The purpose of this paper is to take a comprehensive look at the advantages of close anode to cathode spacing when chromium plating aircraft components. This method of chromium plating, called at various times, reversible rack plating, innovative plating, and conforming anode plating, has been poorly explained and therefore, often misunderstood. As a result, very few aircraft rework or manufacturing facilities are taking advantage of this simple, efficient process.
The Naval Air Rework Facilities at Cherry Point, North Carolina and Pensacola, Florida converted several hard chromium plating tanks to the close anode to cathode spacing arrangement approximately three years ago. Since that time, both facilities have gathered information comparing this system to the conventional, tank anode boxing method, previously used. Significant production advantages were realized with the close anode to cathode spacing.
Close anode to cathode spacing, as it pertains to this paper, shall be defined as a system for hard chromium plating that reduces the space between the anode and cathode to a distance of approximately 0.5 to 1.0 inch (1.3-2.5 cm). Conforming lead mat anodes (see Figure 1) are fabricated for each part. The voltage is measured at the part and is held constant at approximately 4.5 volts. The close spacing between the anode and cathode reduces resistance, increasing the current density. A current density of 430-750A/ft2 (43-75 A/dm2) is typical at 4.5 volts when using this system. A conventional 30 oz/gal (225 g/l) chromic acid, 0.30 oz/gal (22.5 g/l) sulfuric acid plating solution is utilized, operated at a temperature of 140°F (50°C). Parts may be fixtured on a reversible rack (see Figure 2) or plated using in-tank conforming anodes.