KR102194114B1 - Functional chromium layer with improved corrosion resistance - Google Patents

Abstract

The aqueous electroplating bath according to the invention comprises as catalysts chromium (VI) ions, sulfate ions and salts of methane-trisulfonic acid or methane-trisulfonic acid. The functional chromium layer deposited from the aqueous electroplating bath according to the invention has increased corrosion resistance.

Description

Functional chrome layer with improved corrosion resistance {FUNCTIONAL CHROMIUM LAYER WITH IMPROVED CORROSION RESISTANCE}

The present invention relates to plating bath compositions and a method for depositing functional chromium layers by electroplating.

Functional chromium layers deposited by electroplating are used to improve the abrasion and corrosion resistance of products such as shock absorbers, pneumatic pistons and the like.

The plating bath compositions used include chromic acid, sulfate ions, water and salts of alkyl-sulfonic acids or alkyl-sulfonic acids.

Alkyl-sulfonic acid catalysts with a molar ratio S:C ≥ 1:3 are disclosed in EP 0 196 053 B1. Examples of suitable alkyl-sulfonic acids are methyl-sulfonic acid, ethyl-sulfonic acid, propyl-sulfonic acid, methane-disulfonic acid and 1,2-ethane-disulfonic acid. The alkyl-sulfonic acids improve the cathode current efficiency during plating.

The use of alkyl-polysulfonic acids such as methane-disulfonic acid, halogenated alkyl-polysulfonic acids and corresponding salts to reduce corrosion of lead anodes during plating is disclosed in EP 0 452 471 B1.

Aromatic-trisulfonic acids as additives in plating bath compositions for depositing functional chromium layers are disclosed in US 2,195,409. The chromium layers obtained from such plating bath compositions are bright and uniform.

A plating bath composition for depositing a functional chromium layer with improved cathode current efficiency comprising propane-1,2,3-trisulfonic acid is disclosed in DE 43 05 732 A1.

It is an object of the present invention to provide a plating bath composition and a method of using the plating bath composition for depositing functional chromium layers having improved corrosion resistance.

For this purpose,

(i) a source for chromium (VI) ions,

(ii) a source for sulfate ions and

(iii) An aqueous electroplating bath for depositing a layer of functional chromium, comprising a salt of methane-trisulfonic acid or methane-trisulfonic acid.

For this purpose,

(i) providing a metallic substrate,

(ii) contacting the metallic substrate with an aqueous electroplating bath comprising a source for chromium (VI) ions, a source for sulfate ions and a salt of methane-trisulfonic acid or methane-trisulfonic acid, and

(iii) applying an external current to the metallic substrate as a cathode, thereby depositing a layer of functional chromium on the metallic substrate, in this order, as a method for depositing a layer of functional chromium on a metallic substrate It is solved further.

The functional chromium layers deposited from the aqueous plating bath and by the method according to the invention have increased corrosion resistance compared to the functional chromium layers deposited from conventional electroplating bath compositions comprising known alkyl-sulfonic acids. .

The aqueous electroplating bath according to the invention comprises a source for chromium (VI) ions, a source for sulfate ions, a salt of methane-trisulfonic acid or methane-trisulfonic acid, and optionally a surfactant.

The source for chromium(VI) ions is preferably a chromium(VI) compound soluble in a plating bath such as CrO ₃ , Na ₂ Cr ₂ O ₇ and K ₂ Cr ₂ O ₇ , most preferably CrO ₃ . The concentration of chromium (VI) ions in the electroplating bath according to the invention is preferably in the range of 80 to 600 g/l, more preferably 100 to 200 g/l.

The sulfate ions present in the electroplating bath are preferably added in the form of sulfuric acid or soluble sulfate salts such as Na ₂ SO ₄ . The concentration of sulfate ions in the electroplating bath is in the range of 1 to 15 g/l, more preferably 2 to 6 g/l.

The ratio of the concentration in wt% of chromic acid to the sulfate salt is preferably in the range of 25 to 200, more preferably 60 to 150.

