CS254434B1 - Additive to weakly acidic zinc baths, which prevent co-deposition of divalent iron into the zinc coating - Google Patents

Abstract

The additive consists of compounds of the general formula 1, where Rj - H-, n-alkyl, where η = 1 to 8, with a straight or branched chain, Rj - 2-hydroxyphenyl, 4-hydroxyphenyl, 2,4-dihydroxyphenyl, 2-hydroxy-3-sulfophenyl, the group -CH(OH)-CH(OH)-COOH, which are contained therein alone or in mixtures. The additive forms strong complexes in a weakly acidic zinc plating bath, preventing the co-precipitation of iron into the zinc coating, if the Fe2+ concentration does not exceed 35 mg per 1000 ml. This suppresses the increase in internal stress of the coatings, the decrease in ductility and darkening during passivation in chromating preparations. The concentration of the additive must be maintained in the bath in the range of 0.001 to 0.1 g per 1000 ml.

Description

The invention relates to an additive for a weakly acidic zinc bath containing ammonium salts, i.e. based on zinc chloride or zinc sulphate and ammonium chloride or acetate, or both or without ammonium salts, i.e. based on zinc chloride or sulphate, sodium or potassium chloride and boric acid in a wide variety of proportions and flame-retardant additives provide the appearance and performance of the zinc coatings to be deposited.

All weak acid galvanizing baths used so far are sensitive to the content of foreign metals - especially iron, copper, lead and others. Increasing the content over certain concentrations, of the order of tens of milligrams per 1000 ml bath, causes a sharp deterioration of the coating parameters. Thereafter, it is necessary to carry out cleaning operations which are expensive and often require the bath to be taken out of operation for a certain period of time. Among foreign metals, bivalent iron causes the greatest difficulty. It is practically always included in the spa, because the most commonly treated material is structural steel grade. 11. During plating, the material used by the zinc electrolyte is attacked in shielded areas of more complex shapes, where the zinc is no longer deposited, or the goods falling into the bath dissolve during operation. The concentration of divalent iron ions in the operated bath gradually increases until the concentration range 25 to 3 (1 mg per 1 - 100 ml) causes a sharp deterioration of the coating quality - the ductility decreases, the internal stresses are increased and the bathing ability decreases. This critical build-up, iron ions are still present in the solution and consistently contribute to the deterioration of the coatings, resulting in low adhesion of the zinc coatings to the base material, cracking of thicker layers at more current-exposed product sites, and darkening of the coatings during passivation in chromating agents.

These disadvantages are overcome by the additive according to the invention which forms strong complexes in the environment of a weakly acidic zinc bath with divalent iron ions, and which consists of compounds of the general formula

R - CH - C = C - CH, -C - C - R II '

CH C where R _{3 is} -H-, CH ₃ (CH ₃ ) _n -, n = 0 to 7, with both unbranched and branched chains

R, -2-hydroxiphenyl, 4-hydroxiphenyl, 2,4-dihydroxiphenyl, 2-hydroxi-3-sulfophenyl, a group

- CH-CH-CCCH

I I

OH OH contained therein alone or in mixtures.

It has been found that the additive has the ability to suppress the co-deposition of iron into the zinc coating until a critical build-up occurs, where it is necessary to regenerate the bath. The operation of the bath is thus much more balanced and stable than it normally is. The concentration of the additive should be maintained between 0.001 and 0.1 g per 1000 ml bath. It is very advantageous to dispense the additive directly into the brightening agents and to ensure that its concentration is maintained at the same level by regular dosing.

A detailed explanation is given in the following examples.

Example 1

To a bright galvanizing bath of 70 g zinc chloride, 30 g boric acid, 150 g sodium chloride, 5 g sodium benzoate, 20 g nonionic surfactant (3 ethoxy groups per molecule), 3 g ethylene-propylene glycol copolymers (MW 2500), 0 0.05 g of aectothenone and 0.1 g of benzylidenacelone in a 1000 ml bath were successively dosed with bivalent iron and the quality of the deposited coatings was monitored on test panels in a H-cell cell plated at 1 A for 10 min.

In the concentration range of 0 to 12 mg Fe ~ ⁺ , the coatings were glossy and bent well at the highest current densities. In the range of 15 to 25 milligrams of Fe ~ ⁺ , the ductility slowly decreased and the samples began to crack, at higher current densities they were shiny, in the lower the appearance changed to semi-matt to matt. Between 25 and 35 mg Fe ⁺ , the samples were brittle, semi-glossy at higher current densities, and in the lower ones the coating did not precipitate at all. A black band appeared at the edge of the panel at the highest current densities, the color of which increased even more when passivated in the blue chromating agent.

The measurement was again carried out with the bath described above, into which 0.05 g of 2-butin-4-ol-1-sulfosalicilate was metered. Iron (II) was metered into the bath and the results evaluated by Hull cell plating in the same manner as in the previous case. In the concentration range of 0 to 35 mg Fe ^{- +} , the zinc coatings were glossy and well ductile. Between 35 and 40 mg Fe ⁺ , there was a sharp drop in ductility and only low-gloss coatings were excluded at low current densities.

Between 40 and. 45 mg of Fe ⁺ coatings have become brittle, have lost their luster and have not been eliminated at low current density. Passivation In the blue chromating agent, a hint of blackening occurred at the highest current densities.

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Example 2

0.01 g of 2-butyne-4-ol-1-sulfosalicillate and 0.01 g of 2-hexin-4-ol-1-salicilate were metered into the bath of Example 1, ferrous iron was added and the results evaluated by Hull cell plating. as in Example 1.

The zinc coatings were glossy and well ductile in a concentration range of 0 to 30 mg FU ⁺ . In the range of 30 to 40 mg Fe ⁺ , the ductility slowly decreased, in low current densities zinc stopped at all, there was a decrease in gloss even in higher current densities. In the range of 40 to 45 mg Fe ⁺ coatings have become brittle, at higher current densities the gloss has turned into a semi-gloss and in the low the coating has not been eliminated.

The above results show that the main area of action of the additive of the invention is in galvanizing baths where the concentration of iron (II) does not exceed 35 mg per 1000 ml bath. The properties of the bath and the coating parameters are minimally affected here, when the concentration exceeds 35 mg, there is a sharp deterioration and the bath needs / regeneration.

The invention can be used for galvanizing in weakly acid baths in the field of mechanical and electrical engineering companies and production cooperatives.

Claims (1)

SUBJECT OF THE INVENTION Addition to weakly acidic zinc baths to prevent co-precipitation of divalent iron into the zinc coating, applicable to baths based on zinc chloride or zinc sulphate and ammonium chloride or acetate, optionally both or without ammonium salts, ie zinc chloride or zinc sulphate, sodium chloride or potassium and boric acid in various proportions, characterized in that it consists of compounds of the general formula R-CH-C-C-CH-O-C-R 'I II' OH O where R ₍ - H -, CH _g (CH 2) -, n = 0 to 7, with both unbranched and branched R, - 2-hydroxitenyl, 4-hydroxiphenyl, 2,4-dihydroxiphenyl, 2-hydroxyl- 3-sulfophenyl group -CH-CH-COOH I I OH OH contained therein alone or in mixtures.