ZINC ELECTROLYTE DEVOID OF BORIC ACID AND AMMONIUM FOR THE ELECTRODEPOSITION OF ZINC COATINGS
20220064814 · 2022-03-03
Assignee
Inventors
Cpc classification
C25D21/14
CHEMISTRY; METALLURGY
International classification
Abstract
The invention relates to an aqueous electrolyte devoid of boric acid and ammonium for the electrodeposition of zinc coatings and to a method for producing such an electrolyte. The electrolyte comprises (a) Zn.sup.2+ in a concentration of 15 to 70 g/L; (b) Cl.sup.− in a concentration of 100 to 200 g/L; (c) K.sup.+ and/or Na.sup.+ in a total concentration of 0.75 to 6.0 mol/L; (d) acetate in a concentration of 5.0 to 45 g/L; (e) glycine and/or alanine in a total concentration of 0.5 to 30 g/L; and (f) water. The electrolyte has a pH of 4.5 to 6.5. In a preferred variant, the electrolyte contains (g) nicotinic acid and/or (h) ethoxylated thiodiglycol. The invention also relates to a method for producing a component having a zinc coating, which uses the electrolyte.
Claims
1. An aqueous electrolyte devoid of boric acid and ammonium for the electrodeposition of zinc coatings, comprising: (a) Zn.sup.2+ in a concentration of 15 to 70 g/L; (b) Cl.sup.− in a concentration of 100 to 200 g/L; (c) K.sup.+ and/or Na.sup.+ in a total concentration of 0.75 to 6.0 mol/L; (d) acetate in a concentration of 5.0 to 45 g/L; (e) glycine and/or alanine in a total concentration of 0.5 to 30 g/L; and (f) water; wherein the electrolyte has a pH of 4.5 to 6.5.
2. The electrolyte according to claim 1, wherein the concentration of Zn.sup.2+ in the electrolyte is 20 to 60 g/L, preferably 25 to 50 g/L, more preferably 30 to 40 g/L and most preferably 35 g/L.
3. The electrolyte according to claim 1, wherein the concentration of Cl.sup.− in the electrolyte is 120 to 190 g/L, preferably 130 to 180 g/L and more preferably 160 g/L.
4. The electrolyte according to claim 1, wherein the electrolyte contains K.sup.+ and the concentration of K.sup.+ is 0.75 to 6.0 mol/L, preferably 2.7 to 4.8 mol/L and more preferably 3.3 to 4.1 mol/L.
5. The electrolyte according to claim 1, wherein the concentration of acetate in the electrolyte is 7.5 to 30 g/L, preferably 10 to 20 g/L, most preferably 12 g/L.
6. The electrolyte according to claim 1, wherein the total concentration of glycine and/or alanine in the electrolyte is in the range of 0.5 to 20 g/L, preferably 1.0 to 10 g/L, more preferably 1.5 to 5 g/L and most preferably 2.5 g/L.
7. The electrolyte according to claim 1, wherein the electrolyte contains (a) Zn.sup.2+ in a concentration of 20 to 60 g/L; (b) Cl.sup.− in a concentration of 120 to 190 g/L; (c) K.sup.+ in a concentration of 2.7 to 4.8 mol/L and 0 to 0.5 mol/L Nat; (d) acetate in a concentration of 7.5 to 30 g/L; and (e) glycine and/or alanine in a total concentration of 0.5 to 20 g/L.
8. The electrolyte according to claim 1, wherein the electrolyte additionally contains (g) nicotinic acid in a concentration of 0.01 to 2.0 g/L, preferably 0.05 to 1.0 g/L, more preferably 0.08 to 0.5 g/L, most preferably 0.1 g/L.
9. The electrolyte according to claim 1, wherein the electrolyte additionally contains (h) ethoxylated thiodiglycol, with an average of at least 20 structural units derived from ethylene oxide, in a concentration of 0.3 to 10 g/L, preferably 0.5 to 5 g/L, more preferably 1.0 to 3.5 g/L and most preferably 2.1 g/L.
10. The electrolyte according to claim 1, wherein the electrolyte contains (g) nicotinic acid in a concentration of 0.05 to 1.0 g/L, and/or (h) ethoxylated thiodiglycol, with an average of at least 20 structural units derived from ethylene oxide, in a concentration of 0.5 to 5 g/L.
