PL238492B1 - Method for electrolytic zinc coating of steel products intended for application of paint coatings, preferably polymeric coatings - Google Patents

Abstract

The application concerns the method of galvanizing steel products intended for applying paint coatings, especially polymer coatings, and protecting the zinc surface applied in the galvanizing process against corrosion. Steel details are successively subjected to a chemical degreasing bath, pickling bath, rinsing, electrochemical degreasing bath, rinsing, decapsulation, rinsing, activation rinsing, electrochemical zinc coating, recovery rinsing and subsequent rinsing. Then, an organic conversion layer is formed on the zinc surface by applying thereto for at least 5 seconds an aqueous or hydroalcoholic solution containing 0.1 to 10% of a diketone of the general formula wherein R is an alkyl group with a chain length of 1 to 20 carbon atoms, a substituted or unsubstituted benzyl group, a substituted or unsubstituted alkenyl group, made alkaline with aqueous ammonia solution. The aqueous-alcoholic solution uses a water-miscible alcohol, preferably isopropanol, methanol or ethanol, and then, after drying, optionally applies a protective coating, preferably a polymer paint.

Description

Description of the invention

The subject of the invention is a method of electro-galvanizing steel products intended for the application of paint coatings, especially polymer ones, and for the protection of zinc surfaces applied in the galvanizing process against corrosion.

Galvanized zinc coatings are commonly used to protect cast iron and steel products against corrosion. The corrosion resistance of the products thus protected is approximately proportional to the thickness of the applied zinc coating. The corrosion rate of zinc coatings depends on the environment in which they are used, but in general zinc corrodes at least ten times slower than steel [Biuletyn Galwanotechnika No. 21 (36) 2011]. A significant improvement in the resistance of the zinc layer, and thus the corrosion resistance of the entire product, is achieved by producing passive layers on their surface, the so-called conversion coatings.

The process of forming the conversion coating is an artificially induced and directed process which creates a dielectric layer closely bonded to the metal, practically insoluble in water and the process-inducing environment, on the surface of the metal.

The growing anti-corrosion requirements for metal products mean that in industrial production techniques, additional coating of galvanized surfaces with paint coatings (e.g. polymer powder paints) is practiced. An important issue is that even the most expensive and modern paints will not fulfill their role if they are applied to a poorly prepared surface. The adhesion and protective properties of such a coating will be lower than when using ordinary paint, but on a properly prepared substrate. A well-prepared and painted zinc layer fulfills its protective functions very well, even with relatively small thicknesses of this layer. Thus, the steps of applying the zinc surface, the thickness of the layer and the preparation of the surface, including the formation of the conversion layer, determine the entire process and have a significant impact on the quality and durability of the finished products.

There are many methods of producing conversion layers with increased corrosion resistance and increased adhesion to paint coatings in the galvanizing process.

Known methods of preparing galvanized surfaces for painting are described in the US standard [Surfaces for Painting, ASTM D 2092-95: Standard Guide for Preparation of Zinc-Coated (Galvanized) Steel]. Among the chemical methods, the following methods are mentioned: zinc phosphating (creating a conversion crystal coating), chromating with Cr III (creating an amorphous passive coating), water-based organic chromate treatment, chromate-free polymer treatment (producing a vinyl resin film), creating complex oxide layers (using alkaline solutions containing heavy metal ions), polymer treatment without chromate (polyacrylic resins), fluorine-titanium (H2TiF6) or fluorine-zirconium (H2ZrF6).

In the case of zinc coatings obtained by electroplating, chromate methods are used based on the use of the properties of chromium (VI) and (III). For example, patent description PL 208 733 describes a bath for passivation of zinc coatings and its alloys containing an oxidizing agent, organic and inorganic acids or their salts, heavy metal ions and chromium (III) ions, which is characterized by containing chromium ions ( III) at a concentration of 0.1 to 100 g / dm ³ obtained by reducing chromium (VI) compounds by a by-product of the industrial oxidation of cyclohexane to cyclohexanol and cyclohexanone.

