Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 268 of 707

include acid, alkaline, and neutral formulations (see Table VII). In general, an alloy of 15% to 35% zinc with 65% to 85% tin is produced. This range of composition pro- duces optimum corrosion resistance, especially in sulfur dioxide atmospheres, along with excellent solderability (see Table VIII and Fig. 3). As with the other zinc alloys, a conversion coating is required in order to achieve the optimum corrosion protection. In any event, the tin-zinc deposit has good frictional properties, and excellent ductility for use on parts that may be formed after plating; however, being very soft, it is also susceptible to mechanical damage. Electrical contact resistance of the tin-zinc alloy is low, and it is somewhat superior to pure tin for resistance welding of coated mild steel sheet. Additionally, tin-zinc coatings do not under- go bimetallic corrosion, and can be used, for example, on steel fasteners for alu- minum alloy panels. Tin-zinc deposits have good solderability during long periods of storage. This is superior to pure tin. The alloy also does not grow "whiskers" or dendritic crys- tals for periods up to 600 days. Cost factors previously made tin-zinc the least like- ly of the alloy deposits to be considered. Recently, this has changed to where it is in the same cost range as alkaline zinc-nickel. HOW TO SELECT A FINISH Unfortunately, there is no single answer as to the best substitute for zinc or cad- mium. Each application must be examined to determine which parameters in the specification are most important. Compatibility of the process with existing equipment may also be a determining factor. For example, an existing acid chlo- ride zinc line may be readily converted to acid zinc-cobalt, if that finish will meet the requirements of the part to be plated; however, if the part is to be heat treat- ed after plating, zinc-cobalt is not indicated as the preferred deposit. An analysis must be made of cost versus quality, and a decision made based on a company's philosophy. Table IX presents data highlighting some of the areas of differences among the finishes described. CURRENT APPLICATIONS The United States automotive industry has led the way in the industrial use of zinc alloy plating processes. This mirrors past trends that were first seen in Japan and Europe. Many of the first acid baths have yielded to alkaline formulations, which give more uniform alloy deposition and thickness distribution. Some alloy zinc-plated parts processed include fuel rails and lines, injectors, climate con- trol devices, cooling system pumps, coils, and couplers. Some non-automotive uses are electric metering parts, power transmission units, maritime, military, aerospace, bearings, and many more. Testing programs are lengthy, due to long-lived finishes. Specification changes are slow, largely due to the enormous cost of changes in rewrites. Zinc alloys have improved corrosion characteristics as compared to zinc and cadmium electrodeposits and have earned a well-deserved reputation for quality and performance. 267

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