The alkyl-sulfonic acid in the electroplating bath is methane-trisulfonic acid (HC(SO ₂ OH) ₃ ) or a mixture of methane-trisulfonic acid and one or more other alkyl-sulfonic acids. Other alkyl-sulfonic acids suitable in mixtures with methane-trisulfonic acid are methane-sulfonic acid, methane-disulfonic acid, ethane-sulfonic acid, 1,2-ethane-disulfonic acid, propyl sulfonic acid, 1,2-propane-disulfonic acid, 1 ,3-propane-disulfonic acid and 1,2,3-propane-trisulfonic acid. Corresponding salts such as the sodium, potassium and ammonium salts of the sulfonic acids mentioned above can also be used instead of mixtures with free alkyl-sulfonic acids or as mixtures with free alkyl-sulfonic acids.

The precursor of a methane-trisulfonic acid or a salt of methane-trisulfonic acid oxidized to a salt of methane-trisulfonic acid or methane-trisulfonic acid in the electroplating bath according to the present invention is a partial or single source of a salt of methane-trisulfonic acid or methane-trisulfonic acid. Can be used as

The concentration of the salt of methane-trisulfonic acid or methane-trisulfonic acid in the plating bath according to the present invention is preferably in the range of 2 to 80 mmol/l, more preferably 4 to 60 mmol/l.

When a mixture of other alkyl-sulfonic acids and methane-trisulfonic acid is used, the total concentration of methane-trisulfonic acid and other alkyl-sulfonic acids or salts mentioned above is preferably 4 to 160 mmol/l, more preferably 12 to It is in the range of 120 mmol/l.

A large number of micro-cracks are required inside the deposited functional chromium layer, since high corrosion resistance and desirable mechanical properties, for example reduced internal stress, are achieved. In contrast to macro-cracks in the functional chromium layer, micro-cracks do not diffuse to the surface of the underlying substrate and thus do not cause corrosion of the underlying substrate material, which is generally tough.

The components of methane-trisulfonic acid or salts of methane-trisulfonic acid or mixtures with other alkyl-sulfonic acid(s) are identified by an optical microscope after etching in an aqueous solution containing sodium hydroxide and K ₃ [Fe(CN) ₆ ]. Likewise a large number of desired micro-cracks in the range of 200 to 1000 micro-cracks per cm of the functional chromium layer surface, more preferably 450 to 750 micro-cracks are possible. The number of micro-cracks along the lines is counted, where the number of micro-cracks per cm is the formula; Micro-cracks per cm = (average number of cracks per line): calculated as (length when the line is cm).

The number and corrosion resistance of the micro-cracks are compared to the methane-disulfonic acid sodium salt or propane-1,2,3-trisulfonic acid sodium salt as a single alkyl-sulfonic acid as a catalyst compared to the salt of methane-trisulfonic acid or methane-trisulfonic acid. Is increased by This is illustrated in Examples 1 to 3.

Moreover, an increased number of desired micro-cracks are also obtained at higher current densities (Example 3), while the number of micro-cracks is higher current density in the case of known alkyl-sulfonic acids such as methane-disulfonic acid. Is reduced in the field (Example 1). Higher current density values are required during plating because this increases the plating speed.

The electroplating bath according to the present invention optionally further comprises a surfactant that reduces the formation of unwanted foams on top of the plating liquid. Surfactant additives include perfluorinated sulfonate tensides, perfluorinated phosphate tensides, perfluorinated phosphonate tensides, partially fluorinated sulfonate tensides, partially fluorinated phosphate tensides. Sides, partially fluorinated phosphonate tensides and mixtures thereof.

The concentration of the optional surfactant is preferably in the range of 0.05 to 4 g/l, more preferably 0.1 to 2.5 g/l.

The current density applied during plating is preferably in the range of 10 to 250 A/dm 2, more preferably 40 to 200 A/dm 2. The substrate to be plated with the functional chromium layer serves as a cathode during electroplating.

Cathode current efficiency is the percentage of current actually used for the deposition of metal (chrome) at the cathode during electroplating of the functional chromium layer.

The preferred current efficiency of the method according to the invention is at least 22% at a current density of 50 A/dm 2.

