Adhesion of Nickel to Nickel Adhesion of the second layer of nickel will be dependent on having thorough activation of the first nickel layer. Nickel is readily passivated in reverse current alkalies, making the ac- tivation step more difficult. If possible therefore, strip the chromium layer in an acid solution rather than in a reverse- current cleaner. Parts stripped in strong (33-50 pct by volume) hydrochloric acid are frequently sufficiently active to go directly to the nickel tank after rinsing. If the chromium must be stripped in a reverse-current cleaner, use a highly chelated cleaner if possible. This will give faster stripping and hold down passivation. Follow this, if you can, with a direct-current cleaner and a good acid activation. If current is available in the acid tank, use 10-15 pct by volume sulfuric acid, or a suitable proprietary acid, with 50-100 asf direct current. When no current is available, the best procedure usually is to use a proprietary powdered acid with fluoride and other ac- tivators included. Proprietary activators for use with dilute sulfuric acid (5-10 pct) are also available. Remember that once activated, nickel can be readily repas- sivated, even by the dissolved oxygen in rinse water. Therefore after activation, do not delay-use the fastest possible rinsing and transfer times. If all else fails, you might consider the use of a Woods nickel strike. This is made up of approximately 6 oz/gal of nickel as nickel chloride in 10 pct by volume hydrochloric acid, used at room temperature. After a 30-second to one minute strike, the parts are rinsed as rapidly as possible and transferred to the nickel tank. Provided that not too high a volume of work is be- ing processed this way, so that the dragged in strike solution would upset the balance of the main plating tank, the rinse can be eliminated and the work transferred directly from the strike to the plating tank, thus avoiding any possibility of repassiva- tion during the rinsing cycle.
Posted on: Fri, 14 Mar 2014 10:07:42 +0000
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