CS254435B1 - Additive to weakly acidic zinc baths, which significantly reduces the internal stress of the deposited coatings - Google Patents

Abstract

The additive consists of an equimolar mixture of aliphatic derivatives of thiourea of the general formula 1, where Rj represents H-, n-alkyl, where η = 1 to 8, with a straight or branched chain, and derivatives of N,N'-di-alkynyl-thiourea of the general formula 2, where R2 represents aliphatic alkynyls containing 2 to 9 carbons, with a straight or branched chain. It is dosed into weakly acidic galvanic baths for bright zinc plating, where the parameters of the coatings are influenced by the content of aromatic aldehydes and ketones. It suppresses the increase in internal stress of the coatings when decomposition products accumulate during operation of the bath. It is not degraded by the passage of electric current, only the yield decreases from the bath. Its concentration is maintained in the range of 0.0001 to 0.05 g per 1000 ml of bath.

Description

The invention relates to an additive for weakly acidic galvanic baths for bright zinc plating containing ammonium salts, i.e. based on zinc chloride or sulfate and ammonium chloride or acetate, or both, or without ammonium salts, i.e. based on zinc chloride or sulfate, sodium or potassium chloride and boric acid in various ratios, in which gloss-forming additives containing aromatic aldehydes and ketones participate in the formation of the parameters of the deposited zinc coatings.

Gloss-forming additives used in galvanic zinc plating baths contain a number of inorganic and organic compounds that play a decisive role in the gloss formation, leveling ability, depth efficiency and covering ability of the baths, the value of internal stress, ductility, etc. However, during operation, there is a gradual accumulation of decomposition products of these substances, which arise as a result of their sorption onto the surface of the coatings and subsequent decomposition by electric current from more strongly exposed areas of the crystal lattice of the deposit. These decomposition products significantly affect the parameters of the deposited coatings.

Their effect on individual parameters is not the same, but it is always a greater or lesser deterioration. From the point of view of industrial practice, a significant increase in the internal stress of coatings is very unpleasant. Increased tensile stress is manifested by spontaneous cracking of coatings, mostly on the edges and bends of metallized objects. The result is significantly lower corrosion resistance than would correspond to the strength of the deposited coating. Increased compressive stress is the cause of blisters, which appear with greater frequency, the less intensive the surface pretreatment, i.e. rust removal, degreasing and pickling. Therefore, eliminating the influence of decomposition products on the value of internal stress is of extraordinary importance for operational practice.

The described disadvantages of previously used gloss-forming additives are eliminated by the additive according to the invention, the essence of which lies in the fact that the additive consists of an equimolar mixture of aliphatic thiourea derivatives of the general formula

R, - NH - CS - NH,, where Rj means H-, CH ₃ (CH,) _n -, where n = 0 to 7, with straight and branched chain, and derivatives of N, N'-di-alkynyl-thiourea of the general formula

R, - NH - CS - NH - R,, where R, represents aliphatic alkynyls containing 2 to 9 carbons, with both straight and branched chains.

This mixture of thiourea derivatives is dosed into the galvanic zinc plating bath in an amount of 0.0001 to 0.05 grams per 1000 ml. The additive has a significant effect on increasing the value of the internal stress of the coatings depending on the increase in the concentration of decomposition products during operation when using gloss-forming additives containing aromatic aldehydes and ketones. Its value increases three to five times slower than in baths without the additive according to the invention. Even in cases where the accumulation of decomposition products is so high that there is a significant decrease in the appearance of the coatings, typical defects resulting from increased internal stress do not occur. The additive is not degraded by the passage of electric current, only the yield decreases from the bath. Therefore, it is suitable to incorporate it directly into gloss-forming additives that are formulated to cover the yield of other components of the galvanic bath.

A detailed explanation is given in the following examples.

Example 1

A galvanic zinc plating bath with a composition of 70 g zinc chloride, 30 g boric acid, 180 g potassium chloride, 3 g potassium 4-methoxybenzoate, 1.0 g nonionic surfactant (7 ethoxyl groups per molecule), 3 g ethylene and propylene oxide copolymer m.h. 2300, 0.1 g methyl naphthyl ketone, 0.15 g benzylidene acetone and 0.03 g vanillin per 1000 ml was loaded with electric current during the plating of test sheets. After each passage of 30 A.h per 1000 ml, the internal stress value was measured using a dilatometric probe 1S meter according to ČSN 03 8162.

The same measurement was performed in a bath prepared in the same way, to which 0.002 g of an equimolar mixture of thiourea and N, N’-di-(3-methyl-1-butynyl)-thiourea was added. The following results were obtained by measuring the internal stress:

Bath without additives

expired charge/A.h/1/ 0 30 60 90 120 150 internal stress /MPa/ 8 41 59 77 92 104 Bath with an ingredient expired charge /A.h/1/ 0 30 60 90 120 150 internal stress /MPa/ 11 L 22 24 25 24 26 Example 2 Ready for the spa how in example 1 was

0.01 g of an equimolar mixture of isopropylthiourea and N, N'-di-(4-methyl-2-pentynyl)thiourea was added. The internal stress during plating of test sheets as in Example 1 was measured and the following results were obtained:

254,435

Bath with additive expired charge /A.h/1/ internal stress /MPa/

30 60 90 120 150 9 19 23 26 30. 32

From the results given in Examples 1 and 2, a significant reduction in the internal stress value of galvanically deposited zinc coatings upon application of the additive according to the invention is clearly evident.

The invention can be used in galvanic zinc plating of products in the field of engineering and electrical engineering companies and production cooperatives.

Claims (1)

SUBJECT OF THE INVENTION Addition to weakly acidic zinc baths, which substantially reduces the internal stresses of the deposited coatings, applicable to baths based on zinc chloride or zinc sulphate and ammonium chloride or acetate, or both or without ammonium salts, ie zinc chloride or zinc sulphate, sodium chloride or potassium and boric acid in various proportions and containing brightening agents based on aromatic aldehydes and ketones, characterized in that it consists of an equimolar mixture of aliphatic thiourea derivatives of the general formula - NH - CS - NH ₂ , where R 1 is H-, CH ₃ (CH ₇ ) _n -, where n = 0 to 7, both straight and branched, and N, N'-di-alkynyl-thiourea derivatives of general formula R ₂ - NH - CS - NH - R 1, wherein R 1 represents aliphatic alkynyls containing 2 to 9 carbons, both straight and branched chain.