically be formed in the bath by reduction of the hexavalent chromium during use.

To adjust the pH of the chromation solutions of the invention, acids such as chromic acid and sulfuric acid are used when the pH is to be lowered. If the pH is to be raised, alkaline compounds are added, such as alkali metal hydroxide, zinc oxide, zinc carbonate, zinc hydroxide, nickel carbonate, nickel hydroxide, and the like.

In addition to the components already mentioned above, the chromation baths according to the invention may also contain other conventional components, as is known in the art. Examples of such other components are phosphates, which can be added as phosphoric acid, or in the form of alkali metal phosphates and acidic phosphates and low carboxylic acids, such as acetic acid or are bath-soluble salts. However, it has been found that the presence of nitrate in the chromating bath or on the surface of the zinc / nickel alloys tends to hinder the formation of the desired chromate layer on the electrolytically deposited alloys.

Therefore, according to the invention, no nitrate-containing compounds are used in preparing the chromation solutions, and the substrates on which the zinc / nickel alloy is electrolytically deposited should not be treated with a solution of nitric acid before treatment with the chromate solution.

The chromating solution can be applied to the electrolytically deposited zinc / nickel alloy containing up to about 15% nickel by weight. Usually such alloys will contain at least about 1% nickel, with a nickel content of 5-12% by weight being particularly preferred. Typically, the chromation solutions are applied by immersing the electrolytically coated zinc / nickel alloy articles, although other application techniques may also be used, such as spraying, decanting, and the like. When such coating techniques are used, the zinc / nickel alloy is kept in contact with the chromation solution for 8300090 6 sufficient time to form the desired chromate layer on the surface.

In many cases, a contact time of about 10-30 seconds is appropriate, although this time can be varied so that both shorter and longer contact times can be used to obtain the desired chromate layer 5. The temperature of the chromating bath can vary from room temperature, e.g. about 20 ° C to temperatures near the boiling point of the solution. Usually the solution is used at a temperature of 25-60 ° C.

It has been found that the surface of the electrolytically deposited zinc / nickel alloy can become inactive or inert when the surface is exposed to the atmosphere for a significant time before it is exposed to the chromating treatment. In such cases, it has been found desirable to use the chromatography solutions at higher temperatures, such as 35 DEG-55 DEG C. to still form the desired chromate layer. When the electrolytically formed layer of zinc / nickel alloy is treated with the chromating solution immediately after the electrical precipitation of the alloy, one can also use lower treatment temperatures of 25 - 35 ° C and still obtain a satisfactory chromate layer.

In this regard, it has further been found that activation of the electrolytically precipitated zinc / nickel alloy can also be obtained by using the zinc / nickel alloy as an anode and electrolyzing the substrate in the chromating solution at a relatively low current density. Very suitably such an electrolysis, in which the zinc / nickel alloy serves as an anode, is carried out at a current density of 0.01 - 0.2 amp / dm * for up to 10 seconds. Then, the zinc / nickel alloy and its substrate are kept in the chromating solution, without passing a current, until the desired chromate layer is formed on the surface.

It has further been found that in some cases it may be desirable to deposit a thin film of electrolytically formed zinc on the surface of the zinc / nickel alloy before the surface is treated with the chromation solution of the invention. Such electrolytic zinc fleeces will typically have a thickness of less than 1 micron and they can be obtained by electrolytically coating the zinc / nickel alloy in a galvanizing bath at a current density of 0.1-3 amp / dm2 for up to to 5 seconds, while an electrolysis duration of 2-3 seconds is typical. Usually, such film-thin zinc layers will not be used when the substrate to be treated will be electrolyzed in the chromation solution, but only when the substrate will be chromated by immersion or with similar non-electrolytic treatments.

Using the method described above, electrolytically precipitated zinc / nickel alloys containing up to 10 to about 15 wt% nickel are provided with a chromate layer at least about 100 mg / m2 thick. These layers provide a pleasing color and good corrosion resistance of a magnitude which has hitherto not been found to be possible with electrolytically deposited zinc / nickel alloys on the alloy surface. In addition, it has been found that the electrolytically deposited zinc / nickel alloys containing these chromate layers have a corrosion resistance better than that of electrolytic zinc layers provided with a conventional chromation treatment.

The invention is illustrated by the examples below. t

Example I

An aqueous chromation bath was formulated by dissolving the following components in water in the amounts indicated.

25 Na2Cr20? .2H2O 10 g / 1 (Cr + 6 - 3.49 g / 1) H2S04 2 g / 1 (SO4 "2 = 1.96 g / 1) pH 1.8

A steel plate electrolytically coated with a 3 micron thickness zinc / nickel alloy layer containing 8 wt% nickel 30 was chromatographed with this solution by immersing the plate in the solution for 15 seconds at 25 ° C.

