Racks and Fixtures

The following recommendations are offered regarding the design and use of plating racks:

• Racks should be designed to hold workpieces in a favorable position for plating uniformly on significant surfaces and to facilitate racking and unracking.

• Workpieces with protruding sections should be racked so that parts shield each other. If this is not possible, a current thief should be used to reduce current density at the protruding points.

• Electrical contact with the part should be made on an insignificant surface.

• The contact or rack tip should be rigid enough to hold workpieces securely and maintain positive contact. When the work is heavy enough to ensure positive contact, a hook often suffices.

• To minimize solution losses due to dragout, the work should be hung as nearly vertical as possible, with the lower edge of the work tilted from the horizontal to permit runoff at a corner rather than a whole edge. When recessed areas cannot be racked to allow proper runoff, provision should be made for drain holes or perhaps tilting of the rack when it is being withdrawn from the solution.

Although the design of racks and the methods of racking vary greatly, two basic types of racks are generally used. The first type consists of a single high-conductivity bar on which suitable supports have been mounted for holding the work to be plated; this rack is the cathode side of the plating circuit. The second type consists of two elements, the cathode and the anode; the work is held by the cathode and the cathode is attached to, but insulated from, the anode. Both types of racks are illustrated in Fig. 2. To prevent deposition of chromium or attack by the plating solution on parts of the rack that are immersed in the solution, these parts are covered with nonconducting material such as water-resistant tape, special insulating lacquer, or plastisol coatings.

Fig. 2 Racks used in hard chromium plating Surface Preparation

All soils and passive films must be removed from surfaces of ferrous and nonferrous metals before they are hard chromium plated. In addition to cleaning, certain surface-activating processes are often important in preparing the base metal for hard chromium plating. The processes include etching of steel, preplate machining, and nonferrous metals preparation.

Etching of steel before plating is needed to ensure adherence of the chromium deposit. Anodic etching is preferred for this purpose. Slight etching by acid immersion may be used for highly finished surfaces, but with possible sacrifice of maximum adherence.

Steel can be etched anodically in the chromium plating solution at its operating temperature for plating. A reversing switch is used so that the steel to be plated can serve as the anode for 10 s to 1 min (usually 30 s to 1 min) at a current density of about 15 to 45 A/dm2 (1 to 3 A/in2). Tank voltage should ordinarily be 4 to 6 V. Because mixed-catalyst solutions chemically attack the steel, causing etching of the surface, shorter electrochemical etching time frequently is required than is the case with conventional or fluoride-free chemistries. This process has the disadvantage of causing the solution to become contaminated with iron from the work and with copper from the conductors.

As an alternative, steel may be anodically etched in a separate chromic acid solution without sulfate additions and containing 120 to 450 g/L (16 to 60 oz/gal) of chromic acid. Solution temperature may range from room temperature to that of the chromic acid plating solution, or even higher, provided that current density and time of treatment are adjusted to suit the type of work being processed.

A sulfuric acid solution (specific gravity 1.53 to 1.71) may be used for anodic etching, provided that the solution temperature is held below 30 °C (86 °F), and preferably below 25 °C (77 °F). The time of treatment may vary from 30 to 60 s and the current density may vary from about 15 to 45 A/dm2 (1 to 3 A/in2) at tank voltages ordinarily between 4 and 6 V. A lead-lined tank with lead cathodes should be used. With the use of a sulfuric acid solution, however, two difficulties may be encountered. First, if the rinsing following etching is incomplete, the drag-in of sulfuric acid throws the chromium plating solution out of balance with respect to the ratio of chromic acid to sulfate. Second, in handling parts that are difficult to manipulate, there is danger that surfaces exposed to air more than a very short time will rust and that finely finished surfaces will be overetched.

For high-carbon steel, a sulfuric acid solution of 250 to 1000 g/L (33 to 133 oz/gal), used at a temperature of not more than 30 °C (86 °F) and preferably below 25 °C (77 °F), is effective for anodic etching. The addition of 125 g/L (16.6 oz/gal) of sodium sulfate, based on the anhydrous salt, is of benefit for many grades of steel. Anodic treatment in this solution usually does not exceed 1 min at a current density of about 15 A/dm2 (1 A/in2) (range of 15 to 45 A/dm2, or 1 to 3 A/in2). High acid content, high current density, and low temperature (within the ranges specified) minimize the attack on the base metal and produce a smoother surface. This sulfuric acid solution is stable and not appreciably affected by iron buildup.

Preplate Machining. Metal debris on the surface should be removed before etching (an activation procedure). The use of abrasive-coated papers is common, as is the use of successively finer grit stones in honing and grinding. To prepare a sound surface in superfinishing, 600-grit stones may be used. Electropolishing is sometimes used to remove highly stressed metal and metal debris from the surface of cold-worked steel. This process improves bond strength and corrosion resistance of electroplated coatings. It accomplishes this function without formation of smut, which may result from anodic etching. This treatment is not recommended for parts that are subjected to critical fatigue stresses and that are expensive to manufacture.

Preparation of Nonferrous Metals. Aluminum, in common with certain other metals, quickly develops a natural, passive oxide film after exposure to preplating cleaning cycles. This film must be removed before aluminum is plated. The most widely used method of preparing aluminum for plating involves a zincating treatment, which may be followed by a thin 5 ^m (0.2 mil) copper electrodeposit. However, it is possible to plate chromium directly over the zincate.

Aluminum parts used in hydraulic systems require a nickel undercoat before being plated, to provide corrosion protection to all plated surfaces that are not completely and constantly immersed in hydraulic fluid or similarly protective fluids. A minimum thickness of 10 to 15 ^m (0.4 to 0.6 mil) of nickel is usually specified. This undercoat may also be required for steel parts in similar applications.

Titanium and titanium alloys, as well as magnesium, also form a tight, stable oxide coating and are therefore difficult to plate. These metals can be pretreated with an electroless nickel plate or a coating deposited from a high-chloride nickel strike solution.

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