11. A method for producing an aqueous electrolyte according to claim 1, comprising the steps of: A) forming an aqueous solution of (a′) zinc chloride and/or zinc acetate; (b′) potassium chloride and/or sodium chloride; (c′) at least one of potassium acetate, sodium acetate and acetic acid; (d′) at least one from the group consisting of glycine, a salt thereof, alanine and a salt thereof; and (e′) optionally nicotinic acid or a salt thereof; and (f′) optionally ethoxylated thiodiglycol, with an average of at least 20 structural units derived from ethylene oxide; and B) optionally setting the pH to 4.5 to 6.5 by adding hydrochloric acid or by adding potassium hydroxide and/or sodium hydroxide, which can be added as solids or in the form of an aqueous solution.
12. A method for producing a component having a zinc coating, comprising the electrodeposition of zinc on a metal component from an electrolyte according to claim 1.
13. The method according to claim 12, wherein the metal component comprises or consists of iron or an iron alloy.
14. The method according to claim 12, wherein deposition occurs at a temperature of 20 to 50° C., preferably 25 to 40° C., and a current density of 0.2 to 10 A/dm.sup.2, preferably 0.5 to 6 A/dm.sup.2, is used for the deposition.
15. The method according to claim 12, wherein after the electrodeposition the component having a zinc coating is subjected to a passivation treatment.
16. The method according to claim 12, wherein after the electrodeposition the component having a zinc coating is annealed, optionally before or after a passivation treatment.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0162]
[0163]
EXAMPLES
Production of Electrolytes E1 to E6
Procedure:
[0164] The electrolytes were prepared at room temperature by adding deionized water to the other components and stirring. The dissolution of the solids was accelerated by heating them to 60° C. Once the solution had formed, it was cooled to 25° C.
[0165] Where necessary, the pH was set to 5.2 by adding 5 N hydrochloric acid or 50% potassium hydroxide solution (756 g/L KOH).
Reagents Used:
[0166] Technical grade potassium chloride was used.
[0167] Zinc chloride HP from Schlötter was used as the zinc chloride.
[0168] The additives SLOTANIT BSF 1668 (basic additive) and SLOTANIT BSF 1662 (brightener additive), which are both available from Schlötter, were used.
[0169] “Purest” quality potassium acetate was used.
[0170] “Purest” quality glycine was used.
[0171] Technical grade nicotinic acid was used.
[0172] A 70% solution, namely CHE ED 7127 70%, which is available from the company Erbsloh, or Aduxol TDG-027 70%, which is available from Schärer & Schläpfer, was used as the ethoxylated thiodiglycol.
Components for Electrolyte E1 (Base Electrolyte):
[0173]
TABLE-US-00001 73 g/L ZnCl.sub.2 (corresponds to 35 g/L Zn.sup.2+ and 38 g/L Cl.sup.−) 260 g/L KCl (corresponds to 136 g/L K.sup.+ and 124 g/L Cl.sup.−) 35.0 ml/L SLOTANIT BSF 1668 0.3 ml/L SLOTANIT BSF 1662
Components for Electrolyte E2:
[0174]
TABLE-US-00002 73 g/L ZnCl.sub.2 260 g/L KCl 35.0 ml/L SLOTANIT BSF 1668 0.3 ml/L SLOTANIT BSF 1662 20 g/L Potassium acetate (corresponds to 8 g/L K.sup.+ and 12 g/L AcO.sup.−)
Components for Electrolyte E3:
[0175]
TABLE-US-00003 73 g/L ZnCl.sub.2 260 g/L KCl 35.0 ml/L SLOTANIT BSF 1668 0.3 ml/L SLOTANIT BSF 1662 20 g/L Potassium acetate 2.5 g/L Glycine
Components for Electrolyte E4:
[0176]
TABLE-US-00004 73 g/L ZnCl.sub.2 260 g/L KCl 35.0 ml/L SLOTANIT BSF 1668 0.3 ml/L SLOTANIT BSF 1662 20 g/L Potassium acetate 2.5 g/L Glycine 2.11 g/L CHE ED 7127 70% or Aduxol TDG-027 70%
Components for Electrolyte E5:
[0177]
TABLE-US-00005 73 g/L ZnCl.sub.2 260 g/L KCl 35.0 ml/L SLOTANIT BSF 1668 0.3 ml/L SLOTANIT BSF 1662 20 g/L Potassium acetate 2.5 g/L Glycine 0.113 g/L Nicotinic acid
Components for Electrolyte E6:
[0178]
TABLE-US-00006 73 g/L ZnCl.sub.2 260 g/L KCl 35.0 ml/L SLOTANIT BSF 1668 0.3 ml/L SLOTANIT BSF 1662 20 g/L Potassium acetate 2.5 g/L Glycine 2.11 g/L CHE ED 7127 70% or Aduxol TDG-027 70% 0.113 g/L Nicotinic acid
Pre-Treatment of the Steel Sheets
[0179] 1) Decoction degreasing with SLOTOCLEAN 160 (from Schlötter) for 15 minutes at 65° C. and subsequent rinsing with water. [0180] 2) Hydrochloric acid pickling with 19% hydrochloric acid and 40 mL/L pickling degreaser SLOTOCLEAN BEF 30 (from Schlötter) for 7 minutes at 25° C. and subsequent rinsing with water. [0181] 3) Electrolytic degreasing by means of anodic degreasing with SLOTOCLEAN DCG (from Schlötter) at a cathodic current density of 3 to 6 A/dm.sup.2 for 2 minutes at 25° C. and subsequent rinsing with water. [0182] 4) Pickling with diluted hydrochloric acid (1:10 dilution of concentrated hydrochloric acid) for 1 minute at 25° C. and subsequent rinsing with water.
