DE2427408C2 - Electroplating process for producing shiny or semi-shiny zinc layers as well as electrolyte for carrying out the process - Google Patents

Description

The invention relates to an electroplating process for producing shiny or semi-shiny zinc layers on an object using a non-cyanide alkaline electrolyte or those with a low cyanide content, the electrolyte being water, a soluble zinc compound and a Contains brighteners, and the rinse water used before the electroplating of the object and / or the water in the electrolyte contains impurities which prevent the deposition of a shiny or semi-shiny zinc layer, as well as an electrolyte for carrying out this process.

There are three basic types of alkaline zinc electrolytes, namely high cyanide and low cyanide or cyanide-free. The usual high-cyanide alkaline zinc electrolytes based on zinc cyanide contain a large amount of free sodium cyanide, sodium hydroxide and gloss. Still this one Electrolytes have been used for many years, difficulties sometimes arise with respect to the Water hardness and the impurities contained in industrial water.

3u The commercially available cyanide-free alkaline zinc electrolytes contain sodium zincate, sodium hydroxide and shiners. The commercially available electrolytes with a low cyanide content are generally used to compositions in the sense of cyanide-free electrolytes. However, they still contain small amounts of pure sodium cyanide.

Are cyanide-free electrolytes or those with low cyanide content for shiny plating If zinc was used, it was found that the impurities in the rinse water and in the water had a significant influence Water with which the Eiekiroiyi is made. : st the water hardness is high and / or there are significant impurities present, it is often difficult, if not impossible, to obtain glossy coatings. the Difficulties encountered with bright zinc coatings are likely to be related to the hardness of the water and / or the concentration of impurities hanging out in the water. The gloss that can be achieved in each case depends on the performance of the Shimmer off.

Cyanide-free zinc alkaline electrolytes or those with a low cyanide content are because of the problems the sewage disposal preferred to the high eyanidisehen Eiekiroiyten.

Before the usual zinc plating, the item is cleaned and rinsed before it is plated. It it now appeared that. if the rinse water is hard or has high levels of calcium and magnesium compounds a film of water containing these impurities is obtained on the object. Will this damp objects are then immersed in a cyanide-free alkaline zinc electrolyte Calcium, magnesium or other impurities deposit on the metal object as soon as it comes with it is wetted by the electrolyte. This precipitation on the objects leads to dull and smeared Layers. It is also known that one does not obtain corresponding plating if the objects with hard water or water containing contaminants. However, it is made with distilled When rinsed off with water, the same electrolyte gives much shinier layers.

In the case of electrolytes with a high cyanide content, this is obviously not a problem, since the high cyanide content able to dissolve existing calcium, magnesium or other impurities. As a result, no Precipitation and you get shiny layers after rinsing with distilled as well as with hard water.

To overcome these problems with hard water or water containing other impurities, for the production of cyanide-free alkaline zinc electrolytes or those with only a low cyanide content has already been proposed. Chelating agents or complexing agents before the electrolyte and / or the rinse water add to the actual plating, for example salts of ethylenediaminetetraacetic acid, sodium gluconate or potassium sodium tartrate. With these substances one gets reasonably satisfactory shiny zinc layers, however, they lead to another problem in zinc plating, namely, they also lead to one Chelation or complexing with other metallic impurities, which ultimately leads to their accumulation in the electrolyte. It can be iron or copper, and it happens that way extensive enrichment that finally the zinc layers in the usual zinc plating Dipping and chromating stages become matt. In the case of electrolytes containing small amounts of cyanide, this is However, metal enrichment poses less of a problem, obviously due to the low cyanide content still pose some problem.

The above task is achieved with the aid of the method mentioned at the beginning, which is characterized by.

that a soluble silicate is added to the rinse water and / or the electrolyte, and by means of the electrolyte to carry out this method according to claim 4 solved.

Advantageous developments of the method according to claim i are given in claims 2 and 3.

A wide variety of soluble silicates can be used, such as water glass Na ₂ O - χ SiO ₂ (x = 3–5 ); Sodium disilicate Na ₂ Si ₂ Os; Sodium metasilicate Na ₂ SiO ₃ and sodium ethosilicate Na ₄ SiO ₄ . Instead of the sodium silicates, other silicates can of course also be used.

