Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 491 of 707

meters must be controlled, namely, area, amperage, time, and anode efficiency. At 100% anode efficiency, by substituting the mass obtained from Faraday's law into the thickness formula, the thickness becomes T = eit 10/A d. (3) The area to be measured is determined by a flexible rubber gasket. This area can range from 0.13 to 0.32 cm in diameter. The gasket is an integral part of the deplating cell, that holds the solution during the test. The gasket must be flex- ible so that it will prevent leakage of the solution, yet sufficiently rigid for precise maintenance of the area.A constant pressure device is included to aid in con- trolling the gasket pressure. Since this measurement yields weight per area, accurate control of this diameter is essential. On most instruments, the current source and timer are included in a current supply package. This package provides a means for producing a specific constant amperage for each coating to be tested. When the coating is penetrated, there is a voltage change. The rate of change of this voltage is monitored and used to terminate the test. An electronic timer is used to record the time elapsed. A computer processes the time and amperage, then displays the thickness. Also included in the package are electronic controls to modify the current and termination sensitivity. These controls are provided to compensate for minute changes in anode efficiency and area. It is necessary to use a specific electrolyte for each combination of coating and substrate. The electrolyte must satisfy three conditions: 1. The solution must not chemically attack the coating. 2. Anodic dissolution of the coating should be at constant efficiency, ideally 100%. 3. The voltage change on penetration of the coating should be significant. This method is capable of consistently measuring the thickness of a variety of metallic coatings to ±10% of their true value. For certain coating and substrate com- binations, the accuracy can be higher. The most accurate measurements are in the range of 40 to 2,000 microinches; however, chromium can be measured in thicknesses as low as 3 microinches. The accuracy of measurement in a specific thickness range may be increased by calibrating the instrument with standards in that range. Coatings on wire are measured by means of an auxiliary cell. Tests are per- formed on sample lengths from 0.5 to 4.0 in. in length. One advantage of this method is the ability to measure combination coatings such as copper/nickel/chromium and copper/tin. The instrument manufacturer's instructions should be followed precisely for accurate results. Additional guide- lines for achieving accurate measurements are contained in the following ASTM methods: 1. ASTM B 504, standard method for coulometric thickness. 2. ASTM B 298, for silver coatings on copper wire. 3. ASTM B 355, for nickel coatings on copper wire. The coulometric instrument has found application in measuring other qual- ities of metallic coatings. ASTM B 764 describes a procedure for simultaneous thickness and electrochemical potential (STEP) determination for the layers of multilayer nickel deposit. 490

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