Guest Editorial -For Plateworld.com
Don Baudrand, Don Baudrand Consulting, e-mail:firstname.lastname@example.org
Some Thoughts about Chrome Plating
Bright chrome and hard chrome plating solutions have much in common and significant differences. Consider the coverage into low current density areas. Covering power is important in decorative chrome plating to maintain consistent color and a minimum of yellowing of bright nickel. Throwing power is more important when using hard chrome. Covering power (CP) is the ability of the chrome deposit to cover all the critical areas of low current density, that is, recessed areas farther from the anodes than the other areas. Throwing power (TP) is the ability of the solution to build significant thickness in low current density areas such as recessed areas.
Covering power is important in decorative plating to produce
a bright deposit that covers all critical areas, usually over bright nickel. To
improve covering power, add anodes, move anodes more directly in line with
recessed areas if possible. Small changes in the chemistry of the plating
solution can improve CP such as decreasing the chromic or adjusting the ration
of chrome to catalyst, or use a mixed catalyst by adding a small amount
fluoride, or use a proprietary catalyst following the instructions carefully.
Most proprietary chrome plating solutions produce better CP and TP than the
chromic acid/sulfuric plating formulas. In addition the surface finish has an
influence on both CP and TP smooth surfaces are better than rough. Cleaning and
preparation for plating including good cleaning and acid dips can influence both
CP and TP. If plating over bright nickel, impurities in the nickel solution and/or high brightener can lower CP.
Throwing Power (TP) is the ability of the chrome plating solution to deposit nearly uniform thickness over all surfaces of the cathode (work piece). It is rare that the thickness is uniform, but a significant thickness is expected in recessed areas. Uniform thickness is more important for hard chrome plating since the plating is used for its metallurgical properties such as hardness, wear resistance and corrosion protection. Smooth surfaces work better and produce significant thickness. Changes similar to plating conditions noted above for bright plating also apply to hard chrome plating.
If nickel is plated first, impurities or high brighteners in the nickel reduce TP. Increasing temperature of the chrome plating solution helps. Caution, changes in operating temperature can result in lowering the deposit hardness or increasing hardness depending on the temperature selected. Maximum hardness is often required for usage applications for hard chrome deposits. Adjusting the catalyst as for decorative plating improves TP. Lower chromic acid content favors TP. Do not go below 50% of the normal chromic acid content. High ratio or low catalyst decreases the coverage, and produce "rainbow" or brown stains on the uncovered (not plated) areas. This is corrected by adding catalyst in increments of 5% of the total catalyst preferred concentration.
Using a strike: The starting current/voltage can be raised for a short time causing the recessed areas to plate. This strike is effective for many configurations, since it can cover areas that would not be covered with chrome at the usual current density. Once covered plating will continue to build thickness in the strike areas because plating over deposited chome is more active than trying to cover areas not covered with chrome. Various metals accept chrome plating differently. The order of preference is: chrome, steel, brass, nickel, copper and aluminum
The Hull Cell is handy for looking at the covering or throwing power, and the effects of additions to correct the problems.
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