Patent Application
20060096868
This invention details a process, which can be used to electroplate a nickel deposit directly onto zinc or zinc alloy die-castings, thereby eliminating the need to use traditional cyanide based plating baths.
Zinc parts are often produced as zinc based die-castings. It is common for these to be plated with other metals to improve cosmetic appearance and improve corrosion resistance. This coating generally consists of one or more of the following: copper, nickel, chromium, tin and brass. Traditionally these articles are base plated with a thin layer of monovalent copper from a copper cyanide bath. Because of the toxicity of cyanide and it's impact on the environment and hence, expense of treatment and disposal—it is desirable that an alternative plating solution be found.
There are a number of cyanide-free copper electroplating processes detailed in the literature. For example, U.S. Pat. No. 4,521,282 to Tremmel discloses a divalent copper system which utilises organo-phosphate chelating agents. Similarly phosphonic acid based cupric baths have also been disclosed by Tomaszewski et al in U.S. Pat. No. 4,469,569 and Szotek in U.S. Pat. No. 6,054,037 which also details a divalent copper bath with phosphonic acid complexants and copper acetate and halogen based salts. Other patents describe similar processes including U.S. Pat. Nos. 4,904,354; 4,469,569; 5,266,212, 5,006,262 and 4,462,872.
While these electrolytes provide a satisfactory deposition of copper (particularly on steel substrates)—it has been found that these processes can be deficient in terms of adhesion as each is based on divalent copper complexes which will exhibit immersion plating by displacement at least to some extent. This compromises the integrity of the adhesion between the copper and the zinc substrate and is therefore an unsuitable base for subsequent plated layers. These electrolytes have also been found to be deficient from a commercial standpoint due to the wide disparity between anode and cathode efficiency.
Nickel electrolytes can be used to plate a layer directly onto a zinc or zinc alloy substrate, however this is not practised commercially as these solutions exhibit poor coverage in areas of low current density. Also, nickel baths are generally run at low pH which is an unsuitable medium for plating zinc based die-castings since the acidic nature of the electrolyte destroys the article before it can be plated, leading to blistering and poor adhesion of subsequent plated layers. However, the inventors have formulated an additive system which overcomes these difficulties. When added to the nickel salt plating bath, this additive system allows complete coverage plating over a full range of current densities. The additive system is applied to a near neutral pH bath which is suitable for the application of a plated nickel surface directly onto zinc and zinc alloys.
This invention describes the use of a nickel plating bath and method which provide for plating an adherent base layer on zinc and in particular zinc based die-castings. This method comprises the following steps:
This invention describes a method of treating zinc articles to produce an adherent base coating suitable for the plating of subsequent metallic layers. The process of the invention generally includes the steps of:
The cleaning and activating step is preferred to provide a surface of the article that is suitable for plating. Defects such as lack of adhesion, porosity, roughness, dark spots and non-uniform coatings are likely to occur on poorly prepared parts. The surface preparation process also serves to activate the surface of the part so that it is optimally receptive to the deposition of the metal coating.
The zinc die-cast articles are first cleaned in a standard alkaline cleaning solution. The articles are then activated by a short immersion dip in an acid solution. Thorough rinsing is required between cleaning stages and prior to plating on the surface of the zinc article.
The main source of nickel in the present invention are nickel salts. Of particular interest are nickel chloride and nickel sulphate. The concentration of nickel salt in the aqueous solution is generally between about 150 and about 300 grams per liter.
The chloride present in the solution may come from nickel salt or alkali metal salt. The concentration of chloride salt in aqueous solution should be sufficient to cause effective dissolution of the nickel anodes in the bath.
In addition to the nickel salt and the chloride salt, the solution contains a quantity of buffer material in the form of one or more of the following: boric acid, mono-, di- and tri-carboxylic acids such as, but not limited to, acetic acid, malic acid, succinic acid, citric acid or suitable salts thereof. These are present in the aqueous solution at a concentration of about 5 to about 70 grams per liter.
The nickel plating solution is optimally maintained at a temperature between room temperature and about 65 degrees Celsius. The articles are generally immersed in the solution for a minimum time of one minute, at a current density of 1.5 to 8.0 amps per square decimeter.
In addition to the salt additives required, an additive system is added. The additive system is comprised of one or more of the following: a quantity of sulphonic acid or alkali metal salt of a sulphonic acid preferably at a level between about 0.2 and 2 grams per liter; a sulfonated alkoxylate to act as a brightener, preferably added at a level of about 0.1 to about 1.5 grams per liter; a glycerol compound preferably added at a level of about 0.4 grams to about 4 grams per liter, and a quantity of organic acid of one or more of the following: tolylacetic acid, salicylic acid, hydroxybenzoic acid and/or benzyloxyacetone, preferably added at a level of 0 to about 25 grams per liter. The additive system also preferably comprises an ionic surfactant.
The sulphonic acid or sulphonic acid salt are preferably naphthalene sulphonic acids or salts thereof, such as 4-acetomido-5-hydroxy-2,7-naphthalene-disulphonic acid-disodium salt. The sulphonated alkoxylate is preferably selected from the group consisting of alkoxylated bis-phenols, and sulphonated alkoxylates, such as 2-ethylhexanol ethoxylated sulfopropylate. The glycerol or glycerol derivative is preferably selected from the group consisting of alkoxylated glycols, polyols and polyoxy alkoxylated glycols such as Macol® ETG 3590, available from the Chemax Company, and is a polyoxyethylated glycerol deriviative. Lastly, the aromatic carboxylic acid is preferably selected from the group consisting of tolylacetic acid, salicylic acid, benzoic acid, hydroxy benzoic acid, and benzyloxyacetone.
After the nickel plating process is completed, the articles are again rinsed. The resulting nickel coating is sufficiently noble and continuous as to allow subsequent plating of further metallic layers as required.
The bath runs at a pH of between 5 and 6. This is sufficiently alkaline as to prevent the corrosion and dissolution of the zinc on contact with the invention. The pH is maintained using nickel hydroxy carbonate paste and sulphuric acid. The bath requires filtration after the addition of the nickel hydroxy carbonate paste, which also serves as a secondary nickel source in the bath.
The nickel electroplated coating on the zinc article is carried out by standard electroplating techniques and also applicable for use in barrel plating techniques. Barrel plating is suitable for plating many small articles at one time. Parts are tumbled in a cascading motion inside a rotating vessel in the plating bath.
The process of the present invention forms a firmly adherent and uniform coating of nickel onto zinc articles, allowing subsequent metal layers to be plated onto it. The bath provides full coverage of the electroplated article and hence eliminates the need to use cyanide based copper electrolytes to base cover zinc based die cast parts.
The following non-limiting examples demonstrate the attributes of this invention.
In the following examples, a zinc-plated steel panel was plated in a hull cell containing 267 ml of nickel plating solution, prepared as stated. The panel was plated at 1 Amp for ten minutes. The thickness of the deposit was measured by x-ray fluorescence spectroscopy. Adherence was checked by heating the plated article to a temperature of 160° C. for one hour and then plunging it into cold water at a temperature of approximately 10° C. Lack of adhesion was evident when blistering, cracking and peeling of the deposit was observed on contact with the cold water.