Example II

An aqueous chromatography bath was prepared by dissolving the following components in water in the amounts indicated: 35 Na2Cr207.2H20 20 g / 1 (Cr + 6 = 6.98 g / 1)

ZnSO. .7Ho0 1 g / 1) - -2 "-E ,, v", 4, 2 ^, SO. = 0.75 g / 1)

Cr2 (SO4) 3 aqueous solution 1 g / l) 4 8300090

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δ —2 +6 (Cr £ (S <Dj) 3 content 40% (SO ^ / Cr = 0.11) pH 2.1 (adjusted with CrC ^)

A steel plate, which was electrically coated with a 1 micrometer thick layer of a zinc / nickel alloy containing 8 wt% nik-5 boulder, was chromatographed in this solution by immersing the plate in the solution at 50 ° C for 25 seconds.

Example III

A chromatography bath was prepared by dissolving the following components in water in the amounts indicated: 10 CrO 3 2 g / 1 (Cr + 6 = 1.04 g / 1) H 2 SO 4 0.1 g / 1 (SO 4 ~ 2 = 0.098 g / 1) (S04 “2 / Cr + 6 = 0.094) pH 1.8 ....

A steel plate, which was 3 micrometers thick, electrolytically coated with a zinc / nickel alloy containing 8% nickel by weight, was chromatographed by immersing the plate in solution at 40 ° C for 15 seconds.

Example IV

An aqueous chromatography bath was prepared by dissolving the following compositions in water in the amounts indicated:

Na2Cr207.2H20 30 g / 1 (Cr + 6 = 11.82 g / 1) -¾7¾0. ,. 2.2 (SO -2 = 0.96 g / 1)

Cr- (S0J, dissolved with water 1 g / 1 4 ^ ^ 2 +6 25 (Cr2 (S04) 3 40% content) (S04 / Cr = 0.081) pH 2.0

A steel plate, which was electrolytically coated with a 2 micron thick layer of a zinc / nickel alloy containing 12% nickel by weight, was chromatographed by immersing the plate in the solution while electrolyzing the solution with a current density of 0 , 1 amp / dm2 for 5 seconds with the plate as anode. After 5 seconds, the power was turned off and the plate was kept in solution for another 20 seconds. The temperature of the solution was 50 ° C throughout the treatment.

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Example V

An aqueous chromatography solution was formulated by dissolving the following components in water in the amounts indicated: 5 CrO 3 50 g / 1 (Cr + 6 = 26.0 g / 1) H 2 SO 4 10 g / 1 (SO 4 ~ 2 = 9.8 g / 1) (S04 ”2 / Cr + 6 = 0.38)

Na2HPO4 2 g / 1 pH 1.4 (adjusted with NaOH) 10 A steel plate electrolytically coated with a 6 micron thick layer of a zinc / nickel alloy with 10 wt% nickel was chromatographed by immersing the plate in the chromatography bath at 30 ° C for 10 seconds.

Example VI

An aqueous chromatography bath was prepared by dissolving the following components in water in the amounts indicated: Na2Cr207 · 2Η20 150 g / 1 (Cr + 6 = 52.4 g / 1) H2SO4 10 g / 1 (SO4 ~ 2 = 9.8 g / 1) (S04 “2 / Cr + 6 = 0.19) 20 HCOONa 1.5 g / 1 pH 1.6

A steel plate electrolytically coated with a 3 micron thick layer of a zinc / nickel alloy with 12 wt% nickel and then electrolytically coated with a fleece of zinc with a thickness of 0.1 micron was chromatographed by that immerse in this solution at 30 ° C for 15 seconds.

For comparison, the following examples VII-IX were also performed.

Example VII

An aqueous chromatography bath was formulated by dissolving the following components in water in the amounts indicated.

Cr03 100 g / 1 (Cr + 6 = 52.0 g / 1) H2S04 10 g / 1 (S04 “2 9.8 g / 1) pH 0.6 (S04 ~ 2 / Cr + 6 = 0.19) A steel plate electrolytically coated with a 8300090 3 micron thick layer of a zinc / nickel alloy with 10 wt% nickel was chromatographed by immersing the plate in the bath for 10 seconds at 30 ° C.

Example VIII

An aqueous chromatography bath was formulated by dissolving the following components in water in the amounts indicated. CrO ^ 10 g / 1 (Cr + 6 5.2 g / 1) H2S04 1 g / 1 (S04 “2 / Cr + 6 = 0.98 g / 1) pH 1.2 (S04“ 2 / Cr + 6 = 0.19) A steel plate, which was electrolytically coated with a 3 µm thick layer of a zinc / nickel alloy with 10 wt% nickel, was chromatographed by immersing the plate in the chromatography solution at 30 ° C for 30 seconds. - -

Example IX

An aqueous chromatography bath was formulated by dissolving the following components in water in the amounts indicated: NaCr207.2H20 10 g / 1 (Cr + 6 = 3.49 g / 1) H2 SO4 1 g / 1 (SO4 "2 = 0 , 98 g / 1) HN03 2 g / 1 (S04 ~ 2 / Cr + 6 = 0.28) 20 pH 1.7

A steel plate electrolytically coated with a 4 micron thick layer of zinc was chromatographed by immersing the plate in the chromatography solution at 30 ° C for 15 seconds.