More and more often, due to the need to limit the use of harmful chromium compounds, chromate-free methods using titanium, zirconium or silicon compounds are introduced. Currently, the best substitutes for chromate coatings are coatings obtained, among others, from solutions of organo-metallic compounds, zirconium fluoride, titanium fluoride or silanes.

The simplest baths for depositing titanium-containing conversion coatings are based on titanium (III) chloride or titanium (IV) chloride. Advanced baths based on titanium fluorocomplexes contain, in addition to the source of titanium ions, ions such as: Mn ²⁺ , Zn ²⁺ , PO4 ^3- , F ^- . Coatings obtained from this type of bath are mainly composed of metal oxides and phosphates. They do not show self-healing properties and are not very scratch-resistant. However, they increase the adhesion of powder paint coatings to a galvanized substrate ["Chrome-free conversion coatings on a zinc substrate" J. Winiarski Biuletyn Galwanotechnika No. 26 (41) 2013 pp. 9-15].

PL 238 492 B1

There are also known methods of protecting the zinc surface applied in the process of electro-galvanizing, consisting in treatment in an alkaline environment with silanes. The obtained surfaces are characterized by increased corrosion resistance as well as increased adhesion to various types of paints. Polish patent application no. PL 352612 discloses a water-based composition for anti-corrosive treatment of a metal substrate pretreated with a zinc-based protective coating. The composition is in the form of an aqueous silane-based solution containing from 72 to 95% by weight of water, from 0.5 to 10% by weight of silane, from 0.5 to 3% by weight of boric acid, from 0 to 3% by weight of phosphoric acid, from 1 up to 10% by weight of the micronized silica and 0.1 to 3% by weight of a wetting agent, provided that the total sum of the percentages of boric acid and phosphoric acid does not exceed about 5%.

GB 2,032,466 and GB 2,032,963 describe non-toxic zinc passivation baths which contain expensive organic phosphorus (V) compounds, organic dyes, inorganic silicates and an oxidizing agent - hydrogen peroxide. Unfortunately, the corrosion resistance tested in a salt chamber according to the Polish Standard PN-76 / H-04603 does not exceed a dozen or so hours, which is not a good result.

Unexpectedly, it turned out that 3-substituted beta-diketones can be used to create an effective conversion layer that protects galvanized coatings against corrosion, but also increases the adhesion to them of paint coatings, especially polymer coatings.

The method according to the invention, in which steel details are successively subjected to a chemical degreasing bath, an etching bath, rinsing, an electrochemical degreasing bath, rinsing, decapping, rinsing, activation rinsing, electrochemical zinc coating, recovery rinsing and subsequent rinsing. an organic conversion layer is formed on the zinc surface by applying an aqueous or hydroalcoholic solution to it for at least 5 seconds, containing 0.1 to 10% of a diketone of the general formula, in which R is an alkyl group with 1 to 20 carbon atoms in the chain , substituted or unsubstituted benzyl, substituted or unsubstituted alkenyl, basified with aqueous ammonia solution. After drying, an optional protective coating is applied, preferably a polymer paint, for example polyester.

In the present invention, the alcohol used as the alcohol in the hydroalcoholic solution is a water-miscible alcohol, preferably isopropanol, methanol or ethanol.

According to the invention, the zinc surfaces applied by electro-galvanizing are modified with a solution obtained by dissolving a diketone of the general formula in water or a hydroalcoholic solution and adding ammonia. The solution can be applied in a known manner by dipping, spraying or rolling on with a roller.

The conversion solution is prepared in such a way that a diketone is added to the water, which is not soluble in water. Then, while stirring, ammonia water is added until the diketone dissolves ("greasy" stains disappear). 10 to 50% of a water-miscible solvent (e.g., isopropanol) may be added. Adding alcohol to the solution allows the diketone to dissolve faster and shortens the drying time after applying the conversion layer. The solution can be diluted with water and used as a bath or shower. The contact time of the galvanized surface with the solution depends on what thickness of the layer is to be obtained and the longer the lower the percentage of the 1,3 beta-diketone of the general formula in it.