The soluble silicate is used in the manufacture of the zinc electrolyte along with the other ingredients including the shiners resolved

The soluble silicate amounts to be added to the rinse water and / or the electrolyte are dependent on depending on the silicate used, but primarily on the hardness of the water and the concentration Impurities in the water. The amount of soluble silicate is not critical; an excess is in no way disadvantageous. In general, however, most of the hard waters of the United States are reckoned from America for making shiny zinc coatings with a silica additive ranging from 0.00748 to 1436 g / l.

You can add any shine to the electrolyte, such as alkylamines or reaction products of amines containing tertiary or quaternary amino groups with epichlorohydrin, with a molecular weight of over about 250 per recurring unit organic acids and amines such as ethylene diamine, triethylene tetramine (US-PS 27 91 5 ¹ M and 36 55 534) and mixtures thereof, applicable.

Examples of water that normally dull zinc coatings despite the presence of sheen in the Lead zinc electrolyte, which, however, result in shiny layers when carrying out the process mentioned, are waters of Los Angeles. The same problem exists in other parts of the country, such as the Midwest including the states of Michigan, Indiana, and Illinois. Shiny zinc layer in the sense of Example 1 without Silicate can be found in other areas such as New York City. Below is the average chemical Analysis of waters from Los Angeles in the 1971-1972 fiscal year. The analysis of these waters is given Deviations by 25 to 50% from the "mean value"

Tabel

Los Angeles Los Angeles Metropolitan Water District Reservoir) Owens River River Suppiy (Upper Feeder) (Palos Verdes 1243 Aquaeduct Conduit 790 Specific conductivity · 10 ⁶ 320 621 1277 230 Solute (calculated), mg / 1 199 384 802 56 Total hardness (CaCO ₃ ), mg / 1 82 200 151 22nd Calcium (Ca ⁺ +), mg / 1 24 57 36 177 Magnesium (Mg ^{+ +} ), mg / 1 5.1 14th 15th 4.8 Sodium (Na ⁺ ), mg / 1 33 49 216 123 Potassium (K ⁺ ), mg / 1 3.6 3.8 4.9 341 Alkalinity (CaCO ₃ ), mg / 1 113 141 126 105 Sulphates (SO ₄ -), mg /! 28 110 338 1.6 Chloride (Cl-), mg / 1 14th 37 105 8th Nitrates (NO ₃ -), mg / 1 0.8 6.1 2.1 0.01 Silica (SiO ₂ ), mg / 1 22nd 21 8th 0.23 Iron (Fe), mg / 1 0.05 0.02 0.01 036 Boron (B), mg / 1 0.41 037 0.18 Fluoride (F-), mg / 1 0.56 0.48 0.36

It was also found that conventional water softeners have little or no effect on the quality of the Have zinc layers. Matt or smeared zinc layers cannot be improved by adding Carbonates, phosphates (sodium phosphate and pyrophosphate), borates and chelating agents.

The above analysis of the water from Los Angeles shows a silica content of 22 to 8 mg / l. These Silicic acid can be present in water in the most varied of forms, such as silicate, as silica itself or as colloidal silica. It is not known what form the silica in the water from Los Angeles took is present. Two mean analyzes of waters from New York City in 1972 are given below.

Catskill Croton Delaware Hardness CaCO ₃ , mg / 1 18th 66 Calcium (Ca ⁺⁺ ), mg / 1 5.3 12.8 Magnesium (Mg ^{+ +} ), mg / 1 1.5 7.3 Sodium (Na ⁺ ), mg / 1 3.3 9.2 Potassium (K ⁺ ), mg / 1 0.59 1.4

24 27 (Continuation) 408 Croton Catskill 40 Alkalinity (CaCO ₃ ), mg / 1 Delaware 22.7 Sulphates (SO ₄ -), mg / l 8th 20.1 Chloride (CI-), mg / i 12.5 0.22 Nitrates (NO ₃ -), mg / l 5.1 6.0 Silica (SiO ₂ ), mg / 1 022 0.09 Iron (Fe), mg / 1 22nd 0.00 Boron (B), mg / l 0.05 0 ^ 0 Fluoride (F-), mg / 1 0.00 030

All of the contaminants in New York City water are present in much smaller amounts (including silica) than in the waters of Los Angeles with the exception of iron and fluoride, so it is difficult to determine which contamination actually causes the problem with dull zinc coating However, the fact is that the addition of a soluble silica to the waters of Los Angeles or the Miitelwest can solve the problems with qualitatively unsuitable zinc layers.

