Metal Finishing Guide Book

2013

Issue link: https://metalfinishing.epubxp.com/i/218436

Contents of this Issue

Navigation

Page 248 of 843

plating processes, procedures & solutions ZINCATE- OR STANNATE–FREE PLATING OF ALUMINUM AND ITS ALLOYS BY JOHN W. BIBBER, THE LABORATORY AND RESEARCH DIRECTOR AT SANCHEM, INC. CHICAGO, IL For more than 80 years now the most generally accepted process for the preparation of aluminum and it alloys for plating has involved the use of a "zincate" or "stannate" processing solution. Environmental issues relative to the use of these processes are becoming more and more of a problem. The process itself is long and involved and will differ from one alloy to another. This article presents a much more environmentally acceptable alternative that is considerably easier to work with, gives far more consistent results and is more cost effective. Aluminum is light, strong, environmentally acceptable and relatively inexpensive. As such it enjoys wide use in the electronics industry. It is, however, easily corroded, relatively soft and not easily welded or soldered. As a result, it is quite often plated. The process being presented in this article has now become part of "ASTM specification B253"– "Standard Guide for the Preparation of Aluminum Alloys for Electroplating" which also covers the electroless deposition of metals on aluminum and its alloys by the use of "Zincates". Although other methods of preparation are available[i], "zincate" and/or "stannate" have been the most widely used method for over 80 years now[ii]. "Zincates" basically consist of an alkaline solution of zinc as zinc hydroxide and a number of variations to this basic composition are commercially available. "Stannates" consist basically of an alkaline solution of tin as the hydroxide, and a number of variations to this basic composition are commercially available. After an extensive series of cleaning steps, which may or may not require more than one application of a given "zincate" or "stannate" composition, the surface of the aluminum or aluminum alloy ends up being coated with a very thin film of zinc or tin metal which will be dispersed over the surface of the metal in an irregular pattern depending upon the nature of the alloy being processed and / or the characteristics of the particular "zincate" and / or "stannate" solution being used. This then removes or displaces a large portion of the aluminum oxides on the metal and sets up electrochemical cells whereby the aluminum or aluminum alloy will more easily accept the metal being plated on it while at the same time displacing the zinc and / or tin out into the plating solution along with any other metals that the given "zincate" or "stannate" solution may have contained and were deposited on the aluminum or aluminum alloy. In many cases, this will act to shorten the bath life of your plating solution. In particular, "electroless" deposition solutions such as "electroless" nickel. As is the case of metals other, than, aluminum, copper is frequently plated directly onto a given "zincate" or "stannate" prepared surface to facilitate the plating of nickel or chromium as the type of "zincate" or "stannate" composition used will influence the ability of the aluminum or aluminum alloy to accept subsequent metal deposits[iii],[iv] In addition the use of "zincates" and/or "stannates" will require you to use an alkaline cyanide copper plating bath rather than the more environmentally acceptable acid copper plating bath. The "zincate"and/or "stannate" based processes are totally dependent upon generating and continually maintaining very reactive surface conditions. This may be possible under laboratory conditions, but very difficult (if not impossible) to maintain in any given plating shop depending upon the aluminum alloy 241

Articles in this issue

view archives of Metal Finishing Guide Book - 2013