PL243022B1 - Method of electrochemical production of zinc coatings with increased corrosion resistance - Google Patents

Abstract

The invention relates to a method of electrochemical production of zinc coatings with increased corrosion resistance consisting in electrochemical, galvanic application of a zinc coating in a cyanide-free alkaline bath, preparation of a conversion layer in a passivation solution and sealing the zinc coating in a sealing bath. According to the invention, 1 to 10 ml per liter of a given bath, preferably 2 - 5 ml/l, of a benzotriazole solution with a concentration of 0.1 - 1%, preferably 0.2 - 0, is added to the zinc bath and/or to the passivating bath. 5%, in a solution of potassium or sodium hydroxide with a concentration of 0.5 - 2%, preferably 1%.

Description

The subject of the invention is a method of electrochemical production of zinc coatings consisting in electrochemical, galvanic application of a zinc coating in a cyanide-free alkaline bath, preparation of a conversion layer in a passivation solution, and sealing the zinc coating in a sealing bath.

Galvanizing is a widespread galvanic process, mainly used to protect steel products against corrosion and to give them decorative properties. An important advantage of zinc coatings obtained in galvanic baths is their good adhesion to the steel substrate with, at the same time, their greater plasticity compared to coatings obtained by other methods. Several types of galvanic baths are used for galvanizing, depending on the required properties and application of the applied coating. These are acidic, weakly acidic and alkaline cyanide and cyanide-free baths, while the new generation galvanizing is based mainly on electrolytes - cyanide-free alkaline baths. In addition, alkaline cyanide-free galvanizing, characterized by high current efficiency and good distribution of the zinc coating thickness on the steel substrate, is, compared to other galvanizing processes, a more environmentally friendly process.

In the standard galvanizing process, a zinc coating with a thickness of 8-12 μm is applied to the substrate using the electrochemical method, and then, in order to improve corrosion resistance, a conversion layer with a passivation solution based on trivalent chromium compounds, which are definitely less toxic than coatings, is made on its surface. made with hexavalent chromium. Unfortunately, the corrosion resistance of conversion layers based on chromium (III) is several - several times lower than coatings based on toxic chromium (VI).

The properties of the chromium(III) based conversion coating can be significantly improved by using different types of impregnation. In this way, it is possible to modify the anti-corrosion properties, appearance or coefficient of friction specific to a given coating. For this purpose, an inorganic seal, acrylic varnish or various types of lubricants or waxes can be used. After drying, these agents form a transparent organic and/or inorganic film on the surface, which improves the properties of such a coating. The use of impregnation significantly increases the corrosion resistance of the passivation (often up to 3 times) by sealing damages that occur during processing, e.g. as a result of workpieces hitting each other in the drum.

The conversion layer is sealed with a special varnish for additional protection. A detail protected in this way achieves the NSS test in a salt spray chamber made in accordance with the PN-EN ISO 9227 standard "Corrosion tests in artificial atmospheres. Salt spray test” corrosion resistance 120 hours to white corrosion and 240 hours to red corrosion.

There are various methods of producing bright, shiny zinc coatings by galvanic application from cyanide-free alkaline baths and their additional anti-corrosion protection with the elimination of chromium (VI) compounds, and as a result obtaining a zinc coating with better parameters for the end user, without a significant increase in the price of the finished product.

Alkaline cyanide-free baths are known, which are an aqueous solution of zinc ions Zn ²⁺ and sodium hydroxide and also contain brightening and smoothing substances. US patents 4,166,778 and 4,071,419 describe cyanide-free alkaline baths containing organic derivatives of heterocyclic compounds as brighteners. However, zinc coatings produced in these alkaline baths are characterized by too little plasticity, brittleness or poor adhesion to the substrate.

The Polish patent description PL 163805 describes a process which is carried out in a bath consisting of a strongly alkaline aqueous solution of sodium or potassium hydroxide or their mixture in the amount of 100 to 140 g/ ^dm3 , additionally containing a zinc carrier in the form of zincate ions, most preferably Zn(OH )4 ^2- , Zn(OH)3 ^- , Zn(OH)2 ^- , Zn(OH) ⁺ in the total amount calculated as zinc from 9 to 15 g/dm ³ , as well as a brightening substance in the form of a high-molecular copolymer in the amount from 0.1 to 10 g/ ^dm3 .

Polish patent description No. PL 181935 describes a bath for the electrolytic production of shiny zinc coatings, containing a soluble zinc compound, sodium hydroxide and/or potassium hydroxide and a brightening additive, characterized in that it contains 5-50 g/dm ³ of a soluble zinc compound, 50-250 g/dm ³ of sodium hydroxide and/or potassium hydroxide and

0.5-10 g/dm ³ of a brightening additive being a reaction product of ammonia and/or primary aliphatic amine and/or secondary aliphatic amine and/or aliphatic alkanolamine with carbon disulfide, the molar ratio of ammonia to amines and carbon disulfide ranges from 1:1:0.5 to 5:5:2.

