Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/56—Electroplating: Baths therefor from solutions of alloys
  • C25D3/565—Electroplating: Baths therefor from solutions of alloys containing more than 50% by weight of zinc
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
  • C25D11/02—Anodisation
  • C25D11/04—Anodisation of aluminium or alloys based thereon
  • C25D11/06—Anodisation of aluminium or alloys based thereon characterised by the electrolytes used
  • C25D11/10—Anodisation of aluminium or alloys based thereon characterised by the electrolytes used containing organic acids

Definitions

• the invention concerns a method for depositing a zinc-nickel alloy from an electrolyte.
• Metal substrates are electroplated in order to improve corrosion protection. Because zinc-nickel alloys have better corrosion protection than pure zinc coatings there is strong interest in these alloys in many fields.
• the zinc-nickel alloys typically contain nickel from about 10% to about 16%, since this amount affords the best possible corrosion protection.
• To deposit the zinc-nickel alloys essentially two different electrolytes are used in the prior art. One consists of the alkaline zinc-nickel alloy baths and the other consists of the slightly acidic baths containing ammonium and chlorides. However, these baths have considerable disadvantages.
• the alkaline baths have the disadvantage that the deposition rates are low, and there are problems in depositing coatings directly from these baths onto cast iron or hardened steel. Another disadvantage is that these electrolytes contain high concentrations of highly environmentally stressful complex-forming substances, which result the problematic and expensive purification and disposal of highly contaminated wastewater.
• ammonium-containing slightly acid electrolytes have the considerable disadvantage that the ammonium ions also severely contaminate wastewater. Because of this, there can be problems in removing metals from the wastewater. Because ammonium concentration in wastewater is often subject to strict government regulations, expensive wastewater treatments have are required. Strong ammonium-containing baths, however, are particularly useful for the processing of rack goods and mass-produced articles and are needed to achieve the deposition of a zinc-nickel alloy on profiled workpieces.
• the present invention is directed to a process for deposition of a zinc-nickel alloy onto a substrate in which an electrolyte, which is characterized by a first current density range within which a deposited coating of the alloy is substantially continuous, glossy, and provides corrosion protection, is prepared comprising a source of Zn ions and a source of Ni ions.
• a current density broadening agent is incorporated into the electrolyte to modify the electrolyte such that it is characterized by a second current density range within which a deposited coating of said alloy is substantially continuous, glossy, and provides corrosion protection and which is broader than the first current density range.
• the current density broadening agent is selected from the group consisting of an aromatic carboxylic acid, an aromatic carboxylic acid salt, an aromatic carboxylic acid derivative, an aliphatic carboxylic acid, an aliphatic carboxylic acid salt, an aliphatic carboxylic acid derivative, and combinations thereof.
• An external source of electrons is supplied to the electrolyte to electrolytically deposit the zinc-nickel alloy onto the substrate.
• the present invention is further directed to a process for deposition of a zinc-nickel alloy onto a substrate in which an electrolyte is prepared.
• the electrolyte comprises a source of Zn ions, a source of Ni ions, and an additive selected from the group consisting of an aromatic carboxylic acid, an aromatic carboxylic acid salt, an aromatic carboxylic acid derivative, an aminocarboxylic acid, an aminocarboxylic acid salt, an aminocarboxylic acid derivative, an hydroxy(poly)carboxylic acid, an hydroxy(poly)carboxylic acid salt, an hydroxy(poly)carboxylic acid derivative, and combinations thereof.
• An external source of electrons is supplied to the electrolyte to electrolytically deposit the zinc-nickel alloy onto the substrate.
• the present invention is still further directed to an electrolyte for electrolytic deposition of a Zn-Ni alloy which comprises a source of Zn ions, a source of Ni ions, and a current density broadening agent selected from the group consisting of an aromatic carboxylic acid, an aromatic carboxylic acid salt, an aromatic carboxylic acid derivative, an aliphatic carboxylic acid, an aliphatic carboxylic acid salt, an aliphatic carboxylic acid derivative, and combinations thereof.
• a current density broadening agent selected from the group consisting of an aromatic carboxylic acid, an aromatic carboxylic acid salt, an aromatic carboxylic acid derivative, an aliphatic carboxylic acid, an aliphatic carboxylic acid salt, an aliphatic carboxylic acid derivative, and combinations thereof.
• the invention is based on the surprising effect that the addition to a bath of a current density broadening agent results in the ability to deposit zinc-nickel alloys from a zinc-nickel electrolyte over a broad current range.
• the current density broadening agent comprises an aliphatic carboxylic acid, its salts and/or its derivatives or at least one additional aromatic and/or aliphatic carboxylic acid, its salts and/or its derivatives.
• the aromatic carboxylic acids or their derivatives are added to the electrolyte preferably as sodium and/or potassium salt.