The temperature of the electroplating bath according to the invention is preferably maintained during plating in the range of 10 to 80° C., more preferably in the range of 45 to 70° C. and most preferably in the range of 50 to 60° C.

Insoluble anodes are preferably applied in the method according to the invention.

Suitable insoluble anodes are made of, for example, titanium or titanium alloy coated with one or more platinum group metals, their alloys and/or their oxides. The coating preferably consists of platinum metal, iridium oxide or mixtures thereof. Such insoluble anodes enable higher current densities during electroplating and thus a higher plating rate compared to lead anodes.

The plating bath according to the invention can also be operated with conventional lead anodes.

Chromium(lll) ions are formed when using such insoluble anodes. Salts of methane-trisulfonic acid and/or methane-trisulfonic acid as alkyl-sulfonic acids in chromium (VI) ions based on functional chromium electroplating baths are very sensitive to chromium (III) ions.

In a preferred embodiment of the present invention, cations of additional metals such as silver ions, lead ions and mixtures thereof are added to the electroplating bath. Thereby, the negative influence of chromium (lll) ions can be minimized. The concentration of the ions of the additional metal is preferably in the range of 0.005 to 5 g/l, more preferably 0.01 to 3 g/l.

The present invention provides a method for depositing a functional chromium layer on a functional chromium electroplating bath and also on a substrate with increased corrosion resistance obtained at high current densities.

Examples

The invention will now be illustrated with reference to the following non-limiting examples.

The number of micro-cracks was determined by an optical microscope after etching the surface of the chromium layer in an aqueous solution containing sodium hydroxide and K ₃ [Fe(CN) ₆ ]. The number of micro-cracks along several lines of the same length is determined, from which the average number of micro-cracks is calculated to give the "average number of micro-cracks" in cracks/cm, with the line length given in cm. Is divided.

The corrosion resistance of the functional chromium layers was determined according to ISO 9227 NSS (neutral salt spray test).

An aqueous electroplating bath stock solution comprising 250 g/l of CrO ₃ , 3.2 g/l of sulfate ions and 2 ml/1 of surfactant was used throughout Examples 1 to 3. Different amounts of alkyl-sulfonic acids were added to this stock solution prior to depositing the functional chromium layers.

Example 1 (comparative)

The alkyl-sulfonic acid was the disodium salt of methane-disulfonic acid added to the stock solution at a concentration of 2 to 12 g/l (7.6 to 45.4 mmol/l). Such alkyl-sulfonic acids are disclosed in EP 0 452 471 B1.

Table 1 summarizes the average number of micro-cracks determined at different concentrations of methane-disulfonic acid disodium salt as a single alkyl-sulfonic acid (plating bath temperature: 58° C., current density: 50 A/dm 2 ).

A large number of desired micro-cracks are obtained only when a narrow concentration range of methane-disulfonic acid disodium salt as catalyst is used in the stock solution.

Table 2 summarizes the average number of micro-cracks determined at different current densities for an electroplating bath composition with 18.9 mmol/l (5 g/l) of methane-disulfonic acid disodium salt as a single alkyl-sulfonic acid. .

The number of desired micro-cracks is decreasing with increased current density.

The formation of unwanted red rust was discriminated after a 192 h salt spray experiment according to ISO 9227 NSS (>0.1% of the surface area covered with red rust after 192 h).

Example 2 (comparative)

Alkyl-sulfonic acid was the propane-1,2,3-trisulfonic acid trisodium salt added to the stock solution at a concentration of 14.3 mmol/l (5 g/l). Such alkyl-disulfonic acids are disclosed in DE 43 05 732 A1.

The plating bath temperature of 55° C. and the current efficiency at 50 A/dm 2 is 17.4% and the number of micro-cracks in the chromium layer deposited under these conditions is 160 cracks/cm.

The formation of unwanted red rust was already identified after 24 h of salt spray experiments according to ISO 9227 NSS (>0.1% of the surface area covered with red rust after 24 h).

