DE2251103B2 - - Google Patents

Description

The invention relates to an acidic galvanic zinc bath with a content of zinc, ammonium, Sulphate and chloride ions, an ethoxylated organic compound and an organic Acid. This bath is for producing shiny zinc coatings with dense crystal structures suitable under high current densities.

^{It is known to use acidic zinc plating} baths which contain zinc sulfate, zinc chloride or zinc fluoroborate or the like as a source of Zn 2+ ions. To increase the conductivity of the bath, sulfate or chloride is often added to the bath. With such plating baths, it is possible to produce plating of considerably high density when the plating bath is agitated during the plating operation. At high current densities, however, the surface of the coatings is coarse and not shiny. In addition, large crystals are formed. In order to avoid these difficulties, it has been proposed to add small amounts of organic substance, for example liquorice, glue, gum arabic, dextrose, ^ -naphthol or the like, to the bath. The incorporation of such organic substances enables the production of high quality plating within a certain range of the current density, although the practical range of the current density is relatively limited and it is impossible to produce beautiful coatings at high current densities, especially in the range of 50 A / dm ² to 100 A / dm ² .

For example, an acidic galvanic zinc bath containing zinc, ammonium, sulfate and chloride ions is one contains ethoxylated compound and an organic acid as additives, in the German Offenlegungsschrift 1521029 has been described. This bathroom however, cannot be satisfactory when using high current densities due to the formation of coarse coatings.

The invention is therefore based on the object of creating an improved zinc electroplating bath which enables the production of dense and shiny platings in a wide range of current densities from 10 A / dm ² to 100 A / dm ² .

According to the invention, an acidic galvanic zinc bath of the type mentioned at the outset is available provided, which is characterized in that the bath is an aqueous solution of 200 to 650 g / l zinc sulfate, 50 to 200 g / l zinc chloride, 10 to 70 g / l ammonium chloride, 5 to 30 g / l sodium succinate and 0.1 contains up to 10 g / l nicotinic acid amide and / or 0.01 to 0.1 g / l ethoxylated a-naphtholsulfonic acid.

Zinc sulfate and zinc chloride are used as the source of the Zn ²⁺ ions. If the zinc sulfate concentration is less than 200 g / l, it is impossible to use high current densities because the surface of the platings is burned, while if the concentration exceeds 650 g / l, deposits tend to be generated. These precipitates are zinc ammonium sulfates which poison the conductor roller for outputting an electric current or deposit on the surface of the plating film to deteriorate the plating film.

ίο Therefore, zinc sulfate is used in amounts of 200 to 650 g / l used.

If the concentration of zinc chloride is less than 50 g / l, the conductivity becomes too low to generate high current densities, while at a concentration of more than 200 g / l the appearance of the plating deteriorates when treated with a phosphate or chromic acid.

The conductivity should be improved by adding ammonium chloride. With concentration

ao of less than 10 g / l can achieve this goal of ammonium chloride cannot be achieved, while at a concentration above 70 g / l there is a tendency to There is deposition of precipitation. In electrolysis using the electrolysis bath According to the invention, the favorable pH value of the electrolysis bath is between 3.5 and 4.5 and preferably at 4.0. A pH of the electrolysis bath above 4.5 would cause a deposition of iron and Produce zinc compounds. A pH value of the electrolysis bath below 3.5 would lead to the evolution of hydrogen lead in the bath, which is absorbed by the plating film, and pinholes in the Plating film.

The main function of the sodium succinate is to stabilize the pH of the plating bath. If the sodium succinate concentration is less than 5 g / l, its effect is to buffer the pHs inadequate, while sodium succinate is preferred at a concentration above 30 g / l is not increased.

Nicotinic acid amide and ethoxylated Λ-naphtholsulfonic acid are added to obtain dense and shiny plating surfaces. You can can be used individually or in combination. The incorporation creates fine crystals, resulting in smooth, shiny and nicely plated surfaces can be obtained.

An amount of 0.1 to 10 g / l nicotinic acid amide is suitable. A concentration lower than 0.1 g / l is not effective, while a concentration in excess of 10 g / L does not increase the desired benefit. A suitable concentration of ethoxylated α-naphtholsulfonic acid is in the range from 0.01 g / l to 0.1 g / l. While a concentration of less than 0.01 g / L is not effective, a concentration of more will do than 0.1 g / L to form pinholes in the plated surfaces and to opaque plated surfaces with an unfavorable appearance.

If a plating bath having a composition within the range of the above-described limits is used and this bath is vigorously stirred during the plating operation, it is possible to obtain ^{plated surfaces having a dense crystal structure at high current densities of 10 A / dm 2} to 100 A / dm ^2.

For a better understanding of the invention, the following examples are preferred embodiments represent, listed.

example 1

A cold-rolled steel plate was clad under the following conditions.

Composition of the bath

Zinc sulfate (ZnSO ₄ · 7 H ₂ O) 450 g / l

Zinc chloride (ZnCl ₂ ) 150 g / l

Sodium succinate [(CH ₂ COONa) ₂ ■

6H ₂ O)] 15 g / l

Nicotinic acid amide (C ₆ H ₆ N ₂ O ₂ ) 3 g / l

Ammonium chloride (NH ₄ Cl) 40 g / l

Ethoxylated "-naphtholsulfone-

acid (C ₁₂ H ₁₂ O ₅ S) 0.02 g / l

Current density 100 A / dm ²

Plating time 5 seconds

Example 2
Composition of the bath

Zinc sulfate (ZnSO ₄ · 7 H ₂ O) 300 g / l

Zinc chloride (ZnCl ₂ ) 80 g / l

Ammonium chloride (NH ₄ Cl) 20 g / l

Sodium succinate

[(CH ₂ COONa) ₂ -OH ₂ O)] 10 g / l

Nicotinic acid amide (C ₆ H ₆ N ₂ O ₂ ) 5 gl /

Current density 60 A / dm ²

Plating time 8 seconds

Example 3

Composition of the bath
5

Zinc sulfate (ZnSO ₄ · 7 H ₂ O) 350 g / l

Zinc chloride (ZnCl ₂ ) 200 g / l

Ammonium chloride (NH ₄ Cl) 50 g / l

Sodium succinate

[(CH ₂ COONa) ₂ · 6 H ₂ O)] 20 g / l

Ethoxylated a-naphtholsulfonic acid

(C ₁₂ H ₁₂ O ₂ S) 0.05 g / l

Current density 80 A / dm ²

Plating time 4 seconds

The plating coatings produced according to the corresponding examples have dense crystal structures with a good appearance and are smooth

ao and shiny.

The electroplating operation was used as a control or reference test with the compositions of Examples 1 to 3, but without a content of nicotinic acid amide and / or ethoxylated a-naphtholsulfonic acid, and it was found that the films formed were coarse and the surface of the Plating does not become smooth and shiny.

Claims (1)

Claim: Acid galvanic zinc bath with a content of zinc, ammonium, sulfate and chloride ions, an ethoxylated organic compound and an organic acid, characterized in that that the bath is an aqueous solution of 200 to 650 g / l zinc sulfate, 50 to 200 g / l zinc chloride, 10 to 70 g / l ammonium chloride, 5 to 30 g / l sodium succinate and 0.1 to 10 g / l nicotinic acid amide and / or 0.01 to 0.1 g / l contains ethoxylated a-naphtholsulfonic acid.