The diketone compounds contained in the solution for the formation of the conversion layer react with the surface zinc atoms to form strong chelate bonds through the oxygen atoms of the 1,3-diketone group, while the organic group attached to the 1,3-diketone methylene carbon has a high affinity for paints ( greater than the zinc surface). Creating a bridge connecting the zinc surface with the paint increases the paint adhesion.

The conversion layer bath used in the process according to the invention is non-toxic and can be regenerated. The bath used in the invention is cheap and easy to use, and the sewage derived from it is easy to neutralize.

The following examples illustrate the invention.

P r z k ł a d I (comparative)

The application of a zinc coating with increased corrosion resistance and increased adhesion of organic, polymeric paints and varnishes according to the invention consists of a series of successive steps:

PL 238 492 B1

a) chemical degreasing

Before the zinc coating is applied, steel details are subjected to a degreasing bath to remove light contamination in order to improve the quality of the obtained zinc coating. Degreasing is carried out in a special bath in a bath containing 5-15% hydrochloric acid concentration and Eko-Clean S 101 degreasing agent in the amount of 5-15%, at a temperature of 25-30 ° C until a satisfactory effect is obtained, i.e. removal of visible impurities .

b) digestion

After possible rinsing with water, in the next stage the details are subjected to an etching bath with the following composition: aqueous solution of 15-36% hydrochloric acid with 0.5-2% addition of SIV 28 GAL 800028 in order to clean the surfaces of uncoated metal details from corrosion products, such as such as rust or scale. This process is carried out at a temperature of 25-35 ° C, usually for a few to several minutes, until a satisfactory effect is achieved.

Subsequently, the details are rinsed in a two-stage cascade, where, in countercurrent, for 1 min, they are rinsed with water at 15 ° C in the amount of 10% / h of the volume of the rinsing tub.

c) electrochemical degreasing

In the next stage, the parts are electrochemically degreased in an anodic manner, i.e. the part is connected to the positive DC pole, then oxygen is released on the part, which supports the process of removing impurities. This process is carried out at a temperature of 60 ° C with a current of 5 A / dm ^{2 by} immersion in an alkaline bath, which is an aqueous solution of sodium hydroxide with a concentration of 5-15% with the addition of the SIV 51 GAL 800-051 component in the amount of 20-30 g / 1 and SIV 4 GAL 800-810 EM component in an amount of 0.1-0.5%.

Subsequently, the details are rinsed in a two-stage cascade, where, in countercurrent for 1 min, rinsed with water at 15 ° C in a stream of water flow in the amount of 10% / h of the rinsing tub volume.

d) decapping

In the next step, the details are scraped. This is the last step before the actual coating is applied. It consists in the removal of thin oxide films that may have arisen as a result of the treatment that prepares the surface of the metal detail before applying the coating. Removal of the oxides is necessary to ensure the adhesion of the coating to the substrate. The process is carried out by immersion in a bath of hydrochloric acid with a concentration of approx. 5-20% with the addition of the SIV 21 GAL 800-021 component in the amount of 0.1-0.3%.

Subsequently, the details are rinsed in a two-stage cascade, where, in countercurrent for 1 min, rinsed with water at 15 ° C in a stream of water flow in the amount of 10% / h of the rinsing tub volume.

e) activation rinsing and electrochemical galvanization

The next step is activation rinsing with the immersion method in a bath consisting of 2% sodium hydroxide solution at a temperature of 15 ° C.

Then, after surface activation, the parts are electrochemically applied with a zinc coating. The process is carried out by immersion in an alkaline bath with the following composition: sodium hydroxide 110-175 g / l, zinc 10-18 g / l, gloss carrier - LUX SUPER GAL 301-1001 8-12 ml / l, glow additive - LUX SUPER GAL 301-2002B 0.25-3 ml / l and LUX SUPER GAL 301-4001 10-20 ml / l. Galvanizing takes place at a bath temperature of 30 ° C +/- 2 ° C, with the participation of current with a density of 1-2.5 A / dm ² .