Electrodeposition
Deposition Parameters in the Electroplated Zinc Electrolyte:
[0183] Electrolyte volumes: 3.0 L.
[0184] pH of the electrolyte: 5.2.
[0185] Electrolyte temperature: 25° C.
[0186] Stirring speed set on the magnetic stirrer: 300 rpm.
General Procedure:
[0187] The test set-up for electrodeposition with an angular sheet of steel is illustrated in
[0188] The electrolyte (1) is present in a beaker (10) and was kept permanently in motion by means of a magnetic stirrer (15) and a magnetic stirrer bar (length: 40 mm, diameter: 8 mm) (16). A Schott Duran beaker (3.0 L, low form) was used as the beaker. The temperature was controlled and held constant by means of a contact thermometer (17) which is connected to the heating relay of the magnetic stirrer. In the Examples, a magnetic stirrer with a heatable plate, the IKA RET basic model from the company IKA, was used. A stabilizer (rectifier) (20) from the company Gossen Metrawatt, the SLP 240-40 model, served as the power source. A current strength of 3.0 A and a voltage of 2.5 V are illustrated by way of example.
[0189] Two anodes (2, 2′) were used for the electrodeposition. High-grade zinc anodes (99.99% Zn) in accordance with DIN EN 1179 were used as the anode material (length: 10 cm, width: 5 cm, thickness: 1 cm). The anodes were immersed 9 cm deep in the electrolyte.
[0190] Before the cathode sheets were introduced, they were subjected to the pre-treatment for steel sheets described above.
[0191] The cathode sheet (3) was arranged in the middle of the two anodes in the beaker. The distance to the anodes was 6 cm both from the front side and the rear side of the cathode sheet. The cathode sheet is immersed so deeply in the electrolyte that the total immersed area (front and rear sides) is one square decimeter.
[0192] Where necessary, the pH was set to 5.2 by adding 5 N hydrochloric acid or 50% potassium hydroxide solution (756 g/L KOH) before the start of the deposition.
[0193] In each case, the electrodeposition was performed from the respective electrolyte at the current density specified in the Examples.
Passivation:
[0194] After the electrodeposition of zinc, the coated sheets were thoroughly rinsed with water and subsequently brightened in diluted hydrochloric acid (15 mL 25% HCl in 1 L water). Following another rinse with water, passivation was carried out in a thin-film passivation SLOTOPAS Z 20 blau (prepared with the passivation concentrate SLOTOPAS Z 21 BLAU from Schlötter, 35 mL/L) at 25° C., pH=1.9, and 60 seconds immersion time. The coated sheets were subsequently rinsed with water and dried at 80° C. for 15 minutes in a convection oven. Prior to assessment or further treatment of the sheets, they were cooled to room temperature.
Test A): Burn Marks on the Angular Sheet
Examples 1 to 4, Comparative Examples 1 and 2
[0195] Angular sheets having type 2 geometries according to DIN 50957-2 were used. The material of the angular sheets was cold-rolled steel according to DIN EN 10139/10140 (quality: DC03 LC MA RL). The angular sheets were pre-treated in accordance with the procedure described above and then coated. The electrolytes E1 to E6 as specified in Table 1 were used for Comparative Examples 1 and 2 (CE1 and CE2) as well as for Examples 1 to 4. The current density was varied and was 0.25, 1.0, 2.0, 4.0, 6.0 or 8.0 A/dm.sup.2. The deposited layer thicknesses were 10 μm. The sheets were then passivated as described above.