The above patent descriptions do not indicate a significant improvement in the corrosion resistance of the obtained coatings, although in all these solutions there are additives acting as corrosion inhibitors. Therefore, new inhibitory compounds are constantly sought and research is conducted to determine in which process baths and under what conditions their use will yield the best results.

Studies have shown that very good corrosion inhibiting effects are obtained using relatively simple and easily available organic compounds containing nitrogen and sulfur heteroatoms in the molecule, especially benzotriazole [K. Wang, H. W. Pickering, K. G. Weil, "Corrosion Inhibition of Zinc by Benzotriazole with an Electrochemical Quartz Crystal Microbalance" J. Electrochemical Soc., 150, B176-B180 (2003), U.S. Patent No. 2,877,188], but in these studies benzotriazole has not been directly used in the zinc layer production processes. According to the British patent description No. GB2336378, high protection of the zinc surface against corrosion and oxidation was obtained by immersing the galvanized product in an alcohol solution containing benzotriazole in the amount of 0.005-3 mol/liter.

CN patent 107400906 discloses a high corrosion resistance zinc bleaching agent prepared from the following ingredients in parts by weight: 25-35 parts anionic surfactant, 10-15 parts non-ionic surfactant, 1-2 parts benzotriazole, 4-8 parts NNF (disodiummethylenedinaphtalenesulfonate), 10-15 parts sensitizer and 25-50 parts water. It is a complex system of components, mainly various surfactants, in which benzotriazole is only a small part and is intended for galvanizing in baths with zinc chloride, i.e. in acid baths.

CN 111235607 discloses an alkaline cyanide-free galvanizing additive consisting of a main complex, an auxiliary complex, a surfactant and other additives, where the main complex is potassium bitartrate, the auxiliary complex is 2-mercaptobenzimidazole zinc salt. The surfactant is 2-mercaptobenzimidazole and the other additive is 5-butylbenzotriazole. In this case, an alkyl derivative of benzotriazole is used in a complex system of complex compounds.

According to the state of the art, benzotriazole is used in alkaline galvanizing processes, but mainly as a corrosion inhibitor added at the stage of forming the conversion layer in the passivating solution, where it is usually a small addition to other basic ingredients. Therefore, it is difficult to assess whether the inhibitory effect is due to the action of the benzotriazole itself or also other ingredients.

The essence of the invention is a method of electrochemical production of zinc coatings with increased corrosion resistance consisting in electrochemical, galvanic application of a zinc coating in a cyanide-free alkaline bath, preparation of a conversion layer in a passivation solution and sealing the zinc coating in a sealing bath, consisting in the fact that the galvanizing bath and /or to the passivation bath, 1 to 10 ml per liter of a given bath, preferably 2-5 ml per 1 liter of bath, of a solution of benzotriazole with a concentration of 0.1-1%, preferably 0.2-0.5%, in a solution potassium or sodium hydroxide at a concentration of 0.5-2%, preferably 1%.

Unexpectedly, it turned out that benzotriazole in the form of a solution in potassium or sodium hydroxide affects the corrosion resistance both as an addition to the alkaline, cyanide-free galvanizing bath and the passivating bath, while the use of the benzotriazole addition both at the galvanizing and passivation stages leads to an unexpected synergy effect and a significant increase in corrosion resistance compared to the use of benzotriazole in only one of these steps.

While the addition of an alkaline benzotriazole solution to the passivation bath extends the time until the appearance of white and red corrosion by approximately 40% and 67%, respectively, the addition of an alkaline benzotriazole solution to the galvanizing bath by 60% and 70%, respectively, in relation to the analogous galvanizing process without the addition of benzotriazole , then the addition of an alkaline benzotriazole solution to both the galvanizing and passivating baths extends the time until the appearance of white and red corrosion by 100 and 90%, respectively, while the extension of the time of corrosion occurrence is much higher in the case of white corrosion.

Moreover, the zinc layer obtained by the method according to the invention is smooth and has a good gloss.

The invention is illustrated by the following examples.

Example I (comparative)

The example presents a typical technological process of alkaline hot-dip galvanizing (galvanic components produced by P.W. Galkor Sp. z o.o.), which consists of the following stages:

a) Surface preparation

Steel details to be covered with a layer of zinc are subjected to alkaline chemical degreasing by immersion in a water bath, which includes: 50 g/l of sodium hydroxide with the addition of the SIV Perfekt plus 36 AC GAL 836-036 component in the amount of 3% by volume. The process was carried out at a temperature of 50°C and an immersion time of 10 minutes.

