Electroplated Articles

Motor vehicles include a substantial number of chrome-plated parts for both decorative and functional pvuposes. The overall appearance of the vehicle is significantly enhanced by these highly reflective chrome surfaces. 

However, trim elements also serve a functional purpose in that they help to absorb impact when the vehicle is involved in a collision and when the vehicle contacts flying gravel, road debris, roadway abutments or the Uke. Accordingly, plated metal on a trim element preferably must withstand impact without chipping, cracking, or delaminating. 

Conventionally, automotive trim elements have been manufactured from metals. From economic aspects, plateable plastics are an interesting alternative, because they reduce the vehicle weight and have a much greater design flexibility than metals. When parts are formed from plastic materials, a significant cost savings can be realized in comparison to metal parts. 

A wide variety of plated plastics are known, such as acrylonitrile-butadiene-styrene (ABS) and polyfcarbonate) (PC). ABS has been plated to provide decorative articles such as headlamp surrounds, and plumbing and marine hardware. PC has been utilized as the substrate for plated motor vehicle door handles. PPA resins are interesting for high-temperature applications. 

Articles formed from mineral-filled PPA resins can be metallized by electroless and electroplating techniques. Special formulations of PPA 

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Satisfactory service of an electroplated article is not achieved, however, unless adequate care is given to the choice of deposited metal, its thickness, the technique of application, and the design of the article. The choice of metal deposit is primarily determined by the basis metal, i.e. the metal from which the article is made, and the actual conditions to which the plated article will be subjected during service. In addition, however, attractive appearance and reasonable cost are also important considerations. 

Electroplating chemicals n. Copper, gold, silver, chromium, and nickel are generally used as the conductive metal for plating. Acrylonitrile-butadiene-styrene resins have been most widely used for electroplated articles. Others in commercial use for the process include cellulose acetate, some grades of polypropylene, polysulfones, polycarbonate, polyphenylene oxide, nylons, and rigid PVC. 

Fi s. IM4 Jig rack mourning of electroplated articles, (a) A jig for nickel-plating the interiors and exteriors of kettle bodies. Note the use of platinized titanium auxiliary anodes inside the kettle to provide improved current drsiribution. (Courtesy W, Canning Materials Ltd.) (b) A transported-scrviccd electroplating system, which may be semi- or fully automatic in operation and which utilizes rack- or jig-mounting of components. (Courteiy Electroloid Ltd.) ... 

As already mentioned, electroless coatings arc harder than conventional electroplated articles and as a result the wear resistance is increased. This property restricts the ductility of the deposit, with elongation values of 1-3% being found for electroless nickel. The residual stress is far higher than that of a selphamate nickel electroplate and is generally tensile, which can be a serious hazard in certain industrial situations where crack propagation has to be avoided at all costs. 

The ideal electroless solution deposits metal only on an immersed article, never as a film on the sides of the tank or as a fine powder. Room temperature electroless nickel baths closely approach this ideal electroless copper plating is beginning to approach this stabiHty when carefully controUed. Any metal that can be electroplated can theoretically also be deposited by electroless plating. Only a few metals, ie, nickel, copper, gold, palladium, and silver, are used on any significant commercial scale. 

The use of electrodeposited metals to protect corrodible basis metals from their service environments has been well established for many years and accounts for by far the larger part of the activities of the plating industry. There are many reasons for using an electroplated metal finish in preference to an organic finish or to making the articles concerned from inherently corrosion-resistant materials. 

Some of the reasons are aesthetic, and on many larger articles, such as motorcars, an attractive appearance is achieved by a careful visual balance between parts which are finished and protected by electroplating, and parts which are protected by organic finishes. In many other instances the manufacture of parts from corrosion-resistant materials is ruled out because of the relatively high cost of such materials, and in some cases the physical properties of the appropriate corrosion-resistant materials may render them either unsuitable for economic production or unable to perform the function for which the article is required. 

Articles of complex shape may be impossible to electroplate satisfactorily, and electroless nickel (see Section 12.5) may be useful in providing a relatively uniform protective coating. Even so, the considerations of access of cleaning and process solutions, and retention/draining of all process solutions, still apply. 

Reference may be made to BS 4479 Recommendations for the Design of Metal Articles that are to be Coated, Section 2 of which deals with electroplated coatings. 

It will be seen that the design of articles to be electroplated can have a considerable effect on the corrosion resistance of the electrodeposited coating. The chief effects are the result of variations in deposit thickness, but also important are features which can influence the adhesion, porosity and physical properties of the deposit. Good design will also avoid features of the plated article capable of trapping liquids or solid contaminants which might cause more rapid corrosion. 

Metallic articles arrive at the electroplating stage with a surface film of soils derived from earlier production processes. These are divided into three classes ... 

In many ways, chloroaluminate molten salts are ideal solvents for the electrodeposition of transition metal-aluminum alloys because they constitute a reservoir of reducible aluminum-containing species, they are excellent solvents for many transition metal ions, and they exhibit good intrinsic ionic conductivity. In fact, the first organic salt-based chloroaluminate melt, a mixture of aluminum chloride and 1-ethylpyridinium bromide (EtPyBr), was formulated as a solvent for electroplating aluminum [55, 56] and subsequently used as a bath to electroform aluminum waveguides [57], Since these early articles, numerous reports have been published that describe the electrodeposition of aluminum from this and related chloroaluminate systems for examples, see Liao et al. [58] and articles cited therein. 

A great number of measurements have been reported for articles electroplated with zinc. The various aims have been evaluation of the corrosion rate of zinc that had been plated in a number of commercial cyanide-free zinc baths," comparison of the corrosion rate of a composite material (zinc with codeposits of various oxides) and of pure zinc deposits," corrosion testing of various alloyed zinc platings (Zn-Ni, Zn-Co, Zn-Fe), with or without subsequent post-treatment. Most of the work in the last category was only recorded in internal reports. The published work consists of an examination of the corrosion behavior of a ctoomated Zn-Fe... 

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