[0196] The appearance of the deposited zinc coating in the edge region of the angular sheets and the number of burn marks (discolored, dark, amorphous or coarsely crystalline, mostly powdery regions in the coating) which can be caused by increased local current densities were assessed by a visual inspection.
Key:
[0197] Δ no burn marks
∘ mild burn marks
.circle-solid. moderate burn marks
.box-tangle-solidup. major burn marks
[0198] The results are summarized in Table 1.
TABLE-US-00007 TABLE 1 Burn Marks on the Angular Sheet Current Density in A/dm.sup.2 Example/Electrolyte 0.25 1.0 2.0 4.0 6.0 8.0 CE1 E1 Δ Δ Δ ◯ .circle-solid. .box-tangle-solidup. CE2 E2 Δ Δ Δ ◯ ◯ .circle-solid. 1 E3 Δ Δ Δ ◯ ◯ ◯ 2 E4 Δ Δ Δ Δ Δ ◯ 3 E5 Δ Δ Δ Δ Δ ◯ 4 E6 Δ Δ Δ Δ Δ Δ
[0199] At low and average current densities of up to 2 A/dm.sup.2, a usable zinc coating was deposited from all of the electrolytes E1 to E6. At high current densities of 4 or 8 A/dm.sup.2, considerable differences between the individual electrolytes were shown.
[0200] In Comparative Example 1 (CE1), electrolyte E1, which is devoid of boric acid and ammonium, resulted without the buffer substances acetate and glycine in unfit zinc coatings which have massive burn marks on the edge regions thereof. The addition of acetate allowed initial improvements to be achieved in Comparative Example 2 (CE2) with electrolyte E2, but too many burn marks were still observed at 8 A/dm.sup.2.
[0201] In contrast, high-quality zinc coatings were obtained in Example 1 with electrolyte E3 even at high current densities, which coatings exhibited only minor burn marks even at 8 A/dm.sup.2. Thus, electrolyte E3 according to the invention constitutes a veritable boric acid-free and ammonium-free substitute for conventional electrolytes containing boric acid.
[0202] In Examples 2 to 4, a further considerable improvement was then achieved with the additives ethoxylated thiodiglycol and nicotinic acid.
Test B): Annealability
Examples 5 to 8, Comparative Examples 3 and 4
[0203] Straight steel sheets with the dimensions 0.3 mm thickness, 50 mm width and 130 mm length were used. The material of the straight steel sheets was cold-rolled steel according to DIN EN 10139/10140 (quality: DC03 LC MA RL). The steel sheets were pre-treated in accordance with the procedure described above and then coated. The electrolytes E1 to E6 were used for Comparative Examples 3 and 4 (CE3 and CE4) as well as for Examples 5 to 8 (see Tables 2 and 3). The current density was varied and was 0.25, 1.0, 2.0, 4.0, 6.0 or 8.0 A/dm.sup.2. The deposited layer thicknesses were 10 μm.
[0204] After the electrodeposition of zinc, the sheets were then passivated, as described above, stored for 48 hours at room temperature and subsequently annealed for 24 hours at 210° C.
[0205] Then, the bubble formation caused by annealing was assessed at room temperature.
Key:
[0206] Δ no bubble formation
∘ mild bubble formation
.circle-solid. moderate bubble formation
.box-tangle-solidup. major bubble formation
[0207] The results are summarized in Table 2.
TABLE-US-00008 TABLE 2 Bubble Formation After Annealing Current Density in A/dm.sup.2 Example/Electrolyte 0.25 1.0 2.0 4.0 6.0 8.0 CE3 E1 Δ Δ Δ ◯ .box-tangle-solidup. .box-tangle-solidup. CE4 E2 Δ Δ Δ ◯ .box-tangle-solidup. .box-tangle-solidup. 5 E3 Δ Δ Δ .circle-solid. .box-tangle-solidup. .box-tangle-solidup. 6 E4 Δ Δ Δ Δ ◯ .box-tangle-solidup. 7 E5 Δ Δ Δ Δ ◯ ◯ 8 E6 Δ Δ Δ Δ Δ Δ
[0208] In Example 5, zinc coatings with very good annealability which did not exhibit any bubble formation at all were produced with electrolyte E3, which contains potassium acetate and glycine, at conventional current densities between 0.25 and 2 A/dm.sup.2.