Subsequently, the details are subjected to recovery rinsing and rinsing in a two-stage cascade, where, in countercurrent, for 1 min, they are rinsed with water at 15 ° C in a stream of water in the amount of 10% / h of the rinsing tank volume.

f) surface activation and passivation

Then, galvanized details are subjected to the surface activation process in the bath, which is 0.1-2% aqueous solution of nitric acid with 0.5-1% addition of PASIV AKTIV PLUS 900-889 for 30-60 s and at room temperature. In order to protect the coating against further reactions, the details are subjected to a chromate passivation process by immersion in a bath, which is a 3-5% water solution of the PASIV GAL 900-001 component. The process is carried out for 15-60 seconds, at a temperature of 20-40 ° C, with simultaneous aeration of the bath.

Subsequently, the workpieces are rinsed in countercurrent water with a temperature of 15 ° C, during a 0.5-1 minute immersion, and rinsing is performed with demineralized water with a conductivity below 100 µS / cm. The details are then dried at 70 ° C for 7-10 minutes.

PL 238 492 B1

The dried detail was powder coated with Akzo Nobel EL616G polyester-epoxy polymer paint and tested in a salt spray chamber.

The basic NSS corrosion resistance test was carried out in a LIEBISCH salt chamber, CONSTASAL series, type S - 400 MTR, with the use of brine - aqueous sodium chloride solution. The tests were carried out in accordance with the PN-EN ISO 9227: 2012 standard "Corrosion tests in artificial atmospheres. Research in sprayed brine ”. The pH of the saline solution was adjusted so that the pH of the spray solution collected inside the test chamber was 6.5 to 7.2 at 25 ° C 2 ° C. The details were tested by exposing them to salt spray with a concentration of 5%, the concentration of NaCl in the collected condensate was 50 g / l ± 5 g / l. The samples were placed at an angle of 20 ± 5 ° C from the vertical. The temperature inside the chamber oscillated in the following range: 35 ° C ± 2 ° C. The average condensate collection rate for a horizontal area of 80 cm ² was in the range of 1.5 ml / h ± 0.5 ml / h. Then, the salt spray resistance of the coatings was assessed, which consisted in comparing the visual properties before and after aging. Corrosion changes (blistering of the coating according to PN-EN ISO 4628-2: 2005) after 120 h, 240 h, 528 h and 720 h of the test were assessed on the plates placed in the salt spray chamber - the results are presented in Table 1.

After the completion of the corrosion test, the mechanical evaluation of the coating adhesion loss was carried out using the cross-cut method according to the PN-EN ISO 2409: 2008 standard. In accordance with the requirements of the standard for coatings with a thickness of 60 to 120 μm, incisions were made at 2 mm intervals on a hard substrate with a hand-held single-blade knife. Then, loose coating fragments were removed with an adhesive tape and the classification was performed. The test was also carried out for newly prepared samples after the paint had dried without an aging salt spray test. The results are presented in Table 2.

P r z x l a d II (comparative)

The galvanization was performed in the same way as in example 1, except that immediately after electro-galvanizing, the zinc layer is rinsed and dried (without additional activation and passivation). The prepared surface was painted with a polyester-epoxy polymer paint, and then salt spray tests and mechanical evaluation of the loss of adhesion of the coating by means of a cross-cut grid were performed. The results are summarized in Table 1 and Table 2.

P r z x l a d III

A solution for passivating the zinc surface was prepared as follows: to a beaker containing 50 cm ³ of demineralized water was added 2 cm ^{3 of a} 3-substituted diketone of the general formula, where R is a benzyl group, which was insoluble in water forming an oily layer on its surface. Then, while vigorously stirring, concentrated aqueous ammonia solution (density 0.950 g / cm ³ ) was added in portions (about 5 cm ^{3 in} total) so that the oil layer of the diketone was completely dissolved in the water, creating a homogeneous slightly alkaline mixture with a clearly perceptible smell of ammonia.