Example 3 (invention)

The alkyl-sulfonic acid was the methane-trisulfonic acid trisodium salt added to the stock solution at concentrations of 6.2 to 37.2 mmol/l (2 to 12 g/l).

Table 3 summarizes the average number of micro-cracks determined at different concentrations of methane-trisulfonic acid trisodium salt as a single alkyl-sulfonic acid (plating bath temperature: 58° C., current density: 50 A/dm 2 ).

Table 4 summarizes the average number of micro-cracks determined at different current densities for electroplating bath compositions with 24.8 mmol/l (8 g/l) of methane-trisulfonic acid trisodium salt as a single alkyl-sulfonic acid. .

A large number of desired micro-cracks were obtained over the entire applied current density range.

The formation of unwanted red rust was not discriminated until a 552 h salt spray experiment according to ISO 9227 NSS.

Claims (15)

As an aqueous electroplating bath for depositing a layer of functional chromium,
(i) a source for chromium (VI) ions,
(ii) a source for sulfate ions and
(iii) methane-trisulfonic acid or a salt of methane-trisulfonic acid,
The concentration of the salt of methane-trisulfonic acid or methane-trisulfonic acid is in the range of 6.2 to 37.2 mmol/l,
The aqueous electroplating bath for depositing a functional chromium layer further comprising cations of an additional metal selected from the group consisting of silver, lead and mixtures thereof. The method of claim 1,
An aqueous electroplating bath for depositing a functional chromium layer, wherein the concentration of the chromium (VI) ions is in the range of 80 to 600 g/l. The method of claim 1,
An aqueous electroplating bath for depositing a layer of functional chromium, wherein the concentration of the sulfate ions is in the range of 1 to 15 g/l. The method of claim 1,
The aqueous electroplating bath for depositing a layer of functional chromium, wherein the salt of methane-trisulfonic acid is selected from sodium, potassium and ammonium salts. delete The method of claim 1,
Methane-trisulfonic acid and
Methane-sulfonic acid, methane-disulfonic acid, ethane-sulfonic acid, 1,2-ethane-disulfonic acid, propyl sulfonic acid, 1,2-propane-disulfonic acid, 1,3-propane-disulfonic acid and 1,2,3-propane An aqueous electroplating bath for depositing a functional chromium layer comprising one or more other alkyl-sulfonic acids selected from the group comprising trisulfonic acids. The method of claim 1,
The aqueous electroplating bath includes perfluorinated sulfonate tensides, perfluorinated phosphate tensides, perfluorinated phosphonate tensides, partially fluorinated sulfonate tensides, partially fluorinated An aqueous electroplating bath for depositing a functional chromium layer further comprising a surfactant selected from the group consisting of phosphate tensides, partially fluorinated phosphonate tensides and mixtures thereof. The method of claim 7,
An aqueous electroplating bath for depositing a functional chromium layer, wherein the concentration of the surfactant is in the range of 0.05 to 4 g/l. A method for depositing a layer of functional chromium on a metallic substrate, comprising:
(i) providing a metallic substrate,
(ii) contacting the aqueous electroplating bath according to any one of claims 1 to 4 and 6 to 8 and the metallic substrate, and
(iii) applying an external current to the metallic substrate as a cathode, thereby depositing a layer of functional chromium on the metallic substrate, in this order. The method of claim 9,
The method for depositing a layer of functional chromium on a metallic substrate, wherein the aqueous electroplating bath is maintained at a temperature in the range of 10 to 80° C. during use. The method of claim 9,
A method for depositing a layer of functional chromium on a metallic substrate, wherein a current density in the range of 10 to 250 A/dm 2 is applied to the metallic substrate during use. The method of claim 9,
A method for depositing a layer of functional chromium on a metallic substrate, wherein an insoluble anode is used in step (iii) above. The method of claim 12,
A method for depositing a layer of functional chromium on a metallic substrate, wherein the insoluble anode has a surface selected from the group consisting of platinum metal, iridium oxide and mixtures thereof. delete The method of claim 9,
A method for depositing a layer of functional chromium on a metallic substrate, wherein the concentration of cations of the additional metal is in the range of 0.005 to 5 g/l.