[0209] A further improvement in the annealability of zinc coatings deposited at higher current densities was observed for electrolytes E4 and E5 in Examples 6 and 7, which contained ethoxylated thiodiglycol (E4) and nicotinic acid (E5) respectively. By combining both additives in electrolyte E6 a completely bubble-free annealed zinc coating could still be achieved even at a current density of 8 A/dm.sup.2 (see Example 8).
[0210] Moreover, the optical impression (gloss) was assessed at room temperature.
Key:
[0211] Δ no milky appearance
∘ slight milky appearance
.circle-solid. moderate milky appearance
.box-tangle-solidup. strong milky appearance
[0212] The results are summarized in Table 3.
TABLE-US-00009 TABLE 3 Gloss After Annealing Current Density in A/dm.sup.2 Example/Electrolyte 0.25 1.0 2.0 4.0 6.0 8.0 CE3 E1 .box-tangle-solidup. .box-tangle-solidup. .box-tangle-solidup. .box-tangle-solidup. .box-tangle-solidup. .box-tangle-solidup. CE4 E2 .box-tangle-solidup. .box-tangle-solidup. .box-tangle-solidup. .box-tangle-solidup. .box-tangle-solidup. .box-tangle-solidup. 5 E3 .box-tangle-solidup. .circle-solid. .circle-solid. .circle-solid. .circle-solid. .circle-solid. 6 E4 Δ Δ Δ ◯ ◯ ◯ 7 E5 Δ Δ Δ Δ Δ Δ 8 E6 Δ Δ Δ Δ Δ Δ
[0213] In Comparative Examples 3 and 4, electrolytes 1 and 2, which are devoid of boric acid and ammonium, resulted in a very milky appearance with numerous streaks. With electrolyte E3, which contains acetate and glycine, the gloss was improved and the milky appearance was slightly reduced.
[0214] By adding ethoxylated thiodiglycol to electrolyte E4 and in particular by adding nicotinic acid to electrolytes 5 and 6, the gloss of the annealed zinc coatings was improved further still (see Examples 6 to 8). Regardless of the current density used, the zinc coatings only had a slight milky appearance, or no milkiness, cloudiness or streaks at all.
Test C): Thermal Shock Test
Examples 9 to 12, Comparative Examples 5 and 6
[0215] Straight steel sheets with the dimensions 0.3 mm thickness, 50 mm width and 130 mm length were used. The material of the straight steel sheets was cold-rolled steel according to DIN EN 10139/10140 (quality: DC03 LC MA RL). These were pre-treated in accordance with the procedure described above and then coated at a current density of 3.0 A/dm.sup.2. The electrolytes E1 to E6 were used for Comparative Examples 5 and 6 (CE5 and CE6) as well as for Examples 9 to 12 (see Table 4). The deposited layer thicknesses were 10 μm.
[0216] After the electrodeposition of zinc, the sheets were passivated, as described above, stored for 48 hours at room temperature and subsequently stored for 30 minutes at 220° C. and immediately placed in filtered tap water annealed to 20° C.
[0217] First, the tap water was examined and assessed for any flaking.
[0218] After the thermal shock test the sheets were dried for 15 minutes at 80° C. in a convection oven and after cooling to room temperature the adhesiveness was visually inspected again. For this, 4 cm long, 19 mm wide adhesive strips (Tesafilm® Crystal Clear from Tesa SE) were stuck on the sheets and removed again after 60 seconds. The zinc coatings were then inspected for damage.
[0219] The two adhesion tests were deemed to have been passed if no flakes or other particles, chips, bubbles or damage were observed.
Key:
[0220] Δ Passed (no flaking and good adhesion)
[0221] The results are summarized in Table 4.
TABLE-US-00010 TABLE 4 Adhesion After Thermal Shock Test Example/ Current Density Electrolyte 3.0 A/dm.sup.2 CE5 E1 Δ CE6 E2 Δ 9 E3 Δ 10 E4 Δ 11 E5 Δ 12 E6 Δ
[0222] No flaking was observed with any of the electrolytes, and a good adhesion of the zinc coating on the steel sheet was exhibited. Thus, it is possible to produce adhesion-resistant zinc coatings with the electrolyte devoid of boric acid and ammonium.