Thursday, May 1, 2008

Preparation of Basis Metal (Nickel Plating)

In preparation for applying a plated finish to an article, polishing is often used to smooth the surface before more refined finishing procedures. Subsequent buffing is usually used on such metals as copper, brass, zinc die castings, and article aluminum alloys before plating. Because of high cost, steel is seldom buffed before nickel plating. Where a buffed surface is desired, a common practice has been to apply a copper plate before buffing. More recently, the use of high leveling bright nickel baths, or a combination leveling semi bright bath and a bright nickel bath, or a high leveling, bright, ductile acid copper plating bath eliminates or reduces appreciably the amount of buffing required to produce a lustrous finish.

Polishing and Buffing
The nature of surface condition of the metal can have a profound effect on the performance of the nickel plate. Proper preparation of articles to plated with nickel start with the manufacture of the basis metal. Many investigators have shown that rolling scams, slag inclusions, blow holes, sand inclusions, and the like effect the protective value of nickel plate. Moreover the manner in which the metal is processed, either before or after being form into articles of manufacture, has been proved to affect the porosity of the nickel deposit. In general, the result show that rough grinding was detrimental to the protective value of nickel plate, and in addition, that smoothing out and rounding of the tops of scratch, as for example, with a well lubricated, final polishing wheel, was important from the stand point of enhancing the protective value of nickel. A final sisal wheel accomplishes the same result, especially of eliminating slivers.

Cleaning and Pickling
The purpose of cleaning before plating is to remove all the interfering substance from the surface to be plated. Adhesion, smoothness and corrosion resistance of nickel plate depend on a proper cleaning cycle. Diffusion of one metal into other (alloying) is not necessary for adhesion and may be detrimental when the alloy layer or any part of it is weaker or more brittle than the individual metals. Harmful substances which cause poor adhesion are oxides, graphite, oils, grease and immersion coating of copper or lead from contaminated acid dips or from the presence of chromates, even in small concentrations, in the rinse water or in the plating baths. If cathode cleaners are used that are contaminated with dissolved zinc, copper, or lead and thus plate out a non adherent film, or when cathodic cleaners are used in the cleaning or activating sequence for longer than 1 minute without an anodic activation, or when bipolar or intermediate electrode effects occur in the plating steps, poor adhesion will result.