The chromated samples obtained in Examples 25 I - IX were analyzed to determine the chromium content of the chromate coating; their corrosion resistance was determined using the 5% neutral saline spray test and the heat resistance of the chromate coating was determined. The analytical methods and test methods used were as follows: 30 (1) analysis of the chromium content in the chromate layer:

Each sample was immersed in an aqueous acidic solution containing 100 g / l HCl. The chromate layer was completely dissolved from the sample and the amount of chromium dissolved was analyzed quantitatively by atomic absorption spectroscopy. The amount of chromium found was expressed as mg / m2 area of the sample.

8300090 11 (2) Corrosion resistance test.

Each sample was exposed to spraying with a 5% 1s neutral saline solution according to the procedure described in AS1M-D-117 and the time was determined for the development of white corrosion products on the surface of the sample (white rust) and for the development of red corrosion products (red rust), as indicated in this procedure.

(3) Heat resistance-

Each sample was placed in a thermostatically controlled oven 10 and stored there at constant temperatures of resp.

100, 150, 200 and 250 ° C. After each heating, the sample was taken out of the oven and it was visually observed whether the chromate layer had succumbed or faded by the heat.

Using these methods, the test results were obtained, summarized in Table A below.

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8300090 ♦ a 14

Example X.

A concentrated preparation was prepared containing the following components in the amounts indicated: sodium dichromate (Na ^ Cr211 ^ 0) 300 g / l zinc sulfate (ZnSO ^. 711 ^ 0) 15 g / l chromium sulfate (Cr ^ (SO ^) ^.. 40% solution) 30 ml / l chromic acid (CrO ^) 40 g / l water residue

A chromatography solution was made up by adding 60 ml of 10 'per liter of the above concentrate solution to 1 liter of water. The pH of this chromatography solution was 1.8 and steel plates, which were electrolytically coated with a 4 micron thick layer of a zinc / nickel alloy with 8.4% nickel by weight, were chromatographed by placing the plates in the chromatography solution for 15 seconds at a 15 bath temperature of 45 - 55 ° C. The results of the examination of the chromate layer which was formed hereby correspond to the results obtained for Example III described above.

During the operation of the procedure of this example, the chromatography solution was supplemented, if necessary, using a make-up solution containing the following components in the amounts indicated: chromic acid (CrO ^) 300 g / l sulfuric acid (H ^ SO ^ 50 wt%) 100 ml / 1 sg = 1.4 25 water remainder

The pH and CrO2 concentration of the chromatography bath are measured periodically and the above-mentioned make-up solution is added in an amount necessary to restore the pH and Cr content of the bath to the original values.

The above results showed that the chromating solutions according to the invention provide a colored chromate layer of good quality on an electrolytically deposited zinc / nickel layer and that these chromate layers have excellent corrosion resistance and good color fade resistance. In addition, the corrosion resistance of substrates electrolytically coated with 8300090 9 15 a zinc / nickel alloy and then chromated according to the invention is considerably better than that of substrates electrolytically coated with zinc and then chromated by a known method.

8300090

Claims (8)

An aqueous acidic chromatography solution suitable for forming colored chromate layers on an electrolytically deposited zinc / nickel alloy with up to 15% nickel by weight, which solutions have a pH of 1.3 - 2.7 and contain hexavalent chromium in an amount -2 +6 5 of at least 0.5 g / l and sulfate in a SO 2 / Cr weight ratio of 0.025-1.5 / 1. Solution according to claim 1, characterized in that it contains 0.5-100 g hexavalent chromium per liter. 3. Process for forming colored chromate layers on electrolytically deposited zinc / nickel alloys with up to 15% nickel by weight, characterized in that the zinc / nickel alloy is contacted with a chromatography solution according to claim 1 and the solution is dissolved in Maintains contact with the alloy for sufficient time to form the desired colored chromate layer. Process according to claim 3, characterized in that the chromatography solution contains 0.5-100 g hexavalent chromium per liter. 5. Process according to claim 3, characterized in that prior to the treatment of the zinc / nickel alloy with the chromating solution, a zinc layer with a thickness not exceeding 1.0 is electrolytically deposited on the surface of the zinc / nickel alloy. micrometer. 6. Process according to claim 5, characterized in that the chromating solution contains 0.5-100 g hexavalent chromium per liter. 7. Process according to claim 3, characterized in that the electrolytically precipitated zinc / nickel alloy is immersed in the chromating solution, this solution is electrolyzed with the zinc / nickel alloy as anode for a maximum of 10 seconds at a current density of 0.01 - 0, 2 amp./dm2, then the electrolysis stops and leaves the electrolytically precipitated zinc / nickel alloy in the chromatography solution until the desired colored chromate coating is formed. Process according to claim 7, characterized in that the chromatography solution contains 0.5 - 100 g / l hexavalent chromium. 8300090