Subsequently, the details are rinsed in a bath filled with water at ambient temperature. The operation is aimed at removing significant amounts of alkali that remain on the surface of the details. The process is usually carried out in a few minutes.

Subsequently, the details are subjected to a chemical degreasing bath, which is designed to remove light impurities in order to improve the quality of the obtained zinc coating. Degreasing is carried out in a bath filled with a bath containing 10 wt. hydrochloric acid (concentrated) and Eko-Clean S 101 agent in the amount of 10% by volume. The process was carried out at 30°C and for 10 minutes.

When using an acid degreasing bath, the rinsing step can be omitted and successively the details were subjected to a pickling bath with the following composition: 15 wt. hydrochloric acid (concentrated) with 0.5% vol. with the addition of the SIV 28 GAL 800-028 component. This operation is aimed at removing non-metallic substances from the surface of the details, consisting mainly of iron oxides. The process was carried out at a temperature of 30°C and an immersion time of 15 minutes.

Then, the details are rinsed in a two-stage cascade, where they are rinsed with water at ambient temperature in countercurrent for several minutes. The process is designed to remove significant amounts of acids that remain on the surface of the details after etching.

Subsequently, the details are subjected to electrochemical degreasing, which consists in the emulsification of all greasy impurities, i.e. the so-called the formation of a mixture of immiscible liquids in such a way that one of them is evenly dispersed throughout the entire volume of the other liquid. In this stage, fatty impurities of mineral origin are removed. The process was carried out in an anode manner, i.e. the detail is connected to the positive pole of direct current, then oxygen is released on the detail, which supports the process of removing impurities. The operation was carried out in a bath, which was an aqueous solution of sodium hydroxide with a concentration of 130 g/l with the addition of the SIV Perfekt 64 BE GAL 864-064 component in the amount of 3% by volume, for 5 minutes, at a temperature of 60°C, with a current of 4A /dm ² .

Then, the details are rinsed in a two-stage cascade, where they are rinsed with water at ambient temperature in countercurrent for several minutes.

In the next step, the details are subjected to the so-called Decapping consists in removing thin oxide films that may have been formed in the preparation process and may have a negative impact on the adhesion of the coating to the substrate. The process was carried out by immersion in a bath of hydrochloric acid with a concentration of approx. 5% by weight. (concentrated acid) with the addition of the SIV 21 GAL 800-021 component in the amount of 0.1% by volume, at ambient temperature, for 5 minutes.

After that, the details are rinsed in a two-stage cascade, where they are rinsed in counter-current with water at ambient temperature for several minutes.

Then, the details are subjected to activation rinsing by immersion in a bath, which was an aqueous solution of the AKTIV 7 GAL 900-779 component in the amount of 25 g/l. The process was carried out for 60 seconds and at ambient temperature.

b) Galvanizing

Electrogalvanizing is carried out by immersion in a water electrolyte with moderate stirring containing: 150 g/l sodium hydroxide, 13 g/l zinc and organic components, namely: Lux Super GAL 301-1001 (gloss carrier) 16 ml/l, Lux Super GAL 3012002B (gloss additive) 2.5 ml/l and Lux Super GAL 301-4001 16 ml/l. The process was carried out at a current density of 2.2 A/dm ² for 60 minutes. The bath was maintained at 28°C, alkaline pH (above 12).

Subsequently, the details are subjected to recovery rinsing and rinsing in a two-stage cascade, where they are rinsed in counter-current for several minutes with water at ambient temperature, using air mixing.

c) Passivation

Then the galvanized details are subjected to the process of surface activation in a water bath, which includes 0.5 wt. concentrated nitric acid with the addition of the PasivAktiv plus 900-889 component in the amount of 0.75% by volume. The procedure was carried out at ambient temperature for 50 seconds using air agitation. Then, in order to protect the coating against further reactions, the products are immersed in an aqueous solution of chromate passivation based on trivalent chromium compounds. In this case, an aqueous solution of 10% by volume was used. For the Pasivblue GAL 910-010 component, the process was carried out at ambient temperature for 40 seconds, pH 2 with moderate air agitation.

After that, the details were subjected to a double rinse in DEMI water at ambient temperature for usually a few minutes,

d) Sealing/painting

Subsequently, the details were additionally sealed in Lak GAL WA 7000-01 varnish with a concentration of 10% by volume. by immersion for 25 seconds and at ambient temperature.

Then the details were dried at 75°C for 8 minutes.

