Variations in Plate Thickness

Variations in the thickness of hard chromium plate depend primarily on the potential field distribution. Potential field is controlled by the placement of anodes, shields, thieves, and other parts, as well as the relative position of the sides and surface of the tank. Variations in plate thickness also depend on surface preparation, control of solution conditions, and uniformity of the power source.

Methods of Measuring Plate Thickness. Several methods and types of instruments are available for determining the thickness of plate. These include electrolytic stripping, microscopic measurements of cross sections, torsion dynamometer measurements made with magnets of various strengths, measurement by eddy current instruments, and accurate measurement of the dimensions of the part before and after plating to determine thickness by difference.

Electrolytic stripping and microscopic measurements of cross sections are destructive methods that are most frequently used for purposes of verification, calibration, and sampling of production runs. When calibrating instruments with prototype plated parts, using microscopic measurements of cross sections as umpire checks, several calibration reference curves may be required, depending on the parts being plated.

Measurements by properly calibrated eddy current or torsion dynamometer instruments are affected by the surface finish of the deposit, width and thickness of the piece, surface contour, and composition of the base material. With a properly calibrated instrument, thickness measurements are usually within 10% of the actual thickness. Individual thickness measurements should not be used as the basis for acceptance or rejection; however, an average of several determinations from a well-calibrated instrument is an acceptable measure of the mean thickness from a controlled process.

The normal variation in plate thickness that can be expected when plating the outside diameter of cylinders, rods, or round parts racked as cylinders is ±0.2pm/pm (±0.2 mil/mil) of plate intended. This has been determined over a period of several years by average quality level thickness measurements on piston rings racked as cylinders.

This normal variation of 20% was confirmed in an actual production situation. In plating identical parts to a consistent thickness requirement, sample checks from 74 loads (110,000 parts) representing 27 days of operation were made to determine the plating tolerances that could be expected. The plating cycle was set to provide a plate thickness of 200 to 230 pm (8 to 9 mils) to meet a final requirement for a minimum plate thickness of 150 pm (6 mils) after light stock removal during the subsequent finishing operation. Results of this analysis are shown in Fig. 3.

Fig. 3 Variations in hard chromium plate thickness for 74 loads, representing 110,000 parts of the same design plated over a period of 27 days of operation. Target thickness was 200 to 230 pm (8 to 9 mils) of hard chromium. Average thickness for the 74 loads was 215 pm (8.4 mils).

The throwing power of chromium-plating solutions is related to the ratio of chromic acid concentration to the catalyst concentration. Higher ratios give better throwing power at a given temperature and current density. This is evidenced by the fact that when a very low current density is present on certain areas of irregularly shaped parts, the cathode efficiency at that low current density is less for a solution high in sulfate than for a solution with lower sulfate content. Therefore, less metal is deposited on the areas of low current density from a solution of high catalyst content.

The current density at which no metal deposits is greater for high catalyst solutions than for lower catalyst solutions. Also, metal deposits from a solution of low catalyst concentration at a current density that would be too low for depositing from a solution with high catalyst concentration. Thus, the following factors must be considered to ensure successful plating of complex shapes: chemical balance, operating variables, type of anode, and design of fixtures or racks.

Chromium plating requires far more attention to the variables that affect current distribution than cadmium, zinc, copper, or nickel plating. It is theoretically impossible to obtain the same current density at an inside corner as on the flat adjacent to it. An outside corner without shielding or thieving always has the highest current density and hence the greatest plate thickness. Conforming anodes, shields, and thieves may be used to minimize thickness variation, but except on the simplest shapes, they do not eliminate it.

Some metal is deposited at low current densities in most other plating solutions, but in chromic acid solutions there is a minimum current density for a given solution at a given temperature below which no metal is deposited. If an area of an internal or irregular shape receives less than this minimum current density, no deposition of metal occurs in this area. This explains why it is so difficult to chromium plate recesses and internal shapes without special anodes. Special hardware, in the form of thieves or shields, is required for lowering the current density on areas such as edges to prevent excessive buildup of deposit.

In most electroplating solutions, the primary current distribution on an irregular object can be improved by increasing the tank anode-to-cathode distance. However, beyond a minimum distance, which depends on the shape of the part, no further improvement can be attained.

Because of the low throwing power of hard chromium plating solutions, an increase in the anode-to-cathode distance does not result in even plating of sharp reentrant surfaces such as those formed by internal angles. For plating parts containing shapes of this type, conforming anodes and/or current shields must be used.

Figure 4 illustrates the relation between thickness of deposit and distance of the anode from the part being plated. In this instance, an alternative to an increase in the anode distance is the use of an anode contoured to the curvature of the part.

Figure 4 illustrates the relation between thickness of deposit and distance of the anode from the part being plated. In this instance, an alternative to an increase in the anode distance is the use of an anode contoured to the curvature of the part.

Fig. 4 Variation in thickness of chromium plate on feedworm as a function of the distance of the anode from the part. Values of x are about 25 mm (1 in.) or more.

Special Anodes. When the part contains sharp, narrow recesses, such as grooves, a reduction of the anode distance may help to increase the thickness of the deposit at the bottom of the grooves. However, some parts with sharp-cornered grooves, bosses, and undercuts cannot be uniformly covered even when contoured anodes are used. Examples of parts in this category and the areas of heavy deposits are illustrated in Fig. 5.

Fig. 5 Parts difficult to plate uniformly with hard chromium, even with the use of specially contoured anodes. Variations in plate thickness shown are approximately to scale.

Anodes used for plating recesses can be directly connected to the power supply, or they can be bipolar in nature. The bipolar anode has no direct electrical connection and takes advantage of the fact that current follows the path of least resistance. Bipolar anodes are an interesting curiosity that may have application in rare instances; however, direct connection of the anode to the positive direct current through a rheostat and ammeter, if required, is far more controllable.

The deposit on internal shapes can also be affected by the evolution of gas that occurs during plating. Gas can cause streaked deposits or produce a taper in a long bore. To minimize this effect, the parts should be positioned in a manner that permits the gas to move rapidly away from the part.

Because of fabrication problems encountered with lead alloys, complex-shaped anodes are made of steel, then coated with lead to produce the effect of solid lead anodes. These composite anodes are more economical and lighter in weight. However, the base metal can be destroyed if there are pores or throughholes in the lead alloy coatings. Brass or copper should never be used on the anode side, as they dissolve rapidly and seriously contaminate the solution. Low-carbon steel may be used alone for short runs, and lead-coated steel may be used for longer service.

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