Electroplating fasteners

Cadmium. In 1989, U.S. consumption of cadmium for coatings was 1474 t (7), compared to 1552 t in 1970, 2089 t in 1979, and 1230 t in 1985. Cadmium plating amounts to about 15% of total cadmium production (see Cadmiumand cadmium alloys). Of the cadmium being plated in 1989, 30% was for automotive parts, over 22% for electronics, and 18% for industrial fasteners. Because of cadmium s high and weU-pubHcized toxicity and very tight waste restrictions, there are considerable efforts to develop alternative materials, and the quantities of cadmium used in electroplating ate expected to decrease. The price of cadmium anodes in early 1993 was about l/kg. 

Plate thickness is an important factor in electroplating, in terms of both performance and economics. Corrosion resistance, porosity, wear, appearance, and several other properties are proportional to plate thickness. Minimum plate thicknesses are, or should be, specified as should the location, or check-point, where the thickness is to be measured. In some appHcations, such as threaded fasteners, maximum thicknesses should be specified. Root diameters of finer machine threads can be adversely affected by as Htde as 10 p.m of plating. 

If such fasteners are stove-enamelled (which is normally not done), the trapped hydrogen is removed automatically while being stoved. Since the fasteners are used only for the assembly of switchgear or busbar systems, they are used at room temperature only. Therefore, if they are electroplated, they must be tampered, which is time consuming and adds to the cost of production. Moreover this has no technical advantage. The HT hardware must therefore be used as they are. When it is absolutely necessary to electroplate them, tampering will be essential. 

In Western Europe the largest user is the vehicle construction industry where ABS has been used for fascia panels, door covers, door handles, radiator grilles, ventilation system components, heater housings, seat belt fastenings, console panels, loudspeaker housings, interior trim and other uses. For some years there was extensive use of electroplated ABS. Whilst this continues to be used for nameplates, reflectors and other parts where a bright reflecting surface is a requirement, it has tended to fall out of favour simply for decoration. 

Cadmium and zinc electroplating provides galvanic corrosion protection when coated on steel. Deposit thickness can vary between 5 and 25 p,m (0.2 and 1 mil), and typical applications for both coatings are found in Table 10. Cadmium is preferred for the protection of steel in marine environments, whereas zinc is preferred in industrial environments. Cadmium is also preferred for fastening hardware and connectors because its coefficient of friction is less than zinc. Cadmium is toxic and should not be used in parts that will have contact with food. Precautions for minimizing hydrogen embrittlement should be taken because cadmium plating is more susceptible to such embrittlement than any other plated metal. 

With the plating of fasteners, some special considerations apply, particularly in respect to threads (Ref 8). As might be expected, electroplated metals build up faster on apexes of the threads, and coverage can be minimal at the bottom of the grooves. ANSI Specification B 1.1 states that compared to flat surfaces, plating thickness builds up six times faster on the major diameter than the minor diameter and that this results in a fourfold buildup on the pitch diameter, as illustrated in Fig. S. This is known as the Rule of Four and Six. ... 

Variety of coatings available for metal fasteners to improve corrosion resistance, commonly zinc (electroplated and hot-dip), cadmium, chromate, phosphate and bluing. 

Hydrogen embrittlement in electroplated steel fasteners can be problematic and accelerates failure. Stress concentrations in fastener and joint designs should be minimized by incorporating radii, gradual section changes and recesses. 

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