Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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control, analysis, and testing EXAMINING THE HULL CELL BY JOE FOX, FINISHING TEST SUPPLY, INC., PAINESVILLE, OHIO The hull cell was first outlined by R.O. Hull in a 1939 paper titled "Current den- sity characteristics, their determination, and application" (Proceedings of the American Electroplaters' Society). The biggest advantage of the hull cell is its abil- ity to allow a skilled operator to check the deposit properties at varying current densities, and all on one test panel. The hull cell can examine the effects of the following: • Operational variables, such as pH current density, temperature, and agitation • Organics and metallic contamination • Base salt constituents and addition agents • Brightness range of plating deposit • Covering power Unlike quantitative analysis, the hull cell test gives the analyst a snapshot or picture of the solution. In order to interpret the snapshot, other pictures are required for comparison. The aim of this article is to provide standards and training to educate the ana- lyst. More complete information will be available in Part III of this series, slated for the May issue of Metal Finishing. (Part II will appear in the April issue.) DESIGN The hull cell is a miniature plating tank that is trapezoidal in its configuration. The unique shape and dimensions of the hull cell are depicted in Figure 1 for var- ious sizes. Different sizes exist mainly for the sake of convenience. Original cells were based on 1,000 ml, but smaller cells were later developed. The 267-ml size was chosen because a 2-g addition to the hull cell equals 1 oz/gal addition to the plating bath. The small cells are often criticized for being too small because rapid changes in composition and solution temperature can occur. For example, the 267-ml hull cell can have a 10% change in addition agent concentration in just one 5-minute panel. This is dependent on the initial concentration and the consumption rate of the addition agent system. The angle of inclination of the cathode, shape, and spacing are critical to pro- vide the current distribution that previous research has determined to be advan- tageous. The depth of the solution can be altered, but the current must also be changed to allow for area and subsequent current changes. Figure 1 shows a typ- ical hull cell and temperature controller. HULL CELL COMPOSITION The hull cell should be constructed from a non-conductive material that is chemically inert from the solution being tested. Acrylic and polypropylene are 461

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