The thickness of the zinc coating was measured by X-ray fluorescence, the average thickness of the zinc layer was 13-15 μm. Subsequently, the samples were subjected to an aging test in a salt chamber. A basic NSS corrosion resistance test was carried out in a salt spray chamber according to the PN-EN ISO 9227 standard. As a result, the following results were obtained: white corrosion appeared after 120 hours, and red corrosion after 240 hours.

Example II

The process of surface preparation and electrolytic galvanizing was carried out analogically to example 1. Changes were made in the composition of the passivating bath, to which 2 ml/l of a previously prepared solution with a corrosion inhibitor was added, which was an alkaline solution of 0.2% benzotriazole in a 1% solution of potassium hydroxide. The process was carried out at ambient temperature, pH 2, for 40 seconds. The solution was air stirred moderately and remained clear with no visible turbidity. Subsequently, the details were subjected to a double rinse in DEMI water at ambient temperature for several minutes.

Subsequently, the details were additionally sealed in Lak GAL WA 7000-01 varnish with a concentration of 10% by volume. by immersion for 25 s and at ambient temperature.

Then the details were dried at 75°C for 8 minutes.

The thickness of the zinc coating was measured by X-ray fluorescence, the average thickness of the zinc layer was 13-15 μm. Subsequently, the samples were subjected to an aging test in a salt chamber. A basic NSS corrosion resistance test was carried out in a salt spray chamber according to the PN-EN ISO 9227 standard. As a result, the following results were obtained: white corrosion appeared after 168 hours, and red corrosion after 360 hours.

Example III

The process of surface preparation and electrolytic galvanizing was carried out analogically to example 1, while the passivation process was analogous to example 2, using a previously prepared solution of 0.1% benzotriazole in 0.5% sodium hydroxide solution in the amount of 5 ml/l. The effects were analogous to those in example II.

Example IV

The surface preparation process was carried out analogically to example 1. Changes were made to the composition of the galvanizing bath, to which 2 ml/l of a previously prepared solution with a corrosion inhibitor was added, which was a solution of 1% benzotriazole in a 2% sodium hydroxide solution. The process was carried out at a current density of 2.2 A/dm ² for 60 minutes. The bath was maintained at 28°C, alkaline pH (above 12). In the following operations: rinsing, brightening - activation, passivation, rinsing, sealing and drying, the process was analogous to example 1.

The thickness of the zinc coating was measured by X-ray fluorescence, the average thickness of the zinc layer was 13-15 μm. Subsequently, the samples were subjected to an aging test in a salt chamber. A basic NSS corrosion resistance test was carried out in a salt spray chamber according to the PN-EN ISO 9227 standard. As a result, the following results were obtained: white corrosion appeared after 192 hours, and red corrosion after 408 hours.

Example V

The surface preparation process was carried out analogically to example 1. Changes were made in the composition of the galvanizing bath, to which 5 ml/l of a previously prepared solution with a corrosion inhibitor was added, which was a solution of 0.5% benzotriazole in a 1% solution of potassium hydroxide. The process was carried out at a current density of 2.2 A/dm ² for 60 minutes. The bath was maintained at 28°C, alkaline pH (above 12). Rinsing and brightening - activation was carried out in the same way as in example 1. Subsequent changes were made to the composition of the passivating bath, to which 2.5 ml/l of a previously prepared solution with a corrosion inhibitor was added, which was a solution of 0.5% benzotriazole in a 1% hydroxide solution potassium. The process was carried out at ambient temperature, pH 2, for 40 seconds. The solution was air stirred moderately and remained clear with no visible turbidity. Subsequently, the details were subjected to a double rinse in DEMI water at ambient temperature for several minutes.

Subsequently, the details were additionally sealed in Lak GAL WA 7000-01 varnish with a concentration of 10% by volume. by immersion for 25 s and at ambient temperature. Then the details were dried at 75°C for 8 minutes.

The thickness of the zinc coating was measured by X-ray fluorescence, the average thickness of the zinc layer was 13-15 μm. Subsequently, the samples were subjected to an aging test in a salt chamber. A basic NSS corrosion resistance test was carried out in a salt spray chamber according to the PN-EN ISO 9227 standard. As a result, the following results were obtained: white corrosion appeared after 240 hours, and red corrosion after 456 hours.

Claims (1)

1. A method of electrochemical production of zinc coatings with increased corrosion resistance consisting in electrochemical, galvanic application of a zinc coating in a cyanide-free alkaline bath, preparation of a conversion layer in a passivation solution and sealing the zinc coating in a sealing bath, characterized in that the galvanizing bath and/or 1 to 10 ml per liter of a given bath, preferably 2-5 ml/l, of a solution of benzotriazole with a concentration of 0.1-1%, preferably 0.2-0.5%, in a solution of potassium or sodium hydroxide is added to the passivation bath at a concentration of 0.5-2%, preferably 1%.