The analysis of the above waters is not complete, as these waters contain various other impurities contain even smaller amounts including other metal salts and organic substances, which may also be part of the matte layers can lead.

example 1

A zinc electrolyte was prepared from 12 g / l zinc, 120 g / l sodium hydroxide, 1.58 ml / l of a condensation product of epichlorohydrin and dimethylaminopropylamine with a molecular weight of at least 250 (1.6 ml / l) and 0.0105 g / l anisaldehyde bisulfite The above water from Los Angeles was used to make the electrolyte. A steel test plate was cleaned as usual and rinsed with untreated Los Angeles water of the same quality. The rinsed test plate was inserted into the electrolyte and ^{galvanized for 5 min at a current of 2 A in a 267 cm 3} Hull cell. The layer was galvanized dull and smeared in a current density range of 0.1 to 1 3 A / dm ² , but below and above it glossy.

Example 2

0.01% by volume of a sodium silicate solution (28.7% SiO ₂ content) was added to the electrolyte from Example 1. The test plaque, cleaned as usual, was rinsed with untreated Los Angeles water and then galvanized under the process conditions of Example 1. Even in the current density range between 0. t and 1.5 A / dm ² , the test plate galvanized in this way had a shiny coating.

Example 3

In a modification of Example 1, the test plaque was rinsed with Los Angeles water containing 45 g / l sodium metasilicate and then galvanized in the silicate-free electrolyte. The zinc layer was shiny and uniform over a current density range of 0.1 to 8 A / dm ² .

so B e i s ρ i e I 4

There was prepared a background electrolyte containing 9 g / ¹ zinc, 74.8 g / l sodium hydroxide, 9 g / l of sodium cyanide and 4 ml / 1 of a reaction product of diethanolamine, formaldehyde, epichlorohydrin, and ar-picoline (US-PS 36 55 534 ). When galvanizing under the conditions of Example 1, streaky and cloudy layers are obtained within the current density range of 0.5 to 1 A / dm ^2.

Example 5

If, as a modification of Example 4, 3.75 g / l sodium orthosilicate was added to the electrolyte, the result was Flawlessly shiny zinc layers over the entire current density range.

Example 6

An electrolyte containing 7.48 g / l zinc was used. 74.8 g / l sodium hydroxide and 2 ml / l of a reaction product of ethylenediamine and epichlorohydrin (US Pat. No. 2,791,554) and 0.04 g / l anisaldehyde bisulfite and galvanized under the conditions of Example 1. The zinc layers obtained in this way showed a smeared, streaky appearance in the current density range between 0.1 and 1.5 A / dm ^2.

Example 7

If, in a modification of Example 6, 0.57% by volume of a sodium silicate solution (28.7% SiOj content) was added to the electrolyte, perfect layers were obtained over the entire current density range.

In the above examples, the zinc was used as zinc oxide for the production of the electrolyte. The 5th The above low cyanide electrolytes had less than 15 g / L free cyanide.

Claims (4)

Patent claims: 1. Electroplating process for producing shiny or semi-shiny zinc layers on one Article using a cyanide-free alkaline electrolyte or one with a low Cyanide content, whereby the electrolyte contains water, a soluble zinc compound and a brightener, and the rinse water and / or the water of the used prior to electroplating the article Electrolyte contains impurities that cause the deposition of a shiny or semi-shiny Prevent zinc layer, characterized in that the rinse water and / or the electrolyte a soluble silicate is added. 2. The method according to claim 1, characterized in that the soluble silicate in an amount of at least 0.007 g / l is added 3. The method according to claim 1 or 2, characterized in that water glass, sodium disilicate, sodium metasilicate or sodium orthosilicate is added.
^. 4. electrolyte for performing the method according to claims 1 to 3.