P r x l a d IV

A solution for passivation of the zinc surface was prepared as follows: 2 g of a 3-substituted diketone of the general formula, where R is an allyl group, was dissolved in 90 cm ^{3 of} isopropanol, and then 5 cm ^{3 of} concentrated aqueous ammonia solution (density 0.950 g / cm 3) was added. ³ ) and 10 cm ³ of demineralized water. The obtained solution was alkaline with a distinct ammonia odor.

P r z k ł a d V

The galvanizing is carried out in the same way as in Example 1, except that immediately after electro-galvanizing and rinsing, an organic conversion layer is formed on the zinc surface by spraying the solution obtained according to Example 4 on the zinc surface. After applying the solution, the surfaces of the details were dried in an oven at 100 ° C. The prepared surface was painted with polyester-epoxy paint, and then tests were performed in a salt spray chamber and the mechanical loss of adhesion of the coating was assessed by means of a cross-cut grid method. The results are summarized in Table 1 and Table 2.

P r x l a d VI

The conversion coating on the zinc surface was carried out analogously to Example 5, using the solution according to Example III.

PL 238 492 Β1

After drying, the surface prepared in such a way was painted with polyester-epoxy paint, and then tests were performed in a salt spray chamber and the mechanical evaluation of the loss of adhesion of the coating by means of a cross-cut grid method. The results are summarized in Table 1 and Table 2.

Example VII

The conversion coating on the zinc surface was carried out analogously to Example IV, using the solution prepared as in Example III by dissolving 1 g of a 3-substituted diketone of the general formula I, where R is an isobutyl (2-methylpropyl) group, in 50 cm ³ of demineralized water and 50 cm ^{3 of} ethanol. After drying, the surface was painted with polyester-epoxy paint, and then salt spray tests and mechanical evaluation of the loss of adhesion of the coating by the cross-cut method were performed, with results similar to those obtained in Examples 5 and 6.

Table no. 1 Results of tests in the salt chamber

Example no Coating blistering, Classification according to PN-EN ISO 4628-2: 2005 120 hours 240 hours 528 h. 720 h AND No change 2 (S3) 2 (S4) 3 (S5) 11 No change No change 2 (S4) 2 (S5) V No change No change No change 2 (S4) VI No change No change 2 (S2) 2 (S4)

Table No. 2. Assessment of the mechanical loss of coating adhesion using the cross-cut method

Time / Example no Coating adhesion. Classification according to PN-EN ISO 2409: 2008 AND II V VI After the paint is dry 0 0 0 0 After 720 h in the salt spray chamber 2 1 0 1

Patent claims

Claims (4)

Patent claims 1. A method of electro-galvanizing of steel details intended for the application of paint coatings, especially polymer ones, in which steel details are successively subjected to a chemical degreasing bath, etching bath, rinsing, electrochemical degreasing bath, rinsing, decapping, rinsing, active rinsing, electrochemical application of a zinc coating, rinsing recovery and subsequent rinsing, characterized in that an organic conversion layer is formed on the zinc surface by applying to it for at least 5 seconds, an aqueous or hydroalcoholic solution containing 0.1 to 10% of diketone of the general formula, in which R is the group an alkyl group having 1 to 20 carbon atoms in the chain, a substituted or unsubstituted benzyl group, a substituted or unsubstituted alkenyl group, made alkaline with an aqueous ammonia solution, where an alcohol that is miscible with water is used in the hydroalcoholic solution, PL 238 492 Β1 then, after drying, optionally a protective coating, preferably a polymer paint, is applied. 2. The method according to p. A process as claimed in claim 1, characterized in that isopropanopoly is used in the hydroalcoholic solution. 3. The method according to p. The process of claim 1, wherein methanol is used in the hydroalcoholic solution. 4. The method according to p. The process of claim 1, wherein ethanol is used in the hydroalcoholic solution.