• the aromatic and/or aliphatic carboxylic acid, its salt and/or its derivative in this case is preferably chosen so that a shift of the usable current density range into higher and/or lower ranges is possible in a targeted fashion.
• the relevant aromatic and/or aliphatic carboxylic acid, its salts and/or its derivatives of shifting the usable current density range so that the deposition of an alloy coating that satisfies requirements is possible both in higher and also in lower current density ranges.
• the standard bath for deposition of zinc-nickel alloys typically comprises divalent zinc, divalent nickel, chloride, boric acid, surfactants, and brighteners.
• the surfactants may be anionic or nonionic surface-active substances, similar to those that are used in a conventional acid zinc electrolyte. Ethoxylated alcohols or nonylphenols are examples of useful surfactants. Benzal acetone is useful added as a brightener additive. Aldehydes and ketones can also be added as brighteners, similar to those that are used in a conventional acid zinc electrolyte.
• Some baths may contain a carboxylic acid component, such as for example sodium acetate as a pH buffer.
• the process and electrolyte compositions according to the present invention comprise at least one additional aromatic and/or aliphatic carboxylic acid in the case that a carboxylic acid is otherwise present in the bath a purpose which is different than the objects of the present invention.
• the alloys deposited in accordance with the invention preferably have a nickel content of about 10% to about 16%. However, it is understood alloys of greater or lesser nickel content may be deposited if desired.
• the invention advantageously allows one to operate with an ammonium-free electrolyte so that the above disadvantages of wastewater contamination and related costs are avoided.
• the advantages of the method in accordance with the invention therefore show up in particular in the deposition of a zinc-nickel alloy from a preferably ammonium-free bath which comprises a slightly acidic and chloride-containing zinc-nickel electrolyte.
• nicotinic acid, its salts and/or derivatives are added to the electrolyte as an aromatic carboxylic acid. It has been discovered that the addition of nicotinic acid, its salts, or derivatives improves the layer properties of the deposited coating, with some properties being considerably improved in the ranges of greater current densities. For example, the addition of nicotinic acid to baths operated in the range of greater current densities surprisingly led to reduction or even complete prevention of the cracking and flaking of the layer that occurs without this addition. Also, scorching phenomena are diminished.
• the nicotinic acid or its salt and/or its derivatives are added to the electrolyte in an amount of at least about 0.25 g/L. More preferably, the nicotinic acid or its salt and/or its derivatives are added to the electrolyte in an amount of from about 0.25 g/L to about 1 g/L. Still more preferably, the amount is about 0.75 g/L.
• salicylic acid, its salts and/or its derivatives are added to the electrolyte as aromatic carboxylic acid.
• Acetylsalicylic acid for example, can be used as the salicylic acid derivative.
• the zinc-nickel alloy deposited in this way is, furthermore, detectably more uniform over the entire current density range.
• variations, for example, in the alloy composition essentially do not occur, which has an advantageous effect on the quality of the deposited coating.
• the salicylic acid, its salts and/or its derivatives are added to the electrolyte in an amount from about 0.5 to about 1.5 g/L. It proved to be particularly advantageous to add about 1 g/L salicylic acid, its salts and/or its derivatives to the electrolyte.
• both nicotinic acid and salicylic acid, their salts and/or their derivatives are added to the zinc-nickel electrolyte.
• the addition of both these aromatic carboxylic acids or their salts and/or their derivatives results in it being possible to operate over a particularly broad current density range, since the layer properties are improved both in the range of lesser and greater current densities by the addition of both substances.
• the overall effect due to the addition of both substances is better than the relevant separate effects.
• the nicotinic acid and the salicylic acid are preferably added in an amount of about 0.75 g/L and 1 g/L, respectively.
• the zinc-nickel electrolyte in this case is preferably a slightly acid, ammonium-free, chloride containing electrolyte.
• At least one aliphatic carboxylic acid, its salts and/or its derivatives are added to the electrolyte in addition to or instead of the salts of carboxylic acid that are typically already used in practice, for example sodium acetate.
• carboxylic acid typically already used in practice, for example sodium acetate.
• the use of other different aliphatic carboxylic acids, their salts and/or their derivatives leads to particularly intensive avoidance of the formation of basic nickel compounds and to improved pH correction and thus to a considerable improvement of the deposition of a layer in ranges of lesser current densities.
• the zinc-nickel alloys thus deposited are glossy and detectably more uniform over the entire current density range.
• aminocarboxylic acids, their salts and/or derivatives and/or hydroxy(poly)carboxylic acids, their salts and/or derivatives are added to the electrolyte as aliphatic carboxylic acids.
• Aminoacetic acid is a preferred aminocarboxylic acid
• 2-hydroxypropanoic acid is a preferred hydroxy(poly)carboxylic acid.
