Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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electroplating solutions ZINCATE- OR STANNATE-FREE PLATING OF MAGNESIUM, ALUMINUM, AND TITANIUM BY JOHN W. BIBBER, SANCHEM, INC., CHICAGO, ILL. For more than 80 years, the plating industry has been dependent on the use of various zincate and/or stannate solutions to assist in the plating of metals such as titanium, aluminum, and magnesium. It is becoming ever more difficult to use these solutions due to restrictive regulations—plus, they are difficult to work with. This article presents an environmentally acceptable and easy-to-work-with alter- native process that does not require the metalizing of the metals to be plated or extensive cleaning and deoxidization procedures. The process presented in this article eliminates the use of "zincates" and/or "stannates" and also negates a large portion of the extensive cleaning and deox- idation that is often required in the plating of these metals. The examples shown in this article involve the use of electroless nickel, as this is one of the more com- monly plated materials on these metals and is often used as a base for the plat- ing of other metals. The lightness, strength, and high melting point of titanium, aluminum and various magnesium alloys make them quite desirable for use in the aerospace industry or many automotive applications. In many instances the metals need to be plated, but transition metals such as copper, chromium, nickel, etc., do not form good bonds with the light, non-transition elements. For several decades, the plating of these metals has been accomplished by the initial application of a thin film of zinc and/or tin from what are known as zin- cate or stannate solutions.1 Simple zincate or stannate solutions are nothing more than solutions of zinc oxide or tin oxide in strong sodium hydroxide or potassium hydroxide, but for best results cyanide is generally added to the solutions.2 Other variations to the baths include mixing zincate with stannate and chang- ing the pH and/or concentrations of the mixed solutions.3 In addition, copper is often plated out on these metals in order to smooth out rough areas or to improve the bonding characteristics of the metals being plated out. Due to cost considerations, zincate is by far the most widely used of these solutions.4 CLEANING AND DEOXIDATION With plating, more than just about any other type of metal processing, the most important aspect of the process is proper cleaning and deoxidation of the metal or metal alloy to be plated. Cleaning is the removal of surface oils and/or solid organic material, whereas deoxidation is the removal of unwanted inorganic compounds (oxides and hydroxides) that would interfere with the plating process. The exact cleaner selected will depend on the type of soil being removed and/or the metal or metal alloy being processed. In general, a strong alkaline cleaner will give the best results on titanium, alu- minum, or magnesium and their alloys. This is typically followed by a rinse in high-quality demineralized water—the rinse itself being where the actual clean- 162

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