• the addition of aminoacetic acid or 2-hydroxypropanoic acid, their salts and derivatives considerably improves the layer properties of the deposited coatings, particularly for rack goods. This effect can be enhanced, particularly in baths for electroplating rack goods, by agitation of the baths, for example by moderate flooding.
• the deposition of zinc-nickel coatings can be performed over a broader range of current density. It is to be noted that the process in accordance with the invention may be performed temperatures higher than 35° C., as may be required by specific processes, without disadvantages resulting from this. For example, in barrel plating very good results can be achieved at temperatures of about 40° C.
• An ammonium-free, slightly acidic, chloride-containing zinc-nickel electrolyte is prepared according to the following composition: Standard Bath Zinc chloride 115 g/L Nickel chloride (6 H 2 O) 143 g/L Potassium chloride 245 g/L Boric acid 20 g/L Sodium acetate (3 H 2 O) 65 g/L Surfactants 6 g/L Sodium saccharate 1.5 g/L Benzylideneacetone (dissolved in ethyl diglycol) 75 mg/L
• the pH is between 5.3 and 5.7.
• the surfactants can be anionic or nonionic surface-active substances, similar to those that are used in a conventional acid zinc electrolyte. Ethoxylated alcohols or nonylphenols are usually used. Benzal acetone is added as a brightener additive. Aldehydes and ketones can also be added as brighteners, similar to those that are used in a conventional acid zinc electrolyte. For example, besides benzylidene acetone it is also possible to use sodium benzoate, etc. Zinc-nickel alloys that have a nickel content from about 12% to about 16% can be deposited onto a workpiece with this bath.
• Such a bath has the disadvantage that one can operate only in a very limited current density range. These limitations are clearly detectable in a Hull cell test (2 A, 20 min).
• the Hull cell test systematically covers all of the current density ranges that arise in the electroplating of highly profiled workpieces.
• the particular shape of the test cell allows evaluation of the galvanic coating even in a relatively large current density range.
• a specific current density is established at the cathode plate, so that even the very first experiment gives insight into the mode of operation of the electrolyte in the various current density ranges.
• Hull cell test the current density ranges over which an electrolyte can be used without any problem.
• the electrolytes that are composed in accordance with the embodiment examples were tested with a slightly modified Hull cell in order to estimate the current density range over which the electrolytes operate flawlessly.
• the Hull cell that was used was set up for a volume of 500 mL.
• the cathode length was 200 mm.
• the alloy coating has a completely different composition and contains up to 50% nickel instead of 10-16%.
• the standard electrolyte operates only in a very limited current density range. Only narrow current density ranges from about 1.5 to 4.6 A/dm 2 are seen as usable. Because of these restrictions in the usable current density ranges, only workpieces with simple geometry can be coated, which greatly restricts the application range. Presumably, barrel plating is also not possible because of these restrictions.
• the addition of 0.25 g/L nicotinic acid already noticeably improves the deposition in the current density ranges. For instance, the layer has a lesser tendency to flake off and to form cracks. Through the addition of 0.75 g/L nicotinic acid this effect is even more enhanced, as the table shows. Through the addition of nicotinic acid the usable current density range in the HCD range becomes considerably broader.
• both nicotinic acid and salicylic acid results in it being possible to operate over a particularly broad current density range, since the layer properties of the deposited layer are considerably improved both in the low and in the high current density ranges through the addition of both substances.
• the overall effect due to the addition of both substances is better than the relevant separate effects.
• zinc-nickel layers with good quality properties can be deposited in the current density ranges from under 0.2 up to over 6 A/dm 2 due to the addition of 1 g/L nicotinic acid and 1 g/L salicylic acid. In this bath, compared to the standard bath, the electrolyte operates flawlessly over very broad current density ranges.
• the said zinc-nickel electrolytes can be further developed in an advantageous way.
• the zinc-nickel electrolytes listed below as example Baths A and B proved to be particularly advantageous further developments of a weakly acid zinc alloy electrolyte for the rack plating technology because of the very uniform alloy composition over the entire current density range.
• the temperature of the electrolyte in deposition is under 35° C., since in this way the adhesion properties of the layer are improved. This leads to the layers not flaking off even after some time, which increases the quality of the zinc-nickel alloy layers deposited from the electrolyte in accordance with the invention by the method in accordance with the invention.

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• 2001
  • 2001-09-21 DE DE10146559A patent/DE10146559A1/de not_active Withdrawn
• 2002
  • 2002-09-19 EP EP02020926A patent/EP1295967A3/de not_active Withdrawn
  • 2002-09-19 KR KR1020020057221A patent/KR100556604B1/ko not_active Expired - Lifetime
  • 2002-09-20 CN CNB021424853A patent/CN1291068C/zh not_active Expired - Lifetime
  • 2002-09-20 JP JP2002274993A patent/JP4307810B2/ja not_active Expired - Lifetime
  • 2002-09-23 US US10/252,495 patent/US20030085130A1/